How thin wood can/should I use?

[Miika.Nurmi]

Building a 25” ish cat hull. How thin balsa/plywood should i use for skeleton and also outer shell? Durability would be nice but i suppose it would get very heavy soon. How thin for skeleton, and how thin for also the outer shell. I was thinking of fiberglassing the outer shell so it would be much stringer but waiting for your responses and opinions on this!

[Jesse J]

Welcome to the forum!
The bulkheads need to be strong enough to survive the skinning. 1/8 inch balsa is negligibly heavier than 3/32, but either could work.
I would avoid basswood as that does get heavy fast. If you skin with 1/16 ply, you don’t need fiber glass, significant time saver.
What plans are you using?
What performance expectations do you have?
Jesse

[Miika.Nurmi]

Do you mean using 1,5mm:1/16th on outer shell or skeleton? Was planning on using 2mm birch plywood on skeleton and 0.8mm on outer shell. Sorry about the different measurements im from Finland :)

[Miika.Nurmi]

Just a second ill manage the blueprints for ya.

[Miika.Nurmi]

https://share.icloud.com/photos/01a5...tvDc9K3BL9TOKA

[NativePaul]

I've used .8mm and 2mm it was the go to formula for boats of around that size here when folks still raced wooden boats, it worked well.

I also raced against 1 guy who used .4mm and 1.5mm, very fast boats but he treated them as disposable, any contact or even an unlucky blowover or stuff and it would be done and he would have to build a new boat.

There is no need to glass a birch plywood boat. Balsa does need the surface hardening for anything other than experimental prototypes, 25gsm E glass cloth and epoxy is commonly used for this.

I don't think it worth doing for a boat that size, but I have laminated balsa into thick balsaply over temporary birch plywood formers, and used that to make a 36" cat with the only birch ply retained at the end being the ride pads and transom, it resulted in a very stiff and very light boat, but was also pretty expensive and a PITA to make.

[Miika.Nurmi]

Would it be a good idea to fiberglass ride pads or just let it be wood

[NativePaul]

After seeing your blueprints I see that it is a SAWs type cat for twin motors, forget balsa and .4mm ply, it is a skinny boat that isn't going to handle well anyway so weight isn't a big issue, I would go for 1.5mm, and with those skinny formers you are going to need some thickness to them to get any strength, I would probably use 5mm.

It may be worth noting in case you didn't know, that your "steps" aren't really steps, and are just for show. To avoid them being counterproductive you need them to increase the angle of attack of the ride surface behind them, and on a 25" boat they should ideally be deeper.

[Miika.Nurmi]

What are you meaning with skinny formers? Would 0.8mm be better for outer decking? Sorry bout dumb questions such a noob in boats :)

[Miika.Nurmi]

+ you also mentioned it’s made for 2 motors. Is it a huge downgrade if i only go with one? Was planning to go with 4 pole 3180kv running 4s. Wont be trying to go for huge speeds like 80mph its my first one and I love building stuff so kind of just a ”speedboat” to rip around not that fast

[Dirk Jan]

Hi, 80mph is quite ambitious for a 25" hull. Not impossible, but quite a steep number! As you are going for extreme speeds, I would use 1.5mm plywood for the bottom and 0.8mm or 1.0mm for the deck. I would definitely glass the entire hull for durability.

I made quite a lot of wooden rc boats. I always glass them. My 33" F1 tunnel still got stress cracks after several 50+ mph flips, even though it is fully glassed.

50gr/m2 glass works really well and has quite a smooth finish already. 80gr/m2 is also possible for the bottom.

For the frames I would use 3mm thickness. Once you glassed the hull, the frames are of less importance.

[Miika.Nurmi]

In the end now…. Boat gonna be about 68cm. The horizontal/center prts 2,5mm balsa trusting on balsa just becouse 3 of them will be glued together = 7.5mm… The other way hull things dont know the name but cross parts xd. 2,5mm birch ply. Outer decking. Bottom part 1,5mm birch ply and upper 0.8… Will fiberglass some points at least bottom. Does this sound reliable / good idea?

[NativePaul]

Singles work, I have a singe cat this size, twins would have been better, but the single isn't too bad, I assumed you were going for twins as you have big cut outs in your longditudinal formers compromising strength, and the only reason I see to do that is to allow better access to put stuff in the sponsons.

If you are not going for extreme speed those materials will be fine.