Prop Diameter Questions, Help Please

[onealj]

I am in need of a new propeller for my Tohatsu 50 TLDI. I have it on a J-16 Carolina Skiff set up with stick steer from the front of the boat. I currently have a 11.1 x 13 aluminium on it that is almost worn out. This combination runs 6200 RPMs WOT and is too much for this motor. I replaced it with a 11.1 x 15 aluminum and the RPMs went to 5000, too low. Top speed by GPS is 38-39 with the 13 and 37-38 with the 15. My question is... if I replace them with a 10.5 x 14 aluminum would my RPM range fall closer to 5500 and what in theory would happen to my speed? I have no experience with propeller selections and the dealers will not let me "try" different aluminum props to get the right fit. I don't think cavitation would be an issue with the smaller diameter because as is I can only trim the motor until the boat porpoises, not until it cavitates. Please advise. This propeller figuring thing could get expensive.

 

[Starcraft5834]

thinking your correct in your guess here.... less pitch generally means higher rmps.. however your diameter is wider by 2....that will slow it down a little....give it a go.. if your still to hot, you can always throttle down a speck...

 

[steelespike]

Your numbers just don't add up. 13" @ 6200 and 39 mph would be about 5% slip.
Your 15" at 5000 rpm 38 mph + 1% slip.
Typical slip is usually in the low double digits. I usually use 12%
Your 1200 rom drop in 2" of pitch is highly unlikely. My point is your tach is likely suspect.
If we substitute 12% slip with the 15" we get about 5600.
If we substitute 12% with the 13" we ge about 6600 , Suspect.
I'm assuming 1.85 gear ratio,speed is gps,correct prop size.
You need to verify the tach or replace it.
It's not unusual for a tach to be whacky.

 

[alldodge]

I am in need of a new propeller for my Tohatsu 50 TLDI. I have it on a J-16 Carolina Skiff set up with stick steer from the front of the boat. I currently have a 11.1 x 13 aluminium on it that is almost worn out. This combination runs 6200 RPMs WOT and is too much for this motor. I replaced it with a 11.1 x 15 aluminum and the RPMs went to 5000, too low. Top speed by GPS is 38-39 with the 13 and 37-38 with the 15. My question is... if I replace them with a 10.5 x 14 aluminum would my RPM range fall closer to 5500 and what in theory would happen to my speed? I have no experience with propeller selections and the dealers will not let me "try" different aluminum props to get the right fit. I don't think cavitation would be an issue with the smaller diameter because as is I can only trim the motor until the boat porpoises, not until it cavitates. Please advise. This propeller figuring thing could get expensive.

Agree with SS, think you should stick with the 15 pitch, in all things your playing around with 1 mph. Unless your someone which runs at WOT all the time the 15 may save you some gas

Edit: sorry my bad :welcome: to iboats

 

[onealj]

Tach maybe but I know at WOT the engine rev limits. The speeds are by GPS app on my phone...over and over and over. The only descripancy I have thought about is the 13 pitch being so worn out its not really 13 anymore. I tried an 11 x 13 stainless that belongs to a friend and the RPMS hover around 5900 at 38 - 39. I'm just not prepared to put that much money into a propeller. I thought by going to the 15 in the first place I wouldn't lose but a few hundred, definitely not 1000.

 

[Sea Rider]

Assuimg tach is in good working order, you went for a wot spin alone on a lightly loaded well trimmed boat on flat calm no windy waters, you have 2 choices

-If at wot you got 6200 rpm with a 13 and 5000 rpm with a 15, then ideal would be a 14 that will theoretically put you around 5800 which is near max wot rpm factory stated for that engine model.

-If maintaining current 13, throttle to max 5850 rpm checking with tach so not to over rev engine and live with it.

-If alone at 6200 rpm, as soon you add more weight, passengers rpm will drop accordingly.

Happy Boating

 

[steelespike]

Tach maybe but I know at WOT the engine rev limits. The speeds are by GPS app on my phone...over and over and over. The only descripancy I have thought about is the 13 pitch being so worn out its not really 13 anymore. I tried an 11 x 13 stainless that belongs to a friend and the RPMS hover around 5900 at 38 - 39. I'm just not prepared to put that much money into a propeller. I thought by going to the 15 in the first place I wouldn't lose but a few hundred, definitely not 1000.

I have no doubt your speed is as recorded We know your prop sizes are accurate,1.85 gear ratio is from a Tohatsu chart,speed is by gps
Our 2 unknowns are actual rpm and slip We know that slip is usually about 12% Highly unlikely to be 5% or 1%
That indicates the Tach needs to be verified or replaced.
The 50 has 11 sizes of props listed its highly unlikely that a 2" pitch change would net a 1200 rpm change.
And if the 13 was really at 6200 rpm from its defects the slip wouldn't be 5%.

 

[Sea Rider]

If tach is 100% operative this is a clear indication that while maintaining same diam prop and only varying pitch size you can have as much as + 600 rpm. Can't speak for all OB brands but Tohatsu OB's with factory delivered props performs very well on light to medium applications. This extreme more achieved rpm would be my fifth seen after installing larger 30-40-50-70 HP Tohatsu engines.

Tohatsu could have zillion props to choose from, but at 600 revs +. just one less to play with adjacent to factory delivered one. Tohatsu and Iboats sells excellent alum props, go for a 14 pitch same diam prop and check. Engine should rev nicely between 13 and 15 pitch.

BTW, were did 150-200 + gained rpm with one pitch reduction in size went ?

Happy Boating