OMC 4.3 HO re-power is underproped

I have a 1991 Reinell 200 Bermuda hardtop. I replaced the old 4.3 HO 4-barrel with a new crate Vortec 4.3, Vortec intake and a new Holly 4-barrel. It runs fantastic, but I must be making much more horsepower. The outdrive is the 1991 Cobra 800 15 spline variety 1.68:1 ratio (model # 986032 serial # T07060740). It has been rebuilt, but when choosing a prop I don't want to go too aggressive with an out of date unit.

The current prop is 15X17 aluminum 3-blade (#391200). After breaking in the engine, I tried to do a prop test. It pops up quickly and planes out easily at about 3000 rpms. It smoothly increases speed as I increase the throttle, but with a little throttle left and at 4600, it quickly accelerates to just over 5000 before I can throttle back. This is when the boat is the trimmed for maximum speed. It doesn't seem like cavitation because I feel the push at my back.

I'm not looking for speed necessarily, but for an efficient prop that will keep the engine within its power band. Suggestions?

Re: OMC 4.3 HO re-power is underproped

17p is probably to short. A 19 will drop you about 200 RPM and a 21 about 400 RPM.
If you have a local prop shop they may let you borrow a prop to check it out then you can see what each does. If not, id go with the 19p. They are most common choice.

I also sent you a PM.

Re: OMC 4.3 HO re-power is underproped

Thanks. I was wondering about the different options, i.e. 4.5X19, 5.25X18, 4.25X21, etc. Anyone have a similar boat? There isn't a option of testing props here in Central Oregon.

Re: OMC 4.3 HO re-power is underproped

Rpm without speed doesn't mean much.
If the motor is fully broken in you can run it briefly at full rpm to get the speed.

Re: OMC 4.3 HO re-power is underproped

Thanks. I didn't bring my gps and I don't really trust the piot type water pressure speedometer. It was reading about 35 mph at 4600. Could be higher or lower.
The HO system has a rev limiter at about 5000 and it hit that before full throttle. I guess I'll get a 14.5X19 cupped aluminum 3-blade. If that isn't enough, I guess I need a 21 pitch. Or a 20 pitch 4-blade? Does any company have a prop exchange program?

Re: OMC 4.3 HO re-power is underproped

I thought rev limiters worked by turning off the fuel injectors. How does it work on a carburetted engine?

Re: OMC 4.3 HO re-power is underproped

The HO is a electronic ignition. As I understand it, it senses the rpm and backs off the consistent firing of the plugs until the rpm drops. Kind of like a carb pro-stock dragster. Someone correct me if I'm wrong.

Re: OMC 4.3 HO re-power is underproped

iboats used to have a exchange program on certain props.
On some prop calculators a cupped prop adds 1" of pitch.
So your 19 would calculate as a 20.
Disregarding hull form and assuming 5100 rpm w a 17, a cupped 19 computes to 45 mph at 4500 rpm,assuming 10% slip.
Thats a lot of assuming.

Re: OMC 4.3 HO re-power is underproped

Yes some have a exchange programs, but only with ss props, which makes sense. I'll buy a cupped 19 and if it isn't enough, I'll go up from there.
Thanks for the help.

Re: OMC 4.3 HO re-power is underproped

Turning Point has an exchange program. I traded my 13.5x19 for a 13.5x15 for a spare/heavy load prop. Anyway you can exchange any in new condition prop stainless or aluminum, for $20 and shipping the prop back to them. perhaps stainless is more I dont remember.