Propeller Drag - Challenging my intuition

[JordanH]

Originally Posted by flyingwelshman

Jordan, thanks for starting this thread, I obviously hadn't got around to it yet.

For what it's worth, your story the other day made me laugh out loud while I was sitting in a room by myself. I hope you took my comment above in good spirits as I intended.

[HDChopper]

First off I personally dont have a boat yet..

But being a professional mechanic and eng/trans builder , I think you all are missing a point .

If all things are equal then there COULD only be 1 right way to do a thing .

but not all transmissiona & clutch packs (if used) are the same let alone shift systems or if the trans in question has synchronized gears this would go a long way to explaining the different beliefs of this question ....

Personally if there is no valid reason for the trans & shaft to be locked then a neutral spinning prop would be the way to go IMHHO ..


OOpsss Just noticed the question IS only about prop drag .....never mind

[JordanH]

Originally Posted by HDChopper

OOpsss Just noticed the question IS only about prop drag .....never mind

I think we've firmly answered the question about drag so I don't mind if the conversation swings toward recommended options.

I have a Hurth transmission (HBW-50 2R). I don't know anything about it. Any reason for or against locking that in gear?

[flyingwelshman]

Originally Posted by JordanH

For what it's worth, your story the other day made me laugh out loud while I was sitting in a room by myself. I hope you took my comment above in good spirits as I intended.

No worries.

[paul323]

Well, it look like the consensus is that leaving a fixed-blade prop in neutral - free spinning - results in less drag. Towing a 5 gallon bucket is a scary image.....

Downside - it is noisy.

The one thing I am still stuck on is regarding wear. Logic tells me that a free spinning prop will wear out the cutlass bearings (for example) more quickly that a locked prop. Any thoughts on this - or is the incremental wear too small to consider? Although there was a comment about cooling not being active, I can't imagine a disengaged gear would cause much friction (and therefore heat).

[jsaronson]

Many transmissions are lubricated only when the engine is running. Leaving it in gear means you are running it dry. That's why tab mar says to leave a tranny with a fixed prop in neutral.

[paul323]

Ah, thanks. You learn something every day. I have a Hurth V-drive, so it the gears are bathed in transmission fluid the whole time.

[okawbow]

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.

[Wcoast sailor]

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.

[jfdubu]

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