Prop Math - Do I have this right?

[TraceF3]

A boat with a 1.75 gear ratio LU traveling 6000 rpm with a 25p prop and 7% slip should get about 75 mph result?

6000 engine rpm / 1.75 = 3428 prop turns x (25"x.93) = 79701" water / 12" = 6641' traveled / 5280 (a mile) = 12.77 x 60 mph = 75 mph


Thanks in advance

[Cary 29]

Yup, you're correct.

[oldschoolltv]

Do it the easy way

[Euroski]

This works too

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

[Motv18]

76

either slip works though

slipping x mph off theory 76
or slipping x inches off pitch theory. 75

[KIRCHNER]

A boat with a 1.75 gear ratio LU traveling 6000 rpm with a 25p prop and 7% slip should get about 75 mph result?

6000 engine rpm / 1.75 = 3428 prop turns x (25"x.93) = 79701" water / 12" = 6641' traveled / 5280 (a mile) = 12.77 x 60 mph = 75 mph


Thanks in advance

JUST came back from some prop testing, 25''P, 6000 rpm 1.87gear[ i think] gps says 80, old type speedo says 88? doesnt seem correct. conditions were perfect, we were movin but 80? baffled now. 225 pm, trophy up high, bobs lower good pressure. cant seem to get rid of the torque steer, have tab, hyd steer wants to go right, big time, no fun. any answers welcome. my left arm aches!

[Cary 29]

Gear ratio 1.75 I thought

[Cary 29]

With a 1.75 that would be 1 percent slip

[KIRCHNER]

With a 1.75 that would be 1 percent slip

according to mercury....

[Cary 29]

Yes

[JBS]

How do you figure the slip?
An 88 2.4 fishing motor should have 1.87 gears?
A 200 pro max ECU has a limit of 7200?

[Motv18]

With the proper set up you can see positive slip. Cup and progressive pitch plus some jet like thrust from the tail

also pitch is theoretically figured unlike measured screw threads.

[Beak Boater]

What is this positive slip you speak of?? Never heard of that before. Please enlighten me.

[HStream1]

Heres the formula;

Pitch x RPM =
Gear ratio x 1056 =

So:
25 x 6000 = 150000 =81.168 theoretical speed. Theoretical speed - 7% = 75.487 MPH
1.75 x 1056 =1848

A boat with a 1.75 gear ratio LU traveling 6000 rpm with a 25p prop and 7% slip should get about 75 mph result?

6000 engine rpm / 1.75 = 3428 prop turns x (25"x.93) = 79701" water / 12" = 6641' traveled / 5280 (a mile) = 12.77 x 60 mph = 75 mph


Thanks in advance

[TraceF3]

If I use all the decimals I get the same result-

6000 /1.75 = 3428.57 x (25x.93) = 79714.25 /12 / 5280 = 1.2581 x 60 = 75.48

I was just rounding

Thank you for the short cut formula!