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  #41  
Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

Modern Navy ships (gas turbine) with dual shafts do this all the time. During normal cruising, either the port or starboard shaft is being driven by a single engine (each shaft is coupled to two engines). The offline shaft spins freely with no engines on line. This plant alignment is called "Trail Shaft" and it is very efficient.

Obviously there are differences here, but the point is that trailing a propeller still uses less fuel than having two shafts online.

How fast the trailing screw turns will be a result of the vessel's speed, obviously. But the Navy does not lock the shaft. As the trailing screw turns, drag is reduced. If you aren't going fast enough to turn the trailing screw then drag really isn't a concern in the first place.


Now, whether or not you should lock your shaft is a question of how your gearbox is made. Your manufacturer can answer that question. But as to drag, the trailing screw free wheeling is better than being locked in place.



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  #42  
Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

swap your prop for a kiwi prop and this discussion is mute. Main I love your work.
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  #43  
Old 09-14-2012
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Re: Propeller Drag - Challenging my intuition

Quote:
Originally Posted by JordanH View Post
Of course the argument is that I have a slim chance of having that blade lock vertically.
Next time your out of the water, rotate your prop to vertical, go on board and mark your shaft or coupling and then you have it for future reference.
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Re: Propeller Drag - Challenging my intuition

Quote:
Originally Posted by SloopJonB View Post
Next time your out of the water, rotate your prop to vertical, go on board and mark your shaft or coupling and then you have it for future reference.
Yup, that's the plan as per post #27.
Will need to couple it with the idea above for an inspection hatch since I can't actually see it without ripping out the cockpit floor.
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Re: Propeller Drag - Challenging my intuition

Quote:
Originally Posted by SimonV View Post
swap your prop for a kiwi prop and this discussion is mute. Main I love your work.
Hey Simon, I had to look that one up. Another great idea.

I'll have to see if that is even usable with my boat. The prop must spin through a cut-out between the keel and rudder so often other prop types aren't feasible.
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Old 09-15-2012
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Re: spinning prop on Volvo penta

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Originally Posted by Denhan View Post
I have recently read in the volva penta manual that with a reversing transmission the propeller should not be left to spin without the engine running. The transmission is similar to a car auto trans and has a oil cooler that operates the same as the engine heat exchanger. With engine not running no cooling and transmission overheats and damage occurs. Now to find a shaft brake.
This may depend on which "manual" you're talking about.

If you're talking about the 28 +- horsepower Volvo engines of recent history (popular sized engines in the 32'-36' boat range), models MD11, 2003, and the later model D1-30 (which I have), all say that the control lever should/shall be in neutral while under sail:

MD11 Owner's Manual (Pg. 19)
2003 Owner's Manual (Pg. 6)
D1-30 Owner's Manual (Pg. 7)
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Old 09-15-2012
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Yanmar and SailDrive

We ran into a weird situation last week with a new Yanmar and Saildrive on a new 42 foot sailboat. WHile sailing the maxprop continued to rotate even after placing the transmission in reverse or in forward gear. Turned out the zinc was lose and that might have inhibited the blades from properly feathering. More importantly I was surprised that placing the shifter into reverse did not stop the rotation. Upon communicating with the commissioning yard, they decided the problem related to the shifter not being able to change the state of the transmission unless the electronics to the engine were activated. They told me there is an electronic link between the shifter and the transmission and the only way to place the transmission in gear was to either leave it in reverse when I turned off the engine or to nearly simultaneously move the shifter as we shut down the engine. I do not know if this explanation is accurate and until the prop fouls again, I won't be able to test this hypothesis.
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