Paravane stabilisers actual loads.

[Airstream345]

So I have a friend that happens to have a crane scale he can lend me. Now I just need a paravane fish and we can run this experiment.

 

[Simi 60]

So I have a friend that happens to have a crane scale he can lend me. Now I just need a paravane fish and we can run this experiment.

Oh goody

 

[Airstream345]

Put out a call for a fish. Hoping someone around me has one and a sense of humor.

https://www.trawlerforum.com/forums...awler-forum-experiment-66712.html#post1148897

 

[KnotYet]

This is the big question in Beebe's design and many of the assumptions about the forces involved. as Simi pointed out "500 lbs is 500 lbs"....so it's the torque of that 500 lbs out on a long lever. Think of a seesaw...it doesn't get heavier at the fulcrum if you put a 250 lb weight on either end.

The more I think about this the more I'm beginning to consider it's about the torque on the rig and transfering that to the vessel. So in my theorical model it's not pushing down on the deck with 11,000 lbs of force rather it 5500 ft-lbs of torque for each side going in and our of perfect equilibirum (seesaw example)

In the case of the boat, however, the fulcrum is the center of roll. This is
what makes the loading at the deck so high. The 'pounds' at the fish get
multiplied by the ratio of (length from the fish to the attachment point)
divided by (length from the center of roll to the attachment point) to equal
the force at the attachment point.

 

[Airstream345]

So you're saying that if one side is generating 290 lbs at the fish which translates into 5500 lbs of torque at the fulcrum and so is the other this creates opposing forces equal to 11,000 lbs of torque in some state of equalibrium?

I agree, however this still doesn't put 11,000 pounds of downward force on the boat....rather 5,500 ft-lbs of rotational torque on one side of the rig and 5,500 of ft-lbs or torque on the other.

So if the rig is a triangle, all of the torque involved needs to be taken up through either compression or tension in that triangle (this is simplified given there is more than a single triangle involved) but the center of the fulcrum (the center of the triangle, again simplified) has much greater rotational force than actual downward force.

Again, I'm making most of this up as I go but I keep going back to Simi's statement that it can't get heavier however the lever allows it to create a lot of rotational force at the fulcrum.

The other thing I still can't square is how there are many reports of trawlers running just one fish. If we apply the sniff test, if they were generating 5,500 lbs on only one side would that not significantly roll/list the boat to the fish side? Yet people who report doing this typically talk about a slight yaw only due to dragging the fish forward.

I really wish I paid more attention before I dropped out of engineering school to become a pilot.

 

[Tad Roberts]

The force on the fish or on it's towing line is dependent on more than it's size and speed. The work it's doing (stabilizing the boat) is an additional load. That load is dependent on the boat's stability, the stiffer the boat, the more it follows the wave surface (rolling), and the higher the load on the stabilizing system.

Beebe mentions "My research has convinced me that the stabilizer can generate resisting forces up to 10 pounds per square inch (psi) of surface. This is no mean item in a 300-square-inch stabilizer; until we realized what a tiger we had by the tail, and beefed up our gear to handle it, we had problems aboard Passagemaker with our F/S rig."

Now that was written almost 50 years ago, before the advent of the PC and the easy acquiring of stability data on various hulls.

 

[Airstream345]

Tad - yes, you are hitting on the exact problem here. No one seems to have actual measurements of the forces from the fish themselves. We have Beebe's comment (which proposes a 300 sq-in fish creates up to 3000 lbs of downward force on each fish). I created a crude, amatuer, no-idea-what-I'm-talking-about model using an online lift calculator and came up with under 300 lbs for a 300 sq in delta wing in seawater traveling at 8 knots at max coefficient of lift (which is more like 1 PSI).

And your point about dynamic loads as the vessel rolls is also key. The force of the fish as resistance to roll and the drag it creates becomes rotational torque (roll attenuation) and drag (yaw) at the center of the rig.

I'm really hoping with a real world experiment we can get some actual measured results for at least the fish at various speeds and in different sea states. Then we have something to build on.

 

[Airstream345]

I should go on a record and say my wife is now calling my name down this rabbit hole asking if I'm OK.

One more data point. The max load of 3/4" plywood (which many fish are made from) is 80 psf which converts to 0.55 psi.

 

[Jeff F]

I should go on a record and say my wife is now calling my name down this rabbit hole asking if I'm OK.

Tell her I said you're just fine.

 

[KnotYet]

I should go on a record and say my wife is now calling my name down this rabbit hole asking if I'm OK.

One more data point. The max load of 3/4" plywood (which many fish are made from) is 80 psf which converts to 0.55 psi.

I don't think those values from the construction trade are particularly relevant.
A well supported 300 sq in triangle of 3/4" plywood ought to be able to
handle 1000 lbs of evenly distributed force which equates to 3.3 psi, IMO.

 

[Airstream345]

Completely fair, it's what I could find. Still, if Beebe's 10 psi is true - wouldn't plywood fish buckle or break?

 

[Simi 60]

Don't forget most of those ply fish have a slot cut on the centreline imho strength would be compromised where max load would be.

 

[DDW]

You need to use about 1.3 for the Cl in your equation. A flat plate has a Cd of about 1.1, and your fish will turn into a flat plate in drag at some point. A fish might be a bit more than that due to edge effects. In water at 10 knots, dynamic pressure is about 2 psi, so 300 x 2 x 1.3 = 780 lbs. 4x that at 20 knots. If you figure a roll rate of about 20 deg/sec and a boom of 20 ft out from the CL, that is only about 4 knots up and down in the water, which adds very little to the apparent water velocity of the fish, less than a knot. It seems like for a 10 knot boat and 300 sq in the working load would be around 1000 lbs or less. Designing for 3000 lbs breaking is probably prudent.

 

[Jeff F]

You need to use about 1.3 for the Cl in your equation. A flat plate has a Cd of about 1.1, and your fish will turn into a flat plate in drag at some point. A fish might be a bit more than that due to edge effects. In water at 10 knots, dynamic pressure is about 2 psi, so 300 x 2 x 1.3 = 780 lbs. 4x that at 20 knots. If you figure a roll rate of about 20 deg/sec and a boom of 20 ft out from the CL, that is only about 4 knots up and down in the water, which adds very little to the apparent water velocity of the fish, less than a knot. It seems like for a 10 knot boat and 300 sq in the working load would be around 1000 lbs or less. Designing for 3000 lbs breaking is probably prudent.

That's interesting and valuable.

So that 780 is the case where you're dragging the fish sideways, essentially?

 

[Pascall]

I follow this with great interest!

 

[Delfin]

It's what I was thinking
Load on rope is load on rope
It shouldn't increase because it's held outboard by a stick

The load on each rigging point will total the drag on the fish - that's just physics. Illustration - I lift an 800# Boston Whaler off the foredeck using a 4" diameter internal boom that extends from the 6" external boom via a 4' long 3" diameter hydraulic ram cylinder I had built that nests instead the outside boom. When I picked it up, the builder was skeptical, starting that it would fold you like a wet noodle the minute any side load was applied.


Well, it doesn't for the simple reason that the topping lift transfers the 800' load to the top of the mast, which in turn transfers it to the hull via a running back led forward in line with the direction of lift. As the internal boom extends to position the Whaler outboard of the mother ship, there is zero side load since the force wanting to bend the noodle down is previously offset by the topping force counter balancing it.


The strength of the rigging of the passive paravanes needs to be equal to the maximum load, which is cyclic and effected by things like Hollywood's big log, which is why talking about 300# loads on the fish under ideal conditions is irrelevant.

 

[Airstream345]

The strength of the rigging of the passive paravanes needs to be equal to the maximum load, which is cyclic and effected by things like Hollywood's big log, which is why talking about 300# loads on the fish under ideal conditions is irrelevant.

It's totally relevant in order to get a baseline to confirm/correct old assumptions about the amount of force created by the lift and drag a fish creates underway (in ideal conditions). From that baseline more models can be created to account for max loads, breaking strengths, loads on the rigging, etc.

PS - love it when we see Delfin around the Salish Sea. I feel like we're always one day behind you in anchorages. Hope to meet you in person one of these days.

 

[Rob]

Let’s measure this downward force

Fortitude- You’ve got the crane scale, I’ve got a KK42 that is all set up with the riggings and fish. Let’s get together and tie them together to get some real world numbers. If you remember we met over at Port Ludlow in Sept.
Rob & Anne
“Lady Anne” KK42
La Conner, WA

 

[Airstream345]

Fortitude- You’ve got the crane scale, I’ve got a KK42 that is all set up with the riggings and fish. Let’s get together and tie them together to get some real world numbers. If you remember we met over at Port Ludlow in Sept.
Rob & Anne
“Lady Anne” KK42
La Conner, WA

Boom!

Hey Rob, yes, I remember. This is exactly what needs to happen thank you for offer to volunteer! I need to get the scale from my friend in Seattle and then get to you. I'll reach out and we can plan a window.

 

[Delfin]

It's totally relevant in order to get a baseline to confirm/correct old assumptions about the amount of force created by the lift and drag a fish creates underway (in ideal conditions). From that baseline more models can be created to account for max loads, breaking strengths, loads on the rigging, etc.

PS - love it when we see Delfin around the Salish Sea. I feel like we're always one day behind you in anchorages. Hope to meet you in person one of these days.

If you see us, please stop by if you can. I'll be back up to Philbrooks shortly, who are taking care of a couple of things in preparation for a Hawaii crossing later this year so perhaps we'll see you!

 

[Tad Roberts]

Compressive strength perpendicular to grain for Douglas Fir is 870 psi, strength in tension is about half this and strength in sheer and bending is quite a bit higher than this.....so the 10 psi load will not be an issue unless the fish are awkwardly wide......

When I was a lot younger and stronger, hand lifting plywood paravanes of perhaps 400 sq. inches with 30 lb lead weights on them was barely possible. This was with the boat stopped and drifting. Part of that was the slippery 3/8" towing wire. When the boat rolled the wrong way (away from the fish), it was all I could do to hang on, and if it was a big roll I'd give away wire.....

 

[Airstream345]

Cool boat. The poles look to be about mid waterline?

 

[Tad Roberts]

Cool boat. The poles look to be about mid waterline?

Probably about midships, almost all west coast of North America fishing boats have the stabilizer poles set well forward. Which reminds me about another load on the system; with poles forward the fish will damp out a bit of the pitching motion as well.

 

[Simi 60]

Spoke to the owner of this vessel online

Says it has 500 X 400 (19.6 X 15.7) plywood fish made from one inch ply.

Upright arms appear to be tied into flybridge side with the 2 stays visible
Forward load to pole end is attached to hardwood rail with 3 X 2 inch screws
Boat construction is white cedar and I assume, epoxy/glass.

Says she came to Australia from the US on her own bottom with that setup, but not him doing it.
But he has used them here in Oz.

He agrees, loads can't be that big as that rig would have torn itself apart if they were.

 

[Simi 60]

Probably about midships, almost all west coast of North America fishing boats have the stabilizer poles set well forward. Which reminds me about another load on the system; with poles forward the fish will damp out a bit of the pitching motion as well.

IF.... we go ahead the best place structurally for our is on the wheelhouse bulkhead.
Unfortunately, that is slightly fwd of centre as measured by the waterline

Further aft has less supported spans plus removal of solar panels to contend with.

We do have a barn door sized rudder and the vessel tracks like she's on rails so I would hope steering wouldn't be affected.

Pic shown has current flopper stopper arms on cockpit bulkhead, further aft than ideal but they work well
Red being proposed arms done in full 6m length of 80mm x 6mm ally tube

Our cruising style wont change, it'll still be waiting for favourable weather for our run.
But allow us to go point to point instead of tacking to our destination.

 

[Xlantic]

Red being proposed arms done in full 6m length of 80mm x 6mm ally tube.

Simi, I also built my 6 metre flopper-stopper out-riggers out of 80 mm diameter aluminum tubing but mine were only 2mm thick as I didn't want them to be heavier than needed.

I used an on-line calculator for columns under buckling forces which showed that the diameter has a much greater impact than thickness on the compression forces that a tube can handle.

When deployed, you may want to aim for the outrigger to be level with the roof of your upper cabin so the forces on the roof are horizontal. You can attach a chain plate to the underside of the roof to avoid having to re-position the solar panels.

 

[Simi 60]

Simi, I also built my 6 metre flopper-stopper out-riggers out of 80 mm diameter aluminum tubing but mine were only 2mm thick as I didn't want them to be heavier than needed.

Our flopper stoppers are on 80x3
But actual stabilisers/paravane poles I think will warrant heavier tube.

A mate with a 55ft steely has 80x3 in s/s as stabiliser/paravane poles and while they have stood the test of time they do get a bit of flex.

I used an on-line calculator for columns under buckling forces which showed that the diameter has a much greater impact than thickness on the compression forces that a tube can handle.

I agree
But, I don't want to find out the hard way that 3mm wall was not quite up for the job.

When deployed, you may want to aim for the outrigger to be level with the roof of your upper cabin so the forces on the roof are horizontal.

That was the direction I was aiming for

You can attach a chain plate to the underside of the roof to avoid having to re-position the solar panels

.
Solar panels are only an issue if going well aft of centreline.
Not as much structural support - lack of bulkheads- would be more of an issue.

 

[DDW]

That's interesting and valuable.

So that 780 is the case where you're dragging the fish sideways, essentially?

Yes, dragging it though the water would be like flat plate drag, Cd around 1.1. If it generates lift instead, it can be a little bit higher, but you are unlikely to exceed a Cl of around 1.3 with the crude shape of a fish. A properly shaped asymmetric airfoil can get up to maybe 1.6, a lot higher with a bunch of high lift devices - but fish are usually just a flat plate of steel or aluminum.

 

[Xlantic]

Solar panels are only an issue if going well aft of centreline.
Not as much structural support - lack of bulkheads- would be more of an issue.

Yes but bulkheads would be needed for vertical support. But I would think the the roof can handle the horizontal forces involved here.

 

[Airstream345]

When deployed, you may want to aim for the outrigger to be level with the roof of your upper cabin so the forces on the roof are horizontal. You can attach a chain plate to the underside of the roof to avoid having to re-position the solar panels.

A friend has a setup like this....steel boat but when the paravanes are deployed all of the lines ceom from the top of the pilothouse (this is a Beebe style with an aft raised PH).

I may have some pics of his rig. I'll try and dig them up and post here.