NormanS
Well-Known Member
Yes, my palaver is the same. Worth it though, for the Autoprop.
Prop locked or not
- Thread starter Sailfree
- Start date
achwilan
Well-Known Member
You're right. The chopper analogy mentioned above is probably not appropriate as we are considering propellers (ie wings) with very different aspect ratio, and media of different viscosities.
oldharry
Well-Known Member
"I've just had to strip it down to replace the input shaft and flywheel carrier after they had worn down to almost nothing."
Sorry, I cant see that. If its not moving, how does it get worn? Applying steady pressure to a stationary shaft cannot damage it unless the torque is so high that it starts twisting it.
Sorry, I cant see that. If its not moving, how does it get worn? Applying steady pressure to a stationary shaft cannot damage it unless the torque is so high that it starts twisting it.
A
Anonymous
Guest
Surely the 'drag' is the 'pulling' force against motion? N or kgf. NOT torque, Nm. The drag of a boat increases according to its speed by known laws. This study looked at the drag exerted by a prop that was either locked or spinning -- BUT, rather than just measure the drag (the pulling force, in N or kgf, not the torque in Nm) with the prop locked and spinning, they put a brake on the shaft so they could get some in-between points 'out of curiosity'. They did lock the shaft and those are the upper curves with the free-spinning curves at the bottom. This shows that the drag (in N or kgf) was, in those tests, always greater with the shaft locked. The torque is a red-herring and is not really part of the answer.
graham
Well-Known Member
I have tried sail;ing with the prop locked in gear and freewheeling and it makes not one bit of difference to boat speed.
Probably beacause its an outboard lifted clear of the water. /forums/images/graemlins/grin.gif
Probably beacause its an outboard lifted clear of the water. /forums/images/graemlins/grin.gif
Robin
Well-Known Member
Well we have a brilliant Brunton so shut the engine off in gear ahead and it feathers, minimum drag, pefecto! So you lot can follow in our wake with alternate locked or unlocked props and graphs in your hands whilst we just vanish over the horizon.. [/smug]
A
Anonymous
Guest
Very nice, too, but they are not cheap and they are susceptible to damage if caught by rope.
Robin
Well-Known Member
Ours came with the boat (and a spare 2 bladed folder) and so far hasn't been damaged in nearly 20,000 mls so I can still use a [/smug]! /forums/images/graemlins/smile.gif
However the Ambassador Stripper had a bent blade this liftout we had to fix that showed it had been doing sterling service last season.
However the Ambassador Stripper had a bent blade this liftout we had to fix that showed it had been doing sterling service last season.
vyv_cox
Well-Known Member
<<<< Well we have a brilliant Brunton so shut the engine off in gear ahead and it feathers, >>
With my Yanmar 3GM30, stopping in ahead does nothing, the shaft continues to rotate. Shaft stops rotating with gear lever in reverse, but the lever locks in this position after a few seconds. Unlike a previous poster, if I halt the rotation by going into reverse but then move the lever back to neutral before it locks up, the Brunton and shaft will start to rotate again.
So I stop the engine in neutral, move the lever to reverse and leave it there. On restarting, just fire it up at low revs in reverse, upon which the lever is free to move into either neutral or forward.
With my Yanmar 3GM30, stopping in ahead does nothing, the shaft continues to rotate. Shaft stops rotating with gear lever in reverse, but the lever locks in this position after a few seconds. Unlike a previous poster, if I halt the rotation by going into reverse but then move the lever back to neutral before it locks up, the Brunton and shaft will start to rotate again.
So I stop the engine in neutral, move the lever to reverse and leave it there. On restarting, just fire it up at low revs in reverse, upon which the lever is free to move into either neutral or forward.
Robin
Well-Known Member
[ QUOTE ]
With my Yanmar 3GM30, stopping in ahead does nothing, the shaft continues to rotate.
[/ QUOTE ]
Our engine is also a Yanmar, but a 4JHE and ours works fine stopping it in gear. We usually drop the revs to a little over tickover with forward gear still engaged and then pull the stop with the boat probably still moving through the water at a reasonable speed. Unlike the Volvo on our last boat when we restart it doesn't stick in gear and need a blip of the starter to free it. I think Yanmar gearboxes are Hurth or a yanmar version of them?
With my Yanmar 3GM30, stopping in ahead does nothing, the shaft continues to rotate.
[/ QUOTE ]
Our engine is also a Yanmar, but a 4JHE and ours works fine stopping it in gear. We usually drop the revs to a little over tickover with forward gear still engaged and then pull the stop with the boat probably still moving through the water at a reasonable speed. Unlike the Volvo on our last boat when we restart it doesn't stick in gear and need a blip of the starter to free it. I think Yanmar gearboxes are Hurth or a yanmar version of them?
Refueler
Well-Known Member
[ QUOTE ]
Ours came with the boat (and a spare 2 bladed folder) and so far hasn't been damaged in nearly 20,000 mls so I can still use a [/smug]! /forums/images/graemlins/smile.gif
However the Ambassador Stripper had a bent blade this liftout we had to fix that showed it had been doing sterling service last season.
[/ QUOTE ]
Your lucky to have only a bent stripper blade ... mine - the standing part on shaft tube was ripped off, the 'fixed' cutter part twisted nearly 60 deg's from original line. What ever went through it certainly didn't mess about ! The noise of the two sections then rotating was 'orrid - and I had boat lifted to check out what was wrong ...
New one all fitted just before it failed ....
when lifted ...
removed 'fixed' part and left part to rotate with prop ... still seems to work ...
But this is what happened when it was removed some years prior ... picked up passing Folly Inn .... picture taken at Newport Town Quay :
Ours came with the boat (and a spare 2 bladed folder) and so far hasn't been damaged in nearly 20,000 mls so I can still use a [/smug]! /forums/images/graemlins/smile.gif
However the Ambassador Stripper had a bent blade this liftout we had to fix that showed it had been doing sterling service last season.
[/ QUOTE ]
Your lucky to have only a bent stripper blade ... mine - the standing part on shaft tube was ripped off, the 'fixed' cutter part twisted nearly 60 deg's from original line. What ever went through it certainly didn't mess about ! The noise of the two sections then rotating was 'orrid - and I had boat lifted to check out what was wrong ...
New one all fitted just before it failed ....
when lifted ...
removed 'fixed' part and left part to rotate with prop ... still seems to work ...
But this is what happened when it was removed some years prior ... picked up passing Folly Inn .... picture taken at Newport Town Quay :
lw395
Well-Known Member
[ QUOTE ]
Surely the 'drag' is the 'pulling' force against motion? N or kgf. NOT torque, Nm. The drag of a boat increases according to its speed by known laws. This study looked at the drag exerted by a prop that was either locked or spinning -- BUT, rather than just measure the drag (the pulling force, in N or kgf, not the torque in Nm) with the prop locked and spinning, they put a brake on the shaft so they could get some in-between points 'out of curiosity'. They did lock the shaft and those are the upper curves with the free-spinning curves at the bottom. This shows that the drag (in N or kgf) was, in those tests, always greater with the shaft locked. The torque is a red-herring and is not really part of the answer.
[/ QUOTE ]
The torque is not part of the answer, so much as part of the question. The torque (unless I am mistaken) is the rotational drag or braking applied to the prop, so is a key variable.
The study considers three values of this variable, the greatest of which I suspect is way too low for some real yacht set ups. I did not spot any justification for the range of torques used in the paper.
I don't have any direct observations of my own because I have always had folding props.
I can confirm for the low torque case freewheeling is lower drag, from observations of towing a tender with the outboard in the water. but then you can turn an outboard prop in neutral with your little finger. A 3gm gearbox, shaft seal and cutless bearing is a different beast.
I think that a windmilling folder slows the boat noticeably (well you can notice it on the b+G) compared to folded, but it may do this by causing loss of concentration at the moment you realise it isn't folded.
For the Fred Driftahs, a folding prop and a reluctance to motor is a better defence against ropes than any of your gizmo's, but I expect I will get my comeuppance oneday.
Surely the 'drag' is the 'pulling' force against motion? N or kgf. NOT torque, Nm. The drag of a boat increases according to its speed by known laws. This study looked at the drag exerted by a prop that was either locked or spinning -- BUT, rather than just measure the drag (the pulling force, in N or kgf, not the torque in Nm) with the prop locked and spinning, they put a brake on the shaft so they could get some in-between points 'out of curiosity'. They did lock the shaft and those are the upper curves with the free-spinning curves at the bottom. This shows that the drag (in N or kgf) was, in those tests, always greater with the shaft locked. The torque is a red-herring and is not really part of the answer.
[/ QUOTE ]
The torque is not part of the answer, so much as part of the question. The torque (unless I am mistaken) is the rotational drag or braking applied to the prop, so is a key variable.
The study considers three values of this variable, the greatest of which I suspect is way too low for some real yacht set ups. I did not spot any justification for the range of torques used in the paper.
I don't have any direct observations of my own because I have always had folding props.
I can confirm for the low torque case freewheeling is lower drag, from observations of towing a tender with the outboard in the water. but then you can turn an outboard prop in neutral with your little finger. A 3gm gearbox, shaft seal and cutless bearing is a different beast.
I think that a windmilling folder slows the boat noticeably (well you can notice it on the b+G) compared to folded, but it may do this by causing loss of concentration at the moment you realise it isn't folded.
For the Fred Driftahs, a folding prop and a reluctance to motor is a better defence against ropes than any of your gizmo's, but I expect I will get my comeuppance oneday.
vyv_cox
Well-Known Member
It always seems to me that prop fouling is the strongest argument there is for not allowing it to rotate. We must accept that when motoring there is a possibility that rope or other debris might get itself around the prop. Whether the shaft cutter will dispose of it is the subject of another thread!
However, when sailing with the prop rotating there is almost as much possibility that a rope can wind itself around it. It could be argued that a cutter would barely work under these circumstances, so the resulting tangle might be more serious. Locking the prop gives a much reduced likelihood that this will happen.
However, when sailing with the prop rotating there is almost as much possibility that a rope can wind itself around it. It could be argued that a cutter would barely work under these circumstances, so the resulting tangle might be more serious. Locking the prop gives a much reduced likelihood that this will happen.
A
Anonymous
Guest
Firstly, the report is very technical and about the only thing that most non-engineers can really use is the graph showing locked, free and the in-between cases.
A major problem is the units and terminology. Torque is the turning force on the shaft with the prop being driven by water flow and the shaft being braked by a belt. Torque does not affect the speed of the boat through the water. In essence all we really need to consider is shaft locked and shaft reasonably free (i.e. with a representative load on it to simulate the propshaft, bearings, and gearbox).
What we are really interested in is drag on the boat. It's what you'd measure if you took a line to a canal bank, put a spring balance in the line, and ran alongside the boat keeping the spring balance to, say, 10kg, 20kg,...or whatever. The drag is the force you need to keep that spring balance pointing to that particular kg. This is measured on the test rig - let's imagine that they are using a spring balance.
The question that interests us is what the spring balance reads when the prop is stopped and what it reads when it is allowed to rotate. The test figures show that - on the test propellers - the spring balance (i.e. the drag) was far lower when the prop was allowed to rotate. Commonsense tells us that the harder it is for the prop to rotate, the more nearly stalled it becomes so it more nearly represents a stopped prop - and the test results confirm that.
A major problem is the units and terminology. Torque is the turning force on the shaft with the prop being driven by water flow and the shaft being braked by a belt. Torque does not affect the speed of the boat through the water. In essence all we really need to consider is shaft locked and shaft reasonably free (i.e. with a representative load on it to simulate the propshaft, bearings, and gearbox).
What we are really interested in is drag on the boat. It's what you'd measure if you took a line to a canal bank, put a spring balance in the line, and ran alongside the boat keeping the spring balance to, say, 10kg, 20kg,...or whatever. The drag is the force you need to keep that spring balance pointing to that particular kg. This is measured on the test rig - let's imagine that they are using a spring balance.
The question that interests us is what the spring balance reads when the prop is stopped and what it reads when it is allowed to rotate. The test figures show that - on the test propellers - the spring balance (i.e. the drag) was far lower when the prop was allowed to rotate. Commonsense tells us that the harder it is for the prop to rotate, the more nearly stalled it becomes so it more nearly represents a stopped prop - and the test results confirm that.
LadyInBed
Well-Known Member
Re: Pic 1 - Looks to be quite a large gap between the fixed V and the Tongue.
Was that the gap specified?
PS sorry for drift
Was that the gap specified?
PS sorry for drift
lw395
Well-Known Member
"Commonsense tells us that the harder it is for the prop to rotate, the more nearly stalled it becomes so it more nearly represents a stopped prop - and the test results confirm that. "
That's the bit I take issue with, there is no proof that the drag vs torque curve is a steady climb from free to locked, in fact there are many parallels for some value of torque to produce a higher drag before stalling completely sets in and drag then drops to the locked case.
We simply don't know if the torque of the gearbox is within the range they have used or not. It grates a bit as poor science without a reality check.
However I think we might agree the drag is big and unnecessary, as there are folding and feathering props available in both the 20th and 21st centuries.
That's the bit I take issue with, there is no proof that the drag vs torque curve is a steady climb from free to locked, in fact there are many parallels for some value of torque to produce a higher drag before stalling completely sets in and drag then drops to the locked case.
We simply don't know if the torque of the gearbox is within the range they have used or not. It grates a bit as poor science without a reality check.
However I think we might agree the drag is big and unnecessary, as there are folding and feathering props available in both the 20th and 21st centuries.
DownWest
Well-Known Member
Not a personal reply,
Wings (props etc.) retain their lift until they stall. So a stalled prop will offer less resistence than a rotating one. QED for this question.
A
Wings (props etc.) retain their lift until they stall. So a stalled prop will offer less resistence than a rotating one. QED for this question.
A
Sailfree
Well-Known Member
Reply to everyone!!
Thanks for responses.
On latest boat initially had a fixed prop (Yanmar engine & gearbox) and I did not like the whining sound and potential wear when sailing and prop not locked. When engaging reverse the gearbox selection locked and neutral could not be engaged without starting engine with gearbox still in reverse. Enquiries with Yanmar resulted in dire warnings of gearbox damage if astern used to lock shaft when sailing. Interestingly the Jeanneau manual stated the opposite when sailing lock shart/propeller by engageing astern!
I opted for a feathering prop so can no longer do any practical trials on a 43 with the effect on sailing speed with prop locked or freewheeling!
A lot of interesting replies.
Most of us have clean bottoms and props for the start of the season. Any chance of a few people reporting on here the results of sailing in a steady wind and the effect on their boat speed of a rotating or locked prop (obviously fixed prop type!!)
It may well be that a more significant difference will occur on a bigger boat with a bigger engine but I am facinated to learn the truth after so many forum discussions on this point lead me to believe the contary to what this research now states.
Thanks for responses.
On latest boat initially had a fixed prop (Yanmar engine & gearbox) and I did not like the whining sound and potential wear when sailing and prop not locked. When engaging reverse the gearbox selection locked and neutral could not be engaged without starting engine with gearbox still in reverse. Enquiries with Yanmar resulted in dire warnings of gearbox damage if astern used to lock shaft when sailing. Interestingly the Jeanneau manual stated the opposite when sailing lock shart/propeller by engageing astern!
I opted for a feathering prop so can no longer do any practical trials on a 43 with the effect on sailing speed with prop locked or freewheeling!
A lot of interesting replies.
Most of us have clean bottoms and props for the start of the season. Any chance of a few people reporting on here the results of sailing in a steady wind and the effect on their boat speed of a rotating or locked prop (obviously fixed prop type!!)
It may well be that a more significant difference will occur on a bigger boat with a bigger engine but I am facinated to learn the truth after so many forum discussions on this point lead me to believe the contary to what this research now states.
ebbtide
N/A
[ QUOTE ]
It always seems to me that prop fouling is the strongest argument there is for not allowing it to rotate./quote]
Once got snagged on an unmarked, submerged crab pot while running thro the Looe Channel off Selsey with a strong ebb.
There we were, anchored by the stern, with this rope bar taut and impervious to any Stripper; had to launch dinghy and cut it free - took ages.
Never again. No debate about freewheeling props either!
It always seems to me that prop fouling is the strongest argument there is for not allowing it to rotate./quote]
Once got snagged on an unmarked, submerged crab pot while running thro the Looe Channel off Selsey with a strong ebb.
There we were, anchored by the stern, with this rope bar taut and impervious to any Stripper; had to launch dinghy and cut it free - took ages.
Never again. No debate about freewheeling props either!
Aeolus_IV
Well-Known Member
[ QUOTE ]
I have to lock the shaft to prevent the Brunton spinning it at much higher revs than the engine ever does. Evidently the Autoprop can not only repitch to maximize the conversion of a driven shaft's energy into forward thrust, but also repitch to maximise the conversion of forward motion into rotation of a freewheeling shaft. At hull speed, a high-pitched whining can be heard, and the rubber on the shaft seal was on one occasion too hot to touch.
[/ QUOTE ]
I have exactly the same issue with my AP and have to lock the prop with the gearbox - the drag off the AP spinning is huge. However the torque it is putting into the gearbox is not insignificant either. Last year, off Breskens, I thought something had gone seriously wrong with engine or gearbox. Couldn't get the gearbox into neutral, engine wouldn't start. In the end, we lifted the decompression levers and turned the engine crank by hand and gently eased the gearbox into neutral. Once there, everything worked fine. Worrying at the time.
So (back to topic), at least for me, a fixed prop is faster than a spinning one - which is no surprise really.
Regards,
Jeff.
I have to lock the shaft to prevent the Brunton spinning it at much higher revs than the engine ever does. Evidently the Autoprop can not only repitch to maximize the conversion of a driven shaft's energy into forward thrust, but also repitch to maximise the conversion of forward motion into rotation of a freewheeling shaft. At hull speed, a high-pitched whining can be heard, and the rubber on the shaft seal was on one occasion too hot to touch.
[/ QUOTE ]
I have exactly the same issue with my AP and have to lock the prop with the gearbox - the drag off the AP spinning is huge. However the torque it is putting into the gearbox is not insignificant either. Last year, off Breskens, I thought something had gone seriously wrong with engine or gearbox. Couldn't get the gearbox into neutral, engine wouldn't start. In the end, we lifted the decompression levers and turned the engine crank by hand and gently eased the gearbox into neutral. Once there, everything worked fine. Worrying at the time.
So (back to topic), at least for me, a fixed prop is faster than a spinning one - which is no surprise really.
Regards,
Jeff.