The Offshore Voyaging Reference Site

Propeller Efficiency

Antifouled prop - before and after 147 IMG_3950

In the next few chapters of this Online Book, we’ll be looking at high (and low) tech things we can do with a yacht’s drivetrain to make more efficient use of the engine and its fuel.

But before we delve into sophisticated and costly advanced powertrain options, let’s take a look at the efficiency of the propeller.

On most cruising boats, power or sail, our biggest and cheapest gains in powertrain efficiency will come from choosing the propeller size and speed correctly.


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Dick Stevenson

Hi Matt,
How nice to see an article from you. And on a subject that is, somewhat surprisingly, of great interest to this sailor. I look forward to the next chapters as you have in hand such a good start.
A request: Could you please address the issue of prop pitch setting as it pertains to the conflicting desires of engine manufacturers, engine longevity, effective “punch” into seas and winds, and efficient motoring in calm conditions (or motor sailing)? (Or summarize/integrate the AAC previous discussions on this subject.) Thanks.
My best, Dick Stevenson, s/v Alchemy

Richard s (s/v lakota)

my experience with auto feathering props is not good as the one I had failed catastrophically after only a year of use…one of the blades broke off mid-gulf stream rendering the engine useless beyond idle speed…will never use another…violates the kiss rule…a word to the wise…richard in Tampa bay

Dick Stevenson

Hi Richard,
That sounds really lousy. What kind of prop was it and was there any subsequent explanation? Also not sure what you mean by “auto”: some feathering props claim an automatic pitch adjustment to prevailing demands but all feathering props should “auto” feather if stopped correctly. Anecdotally, and very casually, I have heard some operational concerns about robustness and balance issues with auto-pitch props.
I have owned 3 different Maxprops (fixed pitch) on 2 sailboats over 3+ decades and have been very pleased. As to their robustness, at full speed motoring I caught a lobster pot between the blade and the hull. This stopped the engine with an awful bang (and almost stopped my heart). Many dives later I had dug the pot off the blade (buried 2-3 inches) and, with trepidation, started the engine: it was ok. Then I put it in gear: at idle forward it was ok and even at full speed, there was no out-of-balance vibration. I was amazed, and, of course, very pleased.
Feathering props are clearly more complicated but the advantages are worth it for me: faster boat speed sailing (and less likely to pick up stray lines/nets), but (maybe more important) also a far more powerful reverse and better manoeuvrability in marinas which is very important as we choose not to have a bow thruster.
There is clearly no right or wrong here, but I would not want you or any reader to write off feathering props as they have benefitted many sailors for many decades without a bad track record.
My best, Dick Stevenson, s/v Alchemy

John Harries

Hi Dick,

I would second the vote for the Maxprops. I have been using them for coming about 37 years—there’s a scary number—without a problem, and, like you, I have abused them terribly.

Rob Gill

Hi Dick, thanks for the story – we have a 3 bladed Maxprop with an Ambassador rope stripper attached just ahead. I have often wondered if I need it, how much turbulence is caused, and how it affects flow over the prop. Ambassador’s web-site claim little (or even positive) affects, but don’t provide any evidence of this.
We are coming out of the water for our annual scrub down / check up soon, so I wondered about removing the assembly for a year long comparison. Matt, or anyone got any views?
Cheers, Rob

Marc Dacey

After much confusion and delay on the part of the refitter, we have just resumed sailing our full-keeled cutter, which has a new (2.8 hour runtime) Beta 60 turning a 19 x 15 four-bladed VariProp. While I’ve had the engine to WOT just a couple of times, I have certainly noted both that there is less (or less obvious) drag when we switch off and start sailing, and that I can maneuver more positively under power than the rather cumbersome hull shape we have would suggest. Certainly, as long as I give the blades a few seconds to deploy either in forward or reverse, I have far more control with this prop than I did with its three-bladed, fixed pitch predecessor. Matt’s point about gear ratios will bear testing: I have yet to systematically try to maintain hull speed under power while eyeballing RPMs and signs of engine strain that may necessitate a change of pitch, but articles like these (and Dave Gerr’s guidance in book form) help me to understand the issues, for which I thank you.

Colin Speedie

Hi Matt

great, digestible article as always.

I wonder what your thoughts are on 4 bladed props? We recently fitted a 4 bladed Featherstream on a Boreal 55 and I was very favourably impressed. Amazingly smooth, instant acceleration, excellent ‘grunt’ for punching through a short, steep chop, and much more grip astern, it seems as close to perfection that I’ve ever experienced with a prop. Same diameter as the 3 blade, so no problems with fitment, either.

Incidentally, I’m with Dick. One of the first things that I’d fit to any boat to improve it’s performance under power and sail would be a top quality feathering prop, and I’d certainly include the Maxprop, Variprop and our Darglow Featherstream (as in the title pic to this article) as examples. That’s not to say that problems can’t occur, such as Richard encountered – anything can fail, and we carry a spare fixed 3 blade prop just in case. But after many tens of thousands of miles with feathering props I wouldn’t be without one for all the reasons Dick outlines.

Best wishes

Colin

Marc Dacey

Same experience here. Docking in a cross-wind yesterday, I had to “bail” as I was too far off for the crew to jump off with a midship line, and too close with my bowsprit to the stern of a nice big powerboat for peace of mind. So I gave about half-throttle in reverse and was gratified by that same “punch” or as I call it “authority” with which I backed down. Dead slow is, on the other hand, under 2 knots SOG, so that’s fine as well. And, like you, we aren’t chucking the old three-blader: I’ve heard of a few props having issues, just not often and not many, and I suspect skipped maintainance, rather than manufacturing or design flaws, may be the issue in some cases.

Dick Stevenson

Hi Colin,
I love my Valiant and the way she sails, but she does not like powering, especially into seas. It was coming out of Treguier up current and into 20 kn wind/seas (and you know well the rocks on either side) that Ginger turned to me and gave me permission to search for more “Umph”. This lead first to changing my 17 inch 3 bladed Maxprop for a 4 bladed 19 inch Maxprop. Dave Gerr’s Handbook* (mentioned by Matt) was essential in working out the details as was Chris at Darglow, who went so far as saying if we had overdid it, he would shave down the blades and re-balance gratis. The 19 inch worked fine and has given us greater speed and control in adverse conditions. I am not sure whether the extra blade, extra diameter, or some playing with pitch made the most difference, but the results are pleasing. I believe Maxprop also makes a 5 blade.
My best, Dick Stevenson, s/v Alchemy
*Working the formulas in Dave’s book suggested I had not enough distance prop tip to hull. It may be that his figures are for a flat hull above the prop (such as many power boats have): on Alchemy the hull is “Ved” above the prop and I have had no issues that result from too little prop clearance.