Alpha One Gen One - Prop????

I have been member for a couple of years, never needed to post until now. The search engine is great and has always provided a workable resolution in the past.

This is a restoration project I have been working for a while. Last year my upper gears shredded, didn't now why. So I took the whole boat to a well recommended mechanic thinking that if there was issue I had overlooked i.e. shaft alignment, prop size: he would catch the mistake and fix the issue. Well long story short, he swapped the upper from a 1:50 to 1:84 assuring me that it would not affect the performance as long as I had the correct prop. This where I am now.....

1) Excessive cavitation at low rpms, hard to plain, poor hole shot, you have ease the throttle to get it up on plain.
2) Quicksilver - 3 blade - Alum
3) 14.5 x 19P
4) WOT 4000 and speed 32mph one person, 1/2 tank
5) 305 / 4bbl 600cfm /Alpha One, Engine is new, 230hp ish
6) Formula F-18 Thunderbird deep V-Hull, 1984, 18'2" and 3000lbs with one person and fuel

Thanks in advance

Re: Alpha One Gen One - Prop????

I don't understand you have ing ventilation problems unless the outdrive is higher on the transom for some reason.
Is the prop in excellent shape?Is it trimming down all the way?
Slip is about 18% reasonable for the rpm.
But I think the rpm should be off the dial with a 19" prop and a 1.84 ratio.
I think a 1.84 ratio is about what a 190 hp 4.3 would have. Can't speak to your 18 footer but a 4.3 with a 21" or 23" prop typically pushes a 18 footer close to 50..

Re: Alpha One Gen One - Prop????

Is this the same prop you were running before the ratio change?

Changing from a 1.5 to a 1.8 ratio should (using the same prop size) result in increased WOT rpm due to the lower prop speed with the 1.8 ratio(easier for the motor to turn it).

That gear ratio change would normally require a prop change(more diameter/ pitch)to compensate for the slower shaft speed.

Re: Alpha One Gen One - Prop????

steelespike said:

I don't understand you have ing ventilation problems unless the outdrive is higher on the transom for some reason.
Is the prop in excellent shape?Is it trimming down all the way?
Slip is about 18% reasonable for the rpm.
But I think the rpm should be off the dial with a 19" prop and a 1.84 ratio.
I think a 1.84 ratio is about what a 190 hp 4.3 would have. Can't speak to your 18 footer but a 4.3 with a 21" or 23" prop typically pushes a 18 footer close to 50..

Click to expand...

The prop is in excellent shape
I agree 100% on the ratio change, it should rev out.
I am double checking my carb tune and fuel filters. Thinking it is running lean for some reason.

If I run a larger Dia prop, what should the pitch be?

Re: Alpha One Gen One - Prop????

jestor68 said:

Is this the same prop you were running before the ratio change?

Changing from a 1.5 to a 1.8 ratio should (using the same prop size) result in increased WOT rpm due to the lower prop speed with the 1.8 ratio(easier for the motor to turn it).

That gear ratio change would normally require a prop change(more diameter/ pitch)to compensate for the slower shaft speed.

Click to expand...

Yes its the same prop. I knew a prop change was needed, but wanted some data before making a switch.

What prop would you recommend ?