Getting 4000 rev/min on a 9X9 prop = 0.75 ft every rev so if my math is correct my boat will do 4000Rev/Min x 60 min/hr x 0.75 ft = 180,000'
divide that by 5280 feet/mile and the max speed = 34.09 miles/hour.

Assume 1:1 short tail top/bott.

Assuming 70 eff = 34 x .7 = 24 mph

Anyone run the numbers ?

two factors keep us from running the theoretical speed of our props
-prop slippage
-coefficient of slip in the hull.

a prop with worn cup will not 'bite' as well in the water

heavier loads, dents in the hull, bad design increases the hulls coefficient of slip, thereby making the prop kinda 'peel out' instead of pushing us forward.

and surface drives arent 1:1

Who has the link for the conversion...? It was posted a while ago.

well in a perfect test that means no prop slip and really no boat in theory what ever pitch in ur prop means it will travel that far in one revelotion!! 12x10 (10 pitch should go 10 inches in one revolution)

http://www.rbbi.com/folders/prop/propcalc.htm

There u go. That will tell on you if you are trying to tell someone your lawnmower engine is defying the laws of physics.

DANG! That calculator practically nailed my speed. I put in the suggested 0.15 slip factor for my boat....I have no idea what that means.....but it spit out pretty darn close to what I am getting.

Adjusted the prop slip to 0.18 and it was dead on.

Just guessing since I'm pushing that aluminum tank through the water, probably a higher prop slip.

Now someone needs to post the gear ratios for MB, PD, GT & GD so people can input the correct data.

GD 1:1.5

I did this because some folks think that outboard motors are much more efficient than short tail mud motors. Not true in my setup. My Dixie Mud Motor looks to be running at 70% efficiency. That’s the same level that my offshore boat is running 70%. Same thing.

I utilized straight forward dimensional analysis and assumed 70 % efficiency and it’s very accurate.

My analysis is simplistic but will easily tell you the maximum theoretical speed based on the props RPM.

Best of luck !

Ideally, u should only have about 10-15% slip. Any more than that and there are issues with the setup. Issues could be any number of things. Boat drag from a hook, bent up bottom, too much weight up front, prop trimmed toohigh, prop trimmed too low causing the LU to drag.

My buddy has a fast azz PD boat and his slip from entering all other real world numbers is 8%. my PD boat is short wif a big tank up front causing a lot of drag. My prop slip from real world numbers is ~18%.

If you know the numbers of someones fast boat u can compare numbers to confirm that inefficiencies in the setup is the reason your boat isn't seeing higher numbers r if that's all u can expect out of your boat.

Thats pretty interesting, never really did the numbers as a percentage on my boat. According to the math Im at 8% 'slippage', 34 mph at 4600 rpm, 38 top 45 bottom gears. Theoretically I should be going about 37 mph. hmmm.....maybe time for another coat of gator glide. lol

Auto, u not gonna get much better than 8% slip. U gettin about all out of that u gonna get.

I just finished building a SD for my 1548 running a 25 HP Kohler. Will test with 4 different props. Starting with a 9" - 7 pitch, 11" - 10 pitch, 11" - 11 pitch and lastly a 12" - 11 pitch 3 blade. I will post numbers soon. Also regarding ratios. I built mine with a 1.36:1 ratio using a 22 tooth top and 30 tooth bottom running a 2" belt.

Sounds like a cool build and test. Start a new thread on it