Use what I have or buy new battery packs?

[jfrabat]

I am building my first fast e boat. It is a 31" cat scratch built from some plans I found online.

I know typically you would want to use 2 battery packs in the pontoons (at least that is what I have seen online!), but I have some battery packs already. I was planning on using a 786g 4S 8000MAh pack over the tunnel. I did some flotation tests (everything is inside, but not installed properly) to kind of gauge how heavy it would be, and it does not seem to be extremely deep in the water. What do you guys think?

20200418-114706.jpg 20200418-114712.jpg

If at all possible, I prefer to avoid additional expenses, especially with all this COVID 19 issues, which will greatly affect my line of business, and I am not sure I will have a job in a few months time...

Felipe

[785boats]

Hi Felipe.

Nice looking scratch built cat.

I race my 31" cat with the batteries up on the tunnel, using single 4s or 6s packs. Depends which class I'm racing in at the time.
It handles well enough to win most races. So I believe you could use what you have, if the boat is not too narrow.
Here's a couple of pics of the insides of mine.

P1010041.jpg

P1010042.jpg

Cheers.
Paul.

[jfrabat]

Hmm... That's a really nice boat, but drive system with the propellers inside the boat is new to me!

That cat is a lot wider than mine, but I think I will give the batteries I have a shot. As I said, I do not feel the boat sits too deep in the water, so I think it is worth trying it! Now I just need to figure out where each component will go (and I have to start with installing the flex shaft, as soon as I get some epoxy)!

[785boats]

So. Do you think the boat might go a bit faster with the props outside the boat? I'll give it a try.

Lay all the equipment in the boat to get the balance point at 30% of the length of the boat, when measured from the rear of the sponsons. So that's about 9.3 inches
This includes all the gear mounted on the transom, propshafts, props & hatch.
Make sure you have room to move the batteries a bit, both forward & backwards for fine tuning during testing.

[jfrabat]

Will do, thanks! Just waiting to get some epoxy to glue some stuff in and for the servo to arrive to balance everything out.

I also found an old battery pack from my old hexa drone... It is a 6600 MAh pack weighing in at 660g, so 126g weight reduction. This pack is 35C constant, 70C burst, so I think it should be fine.

I think I will use that one, as it is also shorter and wider (fits fine width wise, and gives me more room to balance front and rear). I can also move the ESC basically the length of the pontoon, so that will also help a lot in balancing. Laterally (side to side) balance will be a little off, as the ESC will be on one side, but I plan to use it on the port side to compensate for the motor's torque.

Here is how the main compartment looks (ESC and receiver would be on the left, servo on the bottom):

20200419-123202.jpg

I used almost all the carbon fiber I had left on the motor floor and battery tray. I think the scrap is enough to make a servo mount, though.

PS: In case anyone is interested; these are the big batteries and these are the ones I just found.