Prop question.

I looked but didn't see a prop specific section.

Going from a 14.5/19p aluminium to a 14.5/21 SS prop what kind of decrease in wot can I expect?
I know rpm drops 200 per inch of increased pitch but what effect does changing prop material have? My cars memory wants me to believe that an Al to SS is good for another 50 drop. Yes, no, maybe?

DaveG55 said:

I looked but didn't see a prop specific section.

Going from a 14.5/19p aluminium to a 14.5/21 SS prop what kind of decrease in wot can I expect?
I know rpm drops 200 per inch of increased pitch but what effect does changing prop material have? My cars memory wants me to believe that an Al to SS is good for another 50 drop. Yes, no, maybe?

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Depends on what engine and boat you have.

You have to be careful that you don't over prop and not maximize you rpm's/power zone.

With my Wellcraft élan, 19.7 with a 4.3, 205 hp, the top speed was almost the same at 46 mph with the 19 pitch at 4,600 rpm's. The 21 pitch ran about 44 mph at 4,300.

The engine didn't have enough additional torque to push the boat much faster, not to mention the reduced lag when starting out. Because of the lower HP, I didn't feel any improvement with SS.

If your engine presently runs within the recommended RPM range @ WOT, I would not change the pitch. If the engine runs very near the maximum RPM specified, you may get away with it. As noted above, you run the danger of over propping, a good way to damage an engine.

What are you trying to accomplish?

FWIW, personally, I'm not a fan of SS props for "normal" applications. If I hit something, I refer that the prop get damaged rather than the "innards". Just my personal bias...

Really my question is just how much difference in wot between identical size/pitched props, one being aluminu and the other SS.I realuze now that I phrased it poorly.
Ok, so here's the full story.
20.5 ft bowrider, 5.0l, A1G2, 14.5"/19p SS prop. WOT in the area of 4600RPM @ 48-52MPH depending on water condition and load.
Got water in the outdrive which trashed the gears.Could not afford a new SEI drive so found a nice used drive.
Here's where it gets a bit odd.
The old drive was a 1.65, the "new" one is a 1.81. After checking with several people, including a tech at SEI, the concensus was the 1.81 would work fine with reproping.
The new drive came with a new 14.5", 19p Al prop. My test drive hap me hitting 5000 rpm at WOT. I don't know the top speed as the speedo is dead at the moment.
My target rpm is 4400-4800. This means I'm underproped by about 2".
I have a 14.5 x 21p ventalated SS prop that was previously too much for the boat.
I'm thinking it may be perfect for this new drive.


Personally, I like the SS prop. Been running one for the last 12 years or so with no problems. I think it gives a better bite when pulling skiers and we have a really clean lake that seldom has anything to hit.


So, how is wot affected Al vs SS?

kimcrwbr1 said:

My only question is why havent you put the stainless prop on at tell us how well it performs. You say the stainless is ventilated is it cupped also, all are factors only trial and error will give you the correct answer anyways.

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I totally agree. Put it on a see. I do think the math will work in your favor with the 21.

The biggest difference between the two props... SS looks better out of the water when trailer ing down the road.

I wouldn't expect any noticeable difference in the two types of metal for speed. The pitch will be the final factor for determining which one stays on.

I have not tried it yet because the 21 was 75 miles away. The boat lives at the lake house and, for some reason I don't remember, I took the 21 home. I intend to try the 19p SS and the 21p SS this coming weekend to see which works best. And, yes, it is cupped. The 19 Al won't be staying on as it takes the WOT above specs with a medium load and good water conditions.

If memory serves I switched to the SS prop because it seemed to do better pulling skiers up. Speed is not a huge concern as long as I can get close to what it was before. If I want speed, I'll ride my PWC.

Thanks for the replies so far.

Quick update for anyone interested.Put the 21p SS prop on last weekend with the boat restored to previous conditions. IE, clean, waxed, hydrophoile installed, trim tabs in place, etc.
Perfect water conditions, half tank of fuel and driver. WOT topped out at 4850rpm by the boat tach, 53 mph, again by the boat speedo which i know reads about 3 mph high in that range.
I am unsure why but the boat gets on plane faster, stays on plane at lower speed, rides better and is handling great at all speeds. I also think fuel economy has improved though, at this point, thats seat of the pants and not really a measured thing.
The ride has improved so much that the Admiral even commented on it without prompting, l was a bit surprised and pleased.

Following up on this thread, I see it worked out perfect! That's the overall advantages of a SS prop that is fitted right, over an Alum. prop! Thanks for letting us all know it all worked out.