2 blade prop

[Deep V]

I'm running a 1978 Mariner 40hp w/ a 10 1/2 x 13 Black Max on a 16.5 Alumnacraft.It runs great at 5500 full throttle( which is at the top end of specs.).I,m looking for a spare. I found a 2 blade in the same size.I know it's just a spare, but is there any pros or cons ?

 

[hwsiii]

Re: 2 blade prop

Deep, theoretically you will lose a little speed and gain some RPM, but that strictly depends on how heavy your boat is.

But since it is a spare I don't believe it really matters if it is cheap, if it is not, then try to buy a better prop for your boat than what you have now.



H

 

[Deep V]

Re: 2 blade prop

Thanks hw,It sounds like , if I do use it , it will put me over the top on RPM's.I was also looking at a SS in a 14p(3 blade). That would go more along the lines of your advice ; giving me more options.

 

[hwsiii]

Re: 2 blade prop

Deep, I will need to know what your gear ratio and Maximum WOT RPM should be and what your max WOT speed and RPM are now in order to help make a recommendation.

You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057


H

 

[Deep V]

Re: 2 blade prop

Hi again hw, The manufacturer lists a full throttle range of 4500-5500. With my present prop I'm at 5700 (full load, by my self) and 26 MPH max speed (on my Lowrance speedometer). The gear ratio is 2:1 (26/13).I checked - out your other post (very informative),and if I did the math right, I'm at .26%.I think thats probably good. My anti-vent plate is 1" above my bottom. Since my last post, I did buy that 10 1/4 x 14p SS prop because when I ran the boat by my self, the motor went over max RPM's by a couple hundred RPM's. Besides I couldn't pass up the deal. It is a new-the-box Vengence for $125.Tell me what you think. Thanks again for your response.

 

[hwsiii]

Re: 2 blade prop

Deep, 26% prop slip is exceedingly High, I recommend that people look at changing their prop when the prop slip reaches 15%.

The Vengeance is an entry level stainless prop, but by changing to the 14" pitch and it being stainless you can raise the motor at least one more notch up and gain speed.

I expect you to gain speed to about 30 MPH as well as drop your prop slip a lot, you should be very happy with the results you get.



H

 

[Deep V]

Re: 2 blade prop

hw, No height adjustment on this old outboard. It would require drilling new holes, I,m not sure I like the idea of a hole an inch or two below the bolt. The top is clamped and the bottom holes on the bracket are slotted. Maybe I can get a little out of that. I'll see what I get out of this prop. Thanks.

 

[hwsiii]

Re: 2 blade prop

Deep, you are very welcome.



H

 

[steelespike]

Re: 2 blade prop

Raise the motor as high as your comfortable with the clamps; steady it with a strip of wood till you clamp it.After you test it then rebolt the bottom and put a brass or stainless carriage bolt in the old holes with a little sealant.In the old days this was the only way to raise a motor for racing without major work.My outboard has been like that for 45 years.If you use 1/4 inch strips you can raise or lower it a little at a time if neccesary and test.

 

[Deep V]

Re: 2 blade prop

Well, the results are in. I gained a couple of mph and got my rpm's back down where they need to be (28mph & 5500 rmp).That still leaves me with excessive slip, but I'm not racing. I did gain some lift, it seems. When I look down at my plate it is above the water; a perfect picture. The extra lift really seems to improve the way my boat takes the waves. The biggest improvment is in the reduction of vibration. I can't believe it! My aluminum prop was brand new. It's had to believe that it was that out of balance.The only down side, was that there was more cavilcation while cornering.I'm happy.