prop question, alum vs. stainless

[craze1cars]

What is the real benefit behind a stainless prop? I understand they're much higher strength, but I don't necessarily see this as a benefit since they'll transfer impact stress to the drive if they hit something.<br /><br />Here's my completely uneducated thinking...aluminum gives easier, so it's better able to sacrifice itself to save the drive. Aluminum is a lighter weight, which should theoretically reduce rotating mass thus get more HP to the water.<br /><br />So what properties make stainless a "higher performance" prop? And would these properties have any noticable impact on my low-HP 3.0L runabout?<br /><br />I'm strongly considering getting a second prop so I can have a different pitch and/or diameter for different load/condition/use and so I can carry a spare. I'm seeing very little reason to spend the extra cash for another stainless like I have now...what am I missing here?<br /><br />And here's the next question...I understand that an inch of pitch translates to 150 to 200 rpm change. What about diameter? What would changing from a 14.5 x 17 to a 14 x 19 do? Going to a 19 would theroretically drop my rpms by 300 to 400, right? But it's also a smaller diameter, so how much impact does that have?<br /><br />Thanks for your continuing to help a newbie who's learning the boat thing....

 

[marinemech1]

Re: prop question, alum vs. stainless

you are correct stainless are stronger and more efficent props<br />the blades of a stainless don't flex as much therefore they are more efficent.<br />your also correct about impact aluminum gives or is more forgiving stainless impact almost always means new gears.<br />as for preformance: see if you can borrow one from somebody to try to see how your package reacts.<br />rule of thumb if going from alum. to stainless drop pitch by two.<br />there are many variables not mentioned too many to get into this is just the basics

 

[tommays]

Re: prop question, alum vs. stainless

i am a 3.0 owner and i am great at finding shallow spots i am sure had i been buying stainless props the last ten years i would have a damaged drive instead of a few tweaked alum props<br /><br />BUT and its a BIG BUT nothing gives more performance for the dollar to a boat than a good stainless prop peroid <br /><br />on a 3.0 or any motor for that matter a 14 x 17 and a 14 x 19 is a huge difference <br /><br />the 3.0 has a small performance window and you have to really carefull with what you want do when you pick a prop<br /><br />tommays<br /><br /> http://www.mercurymarine.com/everything_you_need_to_know_about_propellers5

 

[craze1cars]

Re: prop question, alum vs. stainless

OK...all interesting, but wouldn't the added flex in an aluminum prop be a benefit? Kind of like a shifting prop that flexes to a flatter pitch on take-off and then cups back out at higher speed when there's less force on the blades?<br /><br />In my case, my present stainless 14.5 x 17 prop has me over-revving by about 200 rpms. I don't know of anyone or place to borrow a prop.<br /><br />I fear that going to a 14 x 19 will drop me too far. Then again, if that rule of thumb from marinemech1 holds true, and I get an aluminum 14 x 19, it'll act similar to my stainless 14 x 17, right? Now I'm really confused. <br /><br />I'll study the link from Tommy's later when I have more time, but any additional thoughts on this? Thanks to all who can help.

 

[Don S]

Re: prop question, alum vs. stainless

They don't flex that much or that easy or on time and on command.<br />If you want to go fast and never hit anything, go stainless.<br />if you are a normal boater and want to fish, cruise, pull a skier now and then and you also find the bottom on occasion while heading to the beach for some R&R, stick with aluminum. Your wallet and drive will thank you.

 

[Boatist]

Re: prop question, alum vs. stainless

Stainless has thiner blades and turns a little easier. If you have a stainless and a aluminum both 14 x 17 then your rpms will remain the same. The stainless turn a little easier and is often doubled cupped where it get a little better bite. Unless your raise your drive higner I do not belive it will even make 1 MPH difference.<br /><br />Better I just tell you my experience. I have a 21 foot Aluminum Runabout that weights 2800 pounds. I have three props all are 3 blade. 1: 13 1/4 by 17 Aluminum. 2: 13 3/8 by 17 Stainless. 3: 13 by 19 Stainless.<br /><br />When I switch from the 17 Aluminum to the 17 Stainless I can not tell the difference. Both turn 4600 RPMS at WOT and speed is 35. Stainless may be a little quicker to plane. <br />When I switch to the 19 boat turns 4200 RPMS at WOT and has a top speed of 38 MPH. It also gets better fuel Mileage. It is a little slower to plane.<br /><br />On ocean round trip of 60 miles running at a clean plane speed of 20MPH with the 19 pitch it will take 18 gallons to fill tank when I get back. With the 17 pitch same speed and same trip it will take 21 gallons.<br /><br />If you run aground use the aluminum. The Stainless does not get the edge nicks from hitting floating debree. The aluminum does have to be dressed up a little every couple of years.

 

[craze1cars]

Re: prop question, alum vs. stainless

Don, I'm certainly more in the "normal boater" crowd. Fully expecting to knock bottom and do a variety of things with my beater. If I was a "go fast" boater, I sure wouldn't have settled for a 13 year old 3.0L! So I'm leaning toward aluminum, I guess.<br /><br />This is all good info....thanks to all for your thoughts. Thanks, Boatist, for your specific experiences, that helps a lot in telling me what I might expect, though I realize every boat is different. If others out there can offer specific #'s like he did, that would be VERY helpful.<br /><br />I need to make a run with my new hydrofoil this weekend and see if it changes things with my present prop first, and see if I'm going to keep or dump the foil. Then I'm gonna talk to my favorite marina that I've been giving a lot of business to lately....maybe they have some slightly used or trade-in props they'd let me borrow for a weekend to experiment with. Otherwise I might just buy myself a 14x19 aluminum and see how it goes. Found a Michigan Wheel for only $100 right here on I-boats. If it's useless and I don't like it, Ebay will salvage most of my loss.