Over propped or engine problem?

[merrtpa]

I have a 1996 20ft Proline WAC with a 1996 Johnson carbed 150 Ocean Runner. Both are new to me. The hull weighs 2500lbs. It looks like it might be the original prop and if I am reading the numbers right, it is a 15 1/4 x 15. My issue is this: with only 2 of us on the boat, trimmed up, and 30 gal gas, it maxes out at 5000. Most of the "prop wizzards" recommend a 14 1/4 x 17 for top speed and 15 1/4 x 15 for heavy loads. Is there anything else I should be looking at besides the prop (engine?, hull?) before I start trying other props? I would think less than 15p would be getting pretty low.
Thanks for the help.

 

[Dhadley]

Re: Over propped or engine problem?

Not a lot of info to go on but I'd bet the motor is probably all the way down. It'd also help to know if it's on a bracket but we'll assume it's not. If the motor is all the way down we'd start by looking at the AV plate (plate right above the prop) and where it is in relation to the keel when the plate and keel are parallel. With a large diameter prop it's likely the plate can be above the keel a good bit. We'll also assume the prop is stainless.

 

[merrtpa]

Re: Over propped or engine problem?

No bracket and SS prop. By all the way down, I'm assuming you are talking about how it is mounted on the transom and not the tilt? I will look at the relation of the plate to the keel. What should this relation be optimally?

 

[Dhadley]

Re: Over propped or engine problem?

The optimum height or X dimension is a combination between prop design, hull design and your desiginated use.

Ok, no bracket - the motor is mounted on the transom and we do have a stainless prop. Once we "see" where the plate is we can go from there.

 

[merrtpa]

Re: Over propped or engine problem?

The motor is mounted in the second hole from the top which puts the plate dead even with the keel.

 

[Dhadley]

Re: Over propped or engine problem?

I'd start by raising the motor a hole at a time and keep track of the rpm and performance increases with the existing prop. You have a good baseline by rpm, speed and load. Just keep an eye on water pressure.

 

[merrtpa]

Re: Over propped or engine problem?

Thanks, I will give it a try. Just for my own knowledge, what effect does raising or lowering of the motor have on the ride/performance?

 

[Dhadley]

Re: Over propped or engine problem?

Raising the motor will reduce gearcase drag. It'll do some other things but that's the main thing we're after.

 

[merrtpa]

Re: Over propped or engine problem?

I moved it up one hole and the straight edge shows it about 5/8" above the bottom of the keel. I saw no difference in engine speed or actual speed on GPS (37.5 was max @ 5K rpms). Here are the links to the pictures. If I understand it right, I should see the AV plate right at or just above the water line. So it looks like I should go up another hole?

 

[steelespike]

Re: Over propped or engine problem?

Looks like it.I usually suggest raising the motor to a point just before venting becomes a problem.Some props allow more height than others.Sometimes this has little affect but others in the extreme can net 2-3 mph and some rpm.