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Drogue Chain Plates Strength?

Graham_Wright

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[QUOTE
My own solution would be to rig a bridle starting at my samson post (which passes through the grp deck reinforced with 4" of ply and then down through the keel with a stainless plate below) round both bow cleats, round four midships cleats and round both stern cleats. The bridle would be nylon.
and just how long, in the sort of weather where you are going to need this, are you going to spend on deck rigging all this to ensure proper load sharing of all these points? That prospect is a real horror.
I will trust the horizontal chain plates I fitted (500x50x6) on my own wee boatie and have the drogue attached in a couple of minutes.[/QUOTE]

This is the sort of arrangement that can be rigged with no impact on running the ship before the dreaded strikes. The use of nylon allows just a quick hitch on the cleats and middle eye needs to be dropped over the samson post. The bridle would, of course, be pre-prepared. Time? Even in a sea, a couple of minutes.

I have seen so called samson posts ripped out of a foredeck during a tow. The thought of the stern being gradually demolished terrifies me! It just does not seem seamanlike to put such shear loads on a hull. If you must secure to the hull, surely you should spread the load sideways as well as longitudinally.
 
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pvb

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The use of nylon allows just a quick hitch on the cleats and middle eye needs to be dropped over the samson post.
I'm not convinced this does ensure that the load is distributed around the boat. With a hitch on each cleat, surely the load will primarily be on the aft cleats, until they give way, when it will transfer to the mid cleats, and so on?
 

Graham_Wright

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I'm not convinced this does ensure that the load is distributed around the boat. With a hitch on each cleat, surely the load will primarily be on the aft cleats, until they give way, when it will transfer to the mid cleats, and so on?
The elasticity in the rope should distribute the load but I hope never have to find out! I believe my samson post would eventually take the load even if the cleats should all fail.
 

geem

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I have no experience to back me up but all this fills me with horror.

I believe I have read that towing lines, whether to take forward or backward pulls should have their load distributed as widely as possible. Bolts in a line sound like a recipe for disaster. If one fails, the rest would surely follow.

My own solution would be to rig a bridle starting at my samson post (which passes through the grp deck reinforced with 4" of ply and then down through the keel with a stainless plate below) round both bow cleats, round four midships cleats and round both stern cleats. The bridle would be nylon.

Hopefully, this distribution of load would stop bits breaking off.

The same solution I would adopt whether towing or being towed (the latter is what the RNLI advise).
On our boat the aluminium toe rail is massive. It is also bolted through the deck every 3inches. We have 5mm s/s plates made to sandwich the toe rail so we can connect a large shackle to spread the load along the toe rail.
 
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The elasticity can't distribute the load if there's a hitch on each cleat, as you said.
Elasticity can only distribute a proportion of the load to a downstream cleat hitch/turn depending on the elasticity and initial tension of the connecting line. You cannot determine what that tension would be in all cases and will differ each time its rigged.

The only really way to equally distribute the load is with a proper balancing rig using a multi bridle using biocks to ensure the load is equally shared. If you intend to use for-deck Samson's post, center cleat, and aft cleat you would need at least a 3 stage bridle which IMHO is over complicated.

IMHO if you at least match the rigging attachment plate the strength should be Ok as the attachment is not below the water line and if pulled out the boat won't sink in the same way as if rigging attachment plate pull out.
 

Scotty_Tradewind

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Yes comments about absorbing the load so far, make a lot of sense to me.

Its those snatch loads that are extremely high.

Rather like a good thick and long 'elastic' nylon snubber on an anchor chain, as/Vyv Cox in the latest March 2016 YM, you probably need to incorporate a similar design into the drogue or a tow line if possible.

I've thought about tow lines in the past and I think I subscribe to taking the line around the stricken vessels rear end then via strong articles on the way to a bow bridle and then the tow line onto the bridle.

In reverse if you want to string the drogue out behind, take the line around the bows then around every strong fixed position on the boat to the stern bridle/fixings.
I'm thinking it's the major strength of the hull that I would try and rely upon first.

S.
 

northcave

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Generally speaking from Don Jordons notes and from the experiences using a JSD that have been documented, the snatch loads are low. The cones allow a fairly smooth de-acceleration. I can't comment on a single large drogue.
 

Scotty_Tradewind

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Generally speaking from Don Jordons notes and from the experiences using a JSD that have been documented, the snatch loads are low. The cones allow a fairly smooth de-acceleration. I can't comment on a single large drogue.
that sounds comforting, have you some links to online experiences you refer to?

This then shines a different light upon the towing of a Jorden and that of being towed by another vessel.

S.
 
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JimBrown

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I think you're over complicating this.

The suitable JSD for your boat has a load of about 10,000 lbs or a conservative 5 tons. The attachment part of the rode is 5/8th double braid which has a similar (5T) breaking load, so the loads (shock, static, or combination can never exceed this, or otherwise the rode will snap).

If you go into Skenes and look up a chainplate for 5 tons (which is 9/32nd 1x19 wire rigging) it shows a 1/2" shackle pin attached to a 5/16 (8mm) chainplate. Jordon actually recommends a 1/4" (6mm) attachment plate but Skene is very conservative with a good margin of error.

So a single 12mm bolt in shear will adequately attach the plate to the hull. Two 10mm fasteners will more than adequately locate the chainplate, but also stop it rotating and provide another increase in safety margin. A third would help disapait the sheer into the GRP hull.

In fact it's the design of the backing block and reinforcement of the hull that is the only tricky bit of this because you want the reinforcement and the hull laminate to work as one unit. There is also likely to still be some curvature of the hull in that point, so bedding the reinforcement onto something stick (epoxy resin and hard filler (or less good a PU adhesive) ) would work. The reinforcement needs to be a piece 8mm glass plate (300 x 200) with a slightly smaller piece of 8mm aluminium plate a on the inside and then large penny washers under the nuts.
 

northcave

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that sounds comforting, have you some links to online experiences you refer to? This then shines a different light upon the towing of a Jorden and that of being towed by another vessel.
The theory as per Don J:

The overall forces that are generated by a Jordan Series Drogue are lower and the peak shock loading forces that the boat is subjected to is far lower by design. The design of the JSD allows it to gradually increase the resistance applied to the boat as the rode becomes more heavily loaded—and doesn’t have the issues with collapsing and suddenly re-deploying a parachute sea anchor does.

Experiences:


"First, the forces exerted on the boat by the drogue were truly impressive. The pull from the JSD, while gradual, would cause us to stagger if we were not holding on. "

"Further, as the boat is moving the rode is under constant tension so there is no shock loading even as waves strike the stern. "

http://www.cruisersforum.com/forums/f90/jordan-series-drogue-experience-119799.html


"We moved gently downwind at up to one and a half knots. As each crest passed under the yacht we accelerated gently forward until the drogue applied its force. Tailored to the displacement of my yacht, the drogue applied its force softly"
http://www.yachtingworld.com/features/a-jordan-series-drogue-63180
 

thinwater

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a. And now that the rope has stretched on one side, the bridle is no longer centered.

b. The stress on the sampson post side will be no greater than the ratio of load to that lost by friction in one turn, I think you will find that a full turn absorbs at least 90% of the load, so it will not distribute as you think. Notice how easy it is to hold a line with a turn? Below is a capstan evaluation; a cleat transfers load more quickly due to sharper turns. At 10000 pounds, 9000 will remain on the stern cleat.

c. I'm also pretty sure that if any line fails you will want to cut the whole thing free; it will be pulling you sideways through breakers.

d. You also have a whole in the boat were the cleat pulled out.

Number
of turns Coefficient of friction μ
0.1 0.2 0.3 0.4 0.5 0.6 0.7
1 1.9 3.5 6.6 12 23 43 81
2 3.5 12 43 152 535 1881 6661
3 6.6 43 286 1881 12392 81612 437503
4 12 152 1881 23228 286751 3540026 43702631
5 23 535 12392 286751 6635624 153552935 3553321281
 

thinwater

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Simple math, actually. The JSD can only generate so much force at any given speed. Totally different than a ground anchor, no relationship.

I have also taken a running go at many drogue types on non-stretch line. Not much jolt, they just rip through the water, like they should.
 
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prv

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I have no experience to back me up but all this fills me with horror.

I believe I have read that towing lines, whether to take forward or backward pulls should have their load distributed as widely as possible. Bolts in a line sound like a recipe for disaster. If one fails, the rest would surely follow.

My own solution would be to rig a bridle starting at my samson post (which passes through the grp deck reinforced with 4" of ply and then down through the keel with a stainless plate below) round both bow cleats, round four midships cleats and round both stern cleats. The bridle would be nylon.

Hopefully, this distribution of load would stop bits breaking off.

The same solution I would adopt whether towing or being towed (the latter is what the RNLI advise).
So presumably, on the same principle, your shrouds lead to cables running under the hull, a turn round the keel, and up the other side?

The rest of us use chainplates, and the system for dedicated drogue attachments is closely modelled on these.

The RNLI and RYA rightly advise a bridle because most production yachts aren't equipped with the massive strongpoints being discussed in this thread. If they were, they could be used.

Pete
 

Neeves

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Going back to the OP's question. I think it unlikely the yacht was built and it being envisaged that the laminate at the transom would be needed to take an additional 10,000lb of load concentrated over a relatively small area. I think it inevitable that the glass will need beefed up. Because the yacht might veer, hit by a breaking wave, it should be anticipated that all the load will develop on one arm of the bridle and this then means doubling the load bearing capacity to 2 times. Equally if this is accepted then the load bearing area needs to cover the transom itself, as mentioned in an earlier post, and the idea of a chain plate down the aft of the hull and wrapped round to the transom has merit. If the attachment is to an extension of the plate down the hull, as illustrated, I'd want to beef the protrusion up to stop it bending.

The size of the chain plate itself and numbers and sizes of bolts are easy to determine and there is a strong measure of agreement - its the amount of extra glass that is needed that has not been defined (which is what the OP was asking).

It should be possible to engineer the whole assembly and device such that there is a, or are, weak points which would give early indication that loads near the design loads for the laminate or the chain plates are being approached. An obvious point would be the shackles as they tend to be built to specification , and not much more (especially Grade B shackles). However if loads are approaching limits the last thing you would want to do is dump the whole thing but the alternative would be to deploy longer rodes and having blocks on the shackles and lots of spare rode (like 2 times the length) would be useful. Basically Grade B shackles will fail at specification loads, and deform prior (from slightly more than half specification load) - if you see deformation deploy more rode (as you get more elasticity). This means the rodes would need to be secured not on the shackles but, maybe the aft horn cleat - but I might assume they are sufficiently strong enough already.

There are great similarities to anchor snubbers and bridles (and guessing what Vyv has said - it will merit a read, I had not appreciated the March edition has been issued). Also tow ropes - but only if the tow rope is very long and elastic.

Jonathan
 

Graham_Wright

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So presumably, on the same principle, your shrouds lead to cables running under the hull, a turn round the keel, and up the other side?

The rest of us use chainplates, and the system for dedicated drogue attachments is closely modelled on these.

The RNLI and RYA rightly advise a bridle because most production yachts aren't equipped with the massive strongpoints being discussed in this thread. If they were, they could be used.

Pete
Now that's silly!

However, my chain plates are massive as are the bulkheads they are through bolted to and these descend deep into the hull. There is no load on the deck. The surveyor praised them commenting they were the best he had ever seen.

The chain plates being discussed for the towing bridle obviously do not attach to bulkheads and rely on the shear strength of the hull when the pull is in line and the backing plates when the pull is to one side.

Edit;- No that is wrong. If the pull is to one side, one part of the bridle slackens and the other tightens. The first would not then rely on the backing plates and the second would be pulling in to the hull.
 
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Graham_Wright

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a. And now that the rope has stretched on one side, the bridle is no longer centered.

b. The stress on the sampson post side will be no greater than the ratio of load to that lost by friction in one turn, I think you will find that a full turn absorbs at least 90% of the load, so it will not distribute as you think. Notice how easy it is to hold a line with a turn? Below is a capstan evaluation; a cleat transfers load more quickly due to sharper turns. At 10000 pounds, 9000 will remain on the stern cleat.

c. I'm also pretty sure that if any line fails you will want to cut the whole thing free; it will be pulling you sideways through breakers.

d. You also have a whole in the boat were the cleat pulled out.

Number
of turns Coefficient of friction μ
0.1 0.2 0.3 0.4 0.5 0.6 0.7
1 1.9 3.5 6.6 12 23 43 81
2 3.5 12 43 152 535 1881 6661
3 6.6 43 286 1881 12392 81612 437503
4 12 152 1881 23228 286751 3540026 43702631
5 23 535 12392 286751 6635624 153552935 3553321281
I'm not sure if this is directed at me - a,b etc?

I have never tried the setup I suggested and the comments are useful.

a. If there is a bridle, that is true of any arrangement.

b. That is probably true with a constant pull but these lines are (probably) wet and the pull is intermittent so they will probably slip. I think they would equalise.

c. applies to any arrangement and "just" requires a sharp knife.

d. the "whole" (sic) is unlikely. The surveyor's comment on my backing plates was "Wow!"

However, this whole (!) discussion has clarified my thoughts and I think I would trust the samson post to take the total load using the aftermost cleats just for turning the two parts of the bridle and separating them.

The thought remains that the RNLI when taking a vessel in tow is to do what I have suggested, albeit in reverse.
 

pcatterall

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So..... is it reasonable to say that when we buy a drogue ( Jordan series or 'Jimmy Green' single) we must fit a set of drogue 'chain plates' or risk loosing anything we attach the drogue to?

Interesting to read the comments on securing tow rope bridles and 'sharing' the load etc. I have always wondered how this is achieved. As we all know a single turn around a cleat is enough to absorb most of any load.


Also I guess that when chain plates are fitted as originally designed equipment the hull area strength will have been assessed/beefed up
 
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