Drogue Chain Plates Strength?

[thinwater]

@thinwater - You seem to know your stuff. If you were me with my 10mm thick 45cm ling chain plate and a 10mm solid GRP hull.... would you think such a chain plate would be suffice against approx 8 Tonne of breaking wave force?

Or if you were me what additional measures would you take? More epoxy inside? Longer chain plate? Backing plate with right angle up against the transom also?

Note that my Hull is lloyds standard and the layup as good as it gets from building in Falmouth back in the good old days.

I've simply been doing a lot of testing for an up-coming magazine article.

Let's start by remembering that we are talking about breaking strength, not safe working load. Jordan (and others--he is not the only, but his report is available and well-documented) stated the SWL would likely be 500-1000 pounds; wind load plus some minor breaking seas. The 8 ton value was a one time worst-case and is the BS value. Also remember that while a breaking sea might hit you with 8 tons, that dose NOT mean the load on the drogue is 8 tons. Most of that will dissipate as acceleration and hull drag. The drogue cannot apply more load than it can hold by dragging through the water. That max is about 10,000 pounds at about 20 knots, steady state (less while accelerating). Impact force is a slight misnomer when applied to a JSD. Parachutes and ground anchors are different.

Assuming your statement about the lay-up is true (and it sounds true, but engineers say this stuff), each bolt should be able to hold 5000 pounds in shear. I have tested this in 10mm glass. However, they will affect other fasteners within ~ 120mm down line, so the spacing must be at least 120 mm. Closer than that and less is gained per fastener.

IMHO, 4-6 bolts spaced at least 120mm, with a bonded FRP backing plate (8mm x 100mm x 550mm) and ~ 35mm x 4mm washers is more than enough. I would not bother with the transom tie-in; I think there is actually some chance of stress risers there, unless it was done with laid-on glass. A bonded FRP plate will really spread the load; much superior to SS in shear for that reason. Bond it first (one small screw will clamp it), then drill through, and it becomes a very strong spot on the hull, nearly 20mm.

 

[Fascadale]

This is an interesting discussion: I had fabricated and then I fitted the drogue chain plates shown in the OP's first post to my boat

The boat in question was an Invicta 26, a 40 year old, heavily built long keeler, a Van der Stadt out of Southern Shipbuilding.

She weighed, fully loaded about 3 tons. The hull, where I drilled the bolt holes was about 12mm thick. In one of the photos below you can see evidence of original extra layup around the top of the hull.

Sandwiched between the SS backing plate and the hull was a plate made up of than thinish (3mm?) nylon chopping board. The nylock nuts, with penny washers were done up very tightly.

(the shackles were well moused when the bridle was set up for real)

Fortunately I have not yet deployed the drogue so I cannot really comment on the performance of them system

If I were to make another set of chain plates for that boat I might use slightly thicker SS bar and maybe something L shaped to prevent it bending should the boat slew in a wave. I would also have used pan head screws, maybe have had the plate a foot longer with an extra hole.

I was never quite certain how the drogue bridle would interact with the vane, but I decided that damage to the vane would be the least of my problems in a "drogue storm". I worried too that the vane might destroy the bridle but judging by the YouTube footage I have seen: this is a good onehttps://www.youtube.com/watch?v=IUfHUGHwdaw this is unlikely


In thinking about the deployment of the drogue for real I decided I did not want to be hanging over the stern doing up (dropping) shackles, so like the set up on YouTube, I had the bridle rigged before long passages and stowed as shown below

I am now thinking about designing drogue chain plates for my new boat, a Rival 34.

I have read this thread through carefully and as I understand it there are some saying that the chain plates should be much bigger and the hull should be massively strengthened. From what I have read a boat on a deployed drogue feels as if she is on the end of a spring, there is no snatching. Don Jordan did not write about reinforced hulls (I think), he was suspicious of the use of winches and preferred chain plates

My new boat also has a vane and a narrow stern. Without a transom hung rudder like that of the Invicta, the vane stands closer to the stern so hopefully the bridle will keep clear of it.

I would be interested in any comments, suggestions or recommendations.

 

[pcatterall]

"Why not just go sailing when and where you can just derive pleasure not anxiety.?"

Some good answers to that question already posted! We just want the pleasure of sailing, and certainly don't need anxiety! however part of the pleasure is in knowing that you have the equipment ( and a plan) for unexpected eventualities. Some of the very techy stuff is over my head but I do learn bits from our experts so keep it up all of you!!

Sorry! no offence intended. Some people have mucky minds!

 

[thinwater]

This is an interesting discussion: I had fabricated and then I fitted the drogue chain plates shown in the OP's first post to my boat

The boat in question was an Invicta 26, a 40 year old, heavily built long keeler, a Van der Stadt out of Southern Shipbuilding.

She weighed, fully loaded about 3 tons. The hull, where I drilled the bolt holes was about 12mm thick. In one of the photos below you can see evidence of original extra layup around the top of the hull.

Sandwiched between the SS backing plate and the hull was a plate made up of than thinish (3mm?) nylon chopping board. The nylock nuts, with penny washers were done up very tightly.


Fortunately I have not yet deployed the drogue so I cannot really comment on the performance of them system

If I were to make another set of chain plates for that boat I might use slightly thicker SS bar and maybe something L shaped to prevent it bending should the boat slew in a wave. I would also have used pan head screws, maybe have had the plate a foot longer with an extra hole.

I was never quite certain how the drogue bridle would interact with the vane, but I decided that damage to the vane would be the least of my problems in a "drogue storm". I worried too that the vane might destroy the bridle but judging by the YouTube footage I have seen: this is a good onehttps://www.youtube.com/watch?v=IUfHUGHwdaw this is unlikely


In thinking about the deployment of the drogue for real I decided I did not want to be hanging over the stern doing up (dropping) shackles, so like the set up on YouTube, I had the bridle rigged before long passages and stowed as shown below

I am now thinking about designing drogue chain plates for my new boat, a Rival 34.

I have read this thread through carefully and as I understand it there are some saying that the chain plates should be much bigger and the hull should be massively strengthened. From what I have read a boat on a deployed drogue feels as if she is on the end of a spring, there is no snatching. Don Jordan did not write about reinforced hulls (I think), he was suspicious of the use of winches and preferred chain plates

My new boat also has a vane and a narrow stern. Without a transom hung rudder like that of the Invicta, the vane stands closer to the stern so hopefully the bridle will keep clear of it.


I would be interested in any comments, suggestions or recommendations.

Given the light weight of the boat, it looks good. No worries, I wouldn't change it. But if you want a few comments...

I don't understand the "sandwiched" chopping block.
* The plate should be bonded to the hull if possible. This is the best way to share sheer load and even up the hull. Epoxy for FRP, polyurethane for SS. Polyethylene just makes the bolts longer (weaker) and does not bond.
* NEVER us a plastic chopping block or starboard (it was probably polyethylene, not nylon). Doesn't matter how thick. The stuff creeps over time, the nuts will loosen, and it bends. It does not distribute load and it cracks if loaded. Never, never, never. This is not so bad here, since there is SS over it.
* The plate is a bit thin? You can see the dents under the nuts. Thicker SS bonded with PU would have been stronger.

I'm not much for wood either. It has split in too many of my tests, despite the anecdotal success stories. I suppose it works if it is massive enough. But FRP and SS are much better materials for this purpose.

The truth is, very often I fix something on my boat, live with it, research it, and then learn I could have done something a little bit better. Mostly, perfect is the enemy of good enough. This is good enough, better done than still thinking about it!

 

[Deleted member 36384]

.... My new boat also has a vane and a narrow stern. Without a transom hung rudder like that of the Invicta, the vane stands closer to the stern so hopefully the bridle will keep clear of it. .... I would be interested in any comments, suggestions or recommendations.

Your Rival 34 deck sits on a GRP shelf which runs right round the hull. The shelf extends about 12" into the the hull and both the deck and shelf are glued together. The thickness of the combined hull / shelf is at least 25 mm. I would be inclined to remove the cleats and fit bollards, bolted through the shelf with additional backing plate for the nuts. The deck / shelf is far thicker than the hull layup at the quarters. The bollard has a further advantage of lifting any mounting point clear of the gunwale, which would be a problem if relying on the current cleats.

If you join the Rival Owners Association you can access Peter Brets plans and see what I am talking about. On my own Rival, I was surprised at the thickness when drilling to mount U bolts, it is quite a massive layup.

I would want to test the narrowness of the stern, by making the bridle a bit longer than normal so that the angle is quite low between the bridles and then experiment as it might still be too close to the wind vane if there is any shearing. Find out what shear angle does it catch the wind vane. I would suggest that 45 degrees is a reasonable extreme angle that the bridle could see. If it fouls at less than 45 degrees then you need to have have an extension pole braced against the bollards or go for a parachute type sea anchor. On my own Rival the Monitor wind vane gets in the way if I am towing a dingy off the quarter; not a problem as that is light weight.

I wonder what the effect would be if you towed a JSA on a quarter, would it really be that bad, the sterns of the Rivals are very narrow so your only offsetting about 2' on a Rival 34, if even that. Would the dynamics be catastrophic to achieving a safe, comfortable down wind ride? I suspect not as I can't see it being that sensitive considering its long length, the long fin and deep forefoot of the hull. I don't really know.

 

[Tim Good]

Thinwater ... Excuse my ignorance but what is FRP when you refer to a an FRP Backing Plate.

 

[thinwater]

^^ My bad. Engineering slang without proper explanation.

Fiber Reinforce Plastic. Though it can mean many composites, around boats and in my writing it means fiberglass unless I say something else.

You can either by pre-cast sheets, hack a piece from an old boat (abrasive wheel works best for cutting) or a shop scrap pile, or lay-up your own against something like a cutting board (either slippery or coated with PVA or hair spray as a mould release).

 

[Tim Good]

^^ My bad. Engineering slang without proper explanation.

Fiber Reinforce Plastic. Though it can mean many composites, around boats and in my writing it means fiberglass unless I say something else.

You can either by pre-cast sheets, hack a piece from an old boat (abrasive wheel works best for cutting) or a shop scrap pile, or lay-up your own against something like a cutting board (either slippery or coated with PVA or hair spray as a mould release).

Ummm... so from a quick google search it seems different from normal GRP?

http://www.differencebetween.com/difference-between-frp-and-vs-grp/

Are you interchanging FRP and GRP and mean the same thing? I mean a standard 8mm GRP plate surely can't be as good as an 8mm stainless plate? Also when you say "bond" are we talking chemically bond with some epoxy?

 

[Fascadale]

Your Rival 34 deck sits on a GRP shelf which runs right round the hull. The shelf extends about 12" into the the hull and both the deck and shelf are glued together. The thickness of the combined hull / shelf is at least 25 mm. I would be inclined to remove the cleats and fit bollards, bolted through the shelf with additional backing plate for the nuts. The deck / shelf is far thicker than the hull layup at the quarters. The bollard has a further advantage of lifting any mounting point clear of the gunwale, which would be a problem if relying on the current cleats.

If you join the Rival Owners Association you can access Peter Brets plans and see what I am talking about. On my own Rival, I was surprised at the thickness when drilling to mount U bolts, it is quite a massive layup.

I would want to test the narrowness of the stern, by making the bridle a bit longer than normal so that the angle is quite low between the bridles and then experiment as it might still be too close to the wind vane if there is any shearing. Find out what shear angle does it catch the wind vane. I would suggest that 45 degrees is a reasonable extreme angle that the bridle could see. If it fouls at less than 45 degrees then you need to have have an extension pole braced against the bollards or go for a parachute type sea anchor. On my own Rival the Monitor wind vane gets in the way if I am towing a dingy off the quarter; not a problem as that is light weight.

I wonder what the effect would be if you towed a JSA on a quarter, would it really be that bad, the sterns of the Rivals are very narrow so your only offsetting about 2' on a Rival 34, if even that. Would the dynamics be catastrophic to achieving a safe, comfortable down wind ride? I suspect not as I can't see it being that sensitive considering its long length, the long fin and deep forefoot of the hull. I don't really know.

Many thanks for such a considered and thoughtful reply.............................food for thought, and for further investigation. I shall be up at the boat this coming weekend : I may try to remove a cleat and measure the thickness of the lay up under it..............25mm sounds massive, an inch indeed

Given the light weight of the boat, it looks good. No worries, I wouldn't change it. But if you want a few comments...

I don't understand the "sandwiched" chopping block.
* The plate should be bonded to the hull if possible. This is the best way to share sheer load and even up the hull. Epoxy for FRP, polyurethane for SS. Polyethylene just makes the bolts longer (weaker) and does not bond.
* NEVER us a plastic chopping block or starboard (it was probably polyethylene, not nylon). Doesn't matter how thick. The stuff creeps over time, the nuts will loosen, and it bends. It does not distribute load and it cracks if loaded. Never, never, never. This is not so bad here, since there is SS over it.
* The plate is a bit thin? You can see the dents under the nuts. Thicker SS bonded with PU would have been stronger.

I'm not much for wood either. It has split in too many of my tests, despite the anecdotal success stories. I suppose it works if it is massive enough. But FRP and SS are much better materials for this purpose.

The truth is, very often I fix something on my boat, live with it, research it, and then learn I could have done something a little bit better. Mostly, perfect is the enemy of good enough. This is good enough, better done than still thinking about it!

Again, many thanks for such a helpful reply. As a not particularly scientifically minded person, (I gave up at joules and moles, ) I like your advice "Mostly, perfect is the enemy of good enough. This is good enough, better done than still thinking about it!".

What do you think of BoBs comments about bollards vertically through a 25mm layup, for a five ton 34 foot boat? (Good enough will do)

Message to self...no nylon, no polyethylene, no nylon, no polyethylene, no nylon, no polyethylene

 

[thinwater]

Yes, GRP.

Bonding in this context means epoxy for GRP and polyurethane (3M 5200 or similar) for SS.

And actually, GRP can be just as good as SS in this application. Two reasons:
* GRP bonds better. It really becomes a part of the hull and strength goes WAY up.
* This is about stiffness far more than strength (neither will fail due to strength). The stiffness of the GRP is nearly as good, but becomes greater BECAUSE it is bonded to the hull and is a part of the structure.
* PU for the stainless is mostly for even pressure transfer, like grout to level a floor.
* If the GRP were not bonded, it would need to be thicker. Typically a backing plate should be 50% bolt D if SS and 65% Dia plus a 50% Dia oversize washer if GRP and not bonded. That is what testing has shown. With these numbers, the bolt breaks first (all of this assumes SAE grade 2 316 SS bolts--HT is different).

I am not saying that GPR is stronger than stainless (though the difference is not enormous), but rather that GRP functions better because it becomes a part of the hull and works with it. A structure that is 2x as thick is generally 4x as stiff and strong, and so forth. With stainless, even with the PU, they are still 2 separate pieces and do not act as one.

There are 2 things you are fighting here:
* the bolts can sheer through the glass at about 25000psi side pressure. More glass thickness and more bolts help.
* The whole panel can pullout. Backing plates increase the sheer area.

 

[Neeves]

Thinwater,

I think you, also, need to define what you mean by washer. To me a washer is a round thing, quite thin with a circular hole in the middle, commonly used as part of the reinforcing but equally common - totally inadequate because they are too small and too thin.

In earlier posts here you mention washers - but I think talk of cutting them out - this then becomes to me a backing plate, even if its not very big. But a backing plate could be much bigger, surface area and thickness, than any washer I can buy 'off the shelf'.

You also mention 'fender' washers ' I'm taxing my memory now - is that what I call a penny washer (about the size of an old fashioned UK pre-decimalisation coin)? By stainless I assume you mean 316?


I think you mentioned the work you have done will be in Practical Sailor, where I assume all the detail will be defined - do you know when it will be published?

Jonathan

 

[thinwater]

A fender washer (US) would be about 3cm in dia for a 6mm bolt, and only about 1mm thick. Basically useless for pretty much everything. You think they are doing something, but really snug the bolt and you will see them fold up just as they are needed. On wood they bend so badly they foul the wrench. If a fender washer actually works, you didn't actually need it. Have you noticed that they are nearly always bent when they are removed from anything high-load? I have.

In some cases a perfectly good backing plate can be made by cutting washers in ~ 4mm plate with a hole saw. But a single plate is always better.

 

[Tim Good]

Thinwater....

Ok have a look at this and some Q's

1. Bolts are M12. They are spaced 120 apart from their centers. Do they need to be 120mm apart vertically or can I reduce that and make the plate thinner?
2. I have allowed 12mm from the edge of the bolt to the end of the stainless plate. Should I allow more?
3. I plan to use countersunk bolts rather than normal hex head. That ok?
4. I plan to layup the hull inside by another 5mm and approx 150% larger than the outer chainplate rather than just expoxy in a GRP plate. I felt this would be best practice? Total hul thickness would be about 15mm. I'd then put the stainless 5mm backing plate on that which would be about 120% larger than the out chainplate.
5. I went for 6 bolts instead of 4. Why the hell not!
6. Is there any need for the first bolt to be 120mm from the transom? I've allowed about 40-50mm but maybe this needs to be more?

 

[thinwater]

^^ Looks strong. I would go down to 10mm though, I think. ~ 2000kg each in shear.

At this point, probably the most important factor is the bonding of the layup.

 

[Tim Good]

^^ Looks strong. I would go down to 10mm though, I think. ~ 2000kg each in shear.

At this point, probably the most important factor is the bonding of the layup.

What about the vertical distances of the bolts. Do I need to make a triangle or is the 120mm spacing only important in the direction of pull. I.e. could I bring the plate width down?

 

[Tim Good]

Ok so here is the result of our discussions....

- 12mm thick,
- Reinforced inside to bring the hull to around 15mm thick solid GRP
- 5mm backing plate
- Countersunk A4-70 bolts with shank and A2 Nyloc Nuts (Wanted A4 80 bolts but couldn't get countersunk)
- Crosby shackle rated to 7T working load with 5 times breaking factor.

The lower two photos are the first ones I had made professionally. Unfortunately with my new hydrovane they didn't protrude far enough out so I has to make new ones locally that stuck out well back from the stern. As such I will be selling these on a first come basis on the for sale forum. They cost me over £300 to make and that didn't include the CAD drawings. I'll accept £250. They way exceed what is required and would suit up to a 20T boat.

https://scontent-lhr3-1.xx.fbcdn.ne...9_10154031422408162_1914063807381830691_o.jpg

 

[Neeves]

Though they look lovely

I wondered whether you should have had an oval hole so that the hole/slot would take the bow of the shackle rather than the pin? Comment was passed about loads not necessarily being in a straight line, if the load is offset there will be concentrated loading. Everything looks and reads to be strong enough - maybe I'm just picky.

Jonathan

 

[Tim Good]

You're not wrong. I wanted that but time didn't permit. I wonder if it's possible to do in situ.

Having never used a JSD before I wonder how much the boat yaws about or it is remains fairly straight in relation to the drogue?

 

[Neeves]

Thinwater has been doing some trials with drogues, including (I think) the JSD. When the sun rises where he is he might comment - but if he does not appear I'd suggest sending a PM.

You could enlarge in situ and it would be easiest if you could drill another hole 'alongside' and then file out the 'interconnect'. But it would be so much easier with a bench drill and the end result will look so much better. Do not assume that the 2 shackles are the same size, they vary slightly,. The bow is also not 'circular' it has the embossing on it which can impede articulation if you make the slots a too snug fit (been there, done that)

Finally - Starrr in America will ship Peerlift Grade B shackles by American Post, which makes them a viable source for the UK and Oz. I rate Peerlift over Crosby, 6:1 safety and greater range of sizes. Both Crosby and Peerlift meet their own specifications.

Jonathan

 

[thinwater]

You're not wrong. I wanted that but time didn't permit. I wonder if it's possible to do in situ.

Having never used a JSD before I wonder how much the boat yaws about or it is remains fairly straight in relation to the drogue?

The boat will yaw if the waves are irregular.

What matters is whether it yaws outside the angle of the bridle (does a bridle leg go slack?). The causes chafe and snatch loads. If the load stays within the bridle angle, oval holes are not needed. The correlarly to this is counter intuitive, but is that the bridle legs should not be too long; 2-3x transom width maximum has been widely suggested. This matches what I've seen in testing and is not my original idea.

Ideally, the chain plates would match the angle (about a 20-30 degree bend around the corner of the transom, just as chain plates align with shrouds). The most severe loads, in fact, are when the boat is a little sideways (slight broach) and are inwards against the leward plate.

The same issue comes up when lying to 2 anchors; though wider spacing would seem to increase the load, if the spacing is too narrow they will single load and see more shock loading. In both cases, you want to keep some tension on both legs at all times, if possible.

IF this is the case, then the pin should be OK. There WILL be a lot of up-down rotation. Mouse it good.

---

I'm not sure I'd be hooked on the JSD. I've been testing 2 conventional drogues in series, and you get a lot of the advantages with much easier handling and more versatility (can't use a JSD for steering or slowing a little). A few have tried this in storms without mishap. The main risk of a single drogue is pulling out of a wave face; tandem drogues prevent this. Seems to work with all popular drogues. The total arresting force can equal a JSD, but that depends on the drogue selection. At the same time, some would argue the JSD gives too much arresting force.

---

The testing Neeves refers to is for an up-coming article, and while quantitative in nature, testing was limited to near gale conditions and is mostly focused on steering. I'm not going to say anything about ultimate storms since I never have and never want to see one.