Rolling chocks ve Stabilizers

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Hey Jude, those appear to be as nice a job as one could wish for. From a lot of reading this afternoon, it appears the general consensus is that the bilge keels can help the rolling problem to a small degree, but not as much as one would think.

This site probably comes close to one of the best explaination:
Bilge Keels
 
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Hey Jude, those appear to be as nice a job as one could wish for. From a lot of reading this afternoon, it appears the general consensus is that the bilge keels can help the rolling problem to a small degree, but not as much as one would think.

This site probably comes close to one of the best explaination:
Bilge Keels

The link discusses twin keels more so than bilge keels....

The designer also discusses "turbulence" generated by flat plate protrusions. After extensive reading, actual observation and a pretty good background in aerodynamics....I'm not sure discussing turbulence of thin plates at 8 knots or less is even worthy of discussion. Sure wetted surface and drag induced by marine growth on them can start to get significant..but plain old drag on a plate that is aligned with flow...I don't think so. Otherwise there would be a huge push to "airfoil" everything in sight in the marine world and I just don't see it. So his "design" guestimations are already suspect in my mind although I know some of his designs that seem to be very nice and well "designed".

Bilge keels seem to work best on very round bottoms....much less so on hard chines. But Most evey round bilge commercial vessel I've seen lately has them...how well they work is very much up to the individual boat and it's stability to begin with.
 
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Roll Damping on Two New England Trawlers: An Experimental Study

....from another forum

watto99 said:
Hi
I know that this is a long dormant string, but it's a subject that is very much alive

In my own search for this information I tracked down a report and bought it on line for $40 from the US naval archiect association.
It's well worth the money and answers the questions about benefits and optimum design of bilge keels etc

Here's the title/ authors
Roll Damping on Two New England Trawlers: An Experimental Study
Clifford A. Goudey 1 and Madan Venugopal 2
 
Hi there Tidahapah,

I have a 62ft wooden motor yacht in Vancouver, which rolls somewhat.... you mentioned someone in Canada who designs flume tank systems.... can you recall who that might be? I'd be grateful for a link. Eamonn.
 
... you mentioned someone in Canada who designs flume tank systems.... can you recall who that might be? I'd be grateful for a link. Eamonn.

That would probably be Professor Don Bass of St. John's, Newfoundland.
 
One problem with bilge keels is to not cause too much extra drag , they must be aligned with the water flow at cruise speed.

Otherwise they will act as speed brakes, 100% of the time the boat is moving.

Anyone have a unused test tank, and an accurate scale model?

They do work at damping roll while stopped ,so might help at anchor.

I am not willing to cut mine off as they protect the external keel cooler , but I do have the parts to make a retracting mast , and the regular gear to install flopper stoppers -paravanes for occasional blue water trips.

On the to do list,,,,
 
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My rolling chocks protrude about 10" from the hull and run about half the length of the hull, but I don't really think they do a huge amount to limit rolling. The boat still rolls like a pumpkin in a beam sea even though I also have about a ton of ballast.

Luckily the sails stabilize the rolling incredibly well, as long as there 10+ knots of breeze.
 

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Bump.

Been squirrelling around the Internet for a while looking for more information on rolling chocks, and the following is the first scientific study I've found.

I didn't pay to read the whole study, but the abstract confirms that rolling chocks are indeed effective.

"Rolling chock" is a plate-like hull appendage just below the waterline on small-scaled fishing boat (less than 20 GT), and is expected to play a role of damping effect on the rolling motion by fishermen and shipyard engineers with their empirical knowledge. It was assumed that the resistance produced by the rolling chock in the rolling motion is a main factor of the damping effect. Few studies have however been done on damping effect of the rolling chock. This study confirmed the damping effect of the roling chock in the tank tests by using a model ship of Lpp 1.92m with rolling chock of three different widths (0.01, 0.02, and 0.04m) and without. In this study, the damping effect is assumed to be evaluated by the wave-making component of ship resistance. The damping of the rolling motion measured in free roll tests was modeled with the single-degree-of-freedom rolling equation, and then the extinction coefficients were determined by Froude's energy method. On the coefficients, wave-making component b_<F1> of the model ship equipped with rolling-chock was larger than that without rolling chock, and increased similar to the extinction coefficient as the rolling chock became wider. In contrast, no particular relationship of vortex-making component b_<b2> to rolling chock width was found. The maximum resonance amplitude was clearly reduced with equipping the rolling chock in rolling tests in beam waves of three wave slopes (1/10, 1/20, and 1/30). Linear model substituted into damping moment term of the single-degree-of-freedom rolling equation was valid to express the maximum resonance amplitude of the model ship with and without rolling chock.

https://www.jstage.jst.go.jp/article/fisheng/45/2/45_KJ00005715647/_article

We were considering bat wing stabilizers for a while, but seem to have settled on rolling chocks because we like to sneak through tight, skinny water entrances that can have kelp, which means paravanes wouldn't suit us either. They also are the cheapest solution, around $4,000
 
Hi Murray,

Scaling up to real life from those model tests, for a 45' boat the keels would be 3", 6", and 11" wide, no word on length. Without reading the paper I can tell which was most effective and which added the most resistance. (Hint: taller (wider) is better) Making the keels long reduces the form resistance, but increases surface area (and drag). Longer keels increase the likelihood you'll get it wrong, with the keel running across instead of with water flow over the hull surface. Shorter and taller keels will create more roll damping(because the lever is longer), and lessen the chance of going too far wrong.
 
Hi Tad,

Glad you caught this one.

I recently contacted an east coast Vancouver Island firm that has installed "...more than 270 sets on now all the way up to 120 ton vessels..." who suggested 12" wide by about 15' long rolling chocks for our 30' semi displacement boat. That's where the $4,000 estimate came from.

May do this in 2019 or after retiring, which is about 3 years away.

Hoping to slow down the abrupt deceleration/acceleration phase of motion...
 
Hi Tad,

Glad you caught this one.

I recently contacted an east coast Vancouver Island firm that has installed "...more than 270 sets on now all the way up to 120 ton vessels..." who suggested 12" wide by about 15' long rolling chocks for our 30' semi displacement boat. That's where the $4,000 estimate came from.

May do this in 2019 or after retiring, which is about 3 years away.

Hoping to slow down the abrupt deceleration/acceleration phase of motion...

I am considering the same thing when I need to do bottom paint in the next few years. There is a yard in BC that has done a number of them on North Pacifics and the reports are positive. I figure since I'll need to pay to have the boat hauled, prepped and painted anyway, the incremental cost of the rolling chocks isn't all that bad.
 
I'm curious. When doing roll chocks like that do you put them parallel the centerline? Sort of wouldn't think the waterflow is exactly straight down the boat. 15ft long and 12" deep is a big darn thing on a 30ft boat. Wetted area alone not even counting drag from messing with waterflow has got to be significant.
Are these for the Sundowner? That's a hard chine boat so I'm assuming chocks would end up more vertical than horizontal if mounted square to the bottom of the boat. That's a whole lot of basically keels trying to keep the boat going straight. Do you think it will effect maneuverability much?
 
I'm curious. When doing roll chocks like that do you put them parallel the centerline? Sort of wouldn't think the waterflow is exactly straight down the boat. 15ft long and 12" deep is a big darn thing on a 30ft boat. Wetted area alone not even counting drag from messing with waterflow has got to be significant.
Are these for the Sundowner? That's a hard chine boat so I'm assuming chocks would end up more vertical than horizontal if mounted square to the bottom of the boat. That's a whole lot of basically keels trying to keep the boat going straight. Do you think it will effect maneuverability much?

Yes, for a Sundowner Tug.

I agree that 12" seemed pretty big to me as well, but this was my first communication with this firm.

As to how they're fitted, going from the many semi displacement commercial fishing boats about our boat size I've seen, the rolling chocks would be fitted at the hard chine. They would project out at about a 45 degree angle downwards. They would taper towards the hull at both ends of the boat.

To better visualize, imagine seeing a boat from the stern sitting in a hoists lifting straps. The straps wouldn't even touch the boats sides, but rather go from the bottom of the keel, to the rolling chocks.

*Link to photo's two posts down*
 
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I'm curious. When doing roll chocks like that do you put them parallel the centerline? Sort of wouldn't think the waterflow is exactly straight down the boat. 15ft long and 12" deep is a big darn thing on a 30ft boat. Wetted area alone not even counting drag from messing with waterflow has got to be significant.
Are these for the Sundowner? That's a hard chine boat so I'm assuming chocks would end up more vertical than horizontal if mounted square to the bottom of the boat. That's a whole lot of basically keels trying to keep the boat going straight. Do you think it will effect maneuverability much?

These days the effects can likely be reasonably well modelled using CFD. Perhaps talk to an NA. Not to have them do it, but to check whether they are aware of published studies.

There has to be a hit on speed, but at displacement speeds it might be fairly small and far outweighed by the anti-rolling benefits.
 
CFD, that's funny.

These days the effects can likely be reasonably well modelled using CFD. Perhaps talk to an NA. Not to have them do it, but to check whether they are aware of published studies.

There has to be a hit on speed, but at displacement speeds it might be fairly small and far outweighed by the anti-rolling benefits.
 
I've never heard those called 'rolling chocks' before. I've always known them as bilge keels. A rolling chock is a fairlead with rollers in my experience.

I'm probably paranoid, but the way that NP sits in the slings with all that pressure on the wings, makes me a bit nervous. I'd be worried about bending or breaking the keels.

One potential plus to having bilge keels is that, at least on ships, they're somewhat sacrificial. If you bump into something, they can take the damage without necessarily transferring it to the hull plating. Not sure if the same can be said for these ones.
 
I've never heard those called 'rolling chocks' before. I've always known them as bilge keels. A rolling chock is a fairlead with rollers in my experience.

I'm probably paranoid, but the way that NP sits in the slings with all that pressure on the wings, makes me a bit nervous. I'd be worried about bending or breaking the keels.

One potential plus to having bilge keels is that, at leastd on ships, they're somewhat sacrificial. If you bump into something, they can take the damage without necessarily transferring it to the hull plating. Not sure if the same can be said for these ones.

This was my thought as well: how will a travelift be able to lift her? The yard blocks my spray rails to spread the load, so I would assume divers would be necessary to position something on either side of the chock under the strap. Either that or design and place them so they fit between the correct lifting points on the hull, that may or may not be 15' apart on a 30' boat.
 
Only guessing here...rolling chocks are on many commercial fishing boats on the BC coast and I can't see them putting anything weak on their boats that would need fussing over or coddling, such as having to hire a diver every time they pulled their boats out of the water.

Also, the forces applied by the straps are in a direction where the rolling chocks are their strongest...if the forces were directed "up and out" I could see your point.
 
"This was my thought as well: how will a travelift be able to lift her?"

There are usually arrows screwed to the deck with LIFT HERE written on them.

These clear prop shaft , roll chocks and keel coolers .
 
Regarding lifting straps...they're a moot point where I live because the one and only marina has a trailer with hydraulic pads.
 
It sounds counterintuitive, but a 50' fishboat with an empty hold is a lot lighter than a 50' pleasure trawler despite typically being built to heavier scantlings. I cannot imagine a fishing boat with a full hold and tanks being hauled by a travelift in anything short of an emergency, in which case damaging chocks would not be a concern. However, in any sort of planned haul, 12" chocks at some angle to vertical would still concern me since there could be considerable side as well as point loads on them. It would also be helpful if the curve or taper of the chocks did not encourage the lifting straps to move apart during the lift. Everything can be addressed during a planned lift, but can add to the work, time and expense.
 
Only guessing here...rolling chocks are on many commercial fishing boats on the BC coast and I can't see them putting anything weak on their boats that would need fussing over or coddling, such as having to hire a diver every time they pulled their boats out of the water.

Also, the forces applied by the straps are in a direction where the rolling chocks are their strongest...if the forces were directed "up and out" I could see your point.

I'm sure you're right. If they were stuck on tough enough, they might even allow the boat to dry out at low tide and stay more upright and stable. Makes sense for a rough and tumble work/fishing boat.

Honestly, I'd be really interested to see what kind of difference it would make adding bilge keels.
 
So far I’ve not heard of any North Pacific’s with the rolling chocks that have had a problem with the travel lift straps.
 
You can make anything strong enough to take anything, you just need to know what you're doing.
I'm guessing on the BC coast there are more grids, ways and hydraulic trailers than travelifts.
Sure can't see how you could design, build and attach 2 - 15ft X 12" chocks to a fiberglass hull for $4k.
It'd be an interesting project but I think it'd take a lot of thinking to get it right.
 
That "local firm" has installed 270 sets of rolling chocks but any data is anecdotal at best. There is a paper out of New England from 20 years ago comparing the performance of rolling chocks with towed paravanes (flopper stoppers). The biggest chocks reduced rolling anywhere from 15-25 %, while paravanes reduced rolling around 60- 70%. With chocks bigger will be more effective.

If you really want to know about flow on the hull surface build a scale model, add lines of wet paint, and tow at scale speed. It's amazing......

2.jpg
 
How a local mob does rolling chocks.


Fabrication-rolling-chocks2_w800.jpg


rolling-chocks-20_w800.jpg


More here
https://rlmarine.com.au/stabilisers/

What a great idea, the triangulation must make them immensely strong and the long wide base pad will spread any loads over a wide area. The only downside would be the added turbulence from the offset plates: the same surface area as a solid plate, but a bit more drag....

I wonder if there is any tow or tank test data on this design?
 
...There is a paper out of New England from 20 years ago comparing the performance of rolling chocks with towed paravanes (flopper stoppers). The biggest chocks reduced rolling anywhere from 15-25 %, while paravanes reduced rolling around 60- 70%. With chocks bigger will be more effective....

I could live with a 25% reduction as this would slow down the "snap roll" tendency of our hull.

We don't need them for going out in heavy weather...when the wind is seriously up we stay anchored and go hiking. What we need them for is when having to beat through a lumpy section of water at a headland to reach calmer water, or when the wind picks up during a crossing for example.

If we were spending a lot of time offshore, or in larger bodies of water, paravanes would be in order. The north coast of BC is a complex system of islands and fairly narrow waterways...there is usually calmer water nearby.

To sum up why they appeal to me; they would always be there for those short snotty sections, we'd never have to deploy or retrieve them, and they should slip through kelp.

About paravanes and kelp...we've always approached skinny water entrances during calm weather, but there may come a day when things get ugly enroute. Gives me the heebie-jeebies thinking about trying to get paravanes aboard, in lumpy conditions, before passing through a kelpy entrance into calm water.
 
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