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JSD Chainplates - how many holes and where (stress cracking)

Yellow Ballad

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I've designed a chainplate on SketchUp (to learn a bit about 3d design programs during lockdown) and posted them on the Jester Facebook group, two people who's opinions I respect have mentioned about bolt spacing being too close.

In the interests of learning more, I thought I would open this up to the forum for discussion and hopefully pointed to information on what's best.

FB_IMG_1613382546308.jpg

It's 500x75x10mm the three centre bolts are 125mm c2c away from eachother, the doubled up ones 37.5mm c2c and in the middle distance of the three central ones. My thought the offset ones would spread the load onto more area od the grp. The plan was to affix with thickened epoxy oversized 10mm G10 to the inside of the hull (approx 12mm thick) and glass over it, then use 3mm s/s backing pads, I could also attach the actual plate to the outside of the hull using a permanant type sealant as belt and braces. Obviously this is all man maths which was the principal of it it looks like it's strong... but obviously only a fool would ignore warnings.

By Jim Jordan:

The optimum attachment for the drogue is clearly a strap similar to a chain plate, bolted to the hull at the corners of the transom

This arrangement feeds the load directly into the hull and imposes no bending or pullout loads on the hull or deck. For a load of 14,000 lb. a strap 1/4" x 2-1/4" x 18" attached by 6 - 3/8" bolts.......
Now Jim Jordan's straps are thinner, narrower and shorter but recommends nearly as many bolts.

Are any of you guys smart enough to calculate the loads, stresses etc or offer advice either way/post grp being damaged by being ripped out or cracking from bolts being too close together.

Many Thanks.
 

Aeolus

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No expert myself but your design has 7 holes within approx 50 x 300 mm area. Gut feeling that's too many holes through the fibreglass in such a small area. If I was designing it, I would have fewer holes spread along a longer length. Maybe 4 holes over a 500 or 600 length, perhaps in a slight zigzag pattern.
 

Poignard

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No expert myself but your design has 7 holes within approx 50 x 300 mm area. Gut feeling that's too many holes through the fibreglass in such a small area. If I was designing it, I would have fewer holes spread along a longer length. Maybe 4 holes over a 500 or 600 length, perhaps in a slight zigzag pattern.
Yes, it will be like one of those "tear along the dotted line" packets!
 

Yellow Ballad

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Yes, it will be like one of those "tear along the dotted line" packets!
I'm certainly thinking less will be ok, especially seeing Fascadale's three and taking the vertical bolts out but tearing along the dotted line through an inch of glass?

Can anyone suggest an navel architect/structural designer that I could possible speak to?
As mentioned JSD recommend 6 bolts through a shorter strap.

@Fascadale could I ask what dimensions your plates are? And the bolt spacing?
 

Poignard

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If it was me doing it I would just have the 3 holes on the centreline.

Not for any scientific reason but just because it would look right and be what many other boats have.
 

Comrade Red

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The backing plate is just as important, and ideally will be wider. The compression of both plates spreads the load structurally.
 

differentroads

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Apols for posting another response without any engineering foundation, but...

For my JSD I fitted my 500mm long x 50mm wide x 8mm thick stainless straps with four 10mm bolts through to a 3mm backing plate. This was bedded with thickened epoxy onto the topsides which were already 5mm thick and I added a couple of layers of biaxial mat/ epoxy to smooth and strengthen the interior of the topsides and the curve onto my transom. My drogue has 104 cones.

I made calculations based on Jordan's recommendations (can't find my notes now) and thought that the straps might be slightly undersized - one or two more bolts and 250mm short if I remember but I would have to tear out too much cockpit moulding to make it longer. Four 10mm bolts should hold a couple of tons in either plane though, assuming all the pull is on one strap. In conditions that they are not I'll be down below anyway, drunk as a skunk and singing Abide With Me 🤣
 

differentroads

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Btw, the alignment of holes didn't trouble me too much. The hull layup is random directions of glass mat reinforced in my case by biaxial mat. Fairly resistent to 'tear along the dotted line'. Zig zag is better but only offsets the distance slughtly for my straps and bolt holes so I didn't do it
 

Yellow Ballad

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Yellow Ballad also asked his question on the Boat Design Forum in the USA -
JSD Chainplates - how many holes and where

I posted some thoughts re his plates over there.
Ah just replied to you!

Yes thought a design specific group might have calculations to hand. I've done a bit more reading today and think slightly longer plate with 4 bolts spaced at 120mm should be more than sufficient to keep my mind at rest.
 

differentroads

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One thing I wish I'd done on my straps as make them project 10mm further from the transom. Currently when I fit the shackles the eyes rub on the gelcoat of the transom when pulled towards the centreline. I thought shortesf possible would be best to reduce leverage but got it a few mm wrong.

A longer overhang would have also enabled me to make a slot for the shackle - like an anchor has - rather than a round hole. Then I could put the loop end of the shackle through the strap which would articulate better than the pin.
 

Poey50

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I made some drogue plates for Poey50, see post 14 onwards on Making drogues for his explanation of his design
And excellent plates they are too. The design allows the curved part of the shackle (rather than the pin) to bear against the plate. Unfortunately I don't as yet have photos of them as lockdown has prevented that. Beautifully finished and polished, too.
 
Last edited:

thinwater

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If I were unsure of the shear strength of the layup, I would lay up more fiberglass, tapered off over a larger area, until the engineering made sense to me. At the very least, the backing plate would be fiberglass bonded to the hull, to better spread the load.

The other concern is that the load is not always in-line. Common sense and videos will demonstrated that it will vary acording to the steepness of the waves over at least a 30 degree angle, and that the strongest pull will be at a slight downward angle (when the boat is starting to surf).
 

halcyon

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Couple of passing thoughts.

Bolts pass through strap, hull, then backing pad that is glassed in, so bolts are not imposing a load on the hull, but the backing plate. Without the plate the bolt load would be on the bolt centres, thus generating hull stress.

Make backing plate L shaped so the load is on the hull side and transom.

You want a shaped backing plate, a square one will create a stress riser in the hull, the hull is flexible, so a load that swings side ways is constantly bending the hull.

Why not fit a big O ring bolt in the corner of the transom ? this would allow full articulation of the shackle. Again using a L shaped backing plate transfers load to hull as well.

Brian
 

Neeves

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I might move the focus, where all the attention has been so far, to the plates themselves.

No mention has been made as to what they will be made from and it is not clear to me how far 'aft' beyond the hull and the discussed reinforcement the plates will extend.

But I would not skimp and would not use 316 but a duplex grade. At some point the tension will not be in straight line and all the tension, or most of it will be on one plate. I'd be worried it would bend (if it were 316). The weak point is the last or last two bolt holes.

Stainless anchor shanks, or good anchors, are made from duplex.

I'd also check the fit of the shackles in the proposed 'oval' holes. The edges of the holes need to be bevelled (or rounded - I don't know the term) so that you reduce the opportunity for the shackles to lock up in the slot (it happens with anchors it can happen with a chain plate) - and some times when it happens the shackle fails. If necessary I'd take any old piece of steel of the same thickness as proposed) and I'd cut the desired oval holes and actually try the actual shackle in the mock up of the holes - you will, maybe, be surprised at how easy they will lock and then the shackle is side loaded. Shackles loaded at 45 degrees lose 25% or strength (and 50% loss at 90 degrees) and I agree - I've tested it. The shackle holes might be larger than you show on your drawing (you don't mention size of shackle) and the 'least' bit of steel is at the shackle hole. So I'd work out how big the shackle holes need to be and that will determine the amount of steel for both the width and thickness of the plate.

I would pre-bend the chain plate to the appropriate angle to fit the dimensions of the bridle of the drogue.

The plate fits on the side of the transom, obviously, and I might consider welding an end plate, where the plate would be bent as it 'clears' the transom and weld a further plate at right angles to the top of the chain plate to give it more support - if that makes sense. You need a decent welder to deal with duplex.

Jonathan
 

Yellow Ballad

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Yes reinforcing the transom is on the radar, possibly tieing the plates via the backing pad to it as well. I know Roger Fitzgerald strengthened his as he was worried about pinching as the load came on. I have planned on adding g10 backing around the chainplates and the bilge pump outlets anyway. I could have some sort of bar across on the inside between the plates but I have to remind myself that there are a lot of boats out there that use drogues without issues with the hull falling apart. (Susie Goodall's drogue broke before the chainplate ripped out)

Although I went ott on bolts (and have since been corrected) I don't want to over complicate things though, pretty much all plates I've seen are 316 and I've not heard of any issues. The way I'm looking at them is it's a once in a lifetime event and if they keep me alive to finish the crossing then can be replaced/repaired once getting to port.

I do agree having a bend so it matches the bridle angle is a smart thing and somethingI'll be checking.
 
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