Prop for Skiing? Over-rev?

[trail-running]

I recently bought a used boat so I do not know if this is the stock prop on it, ( I doubt it is ).<br />It is a 19 foot bowrider inboard/outboard with a 4.3 L Mercruiser. About 180 H.P.<br />Prop is an aluminum 14.5 x 17. We mainly intend to use the boat to pull skiers and tubes. I have tested the WOT and it is over reving with this prop on it?<br />Engine says 4800 RPM WOT and it is hitting 5300-5400. I know I need to go up to a 19 or 21 prop but will that greatly hurt the performance pulling skiers up? If I went with a Stainless prop at 19 would that bring the RPM down without hurting the hole shot to much? Would a 4 blade prop maintain the hole shot without over reving the engine? Anyone have any suggestions or advice? Thanks.

 

[QC]

Re: Prop for Skiing? Over-rev?

Aluminums often have a better hole shot than a stainless as they flex a little. Hopefully Walleyehed will stop by, but my experience is that the best hole shot prop is a Merc High-Five. You actually need to go to the 21 based on the info above, and your hole shot will definitely suffer. Are you absolutely positive that your tach is right? I would verify that before I did anything. First thing to check is there is usually a little switch in the back of the tach for 4, 6 or 8 cyl. Good luck.<br /><br />Oh and welcome to Iboats!!

 

[TilliamWe]

Re: Prop for Skiing? Over-rev?

I am with QC, confirm that the tach is accurate. Then confirm that the prop info you supplied is indeed accurate. Also, if the prop has been repaired many times, it may not be performing like it should.<br />If all info & prop are absolutely correct as you stated, you will most likely want a 20in 4 blade stainless prop. If you don't have it, get Trailer Boats Magazines July 2005 issue. It has almost everything you ever wanted or didn't want to know about props in there.<br />Basically it breaks down to these rules of thumb:<br /><br />100-200 RPM drop for every inch of pitch increased.<br />100 rpm drop from AL to SS<br />100-200 RPM drop when adding a blade.<br /><br />But since you appear to be WAY underpropped, you may need to experiment a little. Got a good shop close by that will let you try props first? Or better yet, got friends with spare props?

 

[TilliamWe]

Re: Prop for Skiing? Over-rev?

One more qeustion... isn;t it coming out of the hole like a demon now? With that little of prop on it, the holeshot should be amazing! You may be spoiled when it comes to holeshot. Just be prepared that even with the correct sized 4 or 5 blade prop, it may not come out of the hole like it does now. Think of it like you have 4.56 gears now, with bad tires. But when you change to 4.10s with good tires, it may not be as good as you thought it was!

 

[sangerwaker]

Re: Prop for Skiing? Over-rev?

Sometimes, 2 props are necessary to accomplish what you seek.<br /><br />To get your RPMS into the correct range, you definitely need to go up in pitch. Probably something in a 19-20 pitch. You would be amazed how differently 6 different 19 (or any) pitch props perform on the same boat. If you can find a prop shop that will let you "try before you buy", it makes finding the right prop a whole lot less expensive.<br /><br />For skiing, for best holeshot performance, the optimum prop is never tha same as your high speed prop. If the one you have is great for skiing, use it! You must of course be aware of the tach at all times. <br /><br />My recommended max RPM is 5800. Mine is set up to turn about 5900 without touching the trim. I don't worry too much about someone else over-reving while pulling me cuz I tell 'em to not touch the trim at all. If I plan on cruising and not skiing, I change props to one that allows me to trim up and reach close to 6000.<br /><br />Before doing anything, I'd follow the advice above and make sure everything is accurate.

 

[Richard Petersen]

Re: Prop for Skiing? Over-rev?

Do your wallet a big favor. See if you can get a REV LIMITER from the Mfgr. My limiter has saved me countless times when I am air borne. By the time you react damage is being done. Plus, you do not need to look at RPM's. You can hear and feel the rev limiter doing it's job.

 

[ob15]

Re: Prop for Skiing? Over-rev?

sangerwaker,<br /><br />You must remember that trail-running has an IO. RPM ranges are total different.

 

[trail-running]

Re: Prop for Skiing? Over-rev?

I will check the tach and make sure it is correct. Also you are correct "TilliamWe", it seems to be comming out of the hole pretty good but I don't really have much to compare it to. The prop has been repaired at least twice so it may not be performing very well? If I understand it correctly, then will a 4 blade prop give me more pulling power but due to increased resistance hurt the top speed? If that is the case then I might want to switch to a 4 blade prop because I am more interested in pulling power than top speed.

 

[sangerwaker]

Re: Prop for Skiing? Over-rev?

Dave, I understand the RPM ranges are totally different. I was using my set up as an example. Trail-runner has his recommended WOT right in his opening post-4800.

 

[walleyehed]

Re: Prop for Skiing? Over-rev?

T.R. yes, you can get more out of the hole on an I/O, but don't expect any gain on top.<br />If you have 3bld alum, drop 1 inch for a 4bld alum.....if going to a SS 4bld, drop 2" in pitch from the alum.

 

[TilliamWe]

Re: Prop for Skiing? Over-rev?

trail, think of the four blade's increased surface area as givving you more traction, yes. So out of hole is better. Then at top end, it's creating more drag, so usually a little slower. At least on a set up like yours. The 5 blade mentioned earlier gives great out of the hole due to all the blade surface area, but, obviously creates more drag on the top end. Best of luck to you.