proper pitch for my props

P

potter water

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C Dory Year
1997
C Dory Model
R-21 Tug
Vessel Name
Poopsy
Twin Suzuki EFI's 2007 models, 50hp. 22 foot CD cruiser.

I'm going to remove the current pair of 11 and 5/8 by 12 props. They are pretty worn and one has a chip in it.

I'd appreciate any one who's done the prop dance with the CD's and 50 hp twins, particularly Suzukis.

The PO wasn't all that happy with the 12's, but they've been on there since the boat left the factory. The 12 was the pitch selected for Florida. So, the 12 pitch may not be optimum for the 75% of the time I'll be at higher altitudes.

If I have to do the prop dance to find out on my own, I guess I can, but there are so many CD's set up similar to mine who get around to different altitudes, I'm hoping someone can help. It is a new boat to me and I won't get it into the water for another month.

Thanks ahead for any thoughts.
 
I have twin Yamaha F50s on a Venture 23. So it may be a bit of apples and oranges with regard to engine gearing and hulls, but I've definitely done the prop dance. "Ideal" for my engines is WOT RPM in the upper half of the 5000 to 6000 range with the boat loaded as you most often use it (which in our case is HEAVY for extended cruising). Here's my progress:


10 3/8 X 13 Aluminum 3 blade - WOT 5300 light boat, 5100 heavy
10 5/8 X 12 Aluminum 3 blade - WOT 5500 light boat, 5300 heavy
11 5/8 X 11 Aluminum 3 blade - WOT 5700 light boat, 5500 heavy - BUT, developed serious prop burns after less than 200 hours use.

And the winner is:

11 5/8 X 11 Stainlessl 3 blade - WOT 5900 light boat, 5700 heavy.

Of course, the WOT RPM numbers will vary depending on water conditions and engine trim, but these numbers represent generally optimal conditions.

I'm really pleased with the current props. I do not seem to have lost anything in mpg or in performance at lower RPMs.
 
I would check on the gearing of those suzuki's--the larger ones have a much higher gear ratio than other outboards. Also old props can be rebuilt--and even repitched. It would help if you would give the speed and RPM you can achieve with the current props--and the elevation you plan to boat at.
 
The prior owner never ran this boat at sea level. Powell at 3700 feet was as low altitude as he'd seen. He has trouble getting much over 4500 rpm. So I think getting down to an 11 or even 10 pitch may be called for running at these higher altitudes. I'll get a chance to check the wot rpm in a few weeks. I'll hold off buying spare props 'till I get a chance to run the boat for that purpose. I hadn't thought of getting the chipped prop to a refurbisher. It would probably be worth it. I intend to put new props on the motors in any case and will keep the old 12 pitch props as spares.
 
Remember those are 6500 RPM motors . They need to see 6 grand at least at mid altitude .
I think we set that boat up originally .
Might need those 10Ps .
Bob is right on , got to go WOT with trim before making a move .
Marc
 
Did you own the "c-doggie" originally? If so, what are the two brackets hanging down on the port side by the settee. They are a black half round, two of them, looks like they held a gigantic boat hook or long strange fire extinguisher or?????????
 
Props are available down to 7" pitch for those engines. Ideally you should work with someone who would let you try several different props if they first didn't work (for example Prop Gods)--They were right on for my C Dory 25--and the new owner went with that recommendation for Lake Powell--which in my mind seemed too low a pitch at the time....

If you are only getting 4500 RPM currently--dropping 2" will not be enough. Each inch you lower the pitch you will gain 200 RPM. Assuming that the 4500 RPM is correct, dropping 3" will give you 600 more RPM (I suspect that with good trim etc, the boat will pick up a lot more speed, and that you will then be closer to the 6000-6500 RPM needed.) I would go down 3" to a 9" x 12" diameter, especially if you will be running at Powell and higher altitudes. At 4500 WOT you are lugging the engine.

A good source to see what props are available for any engine is at:

http://boatpropellers.iboats.com/

Another thing to think of is that out of balance props are not good for the lower unit and its seals. You can eat up the cost of new props, or repair of an older prop, very easily if you have to repair the lower unit!

At the very least before your run the boat, take a file and clean up the edges of the prop, to be sure that there are no really bad areas. Then get the new props as soon as possible.
 
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