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Opinion on transom core thickness. 1.50" or 2.25”?

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Opinion on transom core thickness. 1.50" or 2.25”?

Old 02-08-2019, 07:58 PM
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Default Opinion on transom core thickness. 1.50" or 2.25”?

My question. Would you recommend 2 layers of 3/4" or 3 layers of 3/4" core material for my transom rebuild?

Backstory. Rebuilding a 1994 Seacat SL5. Total gut job. Transom had wet spots and crushing of core material. Removed all of that and now shopping for replacement core material. Coosa is pretty much impossible to source in Texas. There is a company in Dallas called CoreLite that has a 28# PVC material that would work. Unfortunately they don't stock the material but indicated North American Composites in San Antonio might. Will call them next week. If they don't have what I need I guess I'll be driving to Louisiana or Florida! I'm going to material for the sole also so might as well make one trip somewhere.

I'll be adding knees to the transom or an inner aluminum bracket like World Cat made available after point of sale on some of their earlier models. I’m leaning towards 2 layers because the guys that have already finished their seacats have all said “keep it light”. (Especially when it comes to the sole which will be a future post.)

This hull originally was spec'd for 2 x 150HP. It had 2 x 175HP when I bought it. Prior owner had fitted an aluminum plate that wrapped over top of transom and also tied the side walls of splash well to transom. Worked OK I guess but gel coat cracked where outer splash well corners met top of transom, etc.

As always, thanks for any and all feedback.

Pics of original transom before/after core removal.

ANOTHER SeaCat Rebuild - 1994 SL5
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Old 02-08-2019, 08:05 PM
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Make youR transom twice as strong as you think it should be !!!just need longer bolts that all ! DO IT !!

ME im into solid glass transoms and using shape !Combine that and you have a winner !! at less than 1/3rd the weight of using wood and will last longer than you will live !!
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Old 02-08-2019, 09:39 PM
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Originally Posted by tunnles View Post
Make youR transom twice as strong as you think it should be !!!just need longer bolts that all ! DO IT !!

ME im into solid glass transoms and using shape !Combine that and you have a winner !! at less than 1/3rd the weight of using wood and will last longer than you will live !!
so wait, if I make the transom thicker than normal, and solid fiberglass (which sinks in water) instead of wood (which floats in water) I can still end up at 1/3 the weight vs. a “normal” wood transom? Chinese technology really is amazing.
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Old 02-09-2019, 04:25 AM
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another person that reads and cannot grasp what's written
Yes a solid light weight fiber glass transom could be 1/3 the weight of a normal transom and would be much stronger and never rot !
I will leave you with that thought and think about HOW IT COULD BE DONE !! AND NO CORES OF ANY SORT USED !100% GLASS ONLY
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Old 02-09-2019, 04:37 AM
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I think that 1 1/2" is widely accepted as a suitable thickness on larger monohulls with much wider spans so the smaller spans associated with the cat hull with more lateral supports should be very strong. best of luck
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Old 02-09-2019, 04:43 AM
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I would go a little thicker if I had the choice. See if you can locate airex near you. Mine was done last year with it. Basically similar to coosa. Mine came out great.
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Old 02-09-2019, 04:44 AM
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Originally Posted by Texas 17 View Post


so wait, if I make the transom thicker than normal, and solid fiberglass (which sinks in water) instead of wood (which floats in water) I can still end up at 1/3 the weight vs. a “normal” wood transom? Chinese technology really is amazing.
So with this logic every steel hulled/structure boat should sink right?
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Old 02-09-2019, 05:10 AM
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Originally Posted by Docked Wage View Post
So with this logic every steel hulled/structure boat should sink right?
Did you even read my post and the one I was responding to? Floating is based on displacing water, not the weight of the material used to build the hole in the water.

Unless Tunnles is using something like a corrugated fiberglass core, (in which case he needs to stop saying solid fiberglass,) he has told the OP to make it thicker, and make it out of heavier material, and make it 66% lighter. That’s a neat trick. It also has nothing to do with the ability of the boat to float.
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Last edited by Texas 17; 02-09-2019 at 05:38 AM.
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Old 02-09-2019, 05:24 AM
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4 layers of 1/2 inch Doug fir marine ply , set in epoxy
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Old 02-09-2019, 03:39 PM
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one thing that nearly everyone forgets about is shape !!
introduce shape into something you can triple its ridged strength
its so simple its stupid !!
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Old 02-09-2019, 03:49 PM
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I agree with every. Only thing I add is a layer of 1700 between each piece of ply. I’m tired of hearing about coosa. You couldn’t give it to me. Encapsulated meranti and no screws ever added. It WILL last atleast 40 years. By then I’m dead don’t care.
2-3/4 plys or 3- 1/2
​​​​​​​check out classicmako for good tips.
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Old 02-09-2019, 04:18 PM
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I would look at the overall panel span to determine thickness. Just shooting from the hip and it being a catamaran with a comparatively small transom area two layers would probably be sufficient with proper lamination.


Originally Posted by tunnles View Post
one thing that nearly everyone forgets about is shape !!
introduce shape into something you can triple its ridged strength
its so simple its stupid !!
Stupidity is relative. There is a difference between stiffness and "strength" (as you call it) introducing shape Into a panel doesn't make it any stronger. But it will make it more stiff.


Maybe (rigid strength) is stiffness?
Aren't you the big proponent for flexible is better???

Originally Posted by Tossedabout View Post
I agree with every. Only thing I add is a layer of 1700 between each piece of ply. I’m tired of hearing about coosa. You couldn’t give it to me. Encapsulated meranti and no screws ever added. It WILL last atleast 40 years. By then I’m dead don’t care.
2-3/4 plys or 3- 1/2
​​​​​​​check out classicmako for good tips.
I am no fan of coosa myself. And I wouldn't see any reason to advocate laminate between the layers but, Transom core is a funny thing. Plywood will rot if its compromised, but a high percentage of the overall strength is supported by the core itself. couple that with its compression strength its hard to beat for the cost.

Composite panels don't rot. But they really don't perform as well in compression. You increase the laminate but many times the force of just tightening engine fasteners can compress the core. The result of this is the laminate skin's that you are relying on for strength become compromised.


As for a solid glass transom. I dont see a problem. But to argue that a solid glass laminate will out perform a properly executed sandwich panel..and be lighter?

Well....that's questionable

Last edited by kln; 02-09-2019 at 04:55 PM.
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Old 02-10-2019, 01:58 AM
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crazy and utterly confused ! lack of general and practical knowledge specially engineering basic principle's that can be applied to composites such as shapes to increase rigidity and strength without adding loads of weight !
How many FLAT panels on any car body ?? take a straight edge and find any panel that doesn't have curves, creased bows and shape you probably never noticed the thickness of the steel of modern day automobile's ??
same with the aircraft industry and the automotive industry they are leaving the antiquated almost prehistoric boating industry so far behind its past being funny !
WAKE UP !!
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Old 02-10-2019, 03:24 AM
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I understand what You are saying about shape/strength but I’m not sure it would be a practical application in a transom. How do you add shape to a transom or core materials in a way that works. Water and wind have different effects. Also remember that when a car is impacted(aka crashed) repaired are needed. If a transom got crashed to that same extent the boat would be out of commission .

The solid glass transom does does seem like a bad/heavy idea but I think what gave birth to that way of thinking was the waterlogged transom of long ago. Not sure how much heavier a solid glass transom would be vs a waterlogged one. Products like air-jay are probably what is being mentioned here. Again, I’m not a fan of these newer greater composites. Take coosa for example. I like the idea of it but I think the cons outweigh the pros. When you layup glass and resin on marine ply the resin gets into the wood. It creates a tenacious bond. On coosa it doesn’t bond the same way and I’ve read many reports of delamination with it. It doesn’t have the same fiber type bonding surface as wood. No doubt coosa works well for many but in time how good is the bond? Boats shake vibrate pound and have a few impacts at times. My money is on meranti marine ply properly encapsulated. To each his/her own just learn the best way to use either product.
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Old 02-10-2019, 04:53 AM
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Originally Posted by tx656 View Post
My question. Would you recommend 2 layers of 3/4" or 3 layers of 3/4" core material for my transom rebuild?

Backstory. Rebuilding a 1994 Seacat SL5. Total gut job. Transom had wet spots and crushing of core material. Removed all of that and now shopping for replacement core material. Coosa is pretty much impossible to source in Texas. There is a company in Dallas called CoreLite that has a 28# PVC material that would work. Unfortunately they don't stock the material but indicated North American Composites in San Antonio might. Will call them next week. If they don't have what I need I guess I'll be driving to Louisiana or Florida! I'm going to material for the sole also so might as well make one trip somewhere.

I'll be adding knees to the transom or an inner aluminum bracket like World Cat made available after point of sale on some of their earlier models. I’m leaning towards 2 layers because the guys that have already finished their seacats have all said “keep it light”. (Especially when it comes to the sole which will be a future post.)

This hull originally was spec'd for 2 x 150HP. It had 2 x 175HP when I bought it. Prior owner had fitted an aluminum plate that wrapped over top of transom and also tied the side walls of splash well to transom. Worked OK I guess but gel coat cracked where outer splash well corners met top of transom, etc.

As always, thanks for any and all feedback.

Pics of original transom before/after core removal.

ANOTHER SeaCat Rebuild - 1994 SL5
My boat is a 22' walkaround with a 150 mounted on a gill bracket and it came from the factory with 3 layers of 3/4 plywood. I wouldn't go any less.
I'm no expert but that is a lot of weight hanging back there and 1 more sheet of 3/4 isn't gonna add much more weight.
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Old 02-10-2019, 05:56 AM
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Originally Posted by Texas 17 View Post


Did you even read my post and the one I was responding to? Floating is based on displacing water, not the weight of the material used to build the hole in the water.
I did.

It was misleading and different from your followup post. You said, "solid fiberglass (which sinks in water) instead of wood (which floats in water)" and then followed up with the correct logic of buoyancy in a subsequent post.

Either way your questioning of the weight difference in your next sentence is valid since FRP (Fiberglass and Polyester resin only with no core) has a higher density than most plywoods.
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Old 02-10-2019, 06:23 AM
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Total thickness on my Scout transom is 2.75", about 2" of core and two .375" glass faces. I can tell you from experience that it was challenging to find a good selection of thru hull fittings that went through a transom that thick. Most fittings are designed around a much thinner wall section.

Thicker won't hurt, but for a narrow load bearing area, thinner may be adequate.
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Old 02-10-2019, 06:47 AM
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Whalerboy,that is a thick transom! As long as motor bolts are long enough the stronger the better in my book!
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Old 02-10-2019, 07:27 AM
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Redid mine with 2 sheets of 3/4" coosa, but sterndrive so don't have the weight of any outboards hanging off the back. 28lb density I believe.

If it's not available in your area, I think it would be worth a long drive to pick some up. Shipping is more than the cost of the coosa itself.

And remember, coosa won't rot but it ain't any better than plywood if it gets wet and delaminates.

Something I wish I had done when bedding mine to the outer skin.... there is stuff called Trevira Mat by Prisma. The use it on all there pre-forms. Would be great to lay down over the outer skin before you bed rather than a layer of CSM, fills voids and rough surfaces very well. Maybe use it between layers of core as well.
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Old 02-10-2019, 07:38 AM
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Check out gem lux 4” scuppers with the duckbill.
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