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Old 04-05-2006, 19:05   #16
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I am a firm believer in a folding prop which eliminates the need for unnecessary knowledge. In gear, out of gear, whatever, the speed of the vessel and resulting water pressure folds the blades and hence ...............
Done deal.
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Old 04-05-2006, 20:32   #17
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Yes Sean, it's most important to know what the maker has to say. Some "gear" boxes are asked for a specific gear direction for oil flow purposes. It ensures the flow is being sent into small oil galleries to buricate bearings correctly. Hydraulic boxes are a little different again. Some can't be spun without the engine running, as there is nothing flowing between friction plates. Some can, because the friction plates don't get forced together when the engine isn't turning. And so on and so on. So it really does depend.
Some boxes allow you to free wheel for a certain number of hrs before they need to be engine run. Some can free wheel with no regard to engine at all.
As to the load from the propellor, it's a difficult one. It depends on the design. So props will stall when not turning and will "drag" through the water with little effort. Some, the drag will be too great and free wheeling is best (if it can safely be done). I have been doing a bit of work on the free wheeling of the prop part. Sometime back, several of us talked about this issue here and I don't think we came to a rock solid answer. Well here is what I found.
When a prop is free wheeling, it actually has significantly more "bite" in the water. It is acting much like a helicopter blade does in auto-gyration. this is because, as the prop is spinning, it is also cutting the water in a hydrodynamic way. Thus it has a greater "grab" in the water. I am purposely calling this grab and not Drag at present. It is possible the more correct term is Drag, but I am trying to show this as a different "result" of pull. If the shaft/box can spin freely, then there is little drag, as the energy that would normally be considered a drag, is turned into rotation. A certain amount of loss is experianced. How much is dependant on how much force it takes to spin the shaft/box. So a spinning prop is possible better than a locked shaft as far as Drag is concerned. But it also depends on the box as far as free wheeling safely is concerned.

As for folding props, that is a difficult one when you get to boats with Hp as large as mine. A standard folding prop is not particuly efficient, especially in reverse. The geared props are a little better, but it depends on makes as to how much better. The feathering props are possibly some of the best around, but very expensive and complex. I dunno, I get along just fine with my big three blader and Velvet drive box free wheeling.
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Old 04-05-2006, 20:48   #18
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Just have to remember to put the gear back into neutral before you start the motor again. It is real easy to forget and that can't be too good for things!
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Old 04-05-2006, 21:41   #19
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Hey Darryl, how's the re-rigging coming along??
Actually, it isn't a problem starting while in gear. The engine starts off slow. It's not the same as changing from forward to reverse etc, while the engine is reving. That can cause considerable damage.
Now, I remember someone making a comment about that the other day. Can't remember where, but someone went from forward to reverse with engine in full revs and the result was twisting the shaft. I am suprised that is all he did.
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Old 04-05-2006, 22:21   #20
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Folding

If you go out on a windy day and are motoring into the wind as you raise the main, with a boat speed of 4 to 5 knots, and then you bear off as you shut off the motor after bringing the shifter back to neutral. A folding prop will continue to spin, you have to either slow the boat to a near stop, or put the gearbox in reverse. Just because it folds does not mean it will not spin. It only happens when the speed transfer is a little quicker and maintained after the switch from engine to power. Before you tell me there is something funny with my gearbox, the same thing happened a few weeks ago with a sail drive. No question that reverse may not be as good, but if you enter your moorage at about two knots and give the engine enough revs the boat will stop, even with a strong wind pushing you. My boat has always stopped for the last 26 years, always with a folding prop. I often back out of my moorage about 100 meters if the wind prevents me from turning the boat into the wind.
If the shifter needs to be held in place I am sure a short piece of rope could do that. If the propshaft does need to be locked a pair of vice grips on the shaft with something to protect the shaft should do.
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Old 05-05-2006, 01:54   #21
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Interesting comment Mike. Just so as I am clear, if the blades are closed and the shaft has been spinning, it will continue to spin as long as you maintain a good boat speed? Have I understood that right?
If that is the case, it just goes to show there is still a considerable amount of drag with a folded folding prop that is being turned into a turning force. Obviousely no where near that of the prop opened out of course.
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Old 05-05-2006, 04:52   #22
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Hi Alan,
mast is out so we just have a launch sitting at Waikawa now. I hope Jeff can get it back in soon as I hate having an empty berth at Mana and paying for another at Waikawa. Came down last weekend and it rained all weekend. Spent the two days reading on the Marina.
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Old 05-05-2006, 08:56   #23
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Folding

Wrong impression from my imprecise writing." Just because it folds " should read. Just because it is a folding prop it may not close, it may stay open and continue to spin until the boat is slowed, or the gearbox is put in reverse. Once the prop has folded I do not think it will ever start to spin even if in neutral. On the Laser 28 this happened the first time out and noone on the boat had experienced it before. The captain thought he had a vibration in the rudder. I told them the prop was turning and we clunked it in gear and fixed the problem, now reverse is always used. On my boat I can lock the shaft where I want it.
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Old 05-05-2006, 12:11   #24
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The Volvo user manual for my saildrives - attached to MD2040s - specifically says leave them in neutral.

On the question of drag, whatever the theory says, the following is what happens with my two large, fixed, 3 bladers...

Sailing at 7 knots with props rotating... put them in gear... you sail at 6.5 knots... put them back in neutral... 7 knots

Works every time (just wish they weren't so damn noisy when they rotate at 10 knots)
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Old 05-05-2006, 14:14   #25
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my new volvo 40hp 2040 manual says to put in reverse.
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Old 05-05-2006, 14:18   #26
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Darryl ,it's been a lovely week weather wise. When you pop over again, give me a yell and me and wife could catch up with you.
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Old 05-05-2006, 17:19   #27
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In Gear or Freewheel

I was once on a boat (not mine so I don't know specifics), but engine could not be started in gear, and gearbox was difficult to shift if engine not running. We nearly were blown hard up on a wharf because engine had been shut down in gear, before all lines were secured -(a fundamental error I now know, we all gotta learn some how); we could not start engine in gear, and we could not get gearbox to neutral to start. Almost like a Road Range truck gearbox where engine revs are used to shift gears.

Just thought I would throw that into the mix for comment.

Fair winds

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Old 05-05-2006, 22:54   #28
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Starting in gear

That is the kind of thing that needs to be considered and or repaired at the dock. My motor will start in gear just like it does in neutral, and it also shifts easy with the engine not running. But that is my motor, others may have it set up differently. My manual also says to put it in reverse.
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Old 05-05-2006, 23:05   #29
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Paragon trans

I have to lock mine in forward, it won't lock in reverse. We have a small 2 blade prop, I don't notice a speed chnge, but I can't stand the noise it makes when freewheeling.
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Old 06-05-2006, 01:08   #30
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Lock in reverse as per Hurth book..

Don't think the prop spins in neutral, never heard any noise and been too lazy to go down and check.

Slow boat and big drag regardless.

Been considering a folding/feathering prop, but $3K is a bit much to pay for 1/2 knot.

The fixed pitch prop is probably more efficient anyway so there is some fuel savings by staying with the old 3 wide-blade 16X10 bronze prop.
Had it fine tuned and 'puter measured last year. No vibrate, no maintenance and $3K towards new sails......
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