Prop Question about diameter?

[nhagar]

Ok here is the prop I have destroyed. It is an old Aluminum Michigan prop SMC 380. It is a 15.25" X 21" 3 blade prop.

That is the original prop on the boat as far as I know. The only thing I would like is a better launch for tubing and skiing. Top speed is not a concern for me. I was told that I should go to a 4 blade prop and that would get me better launch and it would possibly slow top end speed down a few MPH.
I am in the understanding that going from a 3 blade to a 4 blade prop means drop 1" of pitch. that would mean I would need a 15.25 X 20. which they make one and sell it here on Iboats.com but it is stainless steel and $438. way more than I would like to spend on a prop. I would prefer to just stay aluminum.

So I looked at aluminum props with the same pitch of 21 as what I currently have. There are some more options for a 4 blade aluminum prop. 3 different brands but all 3 are 14" X 21".

So If I go to a 4 blade stay with same pitch how is the smaller diameter affect being 1.25" less?
What would you recommend I get?
I just can't afford to spend 350+ on a stainless prop right now so it needs to stay aluminum.

 

[rte592]

Re: Prop Question about diameter?

Ok here is the prop I have destroyed. It is an old Aluminum Michigan prop SMC 380. It is a 15.25" X 21" 3 blade prop.

That is the original prop on the boat as far as I know. The only thing I would like is a better launch for tubing and skiing. Top speed is not a concern for me. I was told that I should go to a 4 blade prop and that would get me better launch and it would possibly slow top end speed down a few MPH.
I am in the understanding that going from a 3 blade to a 4 blade prop means drop 1" of pitch. that would mean I would need a 15.25 X 20. which they make one and sell it here on Iboats.com but it is stainless steel and $438. way more than I would like to spend on a prop. I would prefer to just stay aluminum.

So I looked at aluminum props with the same pitch of 21 as what I currently have. There are some more options for a 4 blade aluminum prop. 3 different brands but all 3 are 14" X 21".

So If I go to a 4 blade stay with same pitch how is the smaller diameter affect being 1.25" less?
What would you recommend I get?
I just can't afford to spend 350+ on a stainless prop right now so it needs to stay aluminum.

Might be worth a read here.
http://www.formulapropeller.com/propellers.html

 

[steelespike]

Re: Prop Question about diameter?

Usually adding a blade they reduce the diameter to help maintain rpm.
Answer the questions in the sticks at the top of the prop forum,.
What is the present wot rpm and gps speed?
Depending on your rpm it may be possible to simply reduce rpm to 19.