maxprop v autoprop

[Laminar Flow]

Nope.
Definitely spinning unless 2 blade hidden behind a long keel.

Without any maths just think about this, if the prop fixed offers less resistance than spinning, why does it spin?

Agree.
Everything I have ever read, including testing done in a tank, indicates that a prop left to freewheel reduces resistance by 40%. I'm pretty sure this argument has been resolved in all finality.

My last boat had twin engines with 3bladed fixed props and letting them spin under sail certainly made an appreciable difference. On our current vessel we can, as this is a function of the gearbox, let our prop spin; there is very little discernible noise and certainly none that bothers us.

In regards to Dream believer's observations: The faster a boat's relative speed, the less the loss in speed due to the prop in spite of the increase in resistance. This is because the Boat's overall resistance is considerably greater at higher speeds. Conversely, the loss in speed at slower velocities is the greatest.
At a relative speed of 0.9 speed loss is: for a spinning 3 blade 8%, feathering or 2 blade is 4% and a folder is 1%
At a relative speed of 1.1 the loss is: 3 blade - 6%, feathering or 2 blade about 3% and folder sub 1%.
At hull speed: 3 blade 4%, feathering or 2 blade 2% and folder again sub 1%.
(data from "Motorsegler", Donat, Delius&Klasing)

For us, with a spinning 3 blade ,this would would mean a loss of 0.4 kts at a relative speed of 0.9 (4.8kts). If we locked the prop we would lose 0.7kts. At hull speed (7.12kts), I lose 0.28kts (spinning) or 0.4kts if locked.
If I were to invest in a feathering model (we do not have room for a folder) I could, at lower speeds, gain 0.2 kts, at hull speed 0.14 kts.

For all the arguments concerning prop drag, I have yet to see any analysis of the resistance caused by exposed shafts, p-brackets or the blunt ended shaft housings seen on some AWB, regardless of what type of propellor they are using.

 

[Buck Turgidson]

And here is a paper on prop drag :
https://www.plaisance-pratique.com/IMG/pdf/Drag-Thesis.pdf

skip to page 47 onwards for the results.

 

[Laminar Flow]

And here is a paper on prop drag :
https://www.plaisance-pratique.com/IMG/pdf/Drag-Thesis.pdf

skip to page 47 onwards for the results.

Thank you for that.
As expected/predicted the freewheeling mode for fixed props greatly outperformed the locked one.

Beyond that however, I got the impression that the paper was written by a bunch of underfunded tech students.
To make a proper comparison, all five props should have been designed to meet the parametres of a particular, if hypothetical, vessel. To compare the resistance of props of vastly different blade areas is rather pointless, to say the least.

I'm sure Gerr and Eliasson/Larsson will be relieved to discover their findings/statements have been confirmed.

I do hope the lads got some support from Kiwi props. If they did they sure sent them their grungiest and oldest bit of kit they had laying around.

 

[fblc]

Hi,

I have currently a 3 blades fixed propeller. My boat is an Ovni 345 so quite heavy. I tink about go for autoprop but a bit worried about the performance in heavy weather.

Does anybody have an opinion about this ? Is there an alternative you could recommand ?

Best reagrds

 

[Daydream believer]

I have currently a 3 blades fixed propeller. My boat is an Ovni 345 so quite heavy. I tink about go for autoprop but a bit worried about the performance in heavy weather.
Does anybody have an opinion about this ? Is there an alternative you could recommand ?

I suggest that you read the thread rather than ask forumites to re gurgitate what has already been said. There are several streams of opinion & plenty for you to add to your knowledge base. Most of what has been said is from either forumites experiences or from serious experiment. So unlike many posts it does have some relevance to your question based on fact rather than fiction. You might also speak with the manufacturers as they do actually know how their props perform. They- rather than dealers- can advise on the best size, pitch etc for your boat- provided you give them the right details.
As for alternatives, then a new thread might be better as it will certainly be one of "what I have got is best" plus a bit of ill informed opinion thrown in the mix. It will be up to you to find the answer that you want. I suppose one could liken it to a set of weather forcasts. You pick the one that you want to hear & sail to that regardless of the others forcasting a gale.
happy hunting

 

[Tranona]

Hi,

I have currently a 3 blades fixed propeller. My boat is an Ovni 345 so quite heavy. I tink about go for autoprop but a bit worried about the performance in heavy weather.

Does anybody have an opinion about this ? Is there an alternative you could recommand ?

Best reagrds

As suggested there is no clear simple answer as it depends on what you are looking for and the type of sailing you do.

Much of the discussion here was about the Autoprop, perhaps because it is so different from a fixed pitch prop. The principle behind it is that it repitches according to water speed so that the engine is always fully loaded, unlike a fixed pitch where the only time the engine is fully loaded is at close to maximum revs. This means that under engine revs are lower for a given speed and it avoids excessive light load running which often happens when motorsailing . You therefore find them very popular with people who do a lot of long distance running under motor or a combination of motor and sail - and for boats that are on the heavy end of the motorsailing spectrum. From a sailing point of view they offer a significant reduction in drag leading to increase in sailing speed, particularly in light airs, but not as much as a folding or feathering prop.

There may not be room for a folding prop on your boat, so the choice is a 3 bladed feathering prop of which the most common (in the UK at least) are Maxprop and Featherstream. both have the advantage that you can tailor pitch in both directions to match better to your boat and gearbox - for example the Featherstream I have on order will have a different pitch forward and reverse because of the different reduction ratios in the gearbox. Not necessarily relevant for all applications. As you can see from direct experiences above, both perform well under motor and have similar drag reduction. There are differences in design - the Featherstream has shaped blades instead of flat for example, but I guess its increasing popularity is down to its price advantage.

As to performance in heavy weather, in theory the Autoprop should do better, but some people find the change in RPM while it deals with changes in speed and load disconcerting.

Not surprisingly as I have just ordered one (my second) I would suggest a Featherstream, but before spending the money you need to be clear that you will value what it offers. Same or better performance under motor and improved speed under sail (clearly more of benefit if you undertake long passages under sail). Your prop runs in an aperture, similar to my boat and you may well find you can go up 1" in diameter which will improve motoring performance without incurring any extra drag penalty under sail.

Hope this helps.

 

[franksingleton]

Hi,

I have currently a 3 blades fixed propeller. My boat is an Ovni 345 so quite heavy. I tink about go for autoprop but a bit worried about the performance in heavy weather.

Does anybody have an opinion about this ? Is there an alternative you could recommand ?

Best reagrds

I cannot answer the question but have never heard of any problems in bad weather. My impression is that Brunton recognise that their props will not suit every boat. If your boat is one such, they will say so. Try asking them.

 

[fblc]

Thanks a lot.

My main concern was the performance in bad weather. If I need to use my engine in arsh condition I would be happy to be sure that I can trust in it.

 

[Daydream believer]

Then do not buy a Brunton

 

[Daydream believer]

. This means that under engine revs are lower for a given speed and it avoids excessive light load running which often happens when motorsailing . You therefore find them very popular with people who do a lot of long distance running under motor or a combination of motor and sail - and for boats that are on the heavy end of the motorsailing spectrum.

I have extracted a section of your prose for brevity. I do not think that doing this will alter the meaning . I do not have experience of the last line of your comment. However, I would take you to task on the rest of it. I do a lot of motor sailing & the engine always runs light. Bruntons acknowledge this & make that point that one can. I only have to run at a fast tickover to get significant increase in speed. There is no point motor sailing at upper throttle & hence high load. I generally run fuel as low as 1.3 l per hour when motor sailing against 2 when motoring & 2.3 in rough weather. That shows that the engine is not loaded when motor sailing. I have a relatively small AWB though. Not a heavy displacement lump . But it is only a small engine. I still would expect a larger ( say 40ft) medium displacement yacht to run light.

 

[Tranona]

You misunderstand load. Low revs does not mean low load if the pitch adjusts to require more power. To get 5.5 knots out of my GH requires around 12hp. My engine (Beta 30) can produce that at 1600rpm, but to turn my fixed pitch prop to get that speed engine revs need to be 2200 when it could produce 16hp. The principle with a variable pitch propeller is to adjust the pitch so that the engine runs at rpm closer to the actual power needed. When you are motor sailing part of the speed comes from the sails so the (fixed) prop will not demand the power to achieve the actual speed, but just the incremental speed and that will be far less than the power needed without the sails. So the engine is lightly loaded producing less power than it is capable of at the revs.

That is why the Bruntons is so popular with large engined heavy motorsailors like my mates Nauticat which spends a lot of time motorsailing, and less suitable for pure sailing boats like yours, which would be better off with a 2 blade folder.

 

[Daydream believer]

So the engine is lightly loaded producing less power than it is capable of at the revs.

Which, as I pointed out is the opposite of what you were saying earlier on.in #66 ie You said "and it avoids excessive light load running which often happens when motorsailing "

But I do agree with your other comment-- for other motoring, I probably would be better off with a folding prop. I would catch less weed & debris for a start

 

[Tranona]

Thanks a lot.

My main concern was the performance in bad weather. If I need to use my engine in arsh condition I would be happy to be sure that I can trust in it.

Maybe a bit more explanation is required about motoring in adverse conditions. The key to good motoring performance is (assuming you have enough engine power) prop diameter. blade area and shaft speed . So a large diameter, 3 bladed prop with a high blade area ratio turning slowly will perform best. Unfortunately on a sailing boat the ideal on these parameters is either difficult to achieve or has a negative effect on sailing performance particularly if the prop is running behind a skeg or a long keel in an aperture. Tip clearance may limit diameter, large blade area means more drag under sail and with some modern high speed engines shaft speed tends to be high with standard reduction boxes.

A feathering propeller deals with the drag problems under sail and provided you have the tip clearance and a low(er) shaft speed you can normally increase the diameter which does wonders for blade area. On a Featherstream the blade area increase going from 16" to 17" is close to 20%. The key then is to get the shaft speed right to swing the larger diameter. You are lucky with your Volvo as you have a shaft speed of around 1400 with the standard box rather than the 1800 that many other standard engine box combinations use. On my boat I specified a 2,6:1 reduction to get the shaft speed down below 1400 so I could have a 17" rather than the usual 15 or16" for that engine (a Beta 30). I suspect your Volvo has a 16" prop but I think you could change to a 17" with a Featherstream which would significantly improve motoring performance. Speak to Chris at Darglow and get his recommendations.

Personally I don't think an Autoprop is the right solution for your boat. Their benefits are more clearly shown on bigger heavier boats which spend more time motoring or motorsailing. While your boat is on the heavy side for its size and the standard engine is perhaps marginal, it is primarily a sailing boat and the challenge is to extract the maximum motoring performance out of the power you have, while minimising the impact on sailing performance through drag.