Do I need to prop up? Tuned engine exceeding MFR WOT recommendation

[whitney]

Hi all, first time boat owner here and I'm happy to report that my tuning of the engine (and fixing a bunch of problems) has resulted in a screamer and I'm now exceeding my mfr recommended WOT RPM. Wondering if I should consider a prop change.

This is a bit complicated since the boat is a 92 w/ a 97 4.3 vortec. So it's making a good 15-20 HP more than it had in 92, and the newer motors are rated higher RPM. So I will answer the questions based on the newer motor:

1) Performance issue you are trying to correct.

Exceeding manufacturer WOT RPM. Max recommended is 4800RPM, I'm touching 5kRPM. Interested in trying to do one of the following:

a. reach the same WOT max speed (50) at lower RPM (4800), without seriously affecting the currently good hole shot

or

b. reach WOT at lower speed (~45-47) at 4800 RPM and increase holeshot

2) Current prop manufacturer, model, aluminum or stainless and as a minimum.
Michigan, Al, not sure of a model but it's a "cheap" one from the PO, nearly new

3) Current prop diameter and pitch (required).
14.5x19

4) Wide open throttle RPM and speed with an average load (very helpful)
2 adults, full gas. 5000RPM, 49-50 MPH

5) Engine/drive make, model, year, and HP
OMC/Volvo 97 4.3GL (Vortec heads), appx 190-200hp

6) Boat make model, year, length and weight
1992 Sunbird Corsair open bow 18.5', 2087 lb per manual

Thanks!!!

 

[steelespike]

Re: Do I need to prop up? Tuned engine exceeding MFR WOT recommendation

I can't be too specific but same brand 21" pitch should reduce rpm by 300 to 400.
likely increase top end with a slightly slower hole shot.Or a same brand 19" 4 blade will increase hole shot and lower rpm and speed.
By going with same brand results will be slightly more predictable.

 

[jkust]

Re: Do I need to prop up? Tuned engine exceeding MFR WOT recommendation

Are you testing with your usual load of people, gear and gas. I exceed 5k in my boat with the 4.3 when I am low on gas with one kid but 99.99% of the time I have 4 people and at least 3/4 tank of gas then it is right at the recommended WOT range. If you are, a 21" will take the rpm's right down and you will go faster. Your boat should scream faster yet since 2087lbs is a featherweight compared to contemporary boats with the same engine. A 19" however will give you better hole shot. Get a gps and let us know how it goes. I recently bought a 21" three blade hustler with more cupping than the stock black max 21" and vent holes which are generally only on ss props. I lost several hundred RPM's and lost more than 5mph which tells me it is too aggressive for my package. It sounds a bit louder and bites harder so I don't need to trim like I did before but it's just slower.

 

[Brandon Spurlock]

Re: Do I need to prop up? Tuned engine exceeding MFR WOT recommendation

i am having some of the same prob i have a 91 corsica 198 with a 4.3 it has a 15x17 3 blade prop i hit 5,500 rpm wot with 4 people and by the speedo i am only going 35 to 38 mph. what prop should i run. i was told that the boat should go around 45 to 50 mph with that motor. any help would be great

sorry for the hi-jack

 

[Tail_Gunner]

Re: Do I need to prop up? Tuned engine exceeding MFR WOT recommendation

Would you like a stainless or alum prop

 

[Tail_Gunner]

Re: Do I need to prop up? Tuned engine exceeding MFR WOT recommendation

i am having some of the same prob i have a 91 corsica 198 with a 4.3 it has a 15x17 3 blade prop i hit 5,500 rpm wot with 4 people and by the speedo i am only going 35 to 38 mph. what prop should i run. i was told that the boat should go around 45 to 50 mph with that motor. any help would be great

sorry for the hi-jack

Hello brandon..Could you start a new thread...