how to find the right prop

[m.qualls89]

Ok so everyone here has been very helpful so ive got another thing to ask. I have a 1978 sea star made by glasstek with a 150hp mercury v1500 black max motor. Im havin prop trouble the 24 pitch im running puts to much drag on the motor and only lets it get to 4000 rpm at WOT in the water. I have a smaller prop that the guy i bought it from said is for skiing but it doesnt have the pitch stamped on it. The small prop is to small i get better acceleration but lose about 7 mph and i cant run WOT or it will over rev. I cant find any info on this boat so how can i find the proper pitch prop? I have a service manual for the motor on the way, will it say in that? Ecpensive as they are id really hate to just buy props until one is right. Thanks in advance.

 

[Texasmark]

Re: how to find the right prop

I think the rule of thumb is 200 rpm per inch of pitch. So if you are looking for 5500 rpm's you need to drop your pitch about 1500/200, or 7 1/2 inches using the same brand/style/material, etc. etc. (apples and apples) prop. Since props usually come in 2" increments, you have to decide on backing off and going with 24-6 or 24-8. You can expect the load on the engine to decrease as the boat speed increases and less of the hull is in the water tending to point to more pitch vs less (-6 vs -8). So 24-6 = 18. But the 18 is not a standard pitch whereas a 17-19 is. Soooooo I'd spend my money on a 17P and if you rev up a little over recommended WOT rpm's personally I run all my Mercs that way and have for years. Also this leaves you some room for pulling water toys. Otherwise you could hold the throttle back to get your max rpm limit, or just go with the 19.

HTH,
Mark

 

[m.qualls89]

Re: how to find the right prop

Thanks alot!

 

[steelespike]

Re: how to find the right prop

I think Mark has a handle on this.
I like the idea of starting a little light when trying to pick a prop.
Your much better off being able to rev freely hopefully with rpm range but if it ends up a little high you can just watch the throttle.
At 5500 a 17 could produce about 44 mph.with lots of pep.
Assuming I have the specs right.