Help! Need to Change Prop Diameter

[atoll]

any advice appreaciated,currently preparing to put an 8 hp yanmar into a 26 ft yacht,that previously had a 12 hp in it.

the existing prop is a 16 diam x 14 pitch,by my calculations for the 8 hp i need a 14 1/4 x 14 pitch,since the pitch is the same would it be possable to get the existing prop machined down to the correct diameter?

can a prop shop do this?

[captain58sailin]

Yes they can. It would also be a good idea to ask the prop shop for their recommendations with regard to pitch and diameter, you may be able to change the pitch and not have to cut down your size. My experience has been that these guys have a real good feel for what would make the best route, for your application.

[atoll]

thanks,can understand changing diameter by grinding down but what does changing pitch entail?

[captain58sailin]

If it is a bronze prop, they will take a rose bud torch and heat up the blades, one at a time and then twist them in the proper direction to obtain the pitch desired, in your case they would be easing the pitch. They have a cute little flat table with a cone shaped center piece that they set your prop down on and then use dial indicators to achieve a consistent twist on all of the blades. In your case since you are reducing your power by 25%, you will likely have to reduce the diameter as well, to prevent lugging your engine and causing it to overheat. This is not a big deal for a competent prop shop.

[atoll]

thanks again,will check out the local marine engeneering shop tommorow and see what they can do but changing the pitch may be beyound them.

reducing the overall size will help a bit with drag as well and probaly be quicker and cheaper.