Prop size

[toms4w]

I have a '95 3.0L OMC cobra in a 17.5 ft fiberglass boat and new to boating. Manual calls for 14.5x19 prop as standard, came used with 15x17 prop on. Manual WOT calls for 4,200-4,600 rpm, with the 15x17 I can get to maybe 4,000-4,200. Barely gets 1 average skier (2 skis) out of the water (with 2 adults in boat). Engine was previously rebuilt before purchasing. Isn't the 15x17 prop better than 14.5x19 for hole shot and getting a skier up? Since I can barely get a skier up I'm thinking I should change to maybe 15 pitch, but that is getting pretty far from the 14.5x19 prop the boat is supposed to have. Or would I be better trying the smaller diameter 14.5x19 prop the manufacturer lists? Maybe it lost some power with rebuild? In looking at some of the posts, I should hit a prop shop that lets me try a few. Would any of you experienced boaters recommend going to maybe a 15 pitch or staying put? Also, is there much difference in 14.5 vs 15 diameter? Thanks!

 

[walleyehed]

Re: Prop size

Diameter (larger) helps in a couple of ways...1, it usually holds, or "pushes" a load better, and 2, it can be run at higher engine mounting heights with outboards. Some props come in a given pitch in 2 different diameters, with the larger size being most effective for the heavier sterndrives and believe it or not, the high HP outboards with high mounting or running heights...the smaller diameter pretty much fills the void in-between.<br />First Item I would check on your rig is the engine performance...is it operating properly?<br />Is the engine putting out what it should, in other-words...how much time on the rebuilt engine?<br />How is the condition of the prop you have now??

 

[MRS]

Re: Prop size

The pitch is more important I run a 120 merc. when I got the boat was a real dog it was running a prop 19-pitch I tuned up and went to a 17 much better and the rpms were close to wear they should be. Then with water skiers that were learning two skis had to go to a 5x15 pitch prop.But had to change trim tab because the one I had on would have hit prop so check when changing to bigger prop also revs higher than should so I do watch the tach. so not to over rev also speed alot slower but if they are on two skis speed is usalley not a problem.

 

[toms4w]

Re: Prop size

Thanks for the responses. Walleyehed, to answer your questions, the engine seems to be running properly. My guess is less than 100 hours on rebuild. The prop I have now does have some dings and nicks. What is a good way to tell when the prop is banged up enough for a rebuild or replacement.

 

[stevens]

Re: Prop size

What is a good way to tell when the prop is banged up enough for a rebuild or replacement.

Does it vibrate, shake, make noises, ventilate, spin, loose grip, have excessive slip, or feel out of tune in some way? Or, do you feel like getting a new one? Then it's time!

 

[toms4w]

Re: Prop size

As far as I can tell, it does none of the above (but I'm a novice). But shouldn't a 3.0L in a 17.5 ft boat get a skier out of the water easily?? Since the engine seems to be running fine, I'm looking at the prop since I can't get to the recommended WOT.

 

[MRS]

Re: Prop size

Boats are sure fun but they are not cheap if you want to do right. I carry two props on board at all times. I run the 17 most of the time and carry and extra ajustable one with 15 and 17 blades for it and the 19 that came with the boat. I was stuck once when I hit something that was in the water do not know what it was but it did destroy my prop that problem will not happen again. Something else maybe but not that and always tune up motor each year before boating season starts for you to make sure things are right and dont forget extra nut for prop I have seen people drop them in the water when they have to change on the lake. If things do not work right with your prop you just have to try different set ups till it is right for your boat.

 

[stevens]

Re: Prop size

There are a number of things that may bog you down. Like, is the bottom clean and smooth, is the boat heavily loaded with gas & people & gear, does the engine run properly and get enough fuel and air, is the drive trimmed right, does the prop have sufficient blade area for the load, etc., etc.<br /><br />First place to start would be to verify the tachometer. You don't want to go to a lot of effort and expense if it is based on false readings from the tachometer.