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Old 07-06-2019, 12:11 AM   #1
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Loads on flopper stopper attachment points

Getting some arms made for at anchor flopper stoppers.
Anyone got any idea of approx loads that I could expect on the downhaul, uphaul rope on a 60fter?

These will be 3 m/9 ft arms with rope going up to our roof.

I was thinking of epoxy setting a 1/2 in s/s eyebolt into the roof beams that extend out from a bulkhead position.
These are 3 inch laminated beams that stick out about 2 ft from the cabin side at 16 inch spacings topped with 5/8th ply.

Not all boats have masts or framework as can be seen in these pics so I can't imagine the loads being that horrific but I certainly don't want to go tearing the roof off.

Boat Stabiliser Boom Application
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Old 07-06-2019, 02:03 AM   #2
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This is just a guess here....but I would think the force would be based on the size of the disk in the water, not the boat. You could minimize the risk of roof damage by putting a weak link in the line so that it will break before the stress causes structural damage to the roof.
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Old 07-06-2019, 01:16 PM   #3
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You can get a good estimate of the relative forces if you know the geometry by doing a vector diagram.

Draw the configuration to scale. Assume a certain length for the down force vector. Complete the vector diagram by extending the uphaul line until it intersects with a horizontal line from the bottom of the down force vector. Replicate the angle of the uphaul line from the tip of the down force vector until it intersects the boom. This will give you the relative uphaul tension force and the pole compression force.

Now you only need to determine/estimate the actual resistance force of the flopper stopper and you'll know your other forces.

You'll note that flat uphaul line angles create very large forces relative to the flopper resistance.
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Old 07-06-2019, 01:28 PM   #4
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This is from Beebe's first edition (1975). It gives you an idea of the loading for paravanes. You may be able to apply some of the loading/calculations for your floppier stoppers. He's was convinced that the fish can generate resisting forces up to 10 lbs. per square inch.
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Old 07-06-2019, 03:04 PM   #5
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This is from Beebe's first edition (1975). It gives you an idea of the loading for paravanes. You may be able to apply some of the loading/calculations for your floppier stoppers. He's was convinced that the fish can generate resisting forces up to 10 lbs. per square inch.
Purely for at anchor, not while underway so loading nothing like what fish generate.
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Old 07-06-2019, 03:06 PM   #6
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Kolstrand in Ballard (Seattle) makes them and has been doing so for many decades.
They also make “reel hydraulic anchor winches” and other related fishing gear.
The’ve overseen many many installs and no doubt know exactly what you need to know.
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Old 07-06-2019, 03:18 PM   #7
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Now you only need to determine/estimate the actual resistance force of the flopper stopper and you'll know your other forces..
That's the issue, no one seems to have any idea.
Some say "a lot" but how much is that?

Quote:
You'll note that flat uphaul line angles create very large forces relative to the flopper resistance
Arms from the rubbing strake on the hull, rope to the roof has it at approx 60 degrees.

Load can be worked out on this calculator, but first one needs to know what that load is.

https://www.stren-flex.com/load-calculator
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Old 07-06-2019, 03:21 PM   #8
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Originally Posted by Benthic2 View Post
This is just a guess here....but I would think the force would be based on the size of the disk in the water, not the boat.
That gives me something to work with
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You could minimize the risk of roof damage by putting a weak link in the line so that it will break before the stress causes structural damage to the roof
Good idea.
Used to do the same when racing catamarans
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Old 07-06-2019, 04:03 PM   #9
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The down force is proportional to mass of effective water column above flopper stopper times roll acceleration of the boat. The effective water column would be less than depth times flopper area because the water far away from the flopper surface would be displaced sideways rather than lifted.

Lets make some simplifying assumptions. Say flopper stopper area is 1 square foot, the effective water column above is 2 feet. The mass of the water displaced would be 135 lbs. Let's further assume the roll acceleration is 1 ft/sec2. This yields a force of 42 lbs per square foot of flopper area.

With that, a 10 square foot flopper stopper would produce about 400 lbs of down force, which in turn depending on angle of uphaul line could produce 1,000 +- lbs of uphaul and boom forces.
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Old 07-06-2019, 05:14 PM   #10
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Based on the above I think I'll put my uphaul attachment eye on the roof/cabin side junction point.
Thanks
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Old 07-06-2019, 05:40 PM   #11
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Simi, you said these are just for when anchored. The lever arm advantage that you will get with extended poles will be great. However, from a realistic point of view, how about just attaching them from your deck, and installing oversized f/s hanging from your beam.

Perhaps the f/s poles will double or even triple the leverage, but it is easy enough to just use f/s disks that are double the diameter, yielding the same force.
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