proper prop size?

[Molly]

How do I determine which size prop I am supposed to have? We think ours may be wrong.1986 Morgan 41 Classic44 hp Yanmar 4JHE

 

[Steve Dion]

Go to a prop shop!

Molly:You can go to a prop shop and they can calculate it for you. You should know the make/model of the boat, the engine model/hp and transmission ratio. If you have the old prop specs this will also give him an idea of where you are vs where you should be.

 

[Mike DiMario]

Right here

Molly,If you know your boat's approximate weight, you can log onto the Hunter website. Contained here are specifications for the Hunters. You may be able to get an idea of about where it should be.What are the reasons you think your's may be wrong?Like Steve D suggested, you may be able to check with some prop shops and right here online as well. PYI has provided excellent response, with plenty of detail, to any topic that I have sent them. You may need to provide more information about your Morgan.good luck,Mike D.

 

[Kevin]

Mostly engine based.

Dear Molly,Thank you for your note. A quick check is to allow the engine to fully warm up. Place the engine into gear away from the dock. Push the throttle to wide open. If the RPM’s of the engine obtain the recommended full load rating of the engine you can assume the pitch is correct. If the RPM’s come up shy we can assume the pitch is too great. We can also assume that if the RPM’s exceed the maximum rated amount the pitch is to low. This will allow you to determine the current loads. Once you have figured whether it is correct or not, you can alter the pitch 1” per adjustment. This increase or degrease of 1” pitch will have an apx. impact of 12% to the engines obtainable RPM’s. Add pitch reduce RPM’s, degrease pitch increase RPM’s. Now you can look at the propellers diameter and compare it to the pitch of the propeller. We like to see a 60% pitch to diameter ratio. 18” x .65% = apx. 12” of pitch. Of coarse this does not always work. Your propeller diameter may have restrictions due to the hull configuration. For instance, you will suffer more efficiency problems in trying to place an 18” diameter propeller at 12” of pitch onto an installation that requires a maximum diameter of 16”. Each and every boat/running gear combination has a balance and to determine that balance you will need to look at all items affecting the propeller. Often people don’t realize that when you over load a vessel the propeller may need some altering. If you would like to zap to me your particular concerns I would be happy to look them over for you. kwoody@pyiinc.comHope this helps some. Kevin

 

[Guest]

looks like we have a winner

talked to a prop shop, and based on all our particulars, we are way off with our current prop. Now we have a 17 x 17, they recommend a 18 x 11 or 17 x 12, depending on clearance available. This would explain our rpm problems, and maybe the vibration too.