I realize this question might be a little out the norm for this site but here goes.

I have an inflatable raft with a 15hp Yamaha 2 stroke. Currently it has a 10.5 pitch aluminum prop. The engine is revving to 6300ish with just me riding, max power band rpm is 5500. Slip is 4% according to the calculator, it is hard to hold tach and watch gps speed while holding on LOL.

My question is will an aluminum prop flex on a small motor like this? They sell an 11 pitch stainless prop or I can get a 12 pitch aluminum prop for half the cost. I know the difference is probably very small on something like this but I'm curious.

Thanks

The aluminum prop will flex more.
More important are aggressive geometry the SS will have. Also, cups that don't wash out, blades that can handle a hit and the ability thin/modify the SS propeller.
Your right, the SS is more expensive but the old saying holds true. You get what you pay for.

Brett

I have to say this --
especially your last sentence -- You get what you pay for

I tell that to my wife all the time -- she looks me straight in the eyes and asks

...now tell me -- how much was that marriage license??

enjoy

3pointstar

I used to live on a yacht and had a 12'4"' RIB dingy with a 25 HP and I can tell you that stainless DOES make a different in the to speed.....
Possibly more than on a larger boat... A stainless prop took my dingy from 27 to 33 MPH..

They race 15 hp boats (or at least they used to) I forget the name of the class but there was one for kids that used 15 HP motors. EVERYONE used Merc props. Did not matter if they had an Merc, OMC Etc. they all used Merc props. I am not sure if they will bolt on or need modification but it is worth looking into.(yes, the numbers above were with a Merc prop)

Also what was said about the stainless props holding up better when you smack something is a very good point. I assume that your talking about a Dingy and usually that means when using it your not in a main channel but using it to run to shore, Ect.. and likely to get biffed more often than your main boat.

What BBlades said about ability to take a hit is a good point..... Think about it, when you use your dingy your usually not heading down the main channel, more likely your heading on a shore excursion, exploring, Etc.

Lastly, OK it cost more.. But how much more? Is it $400 more or $100 more? If you buy the aluminum are you going to buy a spare because it is much more likely to get extensive damage? and if so, will it end up costing more than the price of the stainless prop?

Thanks for all the responses!!!

I purchased the stainless Solas Saturn 11 pitch prop.

I did some back to back testing with just me aboard. On the same day-5 minutes apart, same area, same direction.

With the 10.5" aluminum prop the raft did 27-28mph at 6200rpm. Cruise speed at 18 mph was 4200rpm. Hole shot is great. Raft throws you back and jumps right up on plane.

With the Solas Saturn 11" stainless prop the raft did 29 mph at 5400rpm. Cruise at 18mph was 3400rpm. Right away I noticed it lost a little out of the hole. Still very good but after looking at the rpm change I can understand why.

The blades on the stainless prop are much thinner and you can see the added cup. I am very surprised at the rpm difference just going by the manufactures advertised pitch (I know there is a lot more to prop design). The sound of the prop running in the water has a different sound as well. I didn't know if I should buy the 11 or 12 pitch stainless but am glad I didn't buy the 12.

I have to do so more testing. I had pretty much maximized the prop height and trim settings with the aluminum prop. This prop seems to want different settings. I bet there is another 1-2 mph hidden on the setup. I am happy with the stainless prop.

Yesterday I took my son out and it didn't feel quite as fast on the top end. I didn't have the GPS or tach hooked up. It did feel a bit slower on the top end, probably only a couple of mph, but we had a lot of weight aboard and the raft was definitely dragging in the water more. I have to do some more testing with the GPS and tach to get accurate data. To maximize the heavier load with additional people and load I might need to purchase a 10" stainless prop. For now I think I will just play with the setup using the 11" prop. It has great cruise manners. I already have 3 props for this little engine not to mention the different pitch props for my bravo drive on the larger boat. Is a mph or 2 going to kill me??? Probably lol