Vincent DePillis

Integrating carbon in glass structures

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Am replacing slider hatch and seahood on my Corsair F31with a low profile fixed blister.  (better visibility, lighter, better line routing from mast base lines to the winches.)

Blister is carbon foam prefab piece to be bonded in place after removing the existing hatch surround.  

My concern is that bonding the stiff carbon piece into the soft glass deck will create hard spots that will eventually crack.

My plan to mitigate the risk is to spread the load all the way to the cabin top edges.  I would remove all the gel coat on the aft part of the cabin top and laminate over with carbon, stepping down the laminate and fathering carefully as I approach the edge of the cabin top.   I would reinforce the underside of the cabin top as well (currently thinking stringers, because antigrav lamination is a PITA).

I am also planning to reinforce the (really flexy) cockpit sole, with some carbon foam stringers.  In this case I plan on landing the ends of the stringers on structural bulkheads with some wide (100mm) taping.

Interested in advice in general on how to stiffen a boat locally using carbon without creating downstream problems. 

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Vincent,

I'd be very interested in seeing pics of what you are doing. I'm working on a project for my hatch. I just made a new 2 piece hatch from carbon. My hatch will still slide so relative stiffness is not an issue.   I made two piece that comes apart so I can remove  centre section  and I can later make a high profile blister centre section to give stand up headroom inside to be used only when cruising. The racing version centre section  comes off too so one can stand on the seats and get to all the halyard  clutches at the base of the mast rather than go onto the cabin top. Actually very comfy place to stand while sailing and keeps crew weight fwd.

As far as the cockpit sole is concerned I have 20mm think 100mm wide stringers (edges sloped at 45 deg) underneath running from hull side to side and this has made the floor very stiff. They don't have to go all the way and make contact with the side of the hull, just  most of it. One layer of 100mm wide 400gm uni carbon along the stringer. This is covered with   a wider layer of 400gm DB glass to go over the stringer and onto the under side of the sole (50mm each side of the stringer)  to help bond and transfer load from the carbon uni.  They are spaced around 2ft apart.  Certainly does the job for me.

 

Also there is a stringer  same as under cockpit sole but only 10mm thick foam  under cabin roof and down side to stiffen the cabin.

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Under the cockpit sole around 2 ft ie 600mm.

There is only one in the cabin and that just in front of the hatch opening to strengthen the cabin roof.

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Probably the single most important aspect of carbon fiber is its' stiffness.  Loading stress flows like water, so if you can visualize where and how the stiffness changes you will allow for the transition from flexible to stiff without cracking.  Good luck to you.

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On 7/11/2018 at 4:16 PM, Vincent DePillis said:

My plan to mitigate the risk is to spread the load all the way to the cabin top edges.  I would remove all the gel coat on the aft part of the cabin top and laminate over with carbon, stepping down the laminate and fathering carefully as I approach the edge of the cabin top.   I would reinforce the underside of the cabin top as well (currently thinking stringers, because antigrav lamination is a PITA

Probably way overkill. I'd taper it out over 2-3"/50-75mm and call it good. Decks see mostly compression loads due to rig loads. Think of a boat as a badly shaped beam, with the ends being pulled up by backstay (or mainsheet in your case) and forestay, and mast pushing down in the middle. These stresses run along cockpit sole or side deck. But the back edge of the cabin with companionway opening really makes the stress flow go outward because you've got a big hole there. See FEA picture below. I've seen similar stress images in other boats I've been involved in. (The big red transverse load forward of companionway opening looks like a deck beam). On a small boat like your's global bending loads are miniscule. It's all local water pressure and rig loads.

For under cockpit stiffening beam depth is key to stiffness. Deep as will fit. Mr Hatter's light scantlings sounded fine to me. Bond to bulkheads if you want lots of global stiffening. Taper ends before bulkhead if you just want the sole to be less bouncy.

Don't fit them at all if nothing has broken - deflection ain't failure I keep telling my wife. Um, talking about boat bits flexing, rather than stuff of a more personal nature...

vonMises_stress3_nms7sk.jpg

 

 

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3 hours ago, Zonker said:

Probably way overkill. I'd taper it out over 2-3"/50-75mm and call it good. Decks see mostly compression loads due to rig loads. Think of a boat as a badly shaped beam, with the ends being pulled up by backstay (or mainsheet in your case) and forestay, and mast pushing down in the middle. These stresses run along cockpit sole or side deck. But the back edge of the cabin with companionway opening really makes the stress flow go outward because you've got a big hole there. See FEA picture below. I've seen similar stress images in other boats I've been involved in. (The big red transverse load forward of companionway opening looks like a deck beam). On a small boat like your's global bending loads are miniscule. It's all local water pressure and rig loads.

For under cockpit stiffening beam depth is key to stiffness. Deep as will fit. Mr Hatter's light scantlings sounded fine to me. Bond to bulkheads if you want lots of global stiffening. Taper ends before bulkhead if you just want the sole to be less bouncy.

Don't fit them at all if nothing has broken - deflection ain't failure I keep telling my wife. Um, talking about boat bits flexing, rather than stuff of a more personal nature...

vonMises_stress3_nms7sk.jpg

 

 

Nicely done.  Both on the well spoken description of the loads and, yes, deflections.

 

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