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Old 02-07-2008, 13:42   #31
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Putting air aside, there is very little airfoil shape to a boat propeller. The boat propeller creates thrust primarily due to Newton.
I can assure you that a boat Prop is a rotary wing and acts within a fluid with exactly the same principles.
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Old 02-07-2008, 14:25   #32
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Yanmar states to put your transmissions in neutral when sailing.
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Old 02-07-2008, 17:23   #33
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OK, can one of you two blokes please tell me the acronym definition of "LAME" in these posts

I did a search & ummm, "LAME"[1] & "pilot" don't give any good result examples

[1] in-fact, LAME on its own doesn't give a good result example
Licensed Aircraft Maintenance Engineer - Roughly equivalent to the US Airframe & Powerplant Mechanic.

It is the British term for licensed airplane mechanic.
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Old 02-07-2008, 17:28   #34
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That is the pill I can't swallow (yet) - but hey, I am no rocket scientist and would be happy if someone can point out any errors of my logic here.
Aside from parasitic hydrodynamic drag and vortices which I tried to explain, we also have to consider the installation drag which included the packing gland, cutlass bearing and the energy required to turn the transmission should it remain connected to the prop shaft.

Put it another way - if your boat is on the hard and it takes you energy to turn your prop then the water has to impart that same energy when the boat is moving through it.
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Old 02-07-2008, 17:30   #35
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I can assure you that a boat Prop is a rotary wing and acts within a fluid with exactly the same principles.
No disrespect Wheels and while they share the same discipline of physics - fluid dynamics, air and water are two separate mediums and water is incompressible.

So the details have to be different.
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Old 02-07-2008, 18:24   #36
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Yanmar states to put your transmissions in neutral when sailing.
What does the transmissions manufacturer state?

or is that Yanmar as well?

Dave
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Old 02-07-2008, 23:46   #37
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Thanks Wotname (& Ex-Calif) for the explanation. This is an interesting thread to read indeed. I'm keen on the sabb hvp 25 (cpp unit), so I assume that prop drag shouldn't be too much of an issue for me.
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Old 02-07-2008, 23:50   #38
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Put it another way - if your boat is on the hard and it takes you energy to turn your prop then the water has to impart that same energy when the boat is moving through it.
Absolutely.

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So the details have to be different.
I wasn't saying they are the same. I am trying to give a simple visual explanation (as I stated at the time) so as anybody could understand.
However, fluid dynamics work very similarly to aerodynamics. Hence a Wing on a plane uses the Same NAOO numbers as a rudder for a boat, which is also a wing. Just the actual numbers used are different for different speeds, viscosity and lift performance.I I will repeat for the final time, a boat propeller is a rotary wing. It "sucks" itself forward through the water. The shape of the blade is curved. If you look carefully, you will see that the front face of the blade is shaped ever so slightly different to the back side of th blade. I said earlier, a boat props forward motion is about 55% from the front of the blade and about 45% from the rear as thrust. More energy is developed from the front of the blade. Of course there are many different blade shapes out there, so those figures maybe slightly more of less for different makes of propellers.
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Old 03-07-2008, 00:02   #39
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Thanks for all the input. I've always leaned toward locking the prop in reverse as I don't like the noise generated when freewheeling, some a lot louder than others. I sail different boats so don't always have the manufacturers direction and some of the charter companies I've gone with aren't specific. Good sailing all.
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Old 04-07-2008, 03:36   #40
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Originally Posted by Ex-Calif View Post
....
Put it another way - if your boat is on the hard and it takes you energy to turn your prop then the water has to impart that same energy when the boat is moving through it.
Ex,
I understand the above point and I guess I have always figured that then stopping it must take even more force but you have got me thinking differently about the whole issue now. Maybe the force required to prevent it turning is less than the force required to make it turn (especially once it begins to rotate at speed). My head is going round and round on this one (pun not intended) and maybe one day the answer will be apparent.
Thanks for your input (and others too).
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Old 04-07-2008, 03:43   #41
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The big effect - IMO - is going to be the installation drag caused bay turning the shaft and running gear, if the transmission remains engaged.

All things equal it has to take more energy to turn a gear train than not.
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Old 04-07-2008, 07:24   #42
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Originally Posted by Ex-Calif View Post
The big effect - IMO - is going to be the installation drag caused bay turning the shaft and running gear, if the transmission remains engaged.

All things equal it has to take more energy to turn a gear train than not.
OK, I am more confused. How is the shaft and running gear going to turn if the transmission is engaged? By engaged, I am assuming you are meaning the shift lever is in ahead or astern rather than netural.

I see your point about taking more energy to turn than not to turn so looks like I have been thinking wrongly about this for some years (oh well, not the first time and probably not the last - )
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Old 04-07-2008, 08:28   #43
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Feathering props

Slightly OT, but really just a segway:

For those of us with Max-Prop feathering propellers, here's their recommendation:

PROPELLER USE The Max-prop works automatically. By putting the engine in gear the blades will engage in either forward or reverse. The best way to feather the propeller is:

Power at 2 to 3 knots in forward.
Kill the engine while still engaged in forward.
When the engine has stopped, if the shaft is still spinning engage the transmission in reverse to stop the freewheeling.

You can check to see if the propeller is feathered or not by taking the engine out of gear. If the propeller is not feathered the shaft will freewheel like with a fixed blade propeller.

In that case start the engine again and repeat the three steps. If your propeller has been greased properly it will feather in a fraction of a second as soon as you stop the shaft from freewheeling. Once the prop is feathered, you can either leave the transmission in gear or out of gear, it does not matter. DO NOT kill the engine while in reverse. In this case the blades will be in the reverse position and will not feather. You can actually use this feature to drive a shaft alternator.


Their manuals are available online.

Note that a hydraulic transmission like our Velvet Drive will free-wheel whether it's in gear or not - but it's designed to handle it. Because it doesn't lock the shaft, their advice to engage the tranny in reverse to try to help feather the blades doesn't work...
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Old 04-07-2008, 09:17   #44
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Ex,
I understand the above point and I guess I have always figured that then stopping it must take even more force but you have got me thinking differently about the whole issue now. Maybe the force required to prevent it turning is less than the force required to make it turn (especially once it begins to rotate at speed). My head is going round and round on this one (pun not intended) and maybe one day the answer will be apparent.
Thanks for your input (and others too).
There is one way to get the bottom of it:

Experiment!

While sailing at 7 knots, I shoved my transmission into reverse thanks to this thread. I immediately lost 2 knots of speed (compared to freewheeling) and think I nearly lost the tranny too! ha ha

(don't actually try this at home!)

But, if you can sail in the same conditions, on the same point of sail with sails trimmed the same, try one run in neutral and one with the prop locked.

After reading this thread again I think it has lot more to do with the individual transmission, prop and boat configuration.

On a 45' Gulfstar Hirsh with a large 3-blade prop, standard shaft and gland, as well as Hurth tranny, you indeed lose 2 knots at 7 knots boat speed by locking the prop over letting it freewheel. Can't argue that data, since it actually happened and isn't a bit of theory.

Try it on your boat and see, then (taking the tranny manufacturer's advice into account), do what is most efficient and you feel safest with.

With the Yanmar SD-20 saildrive on my new boat, I keep it freewheeling, since the units are a little more complicated (and scary to replace) than the standard transmission and packing gland setup. I freewheel them only because Yanmar says it's the right thing to do in the manual. I haven't tried both ways for efficiency because I'm too scared to break a saildrive.
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Old 04-07-2008, 17:19   #45
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I have an open mind on this and never really seen any authorative collection of knowledge that proves it one way or another. Seems to depend a bit on where one looks. But as has been said, as far as aviation is concerned, is well proven in air that freewheeling props of the aviation type have more drag than those not freewheeling. As said by others it possibly depends on the boat, prop and drive - especially when you see so many sail boats with big elliptical bladed "power boat" type props.

But it is not something that I have concerned myself over much in respect to our own boat. We have a 3 blade 18" diameter high aspect ratio prop (on shaft, not saildrive) so the blades are no bigger than a man's hand with an inch or two of wrist attached. The worst I can imaging is that the drag, when locked, is something of the order of being similar to as if 3 men on board took to dragging one hand each through the water as we sailed along. Not something I would panic about on a cruising boat from a performance point of view - we have no rule on board about not dragging hands in the water .

What has impressed me is just how hard it is to turn our prop by hand and if it is freewheeled it sure absorbs enough energy to get a fair speed up so quite a lot of work being done. Note, this is for a shaft drive.

While I have on a few short occasions tried watching the log comparing with the prop freewheeling and when it is locked, any advantage one way or another was lost in the "noise" of speed fluctuations from waves, wind velocity changes, etc so I quickly lost interest. Might be different if just ghosting along in little wind and flat sea and tried the same test, but that is when you use the engine anyway; Is it not? .

Which leads to we lock it with the gearbox just because of the noise if it freewheels (probably worse on a metal boat especially as we have hard bearings for lifetime reasons on the prop shaft) even though the engine/box manual says to sail with the box in neutral.

I checked this out and was informally told is really no problem locking our box (is a cone clutch type box) manual really just cos people don't know how to sort out the box jamming in reverse on the clutch cones. (Note my comment on that applies ONLY to my gearbox and to me, and I ain't telling you what that is in case yours goes wrong and I get the blame - ask your own gearbox's manufacturer or representatives whose advice should be followed in my view).

So there you go, all those words and no real answer from me . Think it must get down to try it for yourself on yer own boat and work from that. And if wanting to lock the prop with the gearbox, follow the manual's advice or that of the manufacturer.

EPILOGUE:

Err, actually there is in my opinion a real answer, if one has reason to need to be concerned about prop drag, it would seem sensible to fit one that feathers, folds or otherwise makes itself low draggy when sailing rather than losing sleep over the lock or don't lock debate. That at much higher capital and maintenance costs of course and that might give rise to an alternative reason for lost sleep .
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