How much reinforcement?

[notoldbilbo]

I seek guidance.

I need to reinforce the inside rear hull structure of a 1970s GRP offshore sailboat to take loads which will be transmitted into the boat by my deployment of a Jordan Series Drogue in event of a severe storm.

I append a diagram of the JSD arrangement, a process which is well proven over >40 years of experience.

Those loads will transmit progressively’ into the boat via a bridle and a pair of 95mm titanium ‘padeyes’ bolted to the rear quarters of the hull, as shown.





While the original 70s layup is thought robust, I am advised that substantial reinforcement in the vicinity of the bolted padeyes is required, sufficient in itself to carry the extreme Maximum Design Load from extreme wave strike – which would be around 1280 kg each per side, in near-horizontal Shear. Lesser wave strikes should average about 1/10 of the extreme.

The moulded ‘bump’ shown left of the padeye is concave on the inside. The hull shape is also curved – about 1cm over 20cm ‘top to bottom’. I have rigid flat-plate 12mm D10 material, which cannot be bent to shape. It’s unlikely I could use that successfully, as torquing up the bolts to pull the D10 and the hull GRP surface together would likely distort and crush the original layup.

The above shows the general idea, and a bronze plate 'washer'. I can't use the D10 material effectively, I think, due to curvature of the original hull.

I am advised I should use triaxial or even quad-axial cloth reinforcement internally, in sufficient layers. One friend insists I should use a woven ‘glasscloth' with similar physical characteristics to that used in the original hull layup, and certainly not carbon.

Might triaxial cloth 1200gm x 2 layers, or perhaps 600gm x 4, be thought most suitable?

How much epoxy/hardener and what type should one mix up for each notional 600mm x 600mm piece to ensure saturated and very sticky wet-out?

Access – via the rear deck hatch - is certainly not easy. I can get an arm and a head in there, but not both arms simultaneously. I'll prep using an angle grinder and wire brush, then acetone, then a rolled-on layer of epoxy.


If I can make the wetted-out pieces of fabric sticky enough to stay in place when 'presented', using silica or microfibres, I can then get a roller in. I could position any subsequent pieces once a first piece is gelling….

It needs to be strong enough in shear to resist being pulled out, when loaded as above. However, like an aircraft ejection seat it simply MUST work when needed. A ‘cosmetically-refined’ job is not needed. It does not need to look pretty.

What do you think? What do you recommend?

[User 09]

Ugh! I feel for you on this one. I plan to also add a Jordan drogue and subsequent reinforcements of the hull. I also can not access my starboard quarter. I might have to take out the steering quadrant to get in there to do a proper installation, or i will cut an opening in the cockpit and seal it back up.

When I get to mine, I was going to machine a gigantic backing plate from 3/4" thickness polyester FRP laminate, maybe somethng in the order of 12" x 16"'ish rectangular piece.

As you know the hull is rounded in that area, so was going to thicken epoxy with 1/2" chopped strands and milled fibers with a bit of cabosil until its almost a putty consistency and adhere this backing plate to the hull. The thickened epoxy should fill in the curve.

I was then thinking of laminating on top of that with 40oz fiberglass cloth to build up an additional thickness on top of the FRP of about 3/8" thickness at least feathering it into the hull in progressivly larger overlapping pieces. On top of all of this would be a 3/8" thickness stainless backing plate through bolted to a smaller backing plate on the exterior of the hull.

I'm no engineer but would hope this would suffice and send the strike loads from waves hitting the transom into a larger area of the rear part of the hull. Probably way overkill, but i would not want to loose an attachment when the drogue is out.

Also, a side note to consider if you have not already purchased you drogue. Maybe carefully consider oversizing your drogue one or two sizes larger than what your boat requires. Suzy Goodall lost her boat in the GGR race because the line that attaches her drogue snapped right behind the connecting point. It was surmised that her drogue, although sized appropriatly for her boat maybe should have been one size larger to cope with the extreme forces.

[Montanan]

Are you mounting the hardpoint to the transom or the starboard and port sections of the hull?

Are you mounting at the deck or the side of the hull?

[notoldbilbo]

Originally Posted by Montanan

Are you mounting the hardpoint to the transom or the starboard and port sections of the hull?

Are you mounting at the deck or the side of the hull?

The 'Harken Special Titanium Padeyes' ( don't ask - they're irreplaceable! ) are probably good for something north of 20,000lbf. They'll be fitted, as shown, on the quarters.

That is, on the hull sides P&S.

I've done my sums as per Don Jordan's recommendations, working with the MAX LADEN WEIGHT and not the builder's 'light ship' estimate. I'm using 10mm Dyneema throughout, avoiding needless mid-length knots which reduce Breaking Load substantially.

[Jim Cate]

Have you considered doing the reinforcement on the outside of the hull? Removes the risk of poor access to the inside leading to unsuspected weakness in the job.

Jim

[notoldbilbo]

Originally Posted by Jim Cate

Have you considered doing the reinforcement on the outside of the hull?

Yes, thanks, Jim. The size and shapes I envisage would make for a startlingly ugly ( double ) excrescence. Better, I think, that a bodge is hidden inside - provided it is strong enough.

And I'd still need to guesstimate how much 'stuff' I'd need to do the job. I'm thinking that a rigid new layup, about 600mm x 600mm x about 10mm PLUS the existing hull layup may well be quite sufficient.

I should be able to get a doubled layer of 1200g triaxial cloth to stick to the hull insides, even if I use 2mm barbecue meat needles through the original layup, in way of where the bolt holes will be drilled, to hold it in place temporarily.
Should some more competent reader so persuade me, I ought then to be able to get subsequent layers to stick - if required.

Once all that has cured, and with holes drilled for 4 x 12mm Bumax 88 bolts, a bronze washer-plate of 125mm OD ( redundant winch pads ) will be secured under the torqued-up nuts.

Will that be enough? Who knows....?

[wholybee]

Is that a chainplate aft of the padeye? What does that bolt to inside the boat? Chainplates are usually the strongest part of the structure, so I wonder what sort of reinforcement is already there.

Consider the entire operation. How are you going to deploy the drogue, how are you going to adjust the length that is deployed, and how are you going to retrieve it? Is that padeye location suitable for those tasks? Drogues are often retrieved via a winch. The one time I used a drogue, it was attached to a cleat, and I was able to transfer to a winch to retrieve it.

[notoldbilbo]

Originally Posted by wholybee

Is that a chainplate aft of the padeye? What does that bolt to inside the boat?
Consider the entire operation.

Yes, thanks, 'holybee'. There's a pair of them, for the split backstay. The reinforcement within the transom has proven adequate over ~40 years.
I'll ensure the internal nuts remain accessible for surveyor's attention.

As for deployment and recovery of the JSD, I've researched this every which way, and believe I have at least one viable and practicable method sorted.

And a 'Plan B'....

[Jim Cate]

Originally Posted by notoldbilbo

Yes, thanks, Jim. The size and shapes I envisage would make for a startlingly ugly ( double ) excrescence. Better, I think, that a bodge is hidden inside - provided it is strong enough.

?

It needs to be strong enough in shear to resist being pulled out, when loaded as above. However, like an aircraft ejection seat it simply MUST work when needed. A ‘cosmetically-refined’ job is not needed. It does not need to look pretty.

Oops! I thought you really meant this!

But if faired in nicely, it would not need to be really ugly...

Jim

[dlj]

Originally Posted by notoldbilbo

The 'Harken Special Titanium Padeyes' ( don't ask - they're irreplaceable! ) are probably good for something north of 20,000lbf. They'll be fitted, as shown, on the quarters.

That is, on the hull sides P&S.

I've done my sums as per Don Jordan's recommendations, working with the MAX LADEN WEIGHT and not the builder's 'light ship' estimate. I'm using 10mm Dyneema throughout, avoiding needless mid-length knots which reduce Breaking Load substantially.

What size bolts are in those pad eyes and what material are you using for the bolts?

From the looks of that base foot print, I'd think you'd want reinforcement on both the inside and outside. Can you post dimensions of that plate also?

dj

[Don C L]

What size and weight is the boat? Is that an Albin? I can't see in your images and description where a failure might occur; what you describe looks robust for the boat but I am not an engineer. I wonder if Eigenvector is still a member here? He sure helped me out with engineering calculations some years back. The fittings I have seen for drogues have been large chainplates with 4 or 5 bolts, though-bolted to the quarters. I haven't deployed drogues, but I would think you'd need to consider the chance the boat may still broach and possibly pull one of the chainplates out at an angle. I hope we hear from those who have experience with that.

[Wanderbird]

Is the hull cored or solid in the area in question? If cored, I would most certainly want to rebate the core in the area in question before applying a heavy structural layup to a thin internal skin only.
I would suggest you make a pattern to cut glass for the layup, do the whole layup on a small sheet of heavy plastic on a bench with slow epoxy, and then apply the whole layup to the interior of the hull plastic side out, ie instead of applying a single ply at a time apply them all. If you add a peel ply as well it will save you some sanding.
If you have core in the area in question and remove it, you could do a massive interior layup without having a big hump.

[Rucksta]

In a good hand layup
4x600gsm produces a layup 4mm thick
The first layer will require 750 ml resin / sq mt.
Subsequent wet layers about 600ml / sq mt..

Consider laying up a panel on the bench then cut multiple backing plates of diminishing size.a

Peel ply will give even finish that glues well, low gap.

Do a dry install and ensure the bolts can torque down the not yet glued lamination.

Disassemble, coat bolts with release wax and backing plates backing plates with resin.

Install and torque down bolts.

Excess resin should ooze out the edges.
Wipe with rag.

The secondary bonds between the layers will be weaker than a wet layup but stronger than the secondary bond between old and new work..

Diminishing sizes are to reduce any possible stress riser created by a hard edge.

[Wanderbird]

Originally Posted by notoldbilbo

Yes, thanks, Jim. The size and shapes I envisage would make for a startlingly ugly ( double ) excrescence. Better, I think, that a bodge is hidden inside - provided it is strong enough.

And I'd still need to guesstimate how much 'stuff' I'd need to do the job. I'm thinking that a rigid new layup, about 600mm x 600mm x about 10mm PLUS the existing hull layup may well be quite sufficient.

I should be able to get a doubled layer of 1200g triaxial cloth to stick to the hull insides, even if I use 2mm barbecue meat needles through the original layup, in way of where the bolt holes will be drilled, to hold it in place temporarily.
Should some more competent reader so persuade me, I ought then to be able to get subsequent layers to stick - if required.

Once all that has cured, and with holes drilled for 4 x 12mm Bumax 88 bolts, a bronze washer-plate of 125mm OD ( redundant winch pads ) will be secured under the torqued-up nuts.

Will that be enough? Who knows....?

A doubled layer of 1200 is not even in the ballpark, imho. My Reichhold manual calls out a final laminate thickness of less than 1/8" for a decent hand laminate and that laminate schedule. If you use the aforementioned technique of wetting out on a bench, you can apply twelve plies or more (with slow resin) at a time without trouble, as long as you are not working overhead. I recently did new engine beds for someone, the layup was 12 plies of 1200, was almost exactly 3/8". That is more the sort of layup you should be shooting for, or even more, imho.

[notoldbilbo]

First, thank you all for good input... and esp. Rucksta and Wanderbird.

Here's some answers to YOUR questions...

The boat is a '76 Marcon Cutlass 27' listed at 6500lbs and is 'old school' solid layup. I've based my calcs. on 8000lbs loaded with me, my coffee jug and some muesli. There's little likelihood of Southern Ocean misadventures, but I am very aware that Don Jordan was inspired by the Fastnet '79 disaster in the Celtic Sea to come up with his JSD solution - and that's exactly where I expect to sail.

I've 'sat at the feet' of many who have used JSDs over the years e.g. Jean Socrates, Tony and Susanne Huber-Curphey, Susie Goodall... and tried to learn from each. I know that many others simply bolt on a strip of stainless with X number of bolts 'because that's what Don Jordan did'. But aircraft engineer Don simply wanted to do 'proof of concept' with what he had. He expected US to take his concept onwards with better materials....
Not many seem to have considered HOW MANY bolts and what's behind them.

My four 12mm Bumax 88 bolts - 'known as the strongest Stainless Steel fasteners in the world' - each have a minimum tensile strength of 800N/mm² on a 36 sq.mm section.... which may be sufficient. You do the sums.

Running Finite Element Analysis for the loadpaths guesstimated on this boat would, given the array of necessary assumptions, be IMHO a waste of money to provide, at best, an Act of Faith. Better by far to seek experienced opinion.... which is what I'm doing.

One question I don't have any answer to is that of 'preload' on torquing up the bolts.

In extremis, a broach is quite likely - but the purpose of the JSD is to pull the stern round 'end on' so that the peak loads act through the bridle in line.