Warning for Balsa Builders

[crabstick]

I Notice a few people building 4s powered boats with balsa around the place so i thought id share my experience.
I built a 33 inch balsa cat last year, skinned the sponson's with .8mm birch ply and pretty much the rest of the boat was balsa coated with some norski epoxy, I ran 8xl on 4s2p in it, first run it did 76.4kmh, it was crazy fast out of the hole being so light and it ran really nice. about the 3rd run after some tweaking and prop changes it got 86kmh (53mph) and i had a small blow over turning at full throttle (i call it "testing") in windy conditions, which it survived.

Anyhoo a few weeks ago i was running it and it was windy and ill admit i probably shouldn't have been running it but it blew off and smashed a sponson to pieces, fortunately though it was right by the bank of the pond and the pond being knee deep i jumped in and grabbed what was left of it it before it went down.

Long story short - balsa may be ok for smaller craft, but the loads that will get produced doing 100kmh with a 4s boat need something a little stronger, if you hit anything or get an odd bounce it may be all over red rover.
for example of the strength required for up near those speeds, a while back I ran my climate matrix on 12 cell 8xl and bent and cracked 2 fullers 12 cell struts, it was around 50mph and yeah the hardware was too weak for the setup, but you get my drift ?

im currently rebuilding a 35 inch cat but out of ply this time ! just gotta finish the skin on the deck and its 740 grams at the moment.

[andym]

In my Wild thing I made my bulk heads out of balsa but laminated some 1/32 ply to one side for strength, came out light and strong

[crabstick]

yeah I did the same with the engine mount in this last cat and its still strong as.

Laminating thin ply over the balsa is the way to go if building with balsa IMO

[Fluid]

IMO balsa belongs in sailboats and airplanes, not FE boats. The days of needing to have a 'lightweight' hull are over, with today's power weight is an advantage. I have to add weight to my nitro-hulled FE boats to keep them on the water. This isn't your grandfather's FE boat anymore, an extra 6 ounces is meaningless.

I have used basswood and/or plywood for the last several riggers I built, some of them were SAW record holders. Weight in the form of hull strength kills two birds with one stone - so to speak. I hate "light layup" fiberglass hulls too, another old skool idea way past it's time. I have to add lead to them when extra hull strength would be the best solution.....



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[Rumdog]

Fluid, is that not a Ul-1 as your avatar? That would definately be a light layup hull! Good looking boats though!

[Rumdog]

Fluid, i've been considering the Osborne Rigged Insanity for sport running on 4s, have you had any experience hith this hull? I'm new to riggers and any advice on a good kit would be a big help!

[cyberhoops]

I built two different boats with a mixture of plywood and balsa, and both boats failed to hold up. One boat was balsa with 1/32nd plywood while the other was a laminate of 3/32nd balsa with 1/64th plywood. in both cases I ended up tearing a sponson off the boat at about 45 mph.

In both case I think water was a key component in the failures - if the balsa gets even slightly wet it will lose strength and come apart in a hurry.

I have converted over to building boats of plywood and foam with 1/32nd plywood skins over the pink foam. So far that has been durable.

The balsa boats were light. The 28" laminate Pay-n-Pak came in at 3lb 10oz ready to run while the plywood and pink foam replacement is now 4lb 4oz. However a light boat that falls apart is not really worth much and I don't plan to use balsa in anything that might be structural in the future.


On another note: I have built the Rigged Insanity. I thought the kit quality was excellent and that it was easy to build. I have only raced it once and it is still in the process of getting tuned up, but I do like the boat. I would recommend epoxy to seal around the inside of the hull - especially on the seams since I had initial issues with water leaking into the boat.

[HOTWATER]

I built two different boats with a mixture of plywood and balsa, and both boats failed to hold up. One boat was balsa with 1/32nd plywood while the other was a laminate of 3/32nd balsa with 1/64th plywood. in both cases I ended up tearing a sponson off the boat at about 45 mph.

In both case I think water was a key component in the failures - if the balsa gets even slightly wet it will lose strength and come apart in a hurry.

I have converted over to building boats of plywood and foam with 1/32nd plywood skins over the pink foam. So far that has been durable.

The balsa boats were light. The 28" laminate Pay-n-Pak came in at 3lb 10oz ready to run while the plywood and pink foam replacement is now 4lb 4oz. However a light boat that falls apart is not really worth much and I don't plan to use balsa in anything that might be structural in the future.


On another note: I have built the Rigged Insanity. I thought the kit quality was excellent and that it was easy to build. I have only raced it once and it is still in the process of getting tuned up, but I do like the boat. I would recommend epoxy to seal around the inside of the hull - especially on the seams since I had initial issues with water leaking into the boat.

No doubt! Expect water to get in your boat! Seal everything...

[Simon.O.]

I have just re read this thread and can see where the issue is for some of the balsa boats that have been built and damaged.
1/32" or 0.8mm ply is to thin for me to use anywhere on one of my boats. Even the dress skins on the sponson tops of my new rigger are 1.0mm ply expoxied down to 3mm ( close to 1/8" )balsa. That is the thinest balsa on this build. The tub decks are also 3 mm
3/32 or 2.5mm balsa I might use when laminating around curves. 2 layes of 2mm would suit me.
There is a lot of framing in those sponsons including 3 and 4 mm skins.
Tub sides and bottom are 4mm (5/32") reinforced with hardwood stringers.
At the front where the rear boom is there is a lot more wood.

I just wanted you to see that not all balsa boats are made very thin and light like planes.
When I build my next cat it will be my "heavy" balsa style build with a thin skin of glass or silkspan.

[cyberhoops]

Even the dress skins on the sponson tops of my new rigger are 1.0mm ply expoxied down to 3mm ( close to 1/8" )balsa.

Simon - do you laminate on both sides of the balsa with the plywood skin?

[Simon.O.]

Not in the case of those sponsons.
I have done laminating in a number of ways.
ply-balsa-ply
balsa-ply-balsa
balsa-balsa opposite grains
For any balsa to balsa bonding I use balsa cement CA.
For ply to balsa I seal the balsa with sanding sealer and then epoxy.
I have even layered on my varnish coats before adding ply.
As I wrote in my rigger thread I do a lot of destructive bench testing of parts and bonds and sealing techniques. You should see what I do to metal parts