Alpha one prop questions

[spankym13]

Hey guys im still a little new so please work with me this is my first boat! I've got a 1999 18' bowrider, with a 3.0 mercruiser and alpha-one. The prop that was on it when we got it is a 15x15 and at wot puts my Rpm right were it should be. * 4400-4800*. We went to lake powell last week and I got me a 14x21 because most of the websites prop wizard said that was the best prop for that boat with what we had in it. And I got to lake powell and this prop doesn't fit. It hits the trim tab. Any Ideas? Also maybe some reccomedations for a prop for this boat. most lakes are around 4000ft in elevation. I'd like to run a 14". Also were would you say I should cruise around the lake with this motor and boat. 3800-4000 rpm?

 

[Bondo]

Re: Alpha one prop questions

Ayuh,...

The Torque-Tab can be filed or changed to a smaller 1...
But,...
If you're pulling theRequired rpms with the 15" prop,...
A 21" prop is going to Overload the motor,+ It'll run like $h!te....

 

[steelespike]

Re: Alpha one prop questions

First of all each inch change in pitch changes rpm by about 200.higher number lowers rpm lower nimber raises rpm.So your 21 would put you at about
1200 rpm lower. Prop selectors can only respond to the info given and I doubt there is any elevation numbers.19-21 is the typical pitch for normal operation.Load and activities could change that considerably.
I'm not a pro when it comes to running at higher elevations but it seems to me your boat would normally use a slightly higher pitch at 4,000 ft.Its obvious yours is operating at the right rpm.So for your boat 15" is right.
If your right on with the 15 why would you want to go to a 14?You might do a compression check and be sure it is well tuned.
If everything checks out your good to go.Someone may be along with more specifics about higher elevations.Don't be afraid to ask any questions.
Cruising speed is variable with boat/motor design and load.In the absence of a flow meter the best cruising speed is attained by careful trial and error.Best cruising speed can be anywhere from just on plane to about 2,000 rpm higher.

 

[bhammer]

Re: Alpha one prop questions

Don't use that prop wizard as in every case I have tried to use it, it has been wrong. I have a 17p prop on my boat and it always tells me that I should run a 23p for speed and a 21 for water sports. There is no way my boat can spin either of those sizes. Even when I tried it for a merc 470, it was saying a 21p.

If you are at WOT now, I would stick with that size of a prop. The reason you hit the trim tab is because of the higher rake of the new prop. You can buy a new tab that is made for those types of props or you cen get your file and dremel after the esiting tab. Just remember to take more off it to account for blade flex. But, I would just return the prop as you will find you no longer reach WOT.