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Author Topic:   props for twin 250's
Lewis posted 01-17-2003 01:36 PM ET (US)   Profile for Lewis   Send Email to Lewis  
What size and pitch ss props should I use on my 1985 27 whaler with twin yamaha 250's on a bracket? I want to use 4 blades.
John from Madison CT posted 01-17-2003 04:16 PM ET (US)     Profile for John from Madison CT  Send Email to John from Madison CT     
Lewis,

Unless you find someone with the same exact set-up, you will have a very hard time knowing what's right or not.

Yamaha makes a nice 4 blade prop, at least so I hear. They have them in even size pitches, 18" , 20" etc etc.

I would find a dealer that will allow you to return props after a test run, and I would start with the 18" pitch and see how those motors spin.

FYI, I hear those Yamaha 250's last longer if you prop them to turn at least 5500RPM at WOT.

John from Madison, CT

lhg posted 01-17-2003 05:05 PM ET (US)     Profile for lhg    
I would try the mercurymarine.com prop calculator. Both the Yamaha and Mercury 250 EFI's have the same gear ratio of 1.75, so the selector could work for you.

You also might want to use Mercury's props, which can be bought with interchangeable Yamaha hub at no additional cost. Mercury's four bladed "Offshore Performance" series props may work well for you, since they are designed for "tremendous lift for big, heavy twin engine offshore boats, with great holding in rough seas". In three bladed version, you would want the Mirage series. Yamaha may have an equivalent.

Best price I know of for Mercury performance SS props is offshoreperformance.com., in Ft Myers FL. About $400 each, brand new.

Lewis posted 01-17-2003 07:36 PM ET (US)     Profile for Lewis  Send Email to Lewis     


Dia. Ptch Speed for 250
13 3/4 18 Performance 4 Blade 36-49
13 3/4 20 Performance 4 Blade 43-54
13 3/4 22 Performance 4 Blade 47-60
13 3/4 24 Performance 4 Blade 51-67
13 3/4 26 Performance 4 Blade 56-73

I copied the above chart off of the Yamaha prop page. They only offer their 4 blade in a 13 3/4 inch prop. The estimated speed for each different pitch is listed to the far right. The estimated speed increases as the pitch increases. Question - If I am looking for efficiency more so than top end performance (I don't care if I ever go over 49 MPH) should I consider the 18 pitch. Am I correct in assuming that you give up efficiency when you increase pitch?

SSCH posted 01-17-2003 10:03 PM ET (US)     Profile for SSCH  Send Email to SSCH     
I think you will find that a prop pitch that lets your engines run at their most efficient rpm when pushing your boat at its normal cruise speed will give you the greatest fuel economy.

Lower pitch will let the engines run at a higher rpm for a given speed thru the water. It will also give you better acceleration. You may be able to plane the boat on one engine if you chose a low enough pitch and the boat is lightly loaded (though with a 27, I doubt it).

On my twin 200 HPDI Yamaha, 25 Guardian Whaler, I've chosen to run three blade Merc Mirage Plus props (17 inch pitch). I've set the engines well down in the water to avoid cavitation, and am running a pitch that lets me easily plane the boat with either engine (in normal water conditions). With this set up, I have to watch the RPMs when the boat is lightly loaded as the engines can run slighty above recommended max RPM at WOT. When fully loaded, the boat runs right at rated RPMs at full throttle.

If I had been going for max speed, I would have raised the engines two holes, and added at least two inches to the pitch. As is, I can run 42 knots with normal loads at WOT, and have never heard an engine race in a hard turn or a steep following sea. The setup yields better than 2 statute miles per gallon at 30 mph in decent water.

Stick with your idea to pitch lower than for top speed. My guess .... you don't need anything like the prop pitches you listed in your last post. If you pick the Mirage or similar prop, you can carry a spare hub and will be able to repair a spun prop right on your boat. If you pick a conventional prop, that spun hub means a long, slow trip home, and then a trip to the prop shop for repair.

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