Submerged drive newbie question

[wamf]

I have a 18" mono with submerged drive. It likes to dolphin and if I adjust the trim tabs to prevent it the prop will start to cavitate and the boat slows a few mph. I am using a heavy battery, 4900 lipo and the cg is a bit far back. I'm wondering if i can correct the problem by using a flex cable and strut to change the angle of the prop. I know using a lighter battery would help but i'm getting stupid long times at wot with this set up.
Thanks
Mikev

[Mich. Maniac]

some video and pics of the inside and bottom would be great however the only thing I can think of is leave tabs just down a touch and play with cog. submerged are less touchy than surface in my experiance and just like you say if it starts to act like a dolphin its starting okk and sucking air down there. try the tabs just a bit down and cog more forward.

[Ctonez]

If you used flex and made the prop angle closer to parallel compared to the surface, it should help.
your battery doesn't sound too heavy, what motor are you using? 550? ...does the prop pierce the surface or cavitate with every hop?
If so, then you are essentially launching the boat out of the water because of the angle of thrust, then the boat drops a bit, prop bites a bit and shoots the bow out again. smaller prop might help if this is what is happening, but changing to flex will be the better solution...best solution is to convert to surface drive, but that's an extensive reconfig. Probably not worth doing unless you REALLY want to, depends on which hull too.

[wamf]

It's a Dumas deep V10, it weighs 2.18 pounds with battery and 1.6 with out. I've got way more $$ into batteries than the boat. I'm using a Venom 21t 540 and a $25 pro boat esc. It will do 18mph with a octura x431 for at least 20 min. It is fastest when it is hopping out of the water and the motor stays cooler, it just looks kinda stupid on calm water. I won't convert this one to surface drive but can and will buy the stuff to change the prop angle. I will probably buy a BBY 21" deep V and go BL if I get serious. Run time and looks are the main priority for my boat

[FRED]

submerged drive

Didn't take my advise to help him with his boat, so why give any advise at all!

[wamf]

The Battery is a 2s 4900mah lipo. I don't have pictures or vid. It looks like the boat is jumping small waves even if there are none. If the boat rides level it starts making a whiring sound and slows a little bit. My experience with the Venom 21t is that it turns the 431 prop just fine without over heating.
Of course that is probably because the prop is out of the water half the time. The prop extends .8 " behind the transom and about .75" deep to center of shaft.
I'm using a shaft and am thinking about a flex cable and strut it looks like there is room for some adjustment with that system.
MikeV

[FRED]

submerged drive

Your prop extends .8" behind the transom. No wonder you have problems.

[wamf]

I got it running pretty well without going to a surface drive or putting the batteries any place other than they are now except for lowering them. Moved the prop forward so it's about even with the transom can't go any more without major surgery. Raised the rudder bracket , it was probably dragging. Lowered the cg by lowering the battery 3/8" and added four strakes to the front 2/3 of the hull. The boat now planes nicely doesn't cavitate or suck air and goes pretty fast for about 20 min at wot with a 27mm prop. It doesn't turn all that well and is out of control with a 31 mm prop which the 21t motors spins quite well. Now I'm ready for my third boat my second will be a bl mini rio. My third will be something around 20" and brushless.
Thanks everybody for the help. I know pictures would have made it a lot easier but right now I'm spending my money and time on the boats.
MikeV