hi all
i am struggling i have built a 36" multi hull known as a monza
i have a submerged drive in it tried with a 35 mil prop and running a 3505kv brushless motor
and have completely blown almost caught fire 2 speed controllers
1 was a mystery 80 amp water cooled speed controller
2 was a jeti hacker navy 3 opto 99 i believe (cant read as it in cinders)
both times i was running 12 cells
any ideas before i rip it out and stick either a bbturbo 12v
or a 8.4 v bb turbo
both times motor has also been water cooled and boat has been completely dry so know leakage letting in water and water cooling working properly
many thanks in advance

Oh my. This sounds like a SAW boat to me, cuz IMO 3505kv sounds a bit too high.

What are the other motor specs? (size and type)

hi this is the other details i know about the motor
Xpower 540/3660-7 Marine Brushless Motor 3505Kv 88A

ah you seem to look worriesd about my ist can u recomend what size motor i might need to push this

This marine motor doesn't seem to have the torque for a 36" size boat. And @ 88 max amps w/a high kv it'll need a very pricey esc. I'm not so sure about a 120 hydra handling it on 12 cells.

Pricey enough that you might as well get a different set up for the same price.

I don't know about switching to a different prop selection.

I would recommend a KB45XL motor for this boat size.

does a bigger boat need to have smaller kv realy i dont think i realy
understand them
the boat was designed to run a geared 21 nitro or a 45 size nitro

first off, your motor is too small for a 36" boat
2nd im surprized you didnt burn that up first cuz 3500kv x 12cell(14.4V) = 40320rpms(if it has 80% eff.)
3rd if you dont understand something or are clueless on how to set up a boat, just post before you spend a bunch of money on nothing.
4th I usually like to have my boats running around 30000-33000 rpms(with 80% eff. counted in) so get a KBXL or fiegao(10xl-14xl) and use the proper amount of cells to hit 30000-33000 rpms.
Heres the formula to find out what rpms (guestimate, never really dead on)
KV x .8 x (Volts per cell x cells) = rpms
Heres the one to find out how many cells to run on a certain KV
33000 / (KV x .08) = volts

Hope this helps, btw, post some pics. The submerged drive has to go also. Youll never see good performance with one

I'm assuming this is a deep vee when you said you used fully submerged drive? I could be wrong.

Yo custom, you got a good prop formula/calculator for him to pick a good prop?

Nope there isnt one that i know of, if there was, no one would be burning up stuff. I just go by the start small rule. It seems to work, i check temps every 45sec- 1 min. when trying a new prop. That just about all you can do

My bad, didn't know the terminology.

thats okay... it took me a second time when i read it.

after looking around could you tell me if this motor sounds a better coice then i have made so far
https://www.unitedhobbies.com/UNITED...hless_Inrunner

Yes that will get your hull moving pretty good, should go over 50 if you have it setup right. Then on that site they have a 65$ turnigy 100A esc, i would recommend using that. then i would recomend using that with about a 6s lipo(4500+ cuz the KB's are amp hogs) or 18-20cells nimh.

thanks for assistance i have just orderd the motor and speed controller now lets wait and see
will post back ounce i have it and let you know how its going
i had the diagram to set this boat up for a 21 geared nitro and set to that alowing the wait of the batteries it equal a 16oz fuel tank hope it works
it has always looked good for the last 1 miute or so before i blew everything up