propeller advice for 150 hp

[Hank49]

This is more about boats than boat diving but I guess this is the only place to post this. I have a 22 foot boat with a 115 HP four stroke. It has a prop with a pitch of 15. With this set up I get 5,300 RPMs at full throttle, which is what the manufacturer recommends.
My question is this. I'm getting a 150 HP and am not sure what prop pitch to get. I'm increasing HP by 30% but if I increase the pitch by the same percentage I'm looking at a pitch of 19-20. My friend has a 250 HP on a boat the same size and his prop is a 19, I believe. His gets 5200 RPMs. I realize trial and error would be the best way but it's expensive (aluminum props here go for about US$200)not to mention getting the prop down here from Belize City. Is there a formula for this to save me a 5 hour trip or two to the city? Thanks, Hank

 

[Chuck Tribolet]

Beg and borrow.

Will the old prop fit the new motor? If so move it over, and use it
during breakin, being careful not to overrev. Once broken in see
what you can borrow to test with.

Be sure to consider gear ratios. They vary from motor to motor
and affect the prop pitch proportionately.

You are only increasing HP by 30%

 

[Hank49]

Beg and borrow.

Will the old prop fit the new motor? If so move it over, and use it
during breakin, being careful not to overrev. Once broken in see
what you can borrow to test with.

Be sure to consider gear ratios. They vary from motor to motor
and affect the prop pitch proportionately.

You are only increasing HP by 30%

I guess that may be the only way.(thanks for the HP correction) I have a 13 pitch as a spare for the 115 and when I put it on it buries the tach at above 6000 RPMs. This leads me to believe that the 15 is too small of a bite for the 150 HP. Since I am getting a prop with the new motor I'm thinking a 17 and a spare 18?
Also, I've always used aluminum props. Many have stainless here but since they don't bend or break it seems that something is going to give in the lower end if you hit a rock or reef. What are the advantages of a SS prop?

 

[Bob3]

SS do bend, take my word for it. They're just easier to hammer straight again afterward.
You want I should check prices on new SS props for you? Aluminum start at about $60 - $80, SS at a tad over $200. (new)

 

[Hank49]

SS do bend, take my word for it. They're just easier to hammer straight again afterward.
You want I should check prices on new SS props for you? Aluminum start at about $60 - $80, SS at a tad over $200. (new)

Hi Bob,
Sure, as soon as I figure out which pitch I need (I'm 90% sure it's going to be 17) I'll order a couple from you. A new stainless prop in Belize goes for US$650. I don't know how much the duty on one shipped in will be but it can't be 600% !! Aluminum costs $180. Ahhh, the price of paradise......Thanks

 

[DiverRick69]

The motor manufacturer should be able to help with this. E-mail their customer support with all the info you can give, including boat model, current engine and prop info, new engine, etc. They should be able to get you real close on the first try.

 

[Bob3]

Hi Bob,
Sure, as soon as I figure out which pitch I need (I'm 90% sure it's going to be 17) I'll order a couple from you. A new stainless prop in Belize goes for US$650. I don't know how much the duty on one shipped in will be but it can't be 600% !! Aluminum costs $180. Ahhh, the price of paradise......Thanks

This could be one of those cases where a visitor brings one down... maybe wired up as a lamp? The import duties can't be too bad on lamps.