Converting from ALuminum prop to Stainless Steel

[ScottNovack]

I am interested in upgrading my prop from Aluminum to Stainless Steel to enhance the performance of my boat a little bit. I'm surprised that there isn't more information on line about how to recalculate the pitch.
My current specs are as follows: I have a 1972 Formula 233 cuddy. My boat is about 5000 to 5200 pounds loaded up. I'm running a brand new (2020 installed) 357 ci carbureted Mercruiser linked to an SEI outdrive (1:47 gears) comparable to an Alpha One Gen 1 drive (also installed in 2020). I have had the boat for about 9 years and have always used an aluminum Black Diamond 14 1/2" X 19 prop. I am generally happy with the prop, but I am doing more long distance cruising where I go 3 or 4 hours in one direction, and then come back later in the day. I would really love to get more speed out of the boat. At present, I can achieve about 4400 rpm, and am moving at 40 MPH measured via GPS. 40 is great, because on the old motor I could only get 37 or maybe 38 mph.
I just spoke to a Mercury tech who was concerned that 4400 max RPM might be a reason for them to not honor the warranty. He said they really want to see 4700 or 4800, otherwise it means you are over geared and lugging the engine too much.
I don't do anything that makes the hole shot important, like towing skiers. I do boat in very shallow waters a lot, despite always being on the ocean. I regularly have to repaint the prop because I am in sand somewhere and scuff the paint off. Never any rocks, but sand all the time it seems, so I need a replaceable hub in case I need the cushion to not destroy the lower unit. I don't really care too much about fuel efficiency.

If I buy a new prop and end up with higher rpm's but I go slower I will be very disappointed. I would appreciate any input or experience others have had when making the switch from Aluminum.

Thanks

[blaster]

My understanding is when switching from aluminum to steel you drop one inch of pitch. If you are a little over propped dropping a couple inches of pitch should give you back about 400 rpms.

On some similar size SeaRay cruisers many folks switch to a four blade aluminum. Compared to a three blade on those boats the result is similar top end and better hole shot as well as low speed maneuvering.

[DaleH]

Prop pitch and selecting just a SS prop, even between competing brands can be a 'trial & error' scenario! You should seek out Ken from www.propgods.com for comment and advice.

But from what you yourself wrote, you're lugging that motor with that current prop ...

[ken2]

As for a "conversion calculation" between aluminum and stainless, there is not one. There is a LOT more to choosing the right prop, lot of variables.
I could give you two props that are the same material, diameter and pitch, but they could be 1000 RPM different from each other, and perform completely different.

You have a 19 pitch prop turning 4400 at 40 with a 1.47 gear ratio, that is 25% slip
The prop just doesn't have much traction, your slip numbers are likely worse at cruise, a better prop can improve your performance across the board.

I would suggest a Quicksilver Q3 in 16 pitch.
I would expect it to accelerate significantly stronger, AND actually gain a couple MPH on top end.
Same prop as the Mercury Enertia.
If you get one from me and decide you want a different pitch or style, you can swap out for $35 plus shipping.
Its 14.8 diameter 16 pitch, and I actually have them.
they aren't on my website, so just call if you want one, or you can ask whatever questions you like here too !

[DaleH]

Ken is da man, when it comes to props!

[ScottNovack]

So Ken, what is it that makes you think I should go down to a 16. I don't have your credentials with regard to props, but it seems like a big change up. You probably have come across some of the old Formula's. I'm still having a bit of a hard time believing what the Mercury rep told me but I am getting there. I guess at age 56 I'm becoming an old timer. I've worked on cars my whole life, and I have redone this boat from front to back. And when I say I have done it, I mean, my hands, not, I've written a check and someone else has done it. When I do something I like to understand it, and I move slowly so I don't have to unravel work and do it again. I've still got weeks to go before I get on the water. It's 28 degrees here this morning, so I am not in a rush.
Thanks for the input

[ken2]

You have a 1.47 gear ratio. (motor goes around 1.47 times for every single turn of the prop)
you have a 19 pitch prop. (theoretically goes forward 19 inches every rotation)
All props are going to slip a bit, as they are going through water, but some slip more then others. (just alike a bald tire on a wet road)

Yours is slipping 25% (bad)
Q3 is going to slip about 8-10% (good)

"IF" the Q3 16 pitch brings your slip down around 9% as I expect, and "IF" you can turn it 4800 RPM, you'd be running about 45 MPH

That would be over 10% faster on top end, and accelerate a lot stronger throughout the RPM range, as you would have a much lower pitch and actually have traction.

if we can’t gain a decent amount of speed you might need a 15 pitch. But I think you’ll gain the speed

[ScottNovack]

OK that makes sense. What do you think about Solas props?

[ken2]

They are not bad props.
I don't sell them, so not real familiar with which of their styles does what, but they are definitely a little steep on pitch.
so if you go with a Solas brand stainless prop, I'd definitely drop to 15 pitch.
not a good or a bad thing, but I normally expect 3-5% slip with that brand.