YM prop tests again, Autoprop static thrust

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JimC

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I've only just got down to studying the prop tests, sorry if this has already been discussed.
I've been considering getting a Bruntons Autoprop and was surprised to read that the bollard pull of this prop was comparatively poor. I would have expected the prop to have adjusted itself to a fine pitch to maximise static thrust, as being tethered to a bollard is effectively an extreme case of motoring against contrary wind and sea at which the Autoprop is said to excel. I would have expected the stopping times to be better too for the same reason - or am I missing something?
 
I suspect that the 'testbed' tests bear little resemblance to reality and real conditions. I have an Autoprop and having sailed with it for 7 seasons, I would have one on any boat that I owned.

Thrust is outstanding, there is no prop walk, it gives a phenomenal 'crash stop' and it gives faster sailing too.

A good buy and the reason that they are expensive is that the do what they say on the tin.

No connection with the company other than as a satisfied customer.
 
I wouldn't be too bothered about the static pull result.

Having switched from a two-bladed folder to an Autoprop two seasons ago, I found marina manoeuvering different in a couple of ways, neither of which was a really big deal when I got used to it: (1) there was less prop-walk with the Autoprop; (2) switching from forward to reverse requires a bit more welly with the Autoprop - the first experience was a little disconcerting on a windy day in Dickies dock.

The Autoprop's outstanding feature is its ability to increase its pitch when motorsailing. Imagine you have a prop that drives you at say 4 knots at 2000 revs without any sail up. Now put enough sail up to drive you at 4 knots without the engine, and run the engine in forward gear at 2000 revs. A conventional prop adds very little forward drive because most of those revs are keeping it up with the existing forward motion through the water. If you are after 6 knots to make that tidal gate, you'll have to wack up the revs to 3000 or so. The Autoprop acquires a steeper pitch under these conditions and will give you the additional 2 knots at considerably lower revs. Fuel consumption is of course a function of load and revs, but I reckon that the Autoprop could pay for itself in lower fuel consumption during motorsailing and reduced wear and tear on the engine.
 
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I suspect that the 'testbed' tests bear little resemblance to reality and real conditions. I have an Autoprop and having sailed with it for 7 seasons, I would have one on any boat that I owned.


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Have you read the article the OP is referring to?

The "testbed" as you put it was a Beneteau Oceanis 323, fitted with 16 props over the space of two days.

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As you can see in the video we tested fixed, folding and feathering props, with all of them fitted to the same boat, with a static pull ahead and astern, prop walk also measured, speed tests in both directions on the water, and several crash stops for each prop.

So I believe our "testbed" bears much more than the "little resemblance to reality and real conditions" you suggest in your post.

If you have read the article in May's issue of Yachting Monthly, and feel that way, please let us know why? We'd like to hear your feedback
 
The point of the Autoprop is you don't have to run at full revs to get the best out of the engine. If you're happy with the ‘peddle-to-the metal’ then any prop will do – they’re all working efficiently up there. The benefit of the Autoprop is at low revs, particularly motor sailing. The YM test didn't go there so the Autoprop was never really going to stand out. I do own an Autoprop so I’m biased but perhaps the real point is that there isn’t a BEST prop. It depends on your budget and your style of sailing.

Tim does have a point though. Tying your boat to a post and ‘wellying’ the throttle doesn’t really bear much “resemblance to reality and real conditions” does it? Not unless Snooks and the testers at YM use their boats very differently to the way I do...
 
I'd have to agree with that too. We inherited an Autoprop with our current boat and it is VERY impressive. Just a tiny bit of wind assist even with no sail up and the prop ups the pitch a bit and the speed goes up. Brakes are VERY impressive, I've never had the bottle to try the crash stop from 6kts in a boat length but I sure do believe it! Into a headwind/sea the prop pitches down so more revs are needed for the same speed as before, but come whatever load it WILL reach max revs and therefore maximum horsepower. Fuel consumption under power for us is excellent and way better than others with the same boat and engine but fixed or folding props achieve. Under sail it may have a bit more drag than folders but again it gets my vote.

As others, no connection to Brunton other than a very satisfied user.
 
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If you have read the article in May's issue of Yachting Monthly, and feel that way, please let us know why? We'd like to hear your feedback

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Really?

On another thread I pointed out (a) a factual error in the article's functional description of the Kiwiprop, and (b) suggested that differences in measured performance might in fact be testing the closeness of match of a particular propeller to the test boat, rather than testing propeller type v. propeller type.

Publication of the test rpm data would quickly resolve the latter point, but so far there's been no response to either.
 
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If you have read the article in May's issue of Yachting Monthly, and feel that way, please let us know why? We'd like to hear your feedback

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Really?

On another thread I pointed out (a) a factual error in the article's functional description of the Kiwiprop, and (b) suggested that differences in measured performance might in fact be testing the closeness of match of a particular propeller to the test boat, rather than testing propeller type v. propeller type.

Publication of the test rpm data would quickly resolve the latter point, but so far there's been no response to either.

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We always like to get feedback about the magazine, and this forum is read by most of the staff, including the editor.

The RPM data might be going online, I'll check up when I'm next in the office (week after next)

When the next issue comes out we address some of your comments, and comments of other readers...We are after all first and foremost a monthly magazine.

All the manufactures were given the details of the yacht/engine/gear box ratios and asked to supply the prop they recommend for that yacht. If you owned that yacht with that engine they would supply the prop we tested. If they are recommending what you feel is the wrong prop, it is hardly the fault of Yachting Monthly. Likewise we'd tested all the same diameter/pitch then the manufactures would have said we'd have used the wrong prop and they would have recommended a different one.

However my reply was directed to the poster who erroneously thought the props were tested on a testbed, when all 16 were tested on the water on a yacht.
 
Having had one of the original Autoprops for nearly 20 years, I can only concur and suggest the low static thrust is down to experimental error, to measuring it before the pitch had settled down.

How about a mean over 1000 hours of 1.33 litres/hr for a 3YSM20 driving 4.5 tonnes @ a mean of 5.6knots?

When first I had the beast, I nearly had a few heartstopping moments, slowing in reverse. Finally I decided to really give welly, unfortunately the Doubting Thomas on the foredeck, unprepared for such deceleration, described a not-so-neat parabola over the pulpit. I believe JJ had a similar experience when first driving an Autoprop.

If you want to use propwalk, alternate between short bursts of forward and reverse, the propwalk only occurs when the prop is changing the pitch from reverse to ahead mode.

You can simulate a stern-thruster, if not a bow thruster.
 
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I'd have to agree with that too. We inherited an Autoprop with our current boat and it is VERY impressive. Just a tiny bit of wind assist even with no sail up and the prop ups the pitch a bit and the speed goes up. Brakes are VERY impressive, I've never had the bottle to try the crash stop from 6kts in a boat length but I sure do believe it! Into a headwind/sea the prop pitches down so more revs are needed for the same speed as before, but come whatever load it WILL reach max revs and therefore maximum horsepower. Fuel consumption under power for us is excellent and way better than others with the same boat and engine but fixed or folding props achieve. Under sail it may have a bit more drag than folders but again it gets my vote.

As others, no connection to Brunton other than a very satisfied user.

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I suspect not, in a test (or it may have been by calcs) in French magazine I was interested to note that the Maxprop came out as the slipperiest and folders fared fairly badly (though better than fixed).

However unless I lock my shaft the Autoprop always starts windmilling.
 
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However unless I lock my shaft the Autoprop always starts windmilling.

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We always lock the prop by stopping the engine whilst still in gear ahead at tickover, This is on a Yanmar 44 4JHE.

I wonder what effect a stationary pull test has on results as compared to motoring normally through the water. Certainly when sizing props to fit a boat and engine combination, one factor in the calculations is the predicted speed of the water entering the prop which isn't the same as the boatspeed. Tied to a bollard there is no boatspeed whatsoever and the water feed into the prop speed is anyones guess?

In practical terms too from friends using a variety of props on a variety of boats, the Maxprop has rarely worked out of the box and required it's pre-set pitch to be adjusted, in one case many times before it worked correctly. Once set correctly the results were good, but it does beg the question in a comparison trial if apples are being compared with oranges. One Maxprop owner too has switched this winter to a Brunton! Not getting at Maxprop BTW, just questioning the science of comparisons.
 
Bollard pull provides only one point on the standard type of prop performance curve (at zero advance ratio, i.e., at 100% slip). If thrust at zero boatspeed is what you're after, then it's a valid point for comparison but it tells you absolutely nothing else about how a particular prop will perform. For a more scientific study, "Comparison of Ten Sailboat Propellers" by Lurie and Taylor of MIT in the research journal, Marine Technology is where to go. In their (comprehensive) results, the Autoprop produces a very peculiar set of curves, which is not surprising given the self pitching configuration. Generally, the Autoprop appears to have performed almost as well as typical fixed blade props, and at high Advance Ratios (which correspond to low-slip & high-ish boat speed) it is among the best. However, in the one group of static tests published in the paper - reverse thrust - the Autoprop gave about the lowest thrust of all props at low rpm (25% of a fixed 3-blade at 200 rpm) and the highest of all at 600 rpm by a factor of about 1.1.

The paper is possibly downloadable from the publishers or some other agency for a charge.
 
Doing so many tests in such a short period of time is amazing, but a thought occurs to me (as they do from time to time).

How was the location where the static tests were undertake effected by tide/current? As others here will know far better than I, many marinas based in river systems can be subjected to quite substantial currents. Did this have an impact on the results, was this take into account?

Just a thought.
Jeff.
 
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