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.
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
One imperative that must always be met, when selecting a propeller, is that the engine be properly loaded at wide-open throttle.
If (as is usually the case) the maximum RPM is fixed by the engine and gearbox, and the maximum diameter is limited by the propeller tip clearance, then you’re pretty constrained in what you can do with the pitch. Too steep and you’ll overload the engine, leaving it unable to reach maximum RPM; too shallow and the engine will always be in the lower-right corner of the fuel map (that inefficient region where RPM is high, torque is low, and problems like cylinder glazing or carbon buildup become a concern).
As a general rule, fixed-pitch props should be sized to properly match the engine at its maximum rated RPM. If a range is specified (eg. 3800-4200 RPM), I prefer to aim for the bottom of that range, as this will yield a slightly steeper pitch and therefore load the engine more heavily in the 1/2 to 3/4 throttle range.
As long as you’re dealing with fixed-pitch props, though, there is no completely correct and satisfactory answer to these questions. To reconcile those apparently conflicting goals of mid-speed efficiency, low-speed thrust and proper loading of the engine, you need to either go to a controllable-pitch prop, or have some way to vary the relationship between engine RPM and shaft RPM. That’s where we’re going next in this series.
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
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
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.
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
A rope stripper certainly won’t improve the propeller’s efficiency, but I wouldn’t expect it to have a very significant negative effect either. They only affect the flow very close to the shaft and hub; this region of the water flow is already chaotic and contributes very little thrust.
I’ve never used one (they aren’t available for outboard or sterndrive props), but if there is a significant risk of snagging lines, I would happily take the very small potential loss of efficiency in favour of a reduced risk of fouling the prop.
(Also on that note, I usually cut my mooring lines so that, if dropped overboard, they are 6″ shy of the prop. Just in case.)
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.
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
All else being equal, adding more blades tends to reduce the propeller’s efficiency. The more blades you have, the more each one’s flow will be disturbed by the blade ahead of it.
However, all else is *not* equal in situations like yours. When you’re restricted to a relatively small propeller diameter, going to a 4- or 5-bladed design can get you a lot of extra blade area. That tends to reduce the possibility of cavitation, and often yields more thrust when the advance velocity is low.
A good 4-bladed prop will generally be less efficient than a good 2- or 3-bladed one if there is no limit on diameter. If you’re trying to put a fair bit of power through a small diameter, though, it’s often better to use 4 well-proportioned blades than 2 awkwardly shaped ones to get the necessary blade area.
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.
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.
Re. prop tip to hull clearance.
A very rough rule of thumb would be that a tip clearance of 15% to 20% of the diameter is usually plenty, and 10% is workable but a bit tight. Less than that and you run a risk of vibration problems.
Tip clearance issues are generally more problematic on high power, high RPM propellers near flat or nearly flat bottoms, and less so on slower propellers near sharply V-shaped hulls.