Prop advise

[TQ]

I am in the market for a new prop. I own 2 props now, a stainless that is over pitched for my boat and an aluminum prop that is a little chewed up (chipped tips, etc).

Boat characteristics:
21? Rinker
210 hp mercruiser (alpha 1)
Engine rpm 4200 - 4600
Boat weight (without people) approx 3000 lbs

I tested my two props with the follow results:

14 ?? diameter 21? pitch stainless (Mirage) Prop:
Boat had 4 people ? fuel reached 44 mph and only 3700 rpm

14 ?? diameter 19? pitch aluminum Prop:
Boat had 4 people ? fuel reached 47 mph and 4400 rpm

The aluminum prop characteristics seem to be a good fit for my boat. I could care less about top speed, I am looking for fuel efficiency/cruising (with a load of 6 people) and something that will work for water sports. I would like to buy a used stainless steel prop and my 2 questions are:

-Do I go with a 19? pitch or do I need to adjust for any factors when switching from aluminum to stainless.

-What is the max diameter prop an alpha 1 can handle? I can buy a Mirage prop with 19P but it is 15 1/4? diameter. Will it be too big?

Thanks!

 

[Islandbrah]

Re: Prop advise

Sounds like 19 is a good pitch for you. to determine the largest diameter measure from the center of your shaft to the cavitation plate. That radius is 1/2 the diameter of your prop max minus spacing for clearance, Probably about 1/2 inch would be sufficient.

 

[642mx]

Re: Prop advise

Since top speed is not important, but watersports and cruising are.... I would be looking at a Revolution 4. A 19 pitch would probably work just fine. The Rev 4 is a 4 blade thats an excellent cruising prop with a great holeshot.

Typically, Mirage Plus's are 'go fast' props that come out of the water pretty slow, but have excellent top end.

 

[TQ]

Re: Prop advise

Are there any draw backs to a 4 blade prop compared to 3 blade?

I never really consided that since most of what I see out there are 3 blade.

 

[642mx]

Re: Prop advise

The only draw back to a 4 blade is the slower top speed. They typically knock 2-5 mph off the top end. But since your concerns are cruising and watersports, (and not top speed), the 4 blade would be a good choice.

 

[TQ]

Re: Prop advise

Ok, thats fine. You said 19P should be good but when switching from aluminum to stainless or from 3 blade to 4 blade I thought I heard you should drop an extra inch of pitch. Is that true? Also I usually have a load of 4-6 people when using the boat.

Thanks for your help.

 

[642mx]

Re: Prop advise

Ok, thats fine. You said 19P should be good but when switching from aluminum to stainless or from 3 blade to 4 blade I thought I heard you should drop an extra inch of pitch. Is that true? Also I usually have a load of 4-6 people when using the boat.

Thanks for your help.

This is from Mercury:

If RPMs are at the upper end of the range with a 3-blade prop, use a Revolution 4 of the same pitch. If the RPMs are at the mid to low end of the RPM range with a 3-blade prop, use a Revolution 4 of the next lower pitch.

Your kind of on the border with the 19 pitch 3 blade and 4400 rpms. A 4 blade (19 pitch) will probably drop you down to 4100-4200 rpms. A 17 pitch 4 blade might move you closer to 4700-4800 rpms.

If I was you.... I would find a company like BBlades, that does an exchange program on props. Its like a try it before your stuck with it kind of thing.

 

[truckermatt]

Re: Prop advise

i second the www.bblades.com I drive truck and have delivered to that company in WI, and they seem very knowledgable

 

[hwsiii]

Re: Prop advise

Brett Anderson has superb knowledge in props and boats and an ertainly help you.


H

 

[ramster]

Re: Prop advise

If you are on the edge (4400) with a 19" 3 blade, I would go with an 18" SS four blade, like an Apollo, or Stilletto Bay Pro II, I doubt that you would lose more than 1-2 mph, because you will be able to spin it closer to the max rpm (4800). Since you carry a crowd, and do water sports, better to be a little under propped than over, less strain on the motor. You typically drop an inch of pitch when going from a three blade to a four.