Bilge Keel - Rolling Chock installation

[Bmarler]

We had chocks on our troller in SEAK, and they helped some. We had paravanes too as the rigging was already in place for them. The chocks reduced the number of times we dropped the fish, but we still needed them for fishing, especially on the outside. At trolling speed we’d roll a lot going beam to when making the turn to go back through the hole we were fishing. Chocks alone weren’t enough in those instances.
At speed going from one fishing ground to the next the chocks were at their best, allowing us to run hard without having any gear in the water.

 

[Pete Meisinger]

They will most certainly do the job, in fact there may be overkill. However I am very concerned about the lifting straps.Even if you make chocks or wedges for above and below the keels they will have an almost unbelievable amount of pressure on them at lifts. The straps will want to tear them from the boat and if not that will want to crush them into the hull.

Better make good solid plans for the lift and talk to the marine lift operator in advance, they have seen it all. They need to be on board with the plans.

pete

 

[mvweebles]

They will most certainly do the job, in fact there may be overkill. However I am very concerned about the lifting straps.Even if you make chocks or wedges for above and below the keels they will have an almost unbelievable amount of pressure on them at lifts. The straps will want to tear them from the boat and if not that will want to crush them into the hull.

Better make good solid plans for the lift and talk to the marine lift operator in advance, they have seen it all. They need to be on board with the plans.

pete

I understand your thoughts, but all I can tell you is I know of 6 or 7 fiberglass pleasure boats that have had this done with zero issue whatsoever. Again, the yard Kevin is using has been around over 40-years and has a decent reputation. They probably do more fiberglass and paint work in a week than most US yards do in a year. And the same company owns Penninsular Shipyard, one of the few in Mexico that is qualified to work on superyachts (there is a 300-footer there now).

Kevin's boat is in good hands.

Peter

 

[Comodave]

They will most certainly do the job, in fact there may be overkill. However I am very concerned about the lifting straps.Even if you make chocks or wedges for above and below the keels they will have an almost unbelievable amount of pressure on them at lifts. The straps will want to tear them from the boat and if not that will want to crush them into the hull.

Better make good solid plans for the lift and talk to the marine lift operator in advance, they have seen it all. They need to be on board with the plans.

pete

If they lay it up with some structural glass and not just mat it should be ok. I would assume that the yard will know how to glass it.

 

[DDW]

We had chocks on our troller in SEAK, and they helped some. We had paravanes too as the rigging was already in place for them. The chocks reduced the number of times we dropped the fish, but we still needed them for fishing, especially on the outside.

There are chocks and there are chocks. Many I have seen on fishing boats in SEAK are modestly sized in length and draft (or span, or whatever you call it). The ones being put on by the guys in Coombs BC are at least 2/3 the LWL and about 1/4 to 1/3 the draft of the vessel. I think here, size does matter. That said I've no doubt that properly sized and rigged fish do more.

However I am very concerned about the lifting straps.

Put this out of your mind. Again the guys in Coombs have done over 300 of these, mostly on fishing boats but a lot on recreational. No one pays any attention to them at all when hauling with straps, it is simply not a concern.

 

[Mambo42]

Nearly everyone who installs them makes the same statement that they make a “noticeable difference”.

Unfortunately no one seems to have taken roll acceleration and magnitude measurements before and after, in order to quantify that statement.

Like I said, I just had them removed since they really did not do anything for me. Our boat was rocking and rolling violently in the marina or on anchor, it was absolutely horrible.
That is why I changed to electric stabilizers, which also work on anchor and in the marina/port.

Am curious how ksanders will experience his chocks.

 

[dannc]

In regards to the effectiveness of the rolling chocks, I think Kasten has the best answer, it is a range.

I posted it before, but here is the link, Roll Attenuation and Bilge Keels.

Marine Technology is the magazine of the SNAME(Society of Naval Architects and Marine Engineers) so the numbers quoted should be valid.

* Fixed "Bilge" Keels: Long, low aspect ratio bilge keels, per research published in Marine Technology, have been observed to offer possible roll reductions on the order of 35% to 55%. Vessel speed is not important to roll damping. There is some added frictional resistance due to increased wetted surface area. If proportioned correctly, bilge keels offer enhanced directional stability. Very common as a retro-fit. Relatively inexpensive. Relatively simple to build.

Measuring Roll Behavior

In terms of roll attenuation, there are of course many variables. What works well on one boat, may not be as effective on another boat. For example, paravane size relative to boat size / displacement / righting energy will definitely affect the results.

The percentages quoted above relate in many cases to roll amplitude, which is only one component of rolling behavior... One can isolate several components, as follows:

* Amplitude (measured in degrees)
* Period (measured in seconds)
* Acceleration / deceleration (a result of the above, measured in feet or meters per second squared)
* Rate of Decay (number of cycles to rest or to some other benchmark)
​

Among the behavior patterns directly observable from the numerous data sources are the following...

The wildest rolling is referred to as synchronous rolling, i.e. rolling in beam seas when the wave period is close to the natural roll period of the boat in question. For example, when a given boat rolls to some extent in harbor, say on receiving the wake of a passing boat, it might not do so given a slightly different wake or wave pattern. Another boat that did not roll so much at a given wake or wave pattern may roll wildly with a different wave pattern or period.

The higher percentages for roll attenuation quoted above for any given roll attenuation method (paravanes, keels, etc.) are from measurements of the attenuation of synchronous rolling, and appear to have their effect due to putting the boat out of sync with the wave pattern. The lower percentages in the range of effectiveness quoted appear to be an average of the overall effectiveness. In most of the published research, the majority of measurements were of amplitude. The next most common data quoted were measurements of acceleration.

Later,
Dan

 

[ksanders]

In regards to the effectiveness of the rolling chocks, I think Kasten has the best answer, it is a range.

I posted it before, but here is the link, Roll Attenuation and Bilge Keels.

Marine Technology is the magazine of the SNAME(Society of Naval Architects and Marine Engineers) so the numbers quoted should be valid.


Later,
Dan

Dan, what you posted above, synchronous rolling is the main problem with my boat, and is noted by almost all the owners of this hull.

Here are the symptoms...

The boat will start rolling, and each passing wave adds to the rolling until the boat is rolling so much that move about is difficult, and the counters are cleared.

This happens in relatively small seas that you would never expect such a violent roll response.

This roll characteristic dramatically affects my ability to hold course, and makes it necessary to change course for comfort of the crew.

I have measured the natural roll time of my hull to be between 3.6 and 3.9 seconds through testing at the dock.

The goal of my rolling chock addition is to interrupt that synchronous rolling action.

Others with this same hull have reported very positive results. I do not know if the bilge keels change the roll period, or reduce the acceleration, or decrease the number of rolls. I just know that others have reported very positive results.

This and the huge popularity of the "hard chine" modification on the Bayliner 4788 hull make for a very optimistic outlook for my boat.

 

[DDW]

I think Kasten is kind of guessing, as we all are. When the Miklós of Quebec tried to measure this on sisterships with different roll attenuation systems, they had lots of problems duplicating conditions. Even the installation itself affects results: the paravane system was found to attenuate roll in part because of the increase in GMZ and roll inertia even before the fish were launched.

Unfortunately "rolling chocks" is widely applied to little tiny things that look like short spray rails to really huge things like mine.

In my own experience, there is no way the rolling chocks have reduced the amplitude by 55 or even 35%, despite being quite large. The do seem to have reduced the acceleration and jerk (1st derivative of acceleration) and definitely increased dampinging - even at rest. I think they have reduced the synchronous roll amplitude, but this is so condition dependent it is hard to be sure.

 

[ksanders]

In a week or so I will have a very subjective opinion based on 12 years and thousands of sea miles in my boat.

I will post my honest opinion as to the results good or bad.

 

[dannc]

I think Kasten is kind of guessing, as we all are. When the Miklós of Quebec tried to measure this on sisterships with different roll attenuation systems, they had lots of problems duplicating conditions. Even the installation itself affects results: the paravane system was found to attenuate roll in part because of the increase in GMZ and roll inertia even before the fish were launched.

Unfortunately "rolling chocks" is widely applied to little tiny things that look like short spray rails to really huge things like mine.

In my own experience, there is no way the rolling chocks have reduced the amplitude by 55 or even 35%, despite being quite large. The do seem to have reduced the acceleration and jerk (1st derivative of acceleration) and definitely increased dampinging - even at rest. I think they have reduced the synchronous roll amplitude, but this is so condition dependent it is hard to be sure.

I don't think Kasten is guessing, I think he is quoting research, which no matter how good, is never going to be able to duplicate every condition, on every boat, with every chock design. Which is another way of saying that research is guessing. :lol:

One of my concern with rolling chocks, and especially twin bilge keels, is that it would seem best to have naval architect look at the whole hull to figure out the right size, shape, and placement. Course, that would cost a bunch of money, and for rolling chocks, maybe not worth the cost. Just try it and go.

Later,
Dan

 

[DDW]

There isn't a naval architect in the world who has enough experience - or practically any experience - with rolling chocks on recreational vessels to specify an optimum size and location. Roy Brown has put more of them on than anyone else. It really isn't that critical for what they do, and practical installation issues are more important. I have seen at least one SNAME paper, it deals mostly with relatively small chocks on large ships.

 

[Marco Flamingo]

the paravane system was found to attenuate roll in part because of the increase in GMZ and roll inertia even before the fish were launched

I've wondered about this for awhile. One of the raps against anti-roll tanks is that it places weight up high to reduce the roll. But paravanes do the same thing, placing several hundred pounds of gear 10,15, or more feet above the center of gravity. It could be that just the overhead gear reduces roll by 15% and dragging the vanes another 15%. Of course, the weight of the paravane system blocks, wire, aluminum arms, hinges, etc. is pricey per pound if a lead cannon ball on top of a 12' pole would do the same thing. And if the paravanes,once deployed, adds another 15% in roll reduction, that is good because it is also reducing speed by 15% and probably increasing fuel consumption by about the same.

All of this discussion tends to be speculative because of the lack of measurement and also the lack of definitions. What is a 30% reduction in roll? A 10 degree roll is reduced to a 7 degree roll when the system is in use? A 3.6 roll period is increased to a 3.96 roll period? A blend of the two? As some have noted, confirmation bias probably comes into play. The perceived roll reduction increases as the weight of the captain's wallet decreases.

And, were there both a universal defined metric and a way to accurately measure roll reduction, what would be a reasonable "price per percentage"? $1K gets you a 15% roll reduction, $10K gets you 30% reduction and $100K gets you 60% reduction, which one is the bargain? Or necessary? Or desired? How much should one pay for their boat to not even feel like a boat?

I have been meaning to contact the app designers for my phone inclinometer and discuss the posibility of a record feature. Sitting at my helm, the app will indicate realtime motion in 10ths of a degree (the amount of roll and pitch on a certain heading). But there is no record feature (preferably with graph and averaging data). The inclinometer app is basically for static measurements (e.g., Is this picture on the wall crooked?)

With stabilization systems that are "deployable," it would be fairly simple to quantify the system's effect. Take a heading with beam seas, record the roll and pitch, then drop the paravanes, turn on the gyro, fill the anti-roll tank, whatever, and actually measure the effect. Those who market and install stabilization systems might not appreciate the accuracy. Also, this can't really be done with rolling chocks, which would have to rely on dockside before-and-after roll period and abatement measurements. But that is better than nothing. At least one should be able to verify that there is some difference. Then, one gets to make up a percentage, probably based in part on the amount of time and money spent.

 

[Steel N Time]

So….. the question about lifting straps and breaking your new addition has what I think are some false beliefs that it will be fine! A boat is heavy and those straps pull inward and up and will most definitely break those no matter how hefty you make them. 12” with that weight in a small area will tweek and break Coosa and glass.

We have those on our 53’ Hatteras only they were solid mahogany, stainless steel bolted about every 8”, and only protruded about 6” not 12…… When we bought it the previous owner was real clear that boat yards will tell you it will be fine, then when they break they will blame your boat construction. He made wood blocks covered with carpet and tied together with web strapping, so the weight would be on the wood blocks above and below the strakes….

Last summer the bot yard wouldn’t listen to us and lifted without blocks, they ended up bending the stainless bolts and damaging a section of our strake that they had to repair after my insistence. 12” is huge leverage, be warned they will break….

 

[DDW]

If I had wood chocks lag bolted to a wood hull on a heavy vessel I'd be worried too. If properly done, fiberglass ones will be as strong or stronger than the original hull. The two situations are much different. There are many hundreds of these in existence, with thousands of hauls and no problems. Mine has been on a travel lift 8 times since putting them on without the slightest sign of strain. If you do some engineering calculations you will see that is isn't even a theoretical concern.

 

[Steel N Time]

If I had wood chocks lag bolted to a wood hull on a heavy vessel I'd be worried too. If properly done, fiberglass ones will be as strong or stronger than the original hull. The two situations are much different. There are many hundreds of these in existence, with thousands of hauls and no problems. Mine has been on a travel lift 8 times since putting them on without the slightest sign of strain. If you do some engineering calculations you will see that is isn't even a theoretical concern.

A Hatteras is not a wood boat, it is one of the highest quality fiberglass boats made, far superior to a Bayliner. The glass on the Hatteras is very thick and very strong! The Mahogany was the core material, in this application the core material is Coosa, a much weaker product compared to solid Mahogany. As stated mine were half the width of these and the leverage is amplified with a 12” fin….

The only saving Grace might be that this boat is probably much lighter than our Hatteras. We were slightly over 70k lbs.

I am trying to help by stating how to avoid damage by making a simple set of lifting blocks. Simple.

 

[Turtle Blues]

I have side chines. The PO upon sale gave me a set of very elaborate blocks he had made to support under the straps when lifting. Worst idea ever.The blocks actually cause point loading.

Looking at the width and attachment of your chocks my thought is that lifting will never be an issue. They are certainly capable of distributing the load as the straps come up.

You will certainly know the first time you grit your teeth and watch them lift.

 

[DDW]

I've wondered about this for awhile. One of the raps against anti-roll tanks is that it places weight up high to reduce the roll. But paravanes do the same thing, placing several hundred pounds of gear 10,15, or more feet above the center of gravity. It could be that just the overhead gear reduces roll by 15% and dragging the vanes another 15%. Of course, the weight of the paravane system blocks, wire, aluminum arms, hinges, etc. is pricey per pound if a lead cannon ball on top of a 12' pole would do the same thing. And if the paravanes,once deployed, adds another 15% in roll reduction, that is good because it is also reducing speed by 15% and probably increasing fuel consumption by about the same.

All of this discussion tends to be speculative because of the lack of measurement and also the lack of definitions. What is a 30% reduction in roll? A 10 degree roll is reduced to a 7 degree roll when the system is in use? A 3.6 roll period is increased to a 3.96 roll period? A blend of the two? As some have noted, confirmation bias probably comes into play. The perceived roll reduction increases as the weight of the captain's wallet decreases.

And, were there both a universal defined metric and a way to accurately measure roll reduction, what would be a reasonable "price per percentage"? $1K gets you a 15% roll reduction, $10K gets you 30% reduction and $100K gets you 60% reduction, which one is the bargain? Or necessary? Or desired? How much should one pay for their boat to not even feel like a boat?

I have been meaning to contact the app designers for my phone inclinometer and discuss the posibility of a record feature. Sitting at my helm, the app will indicate realtime motion in 10ths of a degree (the amount of roll and pitch on a certain heading). But there is no record feature (preferably with graph and averaging data). The inclinometer app is basically for static measurements (e.g., Is this picture on the wall crooked?)

With stabilization systems that are "deployable," it would be fairly simple to quantify the system's effect. Take a heading with beam seas, record the roll and pitch, then drop the paravanes, turn on the gyro, fill the anti-roll tank, whatever, and actually measure the effect. Those who market and install stabilization systems might not appreciate the accuracy. Also, this can't really be done with rolling chocks, which would have to rely on dockside before-and-after roll period and abatement measurements. But that is better than nothing. At least one should be able to verify that there is some difference. Then, one gets to make up a percentage, probably based in part on the amount of time and money spent.

At the University of Quebec, they decided that just the change in weight and weight distribution accounted for something like 30% of the reduction.

The question how and what do you measure, and furthermore, what measurement is appropriate or applicable is a very good one. I rarely get roll excursions over 12 degrees on my SD hull. Coming from sailboats as I do, this is inconsequential - if the rates of roll, acceleration, and jerk were low like they tend to be on a (monohull) sailboat. But on the SD hull they are not, and it is those factors that cause stuff to start flying, and make it uncomfortable.

There are many phone apps which record these things for both iPhone and Android. You have to be a little careful because a lot of them confuse motions, mistaking for example forward acceleration as downward pitch. This is why using them as an artificial horizon in an airplane is suicide. The really good ones are compensated for that. The newest autopilots (and structured sonar) use MEMS sensors to maintain a true AHRS (attitude and heading reference system). My Raymarine puts this on the N2K backbone once a second, and I use a Yacht Devices voyage recorder to record this to SD card. So I have yaw, pitch and roll, recorded once every second the instrument system was on since 2021, encompassing about 800 hours of operation. Unfortunately none of which was before the rolling chock install.

One day I will write an app to analyze this whole data set, but other than min and max, I'm really not sure what exactly I'm looking for. Two otherwise identical boats driven over the same course would be interesting to compare - one is less interesting.

 

[Steel N Time]

@Turtle Blues. Not sure how yours fit and if they went above and below to essentially remove weight on the chines. Are your chines at the corners of the vessel angled down like these or simply side chines that are inconsequential comparatively?

On ours the back lifting strap hits the keel and doesn’t touch the hull again until it’s at the chine putting all that weight on a fin that is angled… we lifted our boat 7 times and the blocks did their job every time, except the one time they didn’t listen and messed it up…

The block the previous owner had made were about 4’ wide above and below the chine made of solid 6” x 6” wrapped in carpet to protect the hull.

No different point load than if the chine wasn’t there at all, so I don’t get your logic on that one… there is always a point load when lifting a boat, right where the straps are.

When they lifted it to put it on a transport ship to cross from Panama, they jumped right in and had large padded blocks they put above and below without even being asked…. It’s a thing…. These guys are professionals and do it all the time, they used the blocks because they know better…. Will you get away with it sometimes… maybe yes maybe no…. But looking at a 12” fin angled down at a 45 degree and the weight of the boat pulling up sounds like a bad idea from my personal experience.

 

[DDW]

You need to draw the force diagram of the straps, then you will see that it is no different with chocks mounted on the chines (which is how they are done on a hard chine boat). On my boat with no chocks, the strap spans from the keel, touches only on the corner of the chine, then up to the lift spreader. The only points of contact are the bottom of the keel, and the corner of the chine. A majority of the lifting force is at the chine, a minority on the keel. The chine is a very strong and stiff part of the hull due to the angle of intersecting laminate.

With the chocks, it now spans from the keel to the chock and then up to the lift. Since the chock is installed bisecting the chine angle, it is mostly in compression, the forces on it are nearly identical to the no-chock case. The chock laminate is as thick as the hull, and now the chine is reinforced by this new very stiff, very strong beam.

I'd be much more worried about putting blocks against the hull bottom and side, these are much weaker and less stiff than the chine. Things change somewhat in a planked wood boat since it is a collection of loosely fastened pieces. It is also more complicated in a round bilged boat, though generally similar.

I'm not losing any sleep.

 

[Steel N Time]

You need to draw the force diagram of the straps, then you will see that it is no different with chocks mounted on the chines (which is how they are done on a hard chine boat). On my boat with no chocks, the strap spans from the keel, touches only on the corner of the chine, then up to the lift spreader. The only points of contact are the bottom of the keel, and the corner of the chine. A majority of the lifting force is at the chine, a minority on the keel. The chine is a very strong and stiff part of the hull due to the angle of intersecting laminate.

With the chocks, it now spans from the keel to the chock and then up to the lift. Since the chock is installed bisecting the chine angle, it is mostly in compression, the forces on it are nearly identical to the no-chock case. The chock laminate is as thick as the hull, and now the chine is reinforced by this new very stiff, very strong beam.

I'd be much more worried about putting blocks against the hull bottom and side, these are much weaker and less stiff than the chine. Things change somewhat in a planked wood boat since it is a collection of loosely fastened pieces. It is also more complicated in a round bilged boat, though generally similar.

I'm not losing any sleep.

Makes zero sense… the block literally sets at the corner of the vessel where it would be lifted had these chimes not been added at all… your concern would basically apply to every boat ever lifted… The strap comes from the keel to the edge of the bottom and up the side…. The blocks mimic not having this added appendage.

We will find out in the future I guess if it works or if he ends up with some damage. The Coosa is not that hard on a jinka scale like hard woods are, it’s engineered to be light and 30% stronger than plywood, but it is not crush resistant. The 12” out at a 45 degree angle going directly up to the lift is the torque point. Carrying most of the boat weight.

I am not sure if yours are the same angle, same material, same size, is your not as heavy…. Your comments above suggest you’re talking about a sailboat, not a power boat. You could be comparing apples and oranges.

In the past what I have read about this added chines is they should not be at this drastic of an angle.

I can only go based on what I have read, seen and experienced. Your experience is obviously different than mine. I tried to upload a photo, but I am challenged as it is asking for a URL… ��

 

[Mako]

In regards to the effectiveness of the rolling chocks... "offer possible roll reductions on the order of 35% to 55%."

Thanks Dan for the technical references. However I'm surprised by the quoted effectiveness of chocks. I have never read of numbers so high. Depending on how well they are designed, I've read of anywhere between 9% and 35%. My naval architect in Turkey stated his boats achieved (what he believed was) the most effective possible, and that was 35%. Of course that does bring up the question of how and what exactly was measured (amplitude, acceleration, etc.).

 

[Mako]

I have measured the natural roll time of my hull to be between 3.6 and 3.9 seconds through testing at the dock.

Kevin I think you may wish to address your roll period as well, which is quite short and likely high in acceleration levels. I can't recall if it was Gerr or another respectable designer, but I do remember that the target roll period for a boat was 1 second for every meter of beam. That would give your boat a target of about 4.6 seconds, which seems about right for a 50-foot-ish sized vessel.

 

[DDW]

We will find out in the future I guess if it works or if he ends up with some damage. The Coosa is not that hard on a jinka scale like hard woods are, it’s engineered to be light and 30% stronger than plywood, but it is not crush resistant. The 12” out at a 45 degree angle going directly up to the lift is the torque point. Carrying most of the boat weight.

The Coosa is just a form to laminate glass onto. It adds nothing at all to strength. The force on the chock will bisect the angle of the strap pushing directly into the chine.

 

[Steel N Time]

The Coosa is just a form to laminate glass onto. It adds nothing at all to strength. The force on the chock will bisect the angle of the strap pushing directly into the chine.

I don’t think so at a 45 degree angle… The strap will grab the edge and be pulling upward… It’s a 12” long prybar with the strength of the weight of the vessel. I don’t agree that the weight will be re-directed inward. I’ve seen it on my own vessel and they definitely flex upward… To the point of damage when not protected. I’ve seen the opposite of what you state as fact.

 

[rpackard]

I had roll chocks installed on my Willard 40, Lilliana, three years ago in Mexico. My goal was to reduce rolling in the often rolly anchorages in the Sea of Cortez. I copied the design of a computer analysis done by a naval architect firm, that did the analysis for someone else. According to the analysis, the damping effect comes from the turbulence occuring at the outboard edge of the fin. That edge needs to be as SHARP as possible. The design Kevin shows is the opposite of what the analysis indicates.

To minimize drag, the fins should be oriented along laminar flow lines of the hull at cruising speed and be oriented perpendicular to the hull. Without a numerical analysis it is sufficient to have the fins oriented parallel with the water line. My fins did not change the handling characteristics or the speed at fixed rpm.

Since I have no quantitative before and after data I cannot say what the roll reduction is. However, for ocean passages I use the paravanes much less. when I crossed Chatham Strait last month in heavy beam winds I still used the paravanes for more comfort. I would only recommend roll chocks if you do your cruising in area with rolly anchorages.

The downside of the chocks is that on some installations, when heading into a sizable chop, there are occasional loud BANG sounds, like a small explosion. I suspect this is due to a pocket of air being trapped as the hull drops down off a wave. This is just a guess.

A friend had roll chocks on a Selene that he purchased and had them removed because he did not like that sound.

Just because a firm has been installing these things for many years does not mean that they know how to make them most effective. However, any fins must be better than no fins on a rolly hull.

 

[ddw36]

First let my state my prefernce for bks/chocks. My percetion in all this is that modes of stability in all recreational vessels depend on the tolerance of your lady for motion. Boats, by nature are in motion --constantly. There is no letup at anchor at the slip, or underway. Many men can't tolerate it.



I believe the boating stability industry is dependent on the ladies' desire for as much reduction as possible, resulting in all the paravanes, flopperstoppers, powered fins, gyros, etc. It could well be that most TFers disagree w/ me, and I'm not accusing anyone for their choice of stability.


All I'm saying is that with my ideal of a boat build, single as I am, bilge keels/chocks would be the answer to stability for me, and that if you have a lady, I can well understand your alternate choice of stability. But, all this technical discussion of the pros/cons of bks/chocks is irrelevant.

 

[rslifkin]

I don’t think so at a 45 degree angle… The strap will grab the edge and be pulling upward… It’s a 12” long prybar with the strength of the weight of the vessel. I don’t agree that the weight will be re-directed inward. I’ve seen it on my own vessel and they definitely flex upward… To the point of damage when not protected. I’ve seen the opposite of what you state as fact.

It's going to depend a bit on the lift in question. The more the lift straps angle out after touching the chock, the more upward force and the less inward compression the chocks will see. But how much they'll take without breaking depends on how strong the chocks themselves are made, how strong the hull is in that area, and how well the loads from the chocks are spread into the hull. Presumably the yard installing the chocks will be picking up the boat by them to launch it, so that'll be a good first test of strength.

 

[mvweebles]

First let my state my prefernce for bks/chocks. My percetion in all this is that modes of stability in all recreational vessels depend on the tolerance of your lady for motion. Boats, by nature are in motion --constantly. There is no letup at anchor at the slip, or underway. Many men can't tolerate it.



I believe the boating stability industry is dependent on the ladies' desire for as much reduction as possible, resulting in all the paravanes, flopperstoppers, powered fins, gyros, etc. It could well be that most TFers disagree w/ me, and I'm not accusing anyone for their choice of stability.


All I'm saying is that with my ideal of a boat build, single as I am, bilge keels/chocks would be the answer to stability for me, and that if you have a lady, I can well understand your alternate choice of stability. But, all this technical discussion of the pros/cons of bks/chocks is irrelevant.

Perhaps not equally, but both my wife and I are allergic to roll. With or without her, I would not own a distance/coastal cruising boat without some form of stabilization. And I probably have at least a slightly above average tolerance for rolling.

In my opinion, life is too short to waste time on bad coffee/beer/wine, or unstabilized cruising boats.

Peter

 

[ksanders]

I had roll chocks installed on my Willard 40, Lilliana, three years ago in Mexico. My goal was to reduce rolling in the often rolly anchorages in the Sea of Cortez. I copied the design of a computer analysis done by a naval architect firm, that did the analysis for someone else. According to the analysis, the damping effect comes from the turbulence occuring at the outboard edge of the fin. That edge needs to be as SHARP as possible. The design Kevin shows is the opposite of what the analysis indicates.

To minimize drag, the fins should be oriented along laminar flow lines of the hull at cruising speed and be oriented perpendicular to the hull. Without a numerical analysis it is sufficient to have the fins oriented parallel with the water line. My fins did not change the handling characteristics or the speed at fixed rpm.

Since I have no quantitative before and after data I cannot say what the roll reduction is. However, for ocean passages I use the paravanes much less. when I crossed Chatham Strait last month in heavy beam winds I still used the paravanes for more comfort. I would only recommend roll chocks if you do your cruising in area with rolly anchorages.

The downside of the chocks is that on some installations, when heading into a sizable chop, there are occasional loud BANG sounds, like a small explosion. I suspect this is due to a pocket of air being trapped as the hull drops down off a wave. This is just a guess.

A friend had roll chocks on a Selene that he purchased and had them removed because he did not like that sound.

Just because a firm has been installing these things for many years does not mean that they know how to make them most effective. However, any fins must be better than no fins on a rolly hull.

Please clairify a bit...

Do you mean thinner at the tip?
or do you mean no increase of width at the base?

I am very curious.