propeller choice

[hesper]

We're installing a new/used Volvo MD6A in my 22' Marshall catboat. Of course, the engine wants a Left Hand propeller. The replaced engine (vintage 1973 Palmer was a Right Hand prop. With the change in RPM's etc, I'm soliciting opinions on propeller pitch etc.
Boat is 22' long, 10' beam, approx. 6000#. I'm not sure quite what RPM's we'll be running although the Volvo manual says 2450-2550 RPM max. speed. The old Palmer used to cruise at about 1200 and would cavitate at 1500 RPM.
What do you think?
Jim

[Tim]

There are formulae that prop shops use to determine the appropriate pitch for a propeller. They plug in values like displacement, engine HP, gear ratio, aperture size and clearance (if applicable), and so forth.

The correct pitch for your boat will allow the engine to reach maximum rated WOT, or very close. It's impossible to guess at this, so I'd talk to your prop shop, or you can try to figure it out yourself with an online calculator like this one from Michigan Wheel:

Michigan Wheel Prop Estimator

[MikeD]

I thought I'd revisit this thread since it's title is appropriate. I have two questions:

1. I have to have my prop repitched, and may have a chance to get a two-bladed prop instead. What is the difference between a two-blade and a three-blade prop? (Other than 1 blade....)

2. How do you get the propeller off??? I thought it'd be easy since I just had a new shaft installed last summer.

[Jason K]

1. The two blade prop will exert considerably less drag and there won't be a considerable difference in speed under power. Two cons of a 2 blade prop are that they are less effective in rough seas and aren't as good in reverse.

I have a 2 blade prop in my Triton and am happy with it, though I haven't helmed a three blade in a Triton as a basis for comparison.

2. To take the prop off you need a prop puller. It is a simple brace that has a large bolt. As the bolt is tightened, it pulls the prop off. It's a simple concept, but a bad prop puller is an exercise in frustration. I don't take props off very often and can usually find one to borrow. The one I had was junk. They aren't that expensive, though, and if you buy a new one it should work well enough. The new shaft may make things easier, but you definitely need the right tool for this job.

[MikeD]

Thanks for the reply Jason. Does "Not as good in reverse" = more "prop walk" ?

And "less effective is rough seas"? Somehow it seems a two vs. three blade wouldn't be different if, all else being equal, they're sized right for the engine and pushing the boat at hull speed? Maybe not?

Also, I've gotta ask - your Avatar - is it a mint jullip?

[Tim]

A 3-blade prop is more efficient, particularly in an aperture behind a deadwood, because there is more blade surface consistently available to produce thrust. It's a small difference and essentially unnoticeable, but it's there. With a deadwood/aperture, the increase in drag with the 3 blade is negligible, though it is significant when the propeller is attached to an exposed shaft, as in a more modern fin keel boat.

The increased blade area of a 3 blade prop does help in reverse, but these boats are lousy either way.

Mike, I have a prop puller if you want.

[Summersdawn]

My Dad put a Campbell Sailor 3 blade prop on his 27 Catalina. It replaced a 2 bladed prop. The difference in reverse was significant. You might want to check them out. I am not affiliated with them in any way, I just remeber how much better my Dad's boat sailed and motored with one on.

http://www.westbynorth.com/CampbellSailer/Tests.aspx

http://www.vicprop.com/sailor.htm

[Jason K]

Thanks for the reply Jason. Does "Not as good in reverse" = more "prop walk" ?

And "less effective is rough seas"? Somehow it seems a two vs. three blade wouldn't be different if, all else being equal, they're sized right for the engine and pushing the boat at hull speed? Maybe not?

I think this is one of those rare cases where what I thought I knew and the reality of what I don't know (but thought I did) collide. It's rare because reality rarely intrudes. I'll withhold further comments until I can do some research.

Also, I've gotta ask - your Avatar - is it a mint jullip?

Mint Julep! *gasp!* I think not. It is a MOJITO! I'll forgive you this once.

[MikeD]

Mike, I have a prop puller if you want.

Tim, you're the best! I'll email you off-line with a bribe.

Jason, Sorry to offend - I'll have to give that drink a try. I like new things. Microbrews can get a little wearisome. Did I really just say that?!?!? :)

And thanks for the input all.

[hesper]

Mike,

I think we're all probably going to agree that for most cruising boats such as ours, the difference in drag between 2 and 3 blade props is negligible at best. I think that likewise, the diff in thrust is not generally going to be worth buying a new prop one way or the other unless your current prop is worn out or damaged.

That being said, I could not imagine putting on a hi-drag three bladed fixed prop on my tricked-out speedster of a catboat. I mean, really, I'd like to at least pretend that I have a racing boat. You wouldn't put snow tires on a 'Vette would you?

As far as pulling the prop, you probably will need a puller and potentially a little heat. You can probably come up with a few home-made designs on-line if you google it. (Check out Nigel Calder's and or Don Casey's books)

I ended up with a 2 bladed 12x11 prop which on my boat with new/used engine was sufficient. I would tell you that I spent quite a long time discussing/arguing the amount of pitch required with the supplier. I think that on an old engine especially one without know rpms gear ratios etc, there's more art than science involved. In the long run, the prop I chose seems fine. In fact, I powered non-stop from Hull, MA to Plymouth, MA about 2 weeks after launching and boat, engine and prop all handled well. It could probably use a little more pitch but I'm not complaining and I'm not changing either. With the limited amount of powering I do, I'm not real concerned about getting that last little bit of performance or economy from her.

Good luck

Jim

[Tim]

Jason,

Nice website! How long were you going to keep that secret from us?

(I know, I know...it's not done yet.)

[Jason K]

Yup, not done yet. Give me a week and I'll solicit suggestions for improvement from the peanut gallery. :)

[MikeD]

Well, I ran the numbers through a few prop calculators and even emailed with a very helpful guy from Michigan Wheel. All the calculators came back with BIGGER props than what I've got now. The Michigan Wheel fellow also said that my current prop is nearly the same size as the calculated theoretical size based on the info below. Rather than sell me a new and smaller prop, he wondered if there was another issue - perhaps with the engine...

My setup is this:
Boat: 1967 Pearson Ariel
LOA: 25'6"
LWL: 8' 6"
Beam: 8'
Disp: 5200
Hull Speed: 5.76
Engine: Yanmar 2GM Raw water-cooled, rated 15 HP @ 3600 using a 2.62 to 1 reduction gear
Prop: 13x11, RH, 1" shaft

When under power, I reach hull speed at 2100 or 2200 RPMs.

Am I missing something? Could it be some issue with the engine??? I'll check out the tachometer in the Spring to make sure it's reading properly. Anything else?

Thanks,
Mike

[Tim]

Hmm...this puts things in a whole new light. A 13x11 isn't that much off the mark. I run a 12x13 3- blade with a larger, heavier boat and close to the same engine. This particular prop is about ideal in terms of attaining maximum engine RPMs of around 3400 in my case.

Having not been on board to observe your engine in action, I can't say if you have an engine problem or not. It's enlightening to hear that you actually do reach hull speed, but at a lower then expected RPM; I didn't realize that you were getting the speed, but just at a low RPM, as indicated. I assumed that the low engine speed was coupled with a lack of boat speed as well. Certainly a prop of larger pitch would not increase your RPM; it would have the opposite effect.

You should be reaching cruising speed (roughly around hull speed, or a bit less) around 2800 RPM, give or take a little. I wonder if it's as simple as your tach being inaccurate? Certainly those cheap tachs aren't necessarily spot on. I'd confirm the actual engine RPM before making significant changes.

Most engine problems tend to make the engine bog down and deliver reduced performance, with symptoms including low engine RPM, lack of boat speed, exhaust smoke, etc. All you seem to be showing for symptoms is low engine RPM.

The plot thickens...

[MikeD]

Indeed. Looks like I need to add some more items to my Spring commisioning list.