Prop help for a newbie.

I recently purchased a 1988 Bayliner Capri 1750 with the OMC Cobra 2.3 liter engine. With two people, a 70lb GSP and a full tank of gas I was able to get up to 6000 rpm and 33mpg using a 14x17 aluminum prop. The recommened WOT rpm for this engine is 5200-5600. I was debating trying a SS prop and am not sure if the 300-400 rpm rule still applies when going from an aluminum to SS prop? The 13-1/4 x 19 Rapture for $130 won't be such a great deal if my engine can't spin it to the correct rpm. Thanks for your help.<br /><br />Al

Re: Prop help for a newbie.

I don't think you have got many responses because nobody knows. Too many variables in props. Trial and Error is the only way to find out. For $130 buy it and if your not happy with it sell. I doubt you would lose any money. <br /><br />I would guess that you lose a little holeshot with the rapture but gain at lot of speed. Maybe 5mph or more.

Re: Prop help for a newbie.

doc, do you get Trailer Boats Magazine? In their July issue they published an article on what props do, and the effects of changing them. You need that article. Get it, read it, know it, Live it.<br />In your specific case, figure a loss of 100-200 rpm by going to stainless. The increase in pitch will drop some more. You should be able to stay at or above 5200 with the prop you want to try. And if it doesn't work, welcome to boating! Go spend more money on a different prop!

Re: Prop help for a newbie.

buy an adjustable prop made of composite material.

Re: Prop help for a newbie.

I sent an email to Michigan Wheel asking the same question and their response was:<br />"From the information you gave me I would say going to a 19 pitch would be the right choice. If you went with the Vortex in a 19 pitch you would not see as great of an RPM loss as if you were to use a Michigan Match prop 14 X 19. The smaller diameter on the vortex will cause an increase in RPM's as the pitch increasing created a decrease in RPM's. My recommendation would be a 14 X 19 part # 011025. If you prefer vortex it would be recommended to go with the 12.75 X 21. This is necessary because of the effects of diameter has on the RPM's and how much it decreases."<br /><br />So, I went with the alum Vortex in the 12.75 x 21. I am taking the boat out this weekend and will post my results. I will have the same passengers but will be on a different lake.<br /><br />Thanks for all your help,<br />Al

Re: Prop help for a newbie.

I think aluminum is fine for your four cylinder, you may want to try a 4 blade 20 pitch, if already have the XHS hub for your michigan Vortex, for $100 it's a cheap expirement. The four blade will drop your RPM's and be a little smoother.

Re: Prop help for a newbie.

The results are in...<br />With the new prop I was only able to get up to 5000 rpm at 39mph on gps. The water was a little choppy and I have no idea how much that would affect my speed/rpm's since this is my first boat and I have little time logged on it. From what I have seen in props they tend to decrease in diameter as they increase in pitch for a given engine/outdrive combination. Is this common or just a coincidence for the props I have looked at for my boat?

Re: Prop help for a newbie.

Normal in most cases except for some high performance props where the larger diameter helps bite at higher engine mounting locations.