switch from stainless to aluminum

Hello,
I have a '88 searay 270 amberjack w/ twin 4.3L and alpha 1 drives with 19 pitch mercury mirage stainless props. I want to switch to aluminum props and was told that I would need to increase pitch on aluminum props. Is this true?
There are two reasons for the switch: The first being is corrosion issues....the boat is kept in the water and I have some minor corrosion issues in and around the bearing carrier. the second being the river I navigate is shallow and aluminum props are more forgiving and much cheaper to replace/repair.

Thanks for any input.

Re: switch from stainless to aluminum

Probably due to the fact that most SS are cupped and cupping is said to add about 2" of pitch equivalency at the higher rpm's where it's effects are most felt.....I believe it.

On a contrary note, the SS has thinner blades and is therefore more efficient so with that said it could nullify some of the apparent pitch requirement. Less performance from the prop translates to lower engine hp transfer to the water meaning less pitch would be required to maintain the same load on the engine for a given speed.

Soooo all that says to me that the same pitch may be your best bet....but you don't know till you run em and that is where the answer is. If you have a dealer that will let you swap props till you get it right, it is (to me) worth the extra cost that will surely be incurred with that kind of dealer but when finished you should be happy with the results rather than "wishing I had........" And finally, don't be surprised if the best that you can do works out to a couple less mph at WOT but that is the price you may have to pay.

Mark

Re: switch from stainless to aluminum

Showing ignorance here- are you speaking about galvanic corrosion? I see you are in Michigan. Isn't SS more stable than aluminum? Or am I wrong? I believe SS is the choice of saltwater for the corrosion resistance.

Re: switch from stainless to aluminum

ufm82 said:

Showing ignorance here- are you speaking about galvanic corrosion? I see you are in Michigan. Isn't SS more stable than aluminum? Or am I wrong? I believe SS is the choice of saltwater for the corrosion resistance.

Click to expand...

I agree and I just consulted my Materials Compatibility Chart extracted from Mil-Std-454 and it puts SS in the same Group II as Alum. and Alum. alloys. Additionally it says: "SS may be used with all materials in Groups II, III, and IV" which is pretty much everything but Magnesium alloys and the high strength Alum alloys: 5052, 5056, 5356, 6061, and 6063. Those are the kind of high strength materials like you would make an alum boat skin out of. Sort of confusing but that is what it says and it is out of the horses mouth so to speak.

I don't know what material makes the high strength alum high strength, but whatever it is must be what is incompatible with SS. Oh and Zinc is a group III material, copper and it's alloys (bronze) are in Group IV, as is Nickel.

HTH,

Mark

Re: switch from stainless to aluminum

Thanks for all the answers. As far as material compatibility goes........from experience, stainless props will and do cause increased galvanic corrosion....now to what extent, I'm not sure. I'm sure there are other variables also. My previous boat would eat up at least 2 sets of zincs a year with stainless, compared to a two(2) year lifespan with aluminum props. anyway, I think I'll find a good dealer and try a set of 19" aluminums and go from there. Top speed does'nt concern me too much. This is a fishing boat AND with marina gas close to $4 a gallon, who can afford to be running around at wot?? thanks again

Re: switch from stainless to aluminum

Go with the Solas or Twister alum props.
http://www.partspak.com/productcart/pc/viewCategories.asp?idCategory=4412
http://boatpropellers.iboats.com/propeller-showcase/solas/