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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Propeller Drag - Challenging my intuition
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Thread: Propeller Drag - Challenging my intuition Reply to Thread
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Topic Review (Newest First)
09-15-2012 04:23 PM
msogin
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.
09-15-2012 02:00 PM
RocketScience
Re: spinning prop on Volvo penta

Quote:
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)
09-14-2012 11:44 PM
JordanH
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.
09-14-2012 11:43 PM
JordanH
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.
09-14-2012 11:00 PM
SloopJonB
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.
09-14-2012 10:46 PM
SimonV
Re: Propeller Drag - Challenging my intuition

swap your prop for a kiwi prop and this discussion is mute. Main I love your work.
09-14-2012 09:50 PM
ShoalFinder
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.



*This information is public.

https://www.oocea.com/CorporateInformation.aspx

Automated Propulsion Control for Optimum Thrust and Fuel Efficiency
09-14-2012 09:33 PM
jfdubu
Re: Propeller Drag - Challenging my intuition

Ok, What you are experiancing is exactly why on an apature boat, lock in line with the keel. Sailing slowly there is not enough energy in the water flow to spin the prop thru the gap. Once you are up to speed there is enough flow to break it out and spin it though. But, You are still taking energy from the sails to spin the prop that you wouldn't be using with the prop locked. A true test for you would be to let the prop align itself lock it and sail. Unlock it and let it spin, your speed would likely drop as you say. But, now lock it out of the gap and you speed would drop even more. It's all about energy conservation.

John
09-14-2012 09:17 PM
Wcoast sailor
Re: Propeller Drag - Challenging my intuition

The real question is what does the manufacture of your trans recommend. Mine says (Catalina 30) to put it in reverse which locks it so it won't turn. Neutral turns half the trans and it isn't lubed properly an in forward can damage it over time. I have forgotten a few times and so far no damage but I choose to follow the makers recommendation. I don't notice a difference in speed and I'm not racing. Hey it's a SAIL boat, if I wanted to get there fast, and not enjoy the ride as much, I'd have a power boat.
09-14-2012 09:14 PM
okawbow
Re: Propeller Drag - Challenging my intuition

I sail a Bristol 24 with a YSE8 diesel and a 2 blade prop. I put the transmission in neutral when sailing, but the prop rarely spins unless I'm doing hull speed. I have the propshaft marked so I can tell when the prop is vertical. The prop normally aligns itself vertically behind the keel at sailing speed less than 5 knots. However; when going fast enough that the prop spins, I have on several occasions, found the sailing speed to be less than it is when the prop is locked vertically behind the keel cut-out. I've seen from .1 to .5 knots difference immediately when the prop is stopped from spinning and positioned vertically. It's not much, but it's always slightly faster when stopped.

I can see where a boat with an exposed prop would have more drag when the prop is locked, rather than spinning, but for a full keel boat with a 2 blade prop, I don't see an advantage for letting the prop spin.
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