prop information

puffcard · **Posted:** 06 Mar 2020 22:56

We installed a new prop. Went with the Flexofold 17-11-3B RH. I only have a few hours, so jury is still out. We repowered few years ago with the Beta 35, rated 2,800 and have the Hurth 25 transmission 2.14:1 gear. At first kept the old 2-blade, cruising 1,950 RPMs we are making 6.3kts. Now we are running 2,000 RPMs, 6.3kts. Smooth as silk, which was my main reason for switching from a 2-blade Flexofold. Wide open 2,750 RPMs, 7.1kts clean bottom. The old 17-13-2B maybe 7.1-7.2kts. I also think you are over pitched, getting to hull speed so quickly. It will be ok in flat water, gets to be a problem when punching into a head sea will lead to overheating and black exhaust. Isn't the Beta 38 rated for 3,600 RPMs?

Sswartz06 · **Posted:** 15 May 2020 07:54

Shameless plug that we are looking for a "new" 3 blade for ours, if anyone would like to sell theirs off Smile

. Ill post some info from our unique engine/gear/ prop setups below for ref. But First:

Re: Flex-o-fold (or nearly any other "tech"y prop. They are putting a lot of R&D into their props. The blades on a flexofold (et al) have a different design than your standards bronze fixed prop, meaning its a bit of apples and oranges comparing the two by numbers alone. Beta is going to give you a prop rec based on a very standard prop design. When dealign with a (reputable) prop manufacturer, trust that they know what they are telling you, even if it differs from what another manufacturer says. Obviously, sometimes you get a dud, but in general, they are prescribing THEIR best fit, which may well be different from the others, by design.

SO, we bought our boat with a Yanmar 2qm mated to what I presume was the original prop 16x14 2-blade. the gear is a 1:2.15 ( I believe). With that set up we could turn about 1400-1500rmp before we'd run into fuel delivery issues, but she wasn't good for much more than 1600-1700 anyway. That would push us about 4kts (utilizing maybe 8hp?). I later repropped with a 14x11 2-blade (which I conveniently found locally with a 1" bore!) and it mated perfectly with the engine, almost a perfect load match. With that we could do about 3.5-4.0kts on flat water, but had no bite for any adversity.

We are re-powering now with a slightly older Beta 35 ( actually a lower displacement version of the new 35 which is the same engine as the 38) and will be reusing the 16x14 2-Blade to start with, though I imagine I will be over propped a bit.

Velera · **Joined:** 29 Dec 2006 09:38 **Posts:** 656

I have had some experience with adjusting my Autostream 3 blade feathering prop with two different engines and with wildly different load factors (I cruised heavy for a couple years and am light again). Here is my take:
1. Engine and prop manufacturers can only give you range estimates of proper pitch and even size of prop. The actual torque and rpm curve of your engine, modified by the weight and condition of your specific boat is going to make substantial differences in what the "right" pitch and size of prop is.
2. My Yanmar 3JH3E has much more torque than my old W50 at lower rpm and that meant a complete readjustment of the pitch.
3. I had to adjust the pitch once I got back to the states and took all the stuff off the boat.
4. The key was carefully measuring the speed of the boat vs. the max rpm of the engine. You need to be able, under load to hit max rpm at 100% of rated rpm. If you can't you are over pitched. For most engines you want to run at 80% most of the time, but diesels do not mind wide open throttle periodically if they are not run regularly.
5. Prop size is dictated by clearance of the blades on the hull. A Tartan 37 with a standard shaft should be able to swing a 17"--that is what I have. I think you do not lose much with a 16" and it might be a bit quieter as there is a tiny bit of vibration from tip turbulence in my boat.
The source for this information is the Propeller Handbook, by David Gerr and Nigel Calder's books and it all makes some sense after 28 years of fooling with it. This why an externally adjustable prop seems like a good long term investment to me.
Ray Durkee

Tartan37.com

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