Backing plates

[zoidberg]

I'll have several backing plates to make up and install - e.g. under foredeck with 10mm U-bolt/inner stay loads, inside boatlegs 13mm bearing units, and rear deck cleats 4 x 8mm bolts - and I'm wondering about the optimum material....
.....sliced teak, sliced oak, 18mm plywood with epoxy coating, carbon sheet? The hull is solid layup grp about 1.2" thick
There will be broad s/s 'penny washers' too.

 

[RJJ]

I'll have several backing plates to make up and install - e.g. under foredeck with 10mm U-bolt/inner stay loads, inside boatlegs 13mm bearing units, and rear deck cleats 4 x 8mm bolts - and I'm wondering about the optimum material....
.....sliced teak, sliced oak, 18mm plywood with epoxy coating, carbon sheet? The hull is solid layup grp about 1.2" thick
There will be broad s/s 'penny washers' too.

Plywood surely cheapest, even once you epoxied? I would be tempted to upsize the washers or add a piece of stainless sheet metal.

 

[thinwater]

1.2" solid GRP? That's over 30 mm.

There is no calculation that says you need a plate to spread the load to the lay-up per se, other than a bolting washer so spread the local compression load. The bolts are only a fraction of the thickness and it is not cored. They won't pull out before breaking. That said, more info is needed to know what suport is needed for non-designed flex loads. Maybe a rib or bulkhead, maybe nothing. A structure question; keels have broken off because the larger questions were not addressed, only the local backing plate calculation.

If the boat is GRP, the obvious material is prelaminated GRP. You can bond it right to the hull, if you like.

Note: standard penny washers bend before they do you any good at all. Try it with pliers and you'll see.

 

[Plum]

I'll have several backing plates to make up and install - e.g. under foredeck with 10mm U-bolt/inner stay loads, inside boatlegs 13mm bearing units, and rear deck cleats 4 x 8mm bolts - and I'm wondering about the optimum material....
.....sliced teak, sliced oak, 18mm plywood with epoxy coating, carbon sheet? The hull is solid layup grp about 1.2" thick
There will be broad s/s 'penny washers' too.

Most "penny washers" are way too thin. As you tighten the bolt they tend to "cup" which concentrates the load around the bolt: totally the opposite of spreading the load. Need to be at least 3mm thick. Better still, a 3mm stainless plate the same size as the ply backing plate with two holes drilled for the 10mm U-bolt. The ply backing should have filler against the hull. Without this it will only bear on the high spots of any uneven grp layup.

www.solocoastalsailing.co.uk

 

[Graham376]

Inner stays usually need more strengthening than just a backing plate, many have a substantial long rib, others have steelwork bonded in under the deck.

 

[zoidberg]

Inner stays usually need more strengthening than just a backing plate, many have a substantial long rib, others have steelwork bonded in under the deck.

Yes, Graham. I'm trying to figure out how best/easiest to do that. I could glass in a transverse beam.... carried down to the hull sides..... and I could use ply and grp, or wide carbon tape, bonded in.

And thanks to others above. Points noted.... The grp is 'old school' of about 1/2" thick, laidup in the days before builders learned just how thin and flimsy they could go.

 

[NorthRising]

I'll have several backing plates to make up and install - e.g. under foredeck with 10mm U-bolt/inner stay loads, inside boatlegs 13mm bearing units, and rear deck cleats 4 x 8mm bolts - and I'm wondering about the optimum material....
.....sliced teak, sliced oak, 18mm plywood with epoxy coating, carbon sheet? The hull is solid layup grp about 1.2" thick
There will be broad s/s 'penny washers' too.

Delrin sheet

 

[thinwater]

Yes, Graham. I'm trying to figure out how best/easiest to do that. I could glass in a transverse beam.... carried down to the hull sides..... and I could use ply and grp, or wide carbon tape, bonded in.

And thanks to others above. Points noted.... The grp is 'old school' of about 1/2" thick, laidup in the days before builders learned just how thin and flimsy they could go.

Your OP said 1.2". Yes, you are going to need reinforcement (strength goes with the cube of thickness, so the difference is 14 times in strength!

To brace an inner stay your going to need to calculate the stress (wire strength is a good guide) and then Google the engineering for the bracing. The smart answer would be go to an engineer. Basically, you are trying to imagine a cabin-width bridge capable of holding several tons. And be careful using carbon tape; it is so much stiffer than the glass that it will try to carry 100% of the load. Tricky stuff, but very useful for the stiffness it adds.

 

[coopec]

Haven't you got any plastic fabricators (or sign-makers) near you? I go to their scrap bins and get all sorts of nylon, plastic , delrin scrap,

Osprey Composites, Inc
www.ospreycomposites.com
Mailing Address: P.O. Box 380 Deale, MD 20751 ADDRESS: 389 Deale Rd, Tracys Landing, MD 20779 301 261-9201 (Metro) 410 867-3022 (Local) 301 261-5370 (Fax)

 

[Skylark]

Yes, Graham. I'm trying to figure out how best/easiest to do that. I could glass in a transverse beam.... carried down to the hull sides..... and I could use ply and grp, or wide carbon tape, bonded in.

And thanks to others above. Points noted.... The grp is 'old school' of about 1/2" thick, laidup in the days before builders learned just how thin and flimsy they could go.

Modern boats are designed and built based upon calculated individual loads and their collective interaction with the hull as a system. Old school boats would appear to be built upon guesswork.

Just use any material you happen to find in your garage and glue it in place with the remainder of the mastic you still have left over from your last job, it’s a shame to waste it. I’m sure it will be fine ?

 

[Graham376]

Yes, Graham. I'm trying to figure out how best/easiest to do that. I could glass in a transverse beam.... carried down to the hull sides..... and I could use ply and grp, or wide carbon tape, bonded in.

Personally, I would be inclined to use hardwood ply to laminate a full width beam the shape of the coachroof with a steel plate where the stay fitting is located. Obviously all encased and glassed to coachroof but, have no idea how thick it would need to be so would have to obtain advice on that. No doubt there's a boat building forum somewhere with more design and practical expertise than here.

 

[Laminar Flow]

I'll have several backing plates to make up and install - e.g. under foredeck with 10mm U-bolt/inner stay loads, inside boatlegs 13mm bearing units, and rear deck cleats 4 x 8mm bolts - and I'm wondering about the optimum material....
.....sliced teak, sliced oak, 18mm plywood with epoxy coating, carbon sheet? The hull is solid layup grp about 1.2" thick
There will be broad s/s 'penny washers' too.

I think you need to look at the prospective loads and their direction.

The bolts for the yacht legs will be mostly subject to sheer loading and the actual loads will be relatively low. Equally, the bolts holding the aft deck cleats will predominantly be under sheer. For both these applications I would use the 18mm ply bonded to the hull/deck to prevent any oil-canning or warping and to increase the bearing surface.

The u-bolt for the inner stay is under tension only. Unless the stay is carrying a sail, the load is actually quite low as the stay has to only keep the mast in column. If you carry a sail on your inner stay, calculate the load using 1lbs per square foot. Here, I would be more concerned about bringing the forces down to some strong point such as the keel or a bulhead to prevent the deck from lifting. If the load is modest, I would simply also use ply with an oval backing plate in SS that suits the size of the u-bolt (essentially, the size of two heavy duty washers connected across the centre.

 

[doug748]

Most novices go crackers with engineering solutions, including myself. Many builders put cleats on with standard washers because they knew that on a solid deck, with bolts in shear, they would be perfectly adequate. I like to make a plate but you could argue it is better to have a cleat pulled out with an unfair load, than lose a chunk of deck.

With the cap shrouds however you may want to bond in a fillet of some sort, or bracket the thing to an existing bulkhead, otherwise you may be relying on the strength of the deck hull bond which may be ok, maybe not.

My yacht legs are fixed with a 1/2in ply backing pad with a stainless plate over the bolts but there is also a casting on the outside

I beefed up my inner forestay attachment a couple of years ago, with a bracket to the existing bulkhead, Phoenix 316 in Plymouth fabricated it from a model in plastic.

.

 

[thinwater]

Most novices go crackers with engineering solutions....

Actual "engineered solutions" are calculated. That is the "engineered" part. WAG solutions are a different matter.

As several have pointed out, step one is deciding what the actual loads are. For cable loads, you use the strength of the cable. If you really believe that is too far over strength, then down-size the cable. For legs, I would have to consider allowances for unlevel ground and possibly some pounding when taking the ground. The size of the legs might change (there are engineering tables for columns with differing support arrangements, plus add for possible side load). For cleats, use either the strength of half of the bolts or the strength of the rope, whichever is greater.

Only once you decide on the loads do you start calculating. The OP didn't give us enough information to even start.

This way you don't break stuff.

 

[Laminar Flow]

I designed and built some legs with attachments for our 8.5t boat. I went through a whole painful process to determine the projected loads on the legs and, correspondingly the fastenings to the hull ... the boat dries out with a list of, say, 20 degr., the tanks on one side only are filled and we are having a F12 storm blowing, the local rugby team is vising for drinks and insist on all standing to one side only .... etc, etc. Then, I saw a pic of a 100t trawler with what looked like a couple of 4x4s holding her up and decided I was perhaps overthinking things a bit. The real load is the weight two adults walking from side to side, which is enough to make her tip from one leg to the other. So, roughly, 160kg.
I used 4x M10 screws, washers and a 20x30 piece of 3/4" ply backer epoxied to the inside to attach the top bracket.