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.
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
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
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?
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.
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.
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.
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.
First of all I read, but did not understand, Gear’s book on props. At least I think that is the authors name. Didn’t get the math but got a little of the concept.
My 48 Mason came to me with a fixed pitch 3 blade prop 21″x 18″pitch as I recall. Its 4-236 Perkins turning through a Velvet Drive 1.91:1.00 trans produced 7 knots @ 1800 rpm in calm water. I wanted a feathering prop and so purchased a Luke Feathering 3 blade. I believe I increased the pitch by an inch but the shape of the blade was really different.
Result was 7 knots @ 1500 RPM. To be fair , the extra pitch probably contributed most of that, the older prop was probably under pitched. I got 2200 rpm full throttle with the Luke so I didn’t overpitch.
Luke’s are not cheap but less than another engine. A great prop.
Keep up the great work,
I may be mistaken but I is my understanding that the Perkins 4-236 had a max rpm of 2800. If so, than you are most likely overpitched by about 3″. I’ve always been intrigued by the Luke prop for it’s lack of flat blade foils. I have never been able to understand how they get it to feather, do you have to lock the shaft to induce and maintain feathering?
Matt – This is just a small presentation suggestion. It might be easier for the engineering/math minded readers to read if you use the “proportional to” symbol instead of a lower case alpha. The alpha is used so often as a coefficient that it can be confusing when used in this context. With the standard “proportional to” symbol, there is no ambiguity.
Just my $0.02
Good article, btw!
(The HTML encoding for the “proportional to” symbol is ∝ or ∝ (hex))
Thanks, Robert – I agree, the ∝ symbol is better!
Does anyone have any real life experience with Bruntons variable pitch autoprop? I understand that it loads the engine at all rpms by altering its pitch but I don’t know to which degree and how is this determined e.g by the auxiliaries run by the engine, engine rating etc?
The Autoprop’s designers explain it better than I can: https://bruntonspropellers.com/autoprop/
Essentially, the thing varies its pitch so that, for any given RPM and water flow pattern, it will maximize the torque on the shaft. Change the RPM or the boat speed, and the blades swivel to a new pitch. In theory, this should keep the engine near the top border of its fuel map (generally a pretty good place to be).
I’ve never had an opportunity to test one.
I too would be interested in hearing real world experiences. I have really liked the few controllable pitch props that I have used so I am hopeful that an autoprop would suit us. Being someone who has had multiple prop failures and isn’t willing to spend the big money for a really good feathering prop, we use a 3 blade Campbell Sailor which is very much a compromise prop but works well for us.
There have been a few quantitative propeller tests that have included the autoprop. In the MIT study, the efficiency of the autoprop was very close to the highest efficiency props at low advance coefficients and it became the highest by a large margin at high advance coefficients. Interestingly, the Hymar testing showed it to have the highest efficiency in the midrange speeds but average efficiency at high speeds and the worst at low speeds. One thing that I have found interesting is the inconsistencies in results of prop testing and I believe that this is due to testing methods which often compare props that load the engine very differently.
I have two friends, with substantial miles under their belts, with Autoprop experience. One loved the Autoprop, but did lose a blade half way to the Galapagos. (Bruntons replaced the prop and paid all expenses.) They then completed a circumnavigation with said prop.
The other friend liked the Autoprop, but hated trying to get it to stop spinning with a shaft lock when sailing and finally changed to a Maxprop to solve the problem. (Apparently the torque generated by the Autoprop while sailing at even 4 knots was scary and regularly overpowered the shaft brake.)
I’m thinking that an Autoprop may be a more attractive option for those with mechanically activated transmissions, rather than hydraulic activated where a shaft lock will be required.
John, a friend with a Volvo 55 and a similar boat to mine has an Autoprop and it is precisely this issue that steered me to a feathering Variprop as I knew early on that I wanted a hydraulic transmission. A shaft lock is an added complication; the Variprop feathers behind the deadwood of the keel and the shaft is still when we are sailing. It might move were we to sail off a wave, mind you, but early days yet.
Interesting article and interesting comment stream as always.
I switched from a 2-balde Gori to an 3-blade Autoprop this year for my 31ft HR. We sailed from arctic Norway to Madeira so far and I find this one of the best investment into the boat so far. Here is what I fond subjectively:
– Efficiency seems to be better especially when motorsailing although I can not quantitize this.
– Motoring against sea and wind is much better (was almost impossible with the old prop)
– There is a considerable delay when switching the direction of trust and one has to give more thrust initially to rotate the blades. (not really a disadvantage but needs to get used to.)
I have also fitted an Ambassador ropestripper. Out of Galicia and Portugal there is a lot of waste and used fishing gear floating in the sea. I once noticed a decrease in engine rpm and a change in the propellersound. After putting the motor in reverse twice ropes and parts of fishing net emerged at the stern. The ropecutter definitely saved that day.
Both are expensive parts but I have used money on less useful equipement.
PS: I have a mechanically activated transmission.
I have a four-bladed VariProp and a ShaftShark rope cutter on a hydraulic transmission, but my experiences to date mirror yours quite closely. I agree that the seas around Portugal are hard on props and keels and some sort of help is needed there due to the floating or hanging lines and debris.
Hey Matt and everyone,
An interesting “attention” plate attached to my Yanmar 4JH5E (2014) says that it’s “application” was for this variable speed engine be used with fixed pitch props. I have never heard that before, although it makes some sense to vary the power into the water one way only: by pitch change rather than rpm. But what do I know. It certainly suggests to me that a variably pitched prop would be out of its application realm and therefore, likely then a warranty no-no.
|My best, Dick Stevenson, s/v Alchemy
I’ve seen that limitation a few times. What it generally boils down to is that a controllable-pitch prop (which we’ll look at in the next chapter) gives you the option to severely overload the engine, if you don’t know how to use it properly. Overloading is always a bad thing, but some engines (mainly lightweight, high-RPM ones) seem to be more vulnerable to damage from overloading. So I can see why a manufacturer might include that caveat.
Nicolas, There is some synchronicity working here. My recent email crossed yours and both refer to variable pitch props. Dick
re the sail drive i have with my fin keel dufour 433 Matt, and based on this article, from which I understand turbulence generated by the hull structure itself around the prop as the boat pushes through the water is a significant negative efficiency factor, do you concur that sail drive, with its inherent minimal structure below the waterline by definition provides for a higher level of prop efficiency ?
richard s (hunkered down in Tampa bay for a while more)
I believe that sail-drives are, as you say, very efficient. The other benefit is that the thrust is directly aft with no down angle.
Having said that, we don’t generally recommend saildrives for voyaging boats because of their poor reliability record and the difficulty of servicing them.
Yes, I’d say that in general, saildrives place the prop in relatively undisturbed flow and allow it to see an advance velocity that’s much closer to the speed of the boat. From an efficiency standpoint, they’re quite appealing. They are also relatively compact, and easy for “shop gorillas” to install without inducing a warranty claim.
My main concerns about saildrives are from a maintenance standpoint. Many (but not all) of them are very difficult to service with the boat in the water, and the saildrive-to-hull seal is often a “Criticality 1” item (no redundancy, will sink the boat if it fails) that requires periodic replacement.
thanks john and matt, and matt are you thinking replace this seal generally every five years ? or what are your thoughts please ? my first exp3rience with sail drive and don’t recall seeing any ref to this in my Volvo users manual, but I will look again
richard in tampa bay
Volvo-Penta usually specifies that their saildrive-to-hull seals be replaced every 7 years.
I’ve heard of them lasting as long as 16 years, but would not recommend waiting anywhere near that long.
concur ? Non concur ?
As a comment, I worked Dave Gerr’s Propeller Handbook (a superb book) and found his estimates of necessary prop clearance a bit conservative. For ex.
I went from a 3 blade 17 inch Max prop with 78mm clearance to a 19 inch 4 blade w/ 45.3mm clearance. One reason I pushed the limits as the hull above the prop is fairly deeply “V”ed (as are many sailboat hulls) and not flat (as in many power boat hulls). I have been fine for 3 seasons now.
In working out the details, my prop guy was confident enough to say that he would carve down the blades and re-balance if I ran into vibration problems.
Dick Stevenson, s/v Alchemy
Dick, I did the same (went from a 18 x 13 fixed three-blade, now “the spare”, to a 19 x 15 feathering four-blade) and the increase in torque, control and stopping power is impressively greater. We have slightly greater clearance, but I’d have to say that so far. the difference has been worth it, along, of course, with the reduction in drag from the feathering.
Matt (or others)
What are the nominal RPM ranges of prop/ shaft speeds? I am just looking for a ballpark figure and I know there are many variables.
Shaft speeds can be all over the place but a good starting point for a cruising boat is to assume a gear ratio of between 2 and 3.5 to 1 and max rpms between 2500 and 4000 rpm. In my experience, 1000 to 1800 rpm is pretty common for a maximum shaft rpm. Cruising rpm will likely be around 70% of that. There are some major exceptions such as the old A4’s many of which were mated to a transmission with a 1:1 gear ratio. If you want ultimate low speed operation, many ships are turning speeds below 100 rpm at the shaft.
Thanks Eric. Exactly the info I needed!
I am glad you are happy and thanks for the field report. I suspect you noticed the most marked change in the boat’s stopping power in reverse. I am not sure what you mean by “greater clearance”. If prop tip to hull measurement, I would expect the clearance to be less with the 19 inch prop. Did you buy a Max Prop?
My best, Dick Stevenson, s/v Alchemy
No, it’s a VariProp, which I selected because a) when I bought it, the Canadian dollar was higher than the U.S., and b) I could change the pitch in the water if need be. My comment about the clearance refers to the fact that the aperature on our boat could take an even bigger prop were that a good idea, as can be seen here: http://alchemy2009.blogspot.ca/2013/02/propositioning-or-getting-right.html
And yes, the stopping/backing down power is indeed something to behold. I am glad I got the AquaDrive coupling, a hydraulic transmission and soft mounts for the engine, because I think the torque of rapid shifting in close quarters would wear a more typical drive train.
A big vote for the Bruntons Autoprop after 10 years sailing in the Med.
Its main advantage is when going astern we now have good control as the prop swivels complete to present the right angle of attack to the water.
Read the maintenance manual and grease every year and replace the seals and bearings after so many engine hours??? I fear people don’t always do that.
I’m in the expensive process of repowering my 1989 Catalina 42. After 3.5 years and 18,000nm in the South Pacific, I find my Yanmar 44hp is tired and won’t drive my Autoprop like she use to..
I’ve made many of the mistakes outlined that led to the demise of my original Yanmar, like getting 6knts at 1600 rpm @2 liters/hr for days!
The replacement is the 4JH5E (55hp) with a 2.63:1 gearbox and reusing the H5 Autoprop. I’ve got a good installer here in Australia who has done a Catalina42 before.
I was hoping Dick on Alchemy would comment on his experience and installation. And a general Question to this Amazing group. Does my replacement sound right? I’ve got till July to make any changes so any and all input is welcome.
If you were under-loading the old 44 hp, I can’t see the logic of upping the HP to 55, particularly if you are keeping the same prop since that will just make the under-loading problem worse. I had a look at the specs for the Catalina and I would think that 44 hp would be plenty of power, and probably more than you can use. In fact even with 44 hp I’m going to guess that you will need to over-prop a bit to properly load it.
I would also say that trying to reuse the prop may be false economy since if it is too small, and given that relatively high ratio on the gear, you will quickly damage the new engine, particularly since under-loading is particularly destructive when an engine is new.
On the bright side, Autoprops are do help with the under-loading problem since they dynamically change pitch with load.
Summary: it’s complicated, but you will find pretty much all the information you need to make the right decisions in this Online book, by starting at the beginning and reading through slowly and carefully.
Thanks for taking the time to reply. My choices in new Yanmars are the 55hp or the smaller 39 hp. I’m confused by your statement about being under propped. If my existing 44hp got me 6knts @1600rpm, isn’t that over propped or do I have it backwards?
My Aussie Yanmar dealer says they can get either model but are concerned that the 39 hp might be underpowered when I need a little extra…
I think it could be a perfect fit for my Autoprop and my new tough guy approach to engine use.
I’ve read the first 50 hours recommendations and they want you to drive it hard!
Thanks for any input,
The fact that you got to 6 knots at 1600 does not tell us whether you are over or under propped. Determining that is governed by whether or not you reach specified max RPM at wide open throttle.
My guess is, that given the size of the boat and the fact you were reaching 6 knots at just 1600, you will be better with the smaller 39 hp, but that is simply a guess.
To make sure you get this right, you need to calculate how much HP you were using with the old engine at various speed to length ratios and then apply that to the new installation. You will also need to take into account the ratio of the gear.
The good news is that all the information you need to do this is in this online book and in the power curves for the three engines in play (the old one, and the two options for new ones).
Bottom line, you can’t guess this or go on the vague concerns like “might be underpowered when I need a little extra”, if you want to get it right.
I had a MaxProp on my last boat, but after several weeks of research went with an Autoprop on the new one and have been pleased. Here’s one anecdote of someone who had direct experience replacing the MaxProp with an Autoprop on the same boat, for what it’s worth.
Yes, I think there’s a lot to like about the Autoprop. The downside of them is the need to have a prop lock if the boat is fitted with a hydraulically activated transmission. I have also heard reports of faster wear on the Autoprop than the Maxprop. Neither disqualify the Autoprop, in my opinion, but it’s as well to be aware.
(Not sure what the wear problem for the blog writer you linked to was, but that has certainly not been my experience with the Maxprop and I have been using them for some 35 years.)
Hi Again Jim,
On reflection I should clarify that I’m not saying that Maxprops last forever, they don’t, just that they seem to be pretty robust and last a long time. That said, I too have had the issue of maxprop rebuilds not solving all the play and vibration issues in a old and worn prop.