Prop tip speed - check my math

[RedRiverRat]

I'm running a 66" Ultra Prop 2 with 1.68:1 reduction turning 3300 rpm, I've calc. my prop tip speed at 565 fps.

If I was running direct drive, assuming the motor would turn the same rpms, I'm at 950 fps, any thoughts on switching to dd and see what happens

40hp vanguard on a 66"x10' homemade mini, just playing around trying to get the best out of it, already having a blast with how it performs now, just trying to tweak it more.

Thanks for any input

 

[Slidin Gator]

That engine is rated 40 hp at 3,600 rpm, you are missing some performance at 3,300 rpm. I suggest decreasing pitch to get all the engine makes.

As for DD, 66” is too large of a prop for that engine DD, you would need something more like 48” I believe. As it is you would have to take too much pitch out to where the prop is not efficient. If you can delete blades down to a 2 blade it might work, don’t know.

Keep in mind that a larger diameter prop provides more thrust per hp, downsizing the diameter is going backwards.

If you really want to maximize performance it’s the same answer as the big boys, time to hot rod it with better flowing heads, cam and exhaust so you can turn more revs. The mud motor world is where you will find all the hot rod goodies for that engine. $3k for this package to make 52 hp at what is probably a bit over 4,000 RPM.
BEAST Performance Package for 40 EFI

 

[RedRiverRat]

The 3300rpm is on the trailer, forgot to mention that, im going to look into the performance upgrades, thank you for your response

 

[EastBay]

Slidin Gator...glad to see you here and giving good down to earth advice.
Hope you have been Slidin!
EastBay

 

[Slidin Gator]

The 3300rpm is on the trailer, forgot to mention that, im going to look into the performance upgrades, thank you for your response

I would tune prop pitch for 3,600 on the trailer in present form unless you are specifically looking for maximum speed and don't have issues getting stuck on ground. A bit less pitch will let her jump out of the hole better and you will have more thrust when stuck. The downside is a bit more revs at WOT on step, but how long are you gonna hold her there? Does the EFI setup include a rev limiting system?

Slidin Gator...glad to see you here and giving good down to earth advice.
Hope you have been Slidin!
EastBay

No sir, 12 hour days, 7 days a week until October. Pay now, play later. But man I'ma gonna play come October.

 

[RedRiverRat]

I'm not sure if the EFI has a rev limiting system, I'm still playing around with it. I'm going to remove 2 blades from the 4 and try a more aggressive pitch and see how that goes. Probably, in about a month, go for a tad bigger diameter prop. The Mini Whirlwind prop is 69", I can go up to 72" without any major reworking of the cage. Thought about sticking a 3 blade Razor X on it and see how it runs. I've got another mini aluminum hull that I'm going to work on, so I could use the Razor X on the Aluminum hull and just keep the Ultra Prop II's on the glass mini. Playing around, ultimately wanting a small, lightweight airboat that has as much power/performance as I can get. Definitely appreciating any help here. Love this forum, enjoyed readying old post, lots of knowledge. Unfortunately many knowledgeable characters have passed. Guess I'm about 15 years late to the game.