Underpropped?

[lexkyboater]

I had posted this thread http://forums.iboats.com/showthread.php?t=204674 and they suggested I come to this forum for more info on my prop questions. I have a 1984 15 1/2 foot Cheetah bass boat with a 93 Yamaha 115 and it's turning 5800 rpms at WOT. I know it's a 13 1/2 inch diameter, but don't know the pitch. Have had the prop off, and can't find any identifying numbers or marks, inside or out, other than a cursive "C" on the outside between 2 of the blades. Attached are pictures.

I was wondering if 5800 rpm is too fast, or if that's okay? Some of the newer 115 Yamahas have a WOT range of 5100 to 5500, and that's why I'm wondering if 5800 is too fast. Also, my motor height is about 2 inches from the bottom of the boat, and I do have a v-shaped hydrofoil, and am catching a lot of air even on mild turns, especially to the left. Hoping to get recommendations on that too. Have tried removing the fin, but the front end pitches up and down quite a lot at lower plane speeds, didn't try WOT without the fin. Others have suggested props with more cup, or stainless, but I was wondering if a smaller diameter would help, so that the prop is not as close to the surface of the water? Maybe a 13 inch?

Holeshot is great with the current prop, top speed per the speedometer is 43 mph, but I want to maintain good acceleration for skiing/tubing, etc. Had thought about a 4 blade for those reasons, but would prefer to stay cheap. I'm not really interested in spending a lot on a new stainless prop. A local dealer surprisingly said that he thought he would leave it as is. Thanks for any help, Steve

 

[lexkyboater]

Re: Underpropped?

I'm heading to the prop shop Sat to test some props and get them to identify my current prop. Proper Props in Bronston KY at 606-561-6005. I've had 4 different dealers tell me it's everything from a 15 to a 21. Not sure I believe the 15. Each blade has at least 4 inches of pitch (difference from bottom to top) in the middle, so it has to be at least a 17 pitch, but the average of 3 dealers was that it was a 19 pitch. Two said it could be as high as 21, but I argued since it's 13 1/2 inches in diameter and turning it 5800 rpm. Of course, they may prove me wrong based on the calculator below.

I went to http://boatmartusa.com/marinemall/propPage.php and they let you first enter engine manufacturer, HP, model and year, but on the second page they have a prop calculator based on your boat type (I guess they have a database containing boat weight), length, gas, number of people in the boat, etc. and it said I needed a Hustler 13 1/4 x 21 for watersports and 13 1/4 x 23 for speed. I wouldn't have thought it would turn a 23 pitch on an 1100 pound 15 1/2 foot boat? We had a 23 pitch on an old 72 Evinrude 125 and it struggled with that big of a prop on a 72 Steury tri-hull. But with a 17 pitch 4 blade, that 125 was a rocketship!

Anyway, Proper Props has hundreds of used props, and they say I can buy one and return without damage so I can tune it to run the way I want. However, knowing that the engine runs right (I think) seems they will probably have a suggestion that will be close. I'm just nervous about buying a new prop even at $100 and then it not work well for skiing and pulling.