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Re-pitching propellers

Posted: Sat Oct 24, 2020 10:33 am
by Mongoosemagic
We have a 32’ Altair with 2gm20 engines. We have been having a problem with both engines. Recently the mechanic tried to rev the engines up to WOT but could only get to 2800rpm. He said that we must be over propped and recommended that we change props. We currently have Flexofold two blade folding propellers (15” with unknown pitch).

Should we put new three blade propellers or put new blades on the existing propellers?

Re: Re-pitching propellers

Posted: Wed Oct 28, 2020 1:12 pm
by Doubloon
There is good blog about this at labeled Prop Pitch Matters. We have the same engines on ours but have the 2 blade folding Maxprop installed.

Re: Re-pitching propellers

Posted: Thu Oct 29, 2020 10:47 pm
by Jwood1
Thanks for the plug Doubloon. Mongoose, this sounds pretty much like my experience when I bought the boat. However, I had non-folding three blade props, 15" x 12 pitch. The boat could only reach 2800 RPM and there was black smoke indicating the engines were laboring. I had the prop shop re-pitch the props to 9 and that pretty much fixed the problem. No more smoking and I can get close to 3600RPM WOT.

However, fixed blade props are a real drag on sailing performance and even worse on a cat with two of them! I also think the negative effect is worse on a small cat like the PDQ 32. In fact, I was very much hoping to put Flex O Fold props on Terrapin. I bought two 15 x 12 pitch two-blade props from FOF and they would not fit! There wasn't enough clearance and when folded, they overlapped the skeg by about 3 inches. I considered the possibility of shortening the shaft as there was about 3" between the prop hub and the strut, but in the end, I decided to send the props back.

So, I am jealous that you have flex O fold props and I would be very hesitant to switch them out for fixed props. The FOF folks are very knowledgeable and recommended 15 x 12 two blade props for the PDQ LRC with the 2gm20 engine and 2.62:1 gear ratio. So, if you indeed have 15x12 props, they are probably correct for your boat. Those are great props and I would be very hesitant to switch them out for fixed blade props.

When you say the mechanic "tried to rev the engines to WOT", was that while underway in calm water with no wind? Because that is how you need to gauge whether you have the correct pitch. If, under those conditions you can't get close to 3600, I would also check to make sure the engines can reach 3600 RPM in neutral. Also, do you see black smoke when you max out the throttle? That is another sign of being overpropped.

If you do think your props are the incorrect pitch, I would email the folks at FOF. I found them very responsive and helpful. They will probably help you figure out how to fix the problem (I'm not sure whether they can re-pitch those blades).

Finally, If you or Doubloon have your boat on the hard, I would love to see some pictures and maybe a measurement of the distance from the strut to the skeg and the amount of shaft protruding from the strut. I really want to fit folding props on Terrapin!

Re: Re-pitching propellers

Posted: Sun Nov 01, 2020 9:52 pm
by Doubloon
Boats not on the hard now but hopefully this winter if the Marina ever gets space available. This is the best picture I have, taken during the survey. Sorry no measurements. This prop is actually a feathering prop, not folding.