Baby Bootlegger Surge or cavitation question

[Oldskiboat]

I made a Baby Bootlegger from plans. This is not a super fast boat. It is 42 inches long, about 5.5 inches wide and weights about 8LB. The motor is a MFA Torpedio 850 brushed unit. I am using a 6000mAh 11.1 Volt battery. I have had a 45 mm three blade prop that had this issue, a 2 blade x450 that worked a bit better but over heated the motor and now a 2 blade 42mm that I got from OffShore that also surges. What happens - I accerate the motor to about 3/4 and all is good, when I go to full the boat will start to go faster and then create a noise and slow down, and then still at full it will accelerate and go through the same sequence over and over. The motor does not over heat with this prop or with the 3 blade. Any ideas? I incuded an attachment picture of the boat.

[koen]

is your esc suited for a brushed motor ? because the most esc's are for brushless motors

[jkflow]

Sounds like your rear is lifting too much when you try to get up to speed.
What type of prop are you using? Standard or de-tongued?
You might want to try a non-lifting prop (m or x series or alu de-tongued). Mono hulls typically use trim tabs as well to stabilize.
Last resort, stay with 3/4 throttle

[1coopgt]

I made a Baby Bootlegger from plans. This is not a super fast boat. It is 42 inches long, about 5.5 inches wide and weights about 8LB. The motor is a MFA Torpedio 850 brushed unit. I am using a 6000mAh 11.1 Volt battery. I have had a 45 mm three blade prop that had this issue, a 2 blade x450 that worked a bit better but over heated the motor and now a 2 blade 42mm that I got from OffShore that also surges. What happens - I accerate the motor to about 3/4 and all is good, when I go to full the boat will start to go faster and then create a noise and slow down, and then still at full it will accelerate and go through the same sequence over and over. The motor does not over heat with this prop or with the 3 blade. Any ideas? I incuded an attachment picture of the boat.

Beautiful Boat . The only advice I could give is to Email Chris at DASBOATA.com see what he thinks

[Fluid]

The BB is a semi-displacement hydroplane hull which has a tendency to aerate the prop. The x450 prop worked because it is large enough to push enough water to get that draggy boat moving, and because it is designed to work in a partially areated environment. The other props are either too small or don?t work in a cavitating environment. Part of the problem is that his motor has an excessive rpm/volt rating, and matching a prop to his power setup could be a challenge - too small and it will cavitate badly and too large and it overheats the motor.

That motor is only rated to 10 amps, which with only 11 volts doesn?t develop enough power to turn a prop large enough to efficiently move the boat. Pulling more amps increases the power but overheats the motor. That motor is really designed for slow scale boats like fishing trawlers and steamers. A speedboat 42? long is too much load. The OP would be better off if he obtained a motor more suited to his needs.


.

[Oldskiboat]

the props were all X series and the boat doesn't seem to be lifting at the stern, if anything it lifts more at the bow when at full.

[Oldskiboat]

You mentioned that it is a hydroplane, but this is a mono hull. But I hear your points and agree. What motor type would you recommend?

[Oldskiboat]

Thank you for the nice words. It was a fun boat to build and my first RC. I will take your advice and write to Chris.

[Fluid]

The BB was considered a hydroplane when it raced, monos didn’t have square transoms with the prop well behind the transom back then - terminology has changed over the past 95+ years. I’m not conversant in today’s brushed motors, just brushless. A motor which should be able to turn a 50-55 mm prop efficiently without overheating is this one:

https://hobbyking.com/en_us/turnigy-...ucts_analytics

There are others I’m sure but this one should get you between 15-20 mph, well over the boat’s scale speed. It is tough to find an appropriate motor with the rpm/volt for what you need for that size submerged prop and 3S voltage, plus the prop is only mostly submerged at speed, a rather uncommon design for modern R/C models. Chris may have an idea. A 60-100 amp brushless controller will be needed too.




.