Pitch and Diameter

[phorvati]

This is frequent topic on many forums, but I was wondering if you could tell me what is appropriate size pitch and diameter for C&C 38, 16000 lbs, probably more as I am installing criuising gear. I have an 18/10 martek folding prop on a 1" shaft and barely reach 1400RPM under full throttle, this only pushes me 5-5.5kts. I have 2:1 reduction gear. I am hauling out soon and would like the solve this issue. I just don't know what to get for a cruising prop. Fuel efficiency is much more important than drag. Many reccomend the feathering prop?
Sincerely,

Petar

[Don Moyer]

Petar,

All reports that we've heard regarding 2:1 reduction gear propeller sizing indicate that a 15" or 16" prop with a 10" pitch is about standard. This would, of course, explain your rather anemic performance. We've also heard that a folding prop takes a bit more power to turn than an equivalent-sized fixed blade prop. I would anticipate that a 16 X 10 fixed blade prop would boost your RPM to approximately 1900 RPM and yield an increase of approximately 4 HP available from the engine.

Don

[phorvati]

Don, what about using a three blade fixed prop. Would the diameter still be recommended at 14" or 15". and pitch at 10"? Or should I go lower like 13-9
Thanks,
Petar

[Don Moyer]

Petar,

You really have me over a barrel. I have no rules of thumb at my disposal to convert from a fixed two-blade prop to an equivalent fixed three-blade prop.

If we simply double the size of the popular fixed three-bladed 10 X 7.2 Indigo prop used on direct drive Atomic 4's, we'd end up with a 13 X 10 or 13 X 11, but I'd feel better if we wait for someone to check in who actually has a three-bladed prop on a 2 to 1 reduction unit that they really like.

Don

[hanleyclifford]

the subject of prop size with 2:1 reduction is of real interest to me. I am currently using a 3 blade cupped (power boat) prop which is stamped 13x13, i hope correctly. it allows me to cruise at close to 6 knots in still water between 1500-1700 rpm. i can get close to 2000 really putting the juice to her, but i'm not happy with this arrangement. i would really appreciate some advice on this. the boat is wood, full keel, about 24 feet on the waterline.

[Marian Claire]

Can not give you any advice but the MC is similar in size. A little over 24’ lwl, full keel, wood and 12000 lbs displacement. I have a two blade fixed 14” ??pitch prop in an aperture and 2/1 reduction gear. Hull speed should be around 6.5 knots. I get about 4.4 knots at 2000 RPM and 5.5 knots at 2500 RPM. I do not have the #s in front of me so I am working from memory. The A4 does not strain at these RPMs and I have run slightly higher for short periods. The folks at Martec recommended a 15” 10 pitch two blade prop when I gave them my boat specs. Just some #s to compare. Dan S/V Marian Claire

[wlevin]

Over the years I have asked a few propeller companies to recommend a prop for my sailboat. They have been very expert given engine, gearing, boat model etc. Now you can do this online and still for free and then compare prop recommendations.
Bill and Jeanne
T34C #453
Otter

[insatiablei36]

Hello all,This is my first post I have been reading the MM threads for over three years now and thought i would jump in on this topic. I have an Islander 36, original A4 with a 2:1 reduction gear. When I bought the boat three years ago She had a 13x9 martec folding prop and I could get about 2800 rpm's at full throttle, I would only do this for a short while. I could cruise at 2000 rpm's and about 5.8 kts according to gps. Not to mention it was great under sail The only problem I had was in reverse, Almost non existent....Very scary. I changed over to a 13x10 three blade fixed last year, I'm doing 6.0 kts. at 1900 rpm's Great for cruising, not the best under sail, But great in reverse. Ideally, I would opt for a three blade feathering prop, But at a thousand dollars a blade, I'll stay with the fixed for a while. Hope this helps out!
Dave

[Dave Neptune]

Props

I have played with many props over the years on my direct drive A-4 and been involved in many other applications. I just got involved in a Cat 36 diesel application with a 2:1 reduction and I think it's 23 HP.
Any way my friend has used many props and is really excited about his newest prop. It is a bit like the Indigo for the direct drives by design IE the more efficient narrow blades and thicker leading edge. The prop is very different looking and has an incredible "bite" in the water. The props are made by Campbell in Canada, has anyone ever tried one of these on a reduction A-4?

With a 14X8 3 blade he is getting 1600 RPM @ 6.9 knotts and the engine is at full throttle. We need to get some more RPM's and are looking to reduce the diameter. He claims less drag than with his 14X10 2 blade sailor prop. The blade areas look about the same as the 3 blades are "narrow".

You may wish to check one of these out.

Dave Neptune