penultimate
Well-Known Member
Can anyone advise a source/type of shaft position indicator so that a two bladed propellor can be parked vertically behind the deadwood?
Propellor position indicator
- Thread starter penultimate
- Start date
rivalranger
Well-Known Member
How about marking the shaft ? A bit of paint to mark where the blades are, so you can line them up with the deadwood.
pampas
Well-Known Member
/forums/images/graemlins/grin.gif Years ago in the PBO an article appeared, where a disc was fitted to the shaft having a detent cut,, a pin dropped into the detent and was kept locked out by engine oil pressure, when the engine shut down the pin dropped and the passage through the water rotated the prop until it locked home. this I think was a home made device,remember a lot of letters at the time saying "what happens when" never heard of any follow ups on this, todays systems it would be better to use a disc brake or a drum type arranged in such a fashion, that if the system fitted failed if would be released.
thalassa
Well-Known Member
I used to fit a spare spherical anode on the shaft, and lined up the gap between the halves. Also, it could function as a shaft brake by inserting the short end of a PVC L-profile in. Why PVC? It is strong enough to prevent the prop turning, but will break when you start the engine in an emergency. Ended up just letting the propeller turn.
The anode also prevents the shaft falling out, should the coupling give way.
The anode also prevents the shaft falling out, should the coupling give way.
penultimate
Well-Known Member
The trouble with your idea, and that of RivalRanger's, is that I can't see the shaft.
David_Jersey
Well-Known Member
Fixing a jubilee clip onto the shaft will stop it dropping out of the back of the boat if it becomes detached from the gearbox /forums/images/graemlins/shocked.gif. I guess no reason why you couldn't line the Jubilee Clip up with the Prop and DIY something that slots over the Jubilee clip to stop the shaft rotating?
David_Jersey
Well-Known Member
[ QUOTE ]
I can't see the shaft.
[/ QUOTE ]
That's a bit of a b#gger then /forums/images/graemlins/frown.gif
I can't see the shaft.
[/ QUOTE ]
That's a bit of a b#gger then /forums/images/graemlins/frown.gif
allangray3
Well-Known Member
I use a Magnetic Reed switch as used in burglar alarms, £ 1.20 from B&Q to a small Buzzer. Magnet drilled or glued to Shaft and The switch about 10-20mm away when prop in possition. Just lock gearbox when I hear the buzzer, toggle switch in circuit to activate and silence also comes in handy to check shaft rotation.
cliffordpope
Well-Known Member
Someone please explain the reasoning behind two-bladed propellors and alignment.
Surely the prop wants to turn when you are sailing? So why isn't it better to just let it turn? Doesn't braking it put more of a drag?
Also I read recently somewhere that latest research shows that the drag from a propellor isn't from the prop itself, but from the cut-away space it occupies. Even removing the prop still causes just as much drag, so its presence, or orientation, is irrelevant.
Surely the prop wants to turn when you are sailing? So why isn't it better to just let it turn? Doesn't braking it put more of a drag?
Also I read recently somewhere that latest research shows that the drag from a propellor isn't from the prop itself, but from the cut-away space it occupies. Even removing the prop still causes just as much drag, so its presence, or orientation, is irrelevant.
Phoenix of Hamble
Well-Known Member
An age old source of argument....
However, according to theory a locked prop causes less drag than a rotating one....
but it is a very contentious point....
however, if you really want to go into the gory details.....
http://www.goshen.edu/physics/PropellerDrag/thesis.htm
However, according to theory a locked prop causes less drag than a rotating one....
but it is a very contentious point....
however, if you really want to go into the gory details.....
http://www.goshen.edu/physics/PropellerDrag/thesis.htm
Danny
Well-Known Member
[ QUOTE ]
Surely the prop wants to turn when you are sailing? So why isn't it better to just let it turn? Doesn't braking it put more of a drag?
[/ QUOTE ]Ah! but consider the sycamore seed falling from the tree. It spins and falls slowly to the ground - implying a lot of resistance. If it didn't spin I reckon it would fall much quicker.
Surely the prop wants to turn when you are sailing? So why isn't it better to just let it turn? Doesn't braking it put more of a drag?
[/ QUOTE ]Ah! but consider the sycamore seed falling from the tree. It spins and falls slowly to the ground - implying a lot of resistance. If it didn't spin I reckon it would fall much quicker.
cliffordpope
Well-Known Member
"If it didn't spin" - that would either be because it wasn't shaped like a propellor, so didn't want to spin, or something external stopped it from spinning. Consider what happens when you stop rowing and just coast - is it better to hold the blades rigid in the water, or let them drift? Or imagine a series of oar blades, ie a paddle wheel. Lock the wheel, or let it rotate?
Phoenix of Hamble
Well-Known Member
But the oar isn't designed for good waterflow over its surface, in fact quite the reverse....
Energy is absorbed in turning the propellor, and this energy is greater than the drag caused (retarding energy) of a fixed prop....
Energy is absorbed in turning the propellor, and this energy is greater than the drag caused (retarding energy) of a fixed prop....
Danny
Well-Known Member
OK - here's an explanation that convinces me: (but then I'm not a physicist)
If a propeller had blades with no thickness and were perfectly shaped so they had no drag, and the propeller shaft was similarly absolutely free of friction then the prop would spin at exactly the right rate for the speed of the boat through the water and there would be no drag. No 'work' would be done so the system would consume no energy.
In reality the blades have thickness and drag and the shaft is not without friction. The result is that the spinning propeller has to absorb exactly enough energy to overcome the friction of the shaft and the drag of the blades at the speed that it spins (which is dependant on the boat speed, of course). The 'real' propeller spins slower than the imaginary friction and drag free one.
This slower speed of rotation means that the angle of incidence of the water flow over the blades increase from zero and therefore provides lift. This is the source of energy that is needed to rotate the propeller. Of course, this energy is subtracted from the kinetic energy of the boat - IE it slows the boat down.
Now the critical factor is whether the drag of the spinning blades and the friction of the rotating shaft is more or less than the drag of the blades when they are stalled. Who knows?
Maybe the answer is a folding prop.
If a propeller had blades with no thickness and were perfectly shaped so they had no drag, and the propeller shaft was similarly absolutely free of friction then the prop would spin at exactly the right rate for the speed of the boat through the water and there would be no drag. No 'work' would be done so the system would consume no energy.
In reality the blades have thickness and drag and the shaft is not without friction. The result is that the spinning propeller has to absorb exactly enough energy to overcome the friction of the shaft and the drag of the blades at the speed that it spins (which is dependant on the boat speed, of course). The 'real' propeller spins slower than the imaginary friction and drag free one.
This slower speed of rotation means that the angle of incidence of the water flow over the blades increase from zero and therefore provides lift. This is the source of energy that is needed to rotate the propeller. Of course, this energy is subtracted from the kinetic energy of the boat - IE it slows the boat down.
Now the critical factor is whether the drag of the spinning blades and the friction of the rotating shaft is more or less than the drag of the blades when they are stalled. Who knows?
Maybe the answer is a folding prop.
Phoenix of Hamble
Well-Known Member
And the paper that I linked too above answers that one in that the answers depends upon the size and pitch of the prop, and the boat speed... and that there is no right answer, becuase in some cases a rotating prop is better!
geoffatstanpit
Well-Known Member
On an aeroplane you don't have the option of knocking it out of gear. On a boat if you have an inboard engine with lots of friction - even when in neutral - it's still debatable. But with an outboard in a well there's very little friction to a free spinning prop and my tests say its MUCH better to let it spin.
rivalranger
Well-Known Member
One of the reasons for locking the prop is that some gearbox manufactures recommend it to prevent wear.
Phoenix of Hamble
Well-Known Member
Thats the main reason I lock mine.... well actually less to do with concerns over wear, and more to do with the annoying grumbling noise that it makes as it turns....
