maxprop v autoprop

Chae_73 said:
I'm interested in what would cause this, as it would seem to be a significant flaw in the workings of a feathering prop. Do the engine revs drop significantly when you hit a wave? I don't recall this happening with a fixed prop (although obviously the boat does slow down, which is inevitable).
Click to expand...
It's a function of the Brunton prop design which gets loaded when the free stream water speed reduces. The prop design balances centripetal twisting moment which is a function of rpm against hydrodynamic twisting moment which is a function of pressure. thus theoretically at a fixed RPM as the boat accelerates the prop pitch coarsens to provide constant thrust and vice versa.
It's not something anyone managed to do successfully with aircraft propellors so I'm not surprised it's not great on a boat where free stream velocity varies rapidly in rough conditions.
But the theory is sound.
 
Buck Turgidson said:
thus theoretically at a fixed RPM as the boat accelerates the prop pitch coarsens to provide constant thrust and vice versa.
Click to expand...

Thank you for the detailed explanation.

If I'm understanding this right, that should actually increase the effectiveness of the prop in providing propulsion as the boat is slowed by hitting water, compared to a fixed prop?
 
Chae_73 said:
I'm interested in what would cause this, as it would seem to be a significant flaw in the workings of a feathering prop. Do the engine revs drop significantly when you hit a wave? I don't recall this happening with a fixed prop (although obviously the boat does slow down, which is inevitable).
Click to expand...
The revs do not drop . The prop just feathers the pitch because the forward speed drops. As the speed increases the prop pitches up again but not quickly enough to give enough forward motion to push the boat fast enough through the next wave. I am told from the designers that HP from an MD2020 at the prop might only be about 8HP, so it is not enough to get the weight of the boat moving
 
Last edited:
Chae_73 said:
Thank you for the detailed explanation.

If I'm understanding this right, that should actually increase the effectiveness of the prop in providing propulsion as the boat is slowed by hitting water, compared to a fixed prop?
Click to expand...

well, if the Diesel engine wasn't governed to run at a fixed speed but rather a fixed torque it would be great as the rpm would increase initially and the prop pitch would follow but at a fixed rpm the prop goes to fine pitch so there is a further lag before the speed increases and the prop coarsens again.
 
Chae_73 said:
Thanks. I'm interested as contemplating changing to a folding or feathering prop from a fixed two blade.

As per the discussion above, if they are less effective in any kind of seaway, I can't imagine why anyone would want one?
Click to expand...
only the Brunton does this. The other feathering props go to a fixed forward or reverse pitch and stay there. They are designed so that the centripetal twisting moment is the dominant force. edit: or both twisting moments act in the same direction. (It's been a long time since I had to talk about propellors professionally so it's all a bit dusty)
 
Last edited:
I had mine because I knew no better. However, as a motor sailing prop it is absolutely brilliant. If one had a fixed prop & was sailing at , say, 3 kts then to get extra drive from a fixed prop one would have to motor at something like 4 kts. With the Brunton one can run at 1200-1500 RPM & use 1.3 litres per hour ( perhaps a bit less) & get a kt & a half extra drive easily. Because it pitches up & drives . So in light winds I oten get a push from the engine if I need to meet a target
 
Last edited:
Buck Turgidson said:
well, if the Diesel engine wasn't governed to run at a fixed speed but rather a fixed torque it would be great as the rpm would increase initially and the prop pitch would follow but at a fixed rpm the prop goes to fine pitch so there is a further lag before the speed increases and the prop coarsens again.
Click to expand...
I tried playing the throttle ( suggested by Brunton)but it makes little difference. The whole thing is spoiled by the lack of acceleration. That is why I refer to "Chop". This being where waves might only be 1-1,5 M high but close together. Obviously it is worse in bigger seas, but starts in relatively small seas if close together
 
Daydream believer said:
I tried playing the throttle ( suggested by Brunton)but it makes little difference. The whole thing is spoiled by the lack of acceleration. That is why I refer to "Chop". This being where waves might only be 1-1,5 M high but close together. Obviously it is worse in bigger seas, but starts in relatively small seas if close together
Click to expand...
Nice to know they understand the limitations of their design :-)
 
boatmike said:
Having just acquired a rather nice Moody 35cc I am interested to change the fixed 2 blade prop for something a bit more efficient. I have previously had a Brunton autoprop which I found did what it said on the tin but was disappointed that the miniscule anode on the nose didn't last very long. Looking at the maxprop however it doesn't seem much better, in fact very similar. however my previous experience was saildrive related and the Moody is of course a shaft drive so supplementary shaft anodes are possible which may improve things. In addition the Brunton sales pitch seems to indicate annual out of the water maintenance to check bearings etc and the maxprop not so much. I am thinking the Brunton will be more efficient due to finding the "right" pitch for every sea state etc but of course more expensive......... Hmmmmm!! Anyone have any experience with either on a Moody 35 or 346 or similar? If so what are your thoughts???
Click to expand...
Have sent you a PM.
 
Chae_73 said:
Thanks. I'm interested as contemplating changing to a folding or feathering prop from a fixed two blade.

As per the discussion above, if they are less effective in any kind of seaway, I can't imagine why anyone would want one?
Click to expand...
You might be interested to know that if you are changing from a fixed 2 blade there are no appreciable sailing speed gains to be had when switching to a feathering 3 blade.
While there are good reasons for choosing a 3 blade, feathering or fixed, to improve motoring performance, only a folder will offer appreciably less resistance over a fixed 2 blade.
I remember reading a prop test, quite a while back, and quite possibly in French or German, I can't remember, that also measured resistance for various props when folded or feathered; the values for the Autoprop were considerably higher than for the other types.
 
Laminar Flow said:
You might be interested to know that if you are changing from a fixed 2 blade there are no appreciable sailing speed gains to be had when switching to a feathering 3 blade.
While there are good reasons for choosing a 3 blade, feathering or fixed, to improve motoring performance, only a folder will offer appreciably less resistance over a fixed 2 blade.
I remember reading a prop test, quite a while back, and quite possibly in French or German, I can't remember, that also measured resistance for various props when folded or feathered; the values for the Autoprop were considerably higher than for the other types.
Click to expand...

I’m surprised. I can see that this will be the case for a long keeled boat with the propeller in an aperture, where it can line up vertically, but I am very surprised to learn that this is still the case for a propeller sitting behind a P bracket.
 
Laminar Flow said:
You might be interested to know that if you are changing from a fixed 2 blade there are no appreciable sailing speed gains to be had when switching to a feathering 3 blade.
While there are good reasons for choosing a 3 blade, feathering or fixed, to improve motoring performance, only a folder will offer appreciably less resistance over a fixed 2 blade.
I remember reading a prop test, quite a while back, and quite possibly in French or German, I can't remember, that also measured resistance for various props when folded or feathered; the values for the Autoprop were considerably higher than for the other types.
Click to expand...
My saildrive mounted 2 blade Brunton allows me to sail up to circa .6 kts faster than I did with my fixed 2 bladed volvo prop. Of course it depends on conditions being favourable , because one cannot always sail at max averages . However, a lot of my sailing trips average 6kts as can be seen from my 2 round Uk trips - 5.9 & 6.0 kts average overall. In favourable conditions, I can better this on a 60 mile plus trip. So .6 kts is welcome, as 10 hours can easily equate to 4-5 miles or more saving. Fairly handy on a 31 ft yacht.
But that is 2 blade comparison- not 3 blade. Would one expect the difference to be comparable? I do not know.
The yachting Monthly test reported that the drag on a feathering prop was too small to measure & even less for a folding. However they put the Brunton highest at 80% less than the equivalent fixed blade.But this still shows an apreciable reduction. If correct that kind of debunks your comments about drag
YM test
 
Last edited:
This is a two blade fixed prop on a p bracket, fin and spade rudder hull.

The noise of the prop shaft spinning is quite intrusive (probably need to change a bearing) so we have been sailing with it in gear to get some peace.
 
Chae_73 said:
This is a two blade fixed prop on a p bracket, fin and spade rudder hull.

The noise of the prop shaft spinning is quite intrusive (probably need to change a bearing) so we have been sailing with it in gear to get some peace.
Click to expand...

The question of whether to sail with a fixed propeller spinning or locked used up miles of space in the correspondence columns in my youth but I recall that either Des Sleightholme or Denny Desoutter determined to resolve the question, on a scientific basis, with trials, and did so, and if I recall correctly there is less drag with the propeller locked.

So you are doing the right thing anyway.
 
Kukri said:
The question of whether to sail with a fixed propeller spinning or locked used up miles of space in the correspondence columns in my youth but I recall that either Des Sleightholme or Denny Desoutter determined to resolve the question, on a scientific basis, with trials, and did so, and if I recall correctly there is less drag with the propeller locked.

So you are doing the right thing anyway.
Click to expand...

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?
 
Kukri said:
Oh what fun! We can start the controversy all over again!

Pilots seem to think that a fixed pitch prop uses less energy if it’s not windmilling…

Does a windmilling propeller create more drag than a stopped propeller in an engine out scenario?
Click to expand...
The force rotating the propellor is less than the force the fixed propellor would impart on the water flow. That's why it spins. It's about 50% less drag when spinning. Even our very own correspondents showed this in their experiment.


And as a product of reducing drag you are also operating your engine as the manufacturer recommends.
 
I could bump start the MD2 in my gaff cutter. We needed to be reaching fast, though. Lift the decompressors, take her briefly out of gear to get the three blade prop turning, back into gear, the engine would gradually approach starting speed, open the throttle, drop a decompressor and she would start.

This, like reefing, and setting and handing the topsail, was best done on starboard tack, as the quarter mounted propeller was to port, so deeper in the water, and we had right of way while the skipper was distracted.

It certainly beat hand starting it.
 
Last edited:
You must log in or register to reply here.

Share this page

Top