40hp Tohatsu switching from alum. to stainless prop

hazben

Cadet
Joined
May 2, 2006
Messages
14
Just bought a 2020 40hp Tohatsu 4stroke mounted it on a 1652 Aluminum flatbottom boat. Currently running a Tohatsu 11X15 aluminum prop. Heavy loaded boat WOT 4700RPM. Light load WOT 5100RPM. Manufacturer recommends 5000-6000 RPM at WOT.

How many RPM will I gain going from an aluminum prop to a stainless prop. Suggestions on what brand of stainless prop would be best
 

WesNewell

Chief Petty Officer
Joined
Jan 3, 2018
Messages
497
Solas new saturn but you need to drop down in pitch to 13P. Just going from alum to ss won't change rpm much.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,778
Couple of differences when swapping are SS props are usually cupped which correlates to Wes' comment to drop an inch, but cupping is used to get more prop grip in situations where non cupped props will ventilate....good to have. Grabbing better puts more load on the engine so you need to back off the pitch to compensate and maintain your RPMs where they are.

SS props may have "Rake" (blades are bent back from the root rather than emerging perpendicular to it) which is popular on Bass Boats to help in getting the bow up for riding on "The Pad" and getting higher speeds. You won't be going 65 mph like they enjoy, but the Rake can help to grip the water better, resulting in more engine loading, but its also a bow raising mechanism which, in raising the bow (somewhat), while at a good planing speed, will reduce the hull "wetted area" which reduces drag, which allows the engine to pick up RPMs resulting in more speed due to lower "Prop Slip".....the difference in forward travel of a screw turning in wood vs a screw turning in a semi-solid.

They also have thinner blades that are more efficient and don't bend, but on that engine/speed ranges, those aren't issues.
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
What's the max HP motor and number of passengers allowed for that size boat ?

Boating with variable loads as in your particular case is a RPM serial killer. Heavy load to light load accounts for 400 rpm difference, that's one full +,- pitch. Try an OEM Tohatsu 3 blade 13 prop pitch to pull wot rpm towards max wot 6 K rpm range.

Happy Boating
 

hazben

Cadet
Joined
May 2, 2006
Messages
14
Bought the New Saturn 11.25X14 SS prop. Ran it yesterday and it did pretty well. Well loaded it would turn 5100-5200 RPM. Lightly loaded it would run 5700-5800 RPM. I am happy with that range. Did not lose much if any top speed going to smaller pitch. About 34mph lightly loaded and 28mph with 800lb of fish and equipment on board. I am still blown away how much smaller this prop looks compared to my old e-tec prop (113/4X17). But I am going just as fast or faster.

Thanks again folks for the help.
 
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