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JayC

Prop sizing from Stainless to Aluminum

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I have a 4 blade Renegade offshore SS 14.5 x 17 prop on a 200hp engine. Runs perfectly up 5600 rpm at WOT on flat water. Never cavitates, very responsive, but adds a ton of bow lift until I get on plane. Same boat with a 3 blade 14.5 x 19 aluminum and a 200hp engine has much less bow lift, but probably takes longer to get on plane.

 

I'd like to buy either a 3 blade SS prop

or

3 blade aluminum prop

 

What size would I want based on what I have in a 4 blade prop? Not sure if this is a simple answer or not. I'm thinking 14.5 x 19 for aluminum, not sure on the stainless 3 blade.

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I'm running the same setup and use a 14 x 17" aluminum as a spare. I can get another 1-2 MPH on my top end with the aluminum prop by the hole shot and low end torque isn't anything like the 4 blade.

 

I can't say one has more bow lift than the other coming out of the hole. I just drop the tabs a count of 3 and bury the throttle.

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I can't say one has more bow lift than the other coming out of the hole. I just drop the tabs a count of 3 and bury the throttle.

 

 

 

And watch the gas gauge go down with the bow............

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I'm wondering if it's a simple conversion. Maybe not.

 

SS 4 blade = 14.5 x 17

SS 3 blade = 14.5 x ??

Aluminum 3 blade = 14.5 x 19?

 

Can you change the diameter of 14.5? or is that fixed? I know pitch can change, but I'm a newbie at this prop stuff. I just know that my SS 4 blade is working perfectly and I'd like a backup. I figured a 3 blade would be good since I wanted to decrese the bow lift. I have tabs, and they do get me up and gone in no time, just looking for suggestions.

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The basic performance difference in SS versus aluminum is the resistance to flexing is greater in steel. Generally, SS will give 1 or 2 mph in speed all things being equal. Negatives of SS are obvious cost is higher and if you hit a submerged object, the SS may withstand the impact but transmit more shock to lower end gears. SS is more difficult to "true" after abuse versus aluminum. SS blades can be "cupped" for better performance so pitch is usually not an equal number in SS versus aluminum. I have really bent some aluminum props in my time and every one was repaired and performed just fine. I actually ripped half blade off an alum prop and it welded up fine and never failed again. Everything is a trade off.

 

Have I totally confused you?headscratch.gif

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I'll have to see if I can get a marina to loan me a couple used ones, then I can make a educated purchase. My gutt tells me the 14.5 * 19 in aluminum will be a good choice based on the fact I've ridden on an exact model boat with the same HP outboard and it performed well.

 

I'm not sure what to do about a 3 blade SS though. I may try the same size I have in a 4 blade and see how it goes. I imagine testing a used aluminum will be much easier to find then a used SS. I'll ask my buddies what they have and see if any match up.

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As a VERY general rule of thumb, if you are going from an aluminum prop to a SS, assuming diameter stays the same, you can increase the pitch two inches without negatively affecting your WOT. This has to do with the greater flex of aluminum under load, as mentioned above. I myself am not a big fan of four blade props. Then again, I never goose my throttle and could care less how long it takes me to plane. I am only considered with performance at cruise RPMs and how long it takes me to get to the fishing grounds.

 

That said, I think you should be fine with the 14.5 x 19. Good luck.

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