OMC prop selection

[jjones3]

I have a 18.5' 1989 Ski Deck made by Anchor Ind. It is powered by a Ford 302 (5.0L) with a OMC drive (M/N 985687). I currently have a 4 blade 14.75 x 15 that came on the boat when I bought it and a 4 blade 14.5 x 18 a friend gave me. I also have a 3 blade 15 X 17 that was in the boat as a spare. It does need some work though. I slid it on the shaft yesterday and I would not seat proper. I didn't really look into why. Both of the 4 blade props have a WOT of about 4100 - 4200 RPM. WOT speed on the 14.75 x 15 was 36 MPH. I did not get a WOT speed on the other due to dead cell phone. I did notice the engine has a lot louder roar with the 15p than the 18p. I am a little more concerned with fuel economy than power but still want to be able to pull a skier. I am about 250 lb and want to get back up on a slalom ski again. It has been a long time. My question is will one of the prop I currently have meet my needs or should I look for another one. Another note: I prefer aluminum over stainless just because I'd rather trash a prop over an outdrive if I hit something. That not hard to do around here. Thanks in advance for your time and advice.

 

[jjones3]

Re: OMC prop selection

I also have a dolphin fin on the out drive.

 

[steelespike]

Re: OMC prop selection

Your numbers don't quite add up. You can't go 36 at 4200. Is your speed by gps?
If the 36 is accurate rpm would be about 4800 a little high for your Ford 5.0
I'm using 1.66 as your gear ratio.
Couldn't find any Anchor only ankor. only dating to 1980.
Is it a deep V or more like a shallow V to flat. A deep V would have about a 18 degree V at the transom.
The 4 blade 15" prop should work for skiing.
Being a 89 i'm wondering if the motor may be a little tired
The 18,17,15 prop series and and the dole fin make me wonder.
Does it have a strong hole shot? Does it struggle to stay on plane at reduced throttle?

 

[jjones3]

Re: OMC prop selection

The speed was by GPS. I do not really trust the tac. Anchor Ind. common name is Deck Boat. They were made in Gatesville TX and went out of business back in the 90's some time. The are pretty rare these days. I would say some where between a shallow and a deep v. I am attaching a couple of pic from the restoration to give you an idea. The motor I'm sure is a little tired. The block came from a 78 glastron. I couldn't afford much else at the time. Will probably replace in the next few years. Compression is fair though. It also has a new hot coil and rebuilt 2v holley carb I also tuned it at the end of last season. It does plane within about 3-4 sec. with both 4 blade props and two Full grown adults. It starts dropping out of plane around 2000 rpm if I remember right. I did try the slalom last year and could not hold on to the rope. Did get up on two first try though. It's probably been over ten years since I skied. Was about 50 lighter back then. Guess that's what happen when you have kids.

 

[steelespike]

Re: OMC prop selection

The doelfin should help with hole shot and stay on plane better at reduced throttle.
While it helps to stay on plane it does also create some drag.
If its in the water at speed it probably eats up some top end speed.
The best cruising speed would be somewhere from just on plane to about 1000 rpm higher.
If you really want to strive for best mileage a flow meter is the only way to go, some actually do realtime calculations.