Aluminum vs. Stainless

[elwilliams13]

I heard somewhere that a stainless prop's heavier weight puts more strain on the gearcase than the lighter aluminum. Any truth to this?

 

[VistaMan]

Re: Aluminum vs. Stainless

With higher power motors it would put more strain when shifting into gear, especially if it has a high pitch.

 

[elwilliams13]

Re: Aluminum vs. Stainless

I have a 14 1/4 x 18 on a 1991 225 evinrude. It seems to shift pretty smooth not sure if shift switch is working.

 

[VistaMan]

Re: Aluminum vs. Stainless

On racing setups, with pitches of 25" and more it is normal for the motors to be started in gear to make it easier on the lower end. I know of people that have striped gears over a summer like this but shifting into gear after starting, the inertia of the prop weight and high pitch realy does a number on the gears!

 

[walleyehed]

Re: Aluminum vs. Stainless

Under water, (hydrodynamics) The Blade area has way more significance than the Weight of the prop due to the fact that under water, gravity has 2/3rds less effect. 18lb. prop out of water weighs 6lbs. under water.<br />They both have a rubber hub to absorb shift impact, and the weight has very little to do with it.

 

[VistaMan]

Re: Aluminum vs. Stainless

I agree that the weight under water is less but then a SS prop will still have more inertia to overcome, the specific graity of aluminum is about 2.5 and SS is about 8, 3.2 times heavier! But , yes, the blade area is the predominant factor so a high pitch prop will give more 'clunk' to the gear case.

 

[moderator1]

Re: Aluminum vs. Stainless

Moving to General Outboard discussion. This may benefit many.

 

[walleyehed]

Re: Aluminum vs. Stainless

Blade area, however, is much less on a high pitch prop than a low pitch, larger diameter for the same engine. The initial load is the same.

 

[wtrojan]

Re: Aluminum vs. Stainless

To walleyehed:<br />

that under water, gravity has 2/3rds less effect. <br />18lb. prop out of water weighs 6lbs. under water.<br />

Sorry Kenny, but I think this is not true. This depends on specific gravity, so this “2/3rd less“ does not apply, at least not to prop’s materials like aluminum or steel.<br /><br />Any body weights less in fluid only by a weight of fluid it displaces. <br /><br />In case of steel prop in water, 18lb prop will displace about 2.15 pints, ant this much water would be little more than 2.22 lbs. So this prop weights more than 15.5lbs in water. Salt water would not change it much

 

[james082273]

Re: Aluminum vs. Stainless

Well actually it has more to do with the mass of the prop than the weight. And mass doesn't ever change whether you are on the moon or on Saturn (or under water). At least if we're talking about the resistance to the rotation by the shaft.

 

[JasonJ]

Re: Aluminum vs. Stainless

The stainless won't put any more strain on the gearcase than the fact that the gearcase is still being used to transmit the turning force needed to move the boat. What it can do is affect the dogs that engage the gears. All you have to do is make sure you shift crisply, making one good swift shift into gear. There will be a bit more of a "thunk", but thats okay. You don't want to shift slow, causing a grinding affect, that is where the dogs get worn, which can eventually lead to failure or it jumping out of gear. This can happen even with the lightest props if you don't shift into gear properly.

 

[walleyehed]

Re: Aluminum vs. Stainless

JasonJ....Very well stated!!!!

 

[Solittle]

Re: Aluminum vs. Stainless

elwilliams13 - This issue would only be important if gearcases with SS prop on the back end fail. They don't or at least I have never heard of one failing due to any of the reasons stated above. The only possible problem I can think of is if you have a badly out of balance prop. I would think a SS prop would do a number on seals faster than an al prop.

 

[andrewkafp]

Re: Aluminum vs. Stainless

The only reason that I have an Alloy prop is coz I don't have the money to buy SS

 

[seahorse5]

Re: Aluminum vs. Stainless

Because of it's weight, there is more intertia when shifting into gear with a stainless prop, however, since they are so popular, the gearcases are built for them. In fact, in counter rotation gearcases, only stainless props are offered, no aluminums are made for it.

 

[AndyL]

Re: Aluminum vs. Stainless

Just another slant on the argument is that if you impact some underwater hard object with a stainless prop it will transmit more damage to the gearbox as stainless is much less malleable. Aluminium will distort and reach its yield limit a long time before stainless. In that time the de-forming aluminium is absorbing a huge amount of the destructive energy which would have been transmitted to the gearbox in the case of stainless.

 

[Stumpknocker]

Re: Aluminum vs. Stainless

I think Andyl hit the real nail right on the head! This to me is the real issue... underwater objects encountered of the third kind. The immovable object meets the very tough SS prop....something might give and it probably will not be that very hard SS prop. The driveshafts on the smaller engines might snap like a pencil under the right conditions. Very expensive proposition. <br /><br />I found a solution over the metal props with Piranha props in California. Special plastic molded blades. Gave me better performance than my aluminum blade with lighter weight and if one of the blades hits the "big one" it shears off. I carry a spare blade onboard and can switch it out in about 5 minutes. This already happened once. Stuck some concrete and I heard it snap! Changed it out and kept right on going. Saved my rear end $$ and driveshaft I have no doubt.

 

[BillP]

Re: Aluminum vs. Stainless

OMC commercial ob engines have the identical lower unit (gears but not wp)domestic engines do. Commercial guys with SS props do rapid high rpm fwd/reverse shifts for 2000 hrs and don't damage the gears. What gets damaged is the splines on the drive shaft where it plugs into the head...they strip out. That's why OMC commercial drive shafts have more splines on the drive shaft. Besides that, rubber hubs and shear pins take care of it.<br /><br />Bill P.

 

[Lloydramdeen]

Re: Aluminum vs. Stainless

A very timely and controversal topic. I just wrung a SS prop.for a 225hp Yamaha. Shall I spend the money to have it repaired or shall I invest in an Aluminium one? (A 24' Hull with one engine) <br />Please offers your comments.<br /><br />wahoo