River boat mods

[1gr8bldr]

Just found this forum. The boat is a 24 ft SeaArk, custom built by myself in 2004. Yamaha 150f . Only 400 hours on it from taking a 14 year break from fishing. Getting back out on the water again. I'm in need of some experiential knowledge. My boat's due for new grates since I have straightened them so many times, they all now are ( shaped. Can't get that out. I also could benefit from shimming. However, that is not my question. My question is a theoretical question. Thinking of typical outboard props, like gearing, one might make your engine run higher RPM's than recommened at WOT. Be slower top end but have extreme holeshot. The opposite pitch extreme might make you weaker on the hole shot, give you more speed top end but never reach recommended RPM's as if it ran out of power. My Jet... I'm trying to get more from it. In it's prime, grates in good shape, impeller shimmed properly and good edge, it seems as though something is lacking top end. Always been this way. I don't know if it's typical Jet inefficiency or if it's improper pitch of the impeller? The boat in it's prime, load normal, has ran 34+ mph. However, after a certain RPM, I get mostly noise, very little speed gain, and burns an extreme amount of fuel. It's like the last 1500 RPM is worthless. So, I rarely ever go above 3800 rpm at which she cruises very pleasantly, and gets great fuel mileage. Usually 28 mph. The boat is instant on plane, and has no wait time for power. I have a jack plate at which I know exactly where the best performance is. But the best performance heights will slightly spin out on take off. So, I take off from one position on the jack plate and trim angle. Once on plane, I raise it 5/16 inch and trim up the motor just slightly for highest speed. I don't look at the speed gauge. I simply look at my spray off the side of the boat that reveals how much the front is lifted. Higher the boat lifts, the less drag, the faster it goes. But, I feel like I have potential for better performance. What is going on? I used to think it's the wrong pitch prop. But, I don't know? The last 1500 rpm are not wound tight, like to low a pitch are not weak sounding like to high a pitch.... it's like it's .... I can't explain it. That's my question. What is going on. Understanding it will help me to modify things to get the most out of the motor. Thanks

 

[CedarRiverScooter]

after a certain RPM, I get mostly noise, very little speed gain, and burns an extreme amount of fuel. It's like the last 1500 RPM is worthless.

I have an entirely different setup - inboard jetjon. But I have the same experience. Above a certain point, the efficiency falls off. I chalked it up to the pump can push only so much water.

Maybe there are outlet nozzle mods you can do. I know they make extended nozzles for jetskis. I wouldn't mod anything that you can't undo, however.

Good luck.

 

[1gr8bldr]

I also wonder, maybe the entire jet is not large enough for the motor. Foot, prop, bell housing, etc?

 

[redrum]

Just sounds like a jet to me.

 

[eeshaw]

Have you checked to see if there's a bigger pump available. It sounds like it's just hitting a wall performance wise. I'm thinking that pump may be maxed out. Have you checked with the pump manufacturers to see if they offer a bigger one and to see what they recommend?

 

[handyandy]

I concur with what was said I'm wondering if a different larger pump can be mounted on your engine. I don't know your engine all that well but I do know for omc engines the r model pump fits v4 and v6 engines with a driveshaft change, but a larger pump is recommended for the bigger v6 engines I forget which model. You may have the right pump, but should maybe be running a different impeller. I'd start with new intake grates, sharpening and shimming the impeller. Make sure you sharpen the impeller properly the way outboard jets suggest isn't the best way. Don't file on the bottom of the leading edge of the impeller sharpen it so it angles down and grabs water better.