Beefing up the insides for JSD

[bilbobaggins]

I'm planning to fit titanium pad eyes to the quarters of my small sailboat, to take the bridle-arms of a JSD. I reckon the laden 'mid-weight' will not exceed 9000lb, so Don Jordan's calculation of 'Design Loads' for rode and bridle arms at 7200lb and 5040lb each should be readily achievable using modern materials.

I dislike the external 'bolted-on steel strap on the quarters' idea. No work seems to have been done to determine the optimum load paths into the grp hull layup, nor optimum bolt size/number/placement. It's actually quite a complex engineering issue, and JSD users have just gone with 'That looks OK - probably. I hope....'

I want to put the reinforcement on the inside, and bolt on a pair of Harken 95mm titanium padeyes, rated at 20,000lb or better, as shown below. ( I happen to have them lying around ). The BUMAX bolts I've acquired are good for far more - each.

The issue I'm exploring is how best to beef up the inside of the grp hull, which is about 8-9mm thick. It will be seen there is a moulded-in 'rubbing strake' feature. On the inside this results in a 'dished' shape, which I propose to fill flat with thickened West epoxy ( G-flex or Six10 ).... adding a larger sheet of woven glasscloth, epoxied on/rolled flat ( with peel ply ), then a bonded-on  'plank' of grp sheet about 12mm by 700mm long by about 250mm wide..... each side port and starboard. This could be doubled, if thought needed. Access is not easy, but practicable.

I'll also bond on a section of 12mm grp 'plank' transversely into the transom corners, or a couple of layers of woven glasscloth.

The padeyes will be bolted through all this, with a 3mm by 700mm by 95mm strap of s/s bonded onto the insides acting as a long 'washer'.

I can't do a 'test rig' on the likely strength of the reinforcement when/if loaded to near 'Design Load' but am reliant on experienced guesses. Of course I need to make the whole setup 'massively enough' so there's no realistic prospect of catastrophic failure anywhere. A couple of good engineers have looked at the ideas and approved 'in general terms'.

What does the team think?

 

[El Borracho]

12 x 250 x 700 mm GRP backing plate?!?

 

[longy]

These padeyes are to be mounted on the hull (not the transom) correct?? Then your 4 (each side) bolts are holding the load in sheer. Have you looked at how the bridles will attach to the padeyes? What happens when the hull slews to one side & the bridle has to wrap around the transom corner?

Since mount bolts will be sheer loaded, you need to determine the laminate thickness to hold that sheer load, then how much laminate to add to pick up that load & spread it forwards along the hull. Adding 'planks' won't do much, as the only added strength is the bond joint strength under the 'plank'

I'd add considerable laminate from transom corner forwards in a 45 deg fan (centered off horizontal). Considerable thought must be made as to thread alignment - you need uni threads going forwards to carry the loads, and 90 deg threads at the bolts to create sheer strength

 

[MiddayGun]

I've read a fair few reports of people deploying these in anchor. Sometimes off stern cleats. (not recommended)

Has anyone actually managed to rip out their sea anchor chain-plates?

 

[longy]

The possible max loads for a JSD (as per JSD) are the weight of the boat. There is nothing stock mounted on any boat (ex maybe a metal build) that comes anywhere near those numbers. Add in the need for a fair lead for the highly loaded bridle and you can usually eliminate anything not mounted to deck edges. I would guess maybe 1% of these ever get used in anger? So not a large data base.

 

[bilbobaggins]

Thanks for the above. I've had lots of spare time these past two years to research. I've studied everything anyone has written on JSDs - including all Don Jordan's original material - I've scoured the forums and website comments, and I've spoken with all those individuals I could access who'd actually used a JSD - including one young lady who had one break on her. I've also consulted the rope makers, lifting gear manufactures, and the bolt makers in their Tech Departments. From all that, I determined that MY JSD will be way over-engineered, for there must always be assumptions.

Here are a couple of pics which better show what's intended, for joining the bridle legs to the padeyes and to the rode. The soft shackles are built for me from 11mm D12 Dyneema, which is rated 'single line' at 11600lbs. Some will recognise the Improved Soft Shackle - it gives notionally 230% of the basic s'shackle. Mine are made for me by probably the best in the biz here in southern England. I'm content to assume I'll get 200%.... That's more than 4 times the 'Bridle Design Load'.

The second pic shows the bridle-rode connection, before the anti-chafe sleeve tubing was delivered. Some expressed concern that the s'shackle could open during cyclic unloading. I'll use reusable cable ties to prevent that, although the antichafe sleeve also contributes.

As for chafe on the transom corners, there are plenty of glue-on antichafe patches available.

'Longy' is right in suggesting I should seek guidance on the number of plies of S-glass wide tape I would need to laminate in, rather than 'planks' of 12mm grp sheet which I already have. I'll probably go pester the tech director at Wessex Resins ( UK ) again, unless someone from West Systems - who sponsor this - reads the post and pitches in.

As for 'less than 1% of people actually use these', I view this kit in the same way I view my lifejacket and liferaft. The waters I sail include the Celtic Sea and Biscay, and are the exact same waters in which the Fastnet 79 storm took place. That disaster was why Don Jordan developed this solution. My boat is vulnerable to such conditions. It's entirely appropriate for my intended sailing.

 

[longy]

I don't think you want to use any basic level cloth. You have conflicting forces to deal with: sheer load from the bolts will want the threads running vertically (normal to the load) and to carry the loads out across the hull will want fore/aft thread lines radiating out from bolts. I'd be considering an un-woven heavy biax cloth (one layer of thread horizontal, the other vertical) possibly sandwiched by 45 - 45 cloth.

And your bolts need to be smooth shanked for the thickness of the laminate.

 

[CaptainAhab]

Your original description of construction would be suitable for a load pulling nearly square from the transom. Basically adding some thickened filler to get a flat/smooth surface then bonding on a serious backing plate.

As others have noted that is not what you need to engineer. 

Easiest way to imagine this problem is to install your hardware and bridle. Hook the sling onto a crane and pick up the boat. It would be dangling bow down, transom up.  Beyond shock loading, the typical load is the weight of the boat.  What's going to keep the bolts from sawing thru the hull? 

You need to grind whatever texture is there back to bare fibreglass with 80 grit then get good bonding directly to the old glass. What about the hull construction? Glass/foam? Solid glass? 

If its a cored boat then you should grind thru the interior skin, remove the core, then replace all of that with solid glass. Otherwise you are relying on the outer skin and core to not be crushed or torn by the bolts. The load path is line, pad eye, bolts, outer skin, core, interior skin. You are trying to reinforce the wrong area. The bolt/hull interface is the key. 

The simple answer is you need to make that entire corner of the boat solid glass. If you look at your picture it's what's framed by that ladder looking thing. Rounded on the front, has to wrap around the transom. Minimum 100mm solid glass in all directions including around the corner. 

Better yet, pay a composites engineer for their opinion.

 

[bilbobaggins]

It's not a cored layup, Captain Ahab, but thanks. As I stated, it is 'grp'.....

Conventional practice, if there is such a thing, is to take a strap ( or two ) of stainless, drill a number of holes in them, and bolt them with some gunk to the quarters. There's no known relevant 'composites engineering' on record..... as I stated. Don Jordan's comment on this was scanty - and he was an aircraft engineer. I have a bit of background myself in exposure to that discipline, and I know there are always 'other paths'. My task is to find one that fits, and is 'do-able'.

One example of 'that looks about right' without data is shown below, on a steel boat. There's no good reason I can think of why a 'horizontal chainplate' cannot be fitted internally as well as externally.... or both.

What is evident is that the mechanics of bolted-on plates is surprisingly complex. I've hunted for published info far and near, and found none that is directly relevant. I've looked at research papers, 'stress path predictions' and short of a complex/expensive Finite Element Analysis investigation - which I know has to make multiple assumptions for some of the input data - I can't afford to spend £/$thousands on a one-off small boat solution.

I need a tame engineer or three to exercise lifetimes of judgement, agree a consensus, and I'll go with that.

 

[MiddayGun]

The layup on the inside of the hull for where the external chain-plates on my boat was for 7 layers of 600gm biaxial matt. Smallest 150mmx200mm - going out to 500x500mm.  (inside of the existing hull)
Rigging is 6mm 1x19, lowers & caps to the same chain-plate. There should be a good safety factor built in.
Loads seem roughly comparable to what you have stated above. Especially as your hull is thicker to begin with. 

However a strap would be more suited to bearing this kind of load, preferable one with an offset bolt pattern so they aren't all in line. 

 

[CaptainAhab]

View attachment 464797

So the obvious difference between this and your design is the number of fasteners & the fasteners spread the load a far distance towards the bow. They are also up right on the deck where the load is driven directly into the corner & around.

If you go to a decent structural engineer and describe the crane scenario. Change the object to a rectangular box. I'm imagining a 40' shipping container made from 6061-T6 aluminum same weight. Give him the wall thickness, load point locations, number of fasteners. They will be able to calculate the load on the corner. They should be able to give you a description of how much extra plate there would be welded to the interior corner. This would be with a fat safety factor. 

 

[bilbobaggins]

Thanks for that.....

 

[CaptainAhab]

Thanks for that.....

I just added to it. My Dad was a mechanical engineer. He would be all over this problem.

 

[Autonomous]

My amateur thoughts are rather than drill through the new interior 'chainplate' reinforcement, poke through the new fabric-tow to displace the strands around the fasteners before the resin sets up.

Put a generous fillet at the hull side-transom intersection in that area.

 

[longy]

Why? Loads are forwards, no need to re-enforce the transom

 

[Zonker]

5040 lbs = 22384 N

4 x 12mm bolts 

E-glass shear strength = 120 MPa  (semi-unknown CSM/woven roving layup)

So area needed = 22384 N / 120 MPa = 186mm2 

186mm2 / (2 x 12mm) = (not all 4 bolts because they're all so bloody close together) = 7.8mm thick. But that's a failure load.

So relatively little thickness of solid e-glass is needed. If it were my boat I'd want a laminate with a total thickness of 12mm and taper the new material patch over big area, dropping plys every 12mm. Smallest patch is a bit bigger than the padeye (say 200mm square)

S glass is a bit stronger than E glass but if you have some on hand...use it

300gm/m2 glass is about 0.12mm thick when hand laid up. So to get each 1mm extra thickness you'd need 8 layers of 300gm

Signed - a NA/mech eng. who has done a fair bit of composites engineering.

 

[phill_nz]

just my personal opinion / design ideas for fixing and rigging

i wouldn't use a small ring plate for the anchor point i would favour a long strap ( as per the other pic ) .. i would use coach bolts from the outside to another inside plate to avoid the mess of hex heads shown

as close to the waterline as possible backed up inside with 2 - 3 x 24oz woven then csm to fair ( 3-4 x the area of the strap )

the bridle or first few meters made from bungy / shock cord to avoid the sudden high loads that will be the cause of most failures ( laid up to suitable breaking strain plus safety factor )

 

[Autonomous]

Why? Loads are forwards, no need to re-enforce the transom

The bridle will cause some pinching of the transom, maybe not much but the boat will slew and experience significant side loads.

 

[longy]

In the OP's pic up top, the bridle has a very narrow angle, so not much added compression across the transom. If one did want to allow for that, reinforcement would need to run across the transom a bit, not just filling in the corner. A fillet would ease the cloth around the corner, keeping more strength in the lams. Fillets are usually done with a light weight filler are do not add much strength by them selves