MacGregorSailors.com

Hi to everyone.
I just installed a new mercury 60cv 4t ,no big foot, on m 26x. The original propeller is 12x12. Max rpm is 5500-6000.
Well the max rpm i can reach is 4700, mph 16,8.
May be the boat is not well clean underwater, but not very dirty.

With my old mercury 50 cv carburators 4 t , no big foot, with a propeller 12x9 ,and clean boat, the rpm was 5300, 18 mph.

The questions are :
1) Can a boat not perfectly clean underwater reduce the max rpm of the engine?
2) What is, in your experience, the best propeller ?
Thank you in advance.

Glad for an opportunity to answer a question…

One question first... does cv stand for constant velocity? I didn't know there was a cv prop available? If so, what activates it... oil pressure, electric solenoid or centripetal force?

1) Well, if you have barnacles, I’m sure it’d slow it up noticeably… but if it’s just dirty… I wouldn’t think you could detect it. Macs aren’t racing shells that are waxed before every race for that last hundredth of a knot improvement.
2) I asked nearly the same question when I got my boat…

http://macgregorsailors.com/phpBB/viewt ... =7&t=10530

I’ve got a 26M with the Mercury 60 HP EFI 4-stroke Big-Foot and have an 11x14 prop. I was only getting 5200 rpm then. Now, I’m getting 5400 rpm even though I’ve added some weight to the boat and my boat has some lake mud streaks on it. Red Georgia clay can be a b$#%@. Using GPS, I registered 22 mph on a glass smooth lake. Guess the engines finally getting broken in.

I was fretting about changing the prop to get it up to 6000 rpm… especially since I’m expecting my rpm’s to drop as I add “creature comforts” to the boat. Basically someone pointed out… “What are you using it for?” Well, I'm in it for the sailing… so I use the motor to get it off the trailer and put it on the trailer. And… I pull kids on an inner tube. Top speed, bonsai runs really aren’t in the plan. The Mac will never compete against the cigarette boats on the lake.

When I get a new or a back-up prop, I’d probably opt for the next size down. From my old post, I learned that it would be best to keep the 14 diameter and get a shallower pitch. I don’t have a clue what is available, but maybe a pitch of 9 or 10 inches would pop up the rpm to 6000 and with that I’d probably only get half of a mph improvement. Then I’d fret about reducing the longevity of the engine.

So now... I just don’t fret!

You might be running to big a prop but on the other hand at the slower rpm it will be more efficient so unless you speed a lot I would not be in any big hurry to change it unless the person who sold you the motor will take the prop back and give you another. It sounds like you have just changed the engine - not made any other changes that would effect the 50 to 60 comparison, so if you are at your final - most often - weight then when someone has a prop on sale, you might want to go back to a 12 x 9 or 10 and see how it does.

Correction:
the new propeller is a 12x22 : max RPM 4600 , max speed is 16,6 mph.
the new engine is a Mercury 60 cv 4t no big foot.

The old one was a Mercury 50cv 4 t no big foot with a 12x9 prop.
RPM max 5200, speed 18,0 mph.

So the question is always the same: what could be the best propeller with this Mercury?

CV is French for Horsepower, the Italians borrowed it from the French of course


I would say this is a classic case of being overpropped, since your motor isnt a Bigfoot it has a different final drive ratio than the one you are comparing it to - that change from a 9 to 12 inch pitch is simply too much for that motor.