rpm drop with less pitch prop

[sabsab]

Have a 13foot boston whaler with 40hp yamaha.Using a 10 1/4x15 ss cabelas high performance prop.At wot it produces 5500 rpms, right at what is recommended by yamaha and i get about 33 knots from by gps. Needed a spare prop so bought one at iboat.It is a rapture 10 1/2x13.Figured I could use it for heavy loads or pulling. Well took it out with new rapture and it only run at 4800 rpm`s at wot and about 29 knots. I expected lose of speed but surprised at loss of rpm`s. I thought I would gain rpms with less pitch. Need advice.

 

[tal]

Re: rpm drop with less pitch prop

I would have to guess the larger diameter could have some effect too.

 

[steelespike]

Re: rpm drop with less pitch prop

I doubt the diameter has much affect on the rpm.
Unfortunately sometimes when you change brands all bets are off.
I'm no expert on prop geometry but would bet there are some key differences between these props.Some props have stern lift some bow lift.Surely Cabellas doesn't make props they may have info on who makes props for them.Rapture or one of our prop pros may be able to at least tell you the caricturistics of their prop.
A good prop shop may be able to tell you the difference in the props if yopu bring them to them.They may be able to tell if the 15 has been doctored.
My first thought would be to remove some of the cup from the rapture.
But again try to get some expert advice on what they would reccomend.

 

[Silvertip]

Re: rpm drop with less pitch prop

Agree whole heartedly with "Steel". Two props with the same diameter and pitch from two different manufacturers rarely will perform identically. Diameter and pitch are only two design characteristics. Rake, blade shape, cupping, and blade thickness are other factors. One prop may get the boat out of the hole quicker than the other, but the other may be faster. Test the props on a different boat and the results may flip-flop. This is why you can run numbers into a prop calculator and things may not come out as planned. Prop selection is a crap shoot at best unless you do a great deal of research that includes your exact boat, your exact engine and the exact prop you intend to buy. In your exact situation, you might want to experiment with engine mounting height. Your new prop may allow running the engine higher. I don't mean trimming -- I mean physically mounting the engine higher on the transom. Just don't go so high that you lose water pressure.

 

[sabsab]

Re: rpm drop with less pitch prop

Thanks Guys,I have a Bob`s Machine shop manual jack plate on the boat. I have it set to 2 1/2" setback ,can go to 5". Thought 5" would put to much stress on transom. Ventilation plate is about 3/4" above bottom of boat. Will work with height of engine and see if rapture will perform better. Thanks for advice.