What density of foam composite do I need for a transom?

[ADDvanced]

I have found a lead on divinycell foam but I'm not sure if it's the right weight foam for a transom.

[JPEROG]

You don't want to run foam in your transom. If you are looking to go composite then use Coosa board with layers of 1708 sandwiched between the layers and encapsulating the outside. There is NOT a foam that you want for a transom.

Joe

[ADDvanced]

Coosa isn't sold up here, but divinycell is. Coosa board is just dense foam with some fibers added, no?

[ADDvanced]

Also the reason I was considering it was this website:

https://www.screamandfly.com/showthr...om-good-or-bad

https://www.screamandfly.com/showthr...cell-Foam-Core

https://www.screamandfly.com/showthr...cell-questions

[ADDvanced]

Any updates? I found a lead on divinycell H80 foam, which is 5lb / cubic foot.

[ADDvanced]

This is saying H80 for floors/stringers and H120 for transoms?

https://www.screamandfly.com/showthr...52#post1259352

[colinJ]

5 pounds per cubic foot is too light for a transom, this is only about 2.5x the density of floatation foam.

the data sheet is here:

https://www.divinycell.com.br/pt/wp-...Diab-Group.pdf

The compressive strength is very low and is more than 10x lower than that of wood.

This is saying H80 for floors/stringers and H120 for transoms?

https://www.screamandfly.com/showthr...52#post1259352

[ADDvanced]

Yeah, compressive strength sucks, but for any foam you need to drill out/fill with epoxy/redrill, and maybe use compression sleeves as well.

If I did those things, is h80 acceptable? I'm reading that people have used H80 on transoms, but I have not seen evidence it's been used on high HP transoms.

[powerabout]

G10?

[MattGreen]

You're asking good questions. What is your goal with going with a foam core transom ? Less weight ?

You do not want Divinycell H80 for a transom on our kinds of boats (lighter-duty application like a sailboat ? probably yes OK). Nowhere near enough compressive strength for hanging a big outboard on a jack plate. I'm just wrapping up a transom/stringer/floor complete overhaul on a 21 Checkmate. I'm doing it in a manner similar to what JRPEROG mentioned above - the transom is Coosa board and bi- and tri-axial fabrics.
This pic may help put Divinycell versus Coosa into perspective:




On the left is a scrap of Divinycell H-80 (I used this for stringers and bulkheads - laminated in a rudimentary press with a layer of unidirectional and matt). You might need to zoom in on the picture but there's about 5 dents I made in the core with just my fingernail; think about how well it would tolerate the clamping load from motor-mounting bolts.

On the right is a scrap of 3/4" Coosa Bluewater 26 (no laminates). You can see the two layers of roving impregnated into the foam, plus the random fibers distributed throughout. Coosa comes in multiple densities, -26 is the densest and what they recommend for transoms. It is still soft compared to good-quality plywood. In my opinion you need extra skin thickness on both sides of a Coosa-cored transom as compared to a plywood transom, and big backing plate plates too. Considering the cost of Coosa in my area, although I do like it in some ways, I would seriously consider doing it again in plywood.

Coosa is widely available in the US, but you may need to pay for shipping. Failing that, there are at least 3 direct competitors to it; I suspect a good composites wholesaler in your area will carry one of them.

Good luck,
Matt

[colinJ]

The transom core is loaded almost entirely in compression from engine weight and moments of inertia during turns and other maneuvers. This isn’t just a matter of compressive strength were bolts go through the transom. I wouldn’t use anything lighter than 20 pounds per cubic foot. A small boat like this has maybe 1200 square inches of transom area so if you make the core 1.5 inches thick you have right around 1 cubic foot of actual core material. Comparing this to blue water 26 or plywood your weight savings is in the neighborhood of 20 -30 pounds.

I doubly if most people would notice the addition or removal of even 50 pounds down low where this weight is going.

I did my own transom with bluewater 26. My supplier only had .75” thickness and I ended up making the transom over 3” thick including too much fiberglass. It’s overkill but I do have 450 pounds of engine weight hanging 16 inches back on a 14 inch Jack with a 2in spacers. I Probably could have made the entire layup 75 pounds lighter and still had it be strong enough but I seriously doubly if I would even notice the difference.

Yeah, compressive strength sucks, but for any foam you need to drill out/fill with epoxy/redrill, and maybe use compression sleeves as well.

If I did those things, is h80 acceptable? I'm reading that people have used H80 on transoms, but I have not seen evidence it's been used on high HP transoms.

[ADDvanced]

OKay, thanks. H80 is out. Which sucks, because I cannot find anything around Milwaukee and shipping a 4x8 sheet of anything is just obscene. I've tried calling random plastic suppliers that make polyurthane sheets, but so far nobody is making polyurthane foam sheets, excluding upholstery type places where the density is just way, way, way too low.

[25two.stroke]

In my experience with buildings, boats, model building etc. I have always found God's creations to be quite helpful...Wood is incredibly strong for its weight (yes, I know plywood is technically engineered and man-made) in all forms. As you can see from the above comments, the house is divided as far as wood vs. composite. So it is really up to you. If you are building it to sell then you cannot possibly guess your future buyer's preference unless you already know who it is...so pick whatever you want.

I see NO problems with wood and I am about to re-do my V-King with wood and probably a bit of carbon fiber. That being said, I will be having one of the best in the world doing my glass work with me...
And to all y'all who will oppose my use of wood...all I have to say is I am glad its not your boat.

[colinJ]

I had a coworker that build A 36 foot self designed center console fishing boat in the 90s. It was built on a budget and he did the transom using 1/4 inch underlayment plywood, I think it’s called Luan. Anyways he built the core to be 3 inches thick laminating the pieces together with epoxy with a layer of bias between each piece after first soaking the individual layers with thinned epoxy to add additional rot protection. I think he built up the overall transom to be nearly 5 inches thick when it was all glasses in.

Almost 30 years later the boat is still going strong, now with three 200 HP outboards mounted on a 30 inch engine bracket.

I am not telling you to build your boat like this. The point is that wood is hard to beat if you pay attention to the details in terms of eliminating voids and sealing out the water. I didn’t build with wood because at the time I was able to get 2 sheets of coosa for under $300 and I was worried that I wouldn’t be able to do a good job of sealing everything up because it was my first transom job. If I could do it again with the same experience I would probably use wood for the transom but I would use coosa for the floors.

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OKay, thanks. H80 is out. Which sucks, because I cannot find anything around Milwaukee and shipping a 4x8 sheet of anything is just obscene. I've tried calling random plastic suppliers that make polyurthane sheets, but so far nobody is making polyurthane foam sheets, excluding upholstery type places where the density is just way, way, way too low.

[FUJIMO]

...mantex...or coosa...or corecell...or divinycell...just gotta know what density is good for you & your application... anything but plyrot...