need fiberglass advice

[KJM]

I'm going to be replacing my transom and wanted to use a composite core material, but its hard to get locally. A local fiberglass supply guy said to go to a local roofing business and get rigid insulation boards that they use. I think its perlite board. The guys there said they sell a lot of it for people using it on boats. Its only 30 odd dollars a 4x8 sheet which is way cheaper then other composites. My concern is that it doesn't seem strong enough, i can slightly compress it with my fingers.The fiberglass guy said the core material is only to add bulk and the strength comes from the layers of glass on both sides. I know this is somewhat true but i'm still doubtful. Apparently a local boat building company uses this stuff by the truckful. Any thoughts?

 

[JASinIL2006]

So, when a wood transom rots in a fiberglass boat, what happens to the places where the inner transom plate and the gimbal housing are bolted together? The wood compresses and the transom leaks, even though it has fiberglass on both sides. I'm not sure how you would avoid that with the material you are considering.

I could see it working for stringers, etc, where you could apply really, really thick layers of fiberglass (sort of like they do when they make engineered stringers). I don't know how you would prevent compression, though. Maybe the compressive force is distributed enough that it doesn't crush the inner material? I don't know, but well-encapsulated wood will outlast you, if it's cared for, so that's what I'd probably use...

 

[Scott Danforth]

the wood adds strength to the transom, both compressive and flexural. on a deck, the core material separates the top and bottom layers to reduce weight, while maintaining strength.

if your fiberglass was over 3/4" thick on both sides of a core material, then yes, the core is simply adding space.

however in a traditional fiberglass transom, there is 1/4" to 5/16" of fiberglass on the outside, the 1.5" of wood and the 3/16-1/4" of fiberglass on the inside.

if this is the 44 year old boat in your signature, remember, your transom was in a boat designed to last 15 years and you got 44 years out of it. I would rebuild it like it was and go boating

if you want to consider an alternative transom material, you should use a material that has been proven in the application. that is coosa or other high-sthrength core boards.

as indicate by JAS..... you need really really thick fiberglass to not rely on the core.

 

[jbcurt00]

Sea-Cast
sphelps used it on his MFG rebuild, there are other similar products

No cheap composite choices. Plywood is cheap, thats why thousands of boat manufacturers used it for stringers, transoms and decks, and why the significant majority of rebuilds @ iboats resto forum used it too.....

 

[KJM]

Yeah, I think you are right. If it seems too good to be true, it likely is. I'm converting to an outboard so there will be even more weight on the transom. I was also considering a pour in transom {nida}. Anyone have any opinion on those? I don't want wood because it seems to me that water always finds a way in and i want the peace of mind knowing that the transom is impervious to rot.

 

[Scott Danforth]

remember, when you convert to an outboard, you really should go with one of the well-designed and engineered pods.

the loading between an outboard and I/O are completely different.

on an I/O, the load path is down low, trying to push the boat forward at the base of the transom

with an outboard, the load path is up top, there is both the thrust of the prop at the top of the transom, and the moment from the longer lever arm trying to rip the transom from the boat.

 

[KJM]

i phoned a local guy about making a pod from aluminum, he said between 3 and 4 thousand! Thats more then the i'm planning on paying. I'll cut down the transom for a 25 inch outboard and make it extra strong. I'm more into safety then high speed performance, must be getting old.

 

[kcassells]

Even used they cost and arm and a leg and are rarely available.For that reason I am not getting one. Want one though.
Bunch of good answers.

 

[KJM]

I like the look of them, but they are out of my price range, especially considering i have to also buy an outboard as well!

 

[kcassells]

That plus the bigger deck recoup.

 

[tpenfield]

There are some synthetic transom core materials, but Perlite is not one of them . . . Generally, the proper material is going to be very dense with a high resistance to compression. I also have seen some honeycomb material that (supposedly) is suitable for transoms. Keep in mind what Scott Danforth mentioned about the forces and torque on the transom.

You may want to do some more research as to the proper material, and you will probably have to order it online, rather than find it locally.

 

[Scott Danforth]

i phoned a local guy about making a pod from aluminum, he said between 3 and 4 thousand! Thats more then the i'm planning on paying. I'll cut down the transom for a 25 inch outboard and make it extra strong. I'm more into safety then high speed performance, must be getting old.

I like the look of them, but they are out of my price range, especially considering i have to also buy an outboard as well!

reputable ones are pricey because those companies sunk hundreds of thousands of dollars into development and liability insurance.

just an FYI, most DIY transom only conversions that do not go with a reputable pod end up with less than desirable results. over the past decade I have been on iBoats, I believe there were about 40-50 of them. IIRC, only one or two turned out. most simply stop posting and the members simply disappear.

if you dont go with a pod, remember, you need serious kick braces going from the transom back to the stringers, and a splash well

 

[KJM]

Yeah the more I looked into the perlite, it seems that it absorbs water as well as being weak. Forget that idea, no cutting corners. I'll go with a composite made for the job, and yes definitely have lots of bracing and splash well. The outside fiberglass skin is 1/2 inch thick so that should provide a fair bit on strength to start with.

 

[KJM]

Was also considering a pour in transom (nida core) but I did the calculations for the amount I would need and it came to about 36 gallons! Who would have thought such a small looking space could have so much volume! I thought 1 five gallon bucket would be lots. I also have the inner FG skin off so I would have to make a temporary dam of some sort, not sure how I would do that....

 

[kcassells]

PLYWOOD! Move on.

 

[sphelps]

Something don’t seem right on your calc for the pour in stuff .. must have one huge transom . IIRC my MFG took 12/13 gal of Seacast ..

 

[KJM]

I used the calculator provided on the seacast site. The measurements were estimates but it still should have been in the ballpark. Anyway I found a local supplier for Thermolite which is basically the same as coosa. Do you think a double layer of 1/2 inch thermolite, stuck to the original outside 1/2 inch glass, with a layer of glass woven between the core layers and then a couple of more layers of glass on the inside will be enough for the transom? I plan on a 90 hp motor, maybe 110hp.