Do you sail with your transmission in gear?

[markpierce]

We always locked the propeller shaft in gear with the propeller vertical to align with the immediately-in-front keel. If you've a fin keel and the propeller is "free standing," it makes no difference what the angle of propeller is, but keep the shaft locked to prevent wear.

[Jim Cate]

Originally Posted by wsmurdoch

This is what my prop looks like when locked in place vertically and hidden in its aperture behind the keel:


This is what it looks like when locked in place horizontally exposed to the water flow:


Right now I think, from a drag standpoint, that locking the propeller in place horizontally is not good and vertically is good, but should I let it rotate? All of the testing of rotating vs. not rotating that I have seen is for fully exposed propellers. What about those propellers that are hidden behind the keel?

A reasonable question! I suspect that it varies between specific boats and their props... where they lie in the flow at the aft end of the keel, and the shape of the keel, and the shape of the aperture... and so on.

In your position, I'd make the ad hoc experiment rather than depend on the "advice" of internet "experts'. Not possible to generalize, IMO.

If and when you do the experiment, please let us know how it turns out.

Cheers,

Jim

[Wotname]

Originally Posted by wsmurdoch

This is what my prop looks like when locked in place vertically and hidden in its aperture behind the keel:


This is what it looks like when locked in place horizontally exposed to the water flow:


Right now I think, from a drag standpoint, that locking the propeller in place horizontally is not good and vertically is good, but should I let it rotate? All of the testing of rotating vs. not rotating that I have seen is for fully exposed propellers. What about those propellers that are hidden behind the keel?

Well I'm with Jim on this one.

However a quick and dirty test would be simple enough to give you a good idea as to which is best for your boat.

Go sailing, aligh and lock the prop I the vertical postion and then unlock it. Left to its own devices, it will default to the least drag condition. So if it stays wertical behind the keel, then that is the least drag postion and conversely, if it starts to rotate, then that is the least drag position. I suspect however, it will also be dependent on boat speed.

[Wotname]

Originally Posted by transmitterdan

Technically once the prop is locked no energy (heat) is expended by the locking device. All the waste energy is in the turbulence of the water. But the idea is right.

I agree with no heat is expended by the locking device but disagree with all the waste energy is in the turbulence. Some of it is but the waterflow over the locked blade induces a torque on the shaft which is counteracted by the hull/keel. So the hull is healed over in the opposite direction to that which the prop would like rotate. Not much of course, but nevertheless some of the wasted energy is dissipated by holding the hull healed over.

Again a vector analysis is the easy way to understand this IMO.

[mstrebe]

As for the vertical locking, definitely test. But please dive and clean your hull first for a valid test.

The hydrodynamics of foils (like your keel) are not obvious.

For your keel to "block" turbulent water flow on behalf of your prop, it would have to create as much turbulence as the prop in the same cross-sectional area. Unless it's a really crappy keel or encrusted with barnacles, it is far more efficient than that.

The entire point of shaping a keel like a foil is to eliminated turbulent drag behind it. So the water that reaches your vertical prop is very nearly as smooth and moving as fast as the water ahead of the keel. Hence, it will have very nearly as much drag vertically as horizontally if your keel is efficient.

That said, Keels do have shear induced drag, the same sort of drag that the entire hull has. Shear drag is the drag induced by the moving water over the surface. Essentially, the water in contact with the hull moves at the speed of the hull, while the water just micrometers away is not moving at all. Shear drag is the drag caused by the contact water's inertia (it's resistance to being sped up to some fraction of the boats speed depending on its distance from the hull surface).

It's an entirely different source of drag than turbulent drag, but of about the same magnitude.

Boat designers themselves did not really understand the magnitude of shear drag until the 1990s. It was assumed that turbulent drag was always of greater magnitude, and so sterns were built to have as low a turbulent drag as possible, resulting in nearly wake-less very narrow Double-Enders. As it turns out, the extra hull length required to re-shape the stern water flow has nearly as much shear drag as a cut-off stern has turbulent drag, so two boats of the same length will have similar total drag irrespective of how the stern is shaped with regards to drag. So now we have boats like mine that go through the water making big turbulent wakes yet are just as fast as double-Enders of the same length. It's definitely strange.

So what this all means for your prop is that it may be close enough to your keel for the keel's shear drag to make a difference, but it's more likely that it's not because that area will be very small if your keel is efficient. The laminar flows across your keel should reattach right at the trailing edge and eliminate turbulent flow just ahead of your prop, because that's the entire point of being shaped like a foil.

My 26 foot boat has a daggerboard that can be raised to any height while sailing, and kick-up rudders. One of the speed tricks in that model is to pull up the daggerboard by whatever % you are off of the close-hauled tack angle, so you'd be 100% down when pointing and 100% up when running. That daggerboard is shaped like a NACA 0012 foil, very efficient. But having it pulled up while running dead down wind to eliminate its shear drag was worth nearly a half-knot. We would also pull up the windward rudder when tacking. If going dead down wind, I could pull up both rudders and the prop (outboard) and get a full extra knot out of her, managing heading just with the sheets and headsail. Lots of fun those tricks, and a good way to learn the inherent costs of shear drag.


Sent from my iPhone using Cruisers Sailing Forum

[Maine Sail]

On our old Cape Dory 27, when locked in reverse with the two blade prop vertical behind the dead wood, if we placed it back in neutral the prop took the path of least resistance and began spinning again. Clearly was not as hidden as we thought it was on that particular full keeler.

[wsmurdoch]

I should have included a photo of my propeller in its aperture taken from the side.


I have several times sailed with the propeller locked vertically then freed it to spin. The difference in speed is not really noticeable except in flat water with low wind speeds. Nearing hull speed, with any sea running, and with the usual gustyness in the wind any difference is lost in the normal variation in boat speed as measured by either the paddle wheel or GPS.

Changing from propeller spinning to propeller locked vertically is somewhat involved. I have to make several changes from neutral to reverse to (by luck) catch the propeller vertical which I do by looking at marks on the shaft coupling. The general bother of doing that keeps me from more thorough testing.

My feeling is that vertical locked beats spinning, but I do not have good measured data.

Our habit has been to let it spin unless we are sailing for more than a few hours in which case we will go to the trouble of locking the propeller vertically.

[mstrebe]

Originally Posted by wsmurdoch

I should have included a photo of my propeller in its aperture taken from the side.


I have several times sailed with the propeller locked vertically then freed it to spin. The difference in speed is not really noticeable except in flat water with low wind speeds. Nearing hull speed, with any sea running, and with the usual gustyness in the wind any difference is lost in the normal variation in boat speed as measured by either the paddle wheel or GPS.

Changing from propeller spinning to propeller locked vertically is somewhat involved. I have to make several changes from neutral to reverse to (by luck) catch the propeller vertical which I do by looking at marks on the shaft coupling. The general bother of doing that keeps me from more thorough testing.

My feeling is that vertical locked beats spinning, but I do not have good measured data.

Our habit has been to let it spin unless we are sailing for more than a few hours in which case we will go to the trouble of locking the propeller vertically.

Ah, so your keel is partially hydrodynamic before your prop it seems. It appears to me that the fairing to an edge before your rudder is too short to allow for laminar flow, so turbulence is probably normal there. The curve before the prop would have to go back at least a foot to be gradual enough to prevent de-lamination of flow. So the turbulence behind it will be about the same as the turbulence of the prop, and of the rudder ahead of it.

The difference between locked vertically and spinning won't be much, if its detectable at all, in this keel configuration.

[Bill Seal]

Originally Posted by charliehows

having personally dismantled my gearbox (yanmar 2qm) I can safely assert that the potential for wear caused by a freewheeling prop on the massive industrial strength internals of the box is about 0. Agree with the above too, its just intuitive that a freewheeling fixed prop is going to have less drag - if there was no difference the bloody prop wouldnt push the boat when it was running. Only caveat - as pointed out exhaustively in the previous posts - it depends on the type of clutch - mines a flat 2 single plates job that doesnt have any activation in neutral, hence no problem, but cone or multiplate clutches may have some activation depending on design, which could cause problems.

I believe the part about the torque locking is what the leaving in reverse is all about. If you leave the trans in foreward with the engine off, the over running prop is going to loosen up the clutch, causing wear. After reading this, I'll leave my Volvo MD11C in reverse. Sure don't want to pay those Lamborghini-esque repair prices.

[ilana]

Originally Posted by Maine Sail

On our old Cape Dory 27, when locked in reverse with the two blade prop vertical behind the dead wood, if we placed it back in neutral the prop took the path of least resistance and began spinning again. Clearly was not as hidden as we thought it was on that particular full keeler.

You can check if a two propeller blades has dragged less vertical than horizontal because of ballast by simple trial.
Sail by sails and brake the axis by hands.
Slowly rotate the axis and check the point that the rotational force is the smallest.
A sign that you found the desired point
Just be careful with hands.

Analog man in a digital world.
Ilan

[charliehows]

Re: Do you sail with your transmission in gear?
Quote:
Originally Posted by charliehows
having personally dismantled my gearbox (yanmar 2qm) I can safely assert that the potential for wear caused by a freewheeling prop on the massive industrial strength internals of the box is about 0. Agree with the above too, its just intuitive that a freewheeling fixed prop is going to have less drag - if there was no difference the bloody prop wouldnt push the boat when it was running. Only caveat - as pointed out exhaustively in the previous posts - it depends on the type of clutch - mines a flat 2 single plates job that doesnt have any activation in neutral, hence no problem, but cone or multiplate clutches may have some activation depending on design, which could cause problems.

I believe the part about the torque locking is what the leaving in reverse is all about. If you leave the trans in foreward with the engine off, the over running prop is going to loosen up the clutch, causing wear. After reading this, I'll leave my Volvo MD11C in reverse. Sure don't want to pay those Lamborghini-esque repair prices.

not sure what your point is but - when i did have to replace my 30 yr old clutch it was the forward clutch that was worn out. The reverse clutch looked perfect. So theres another good reason for leaving it in reverse if you're going to leave it in gear - the reverse clutch gets a lot less use, generally.

[Gboy]

Depends on the drive set up....but for most sail drives, in neutral for solid props and reverse for folding.


Sent from my iPad using Cruisers Sailing Forum

[kas_1611]

Here's the text from the Volvo Penta Manual for the D1 and D2 series engines:

"When sailing, set the control lever to astern if a folding propeller is fitted.
If a fixed propeller is fitted the control lever should be set in neutral or reverse. When using a fixed propeller and sailing with the control lever set to reverse the speed is slowed down though less noise is being made."

Pretty conclusive that a fixed prop locked in reverse is slower ie produces more drag than allowing to free rotate in neutral

Keiron

[Tropic Cat]

Here it is again.... Bad info is back again.

All of this has been written years ago and is here:

Yanmar says sailing in any gear violates the engine warranty.
http://www.catamaransite.com/files/Y...al_Sailing.pdf

Volvo, on page 21 of their operating manual, says to free wheel the prop.
http://www.catamaransite.com/files/V...ction_Book.pdf

The MIT test showed that a free wheeling prop has less drag. The paper is here:


http://www.catamaransite.com/files/propeller.pdf



http://www.catamaransite.com/files/S...eller_test.pdf



4. A real world test was done showing beyond all doubt that a free wheeling prop has less drag. Here's the video
[URL="

[Tropic Cat]

I had to continue my post here.... Lastly, there are more reports showing the same dang thing. That a free wheeling prop has less drag than a fixed prop. There is no report anywhere that documents all of these sources I listed in the above post are wrong. Quite the opposite. I tested this on my catamaran and verified the results before I published the page reference.

Prop feathering on an aircraft, reduces the pitch of the propeller. If you have feathering props on your boat, you're right, this is possibly the least amount of drag you can obtain. If you don't own feathering boat props, the point is moot and the data above stands...

The actual difference ....measured in pounds of drag... is listed in a table here.
http://www.catamaransite.com/propeller_drag_test.html

And before anyone comments, that page has been sitting there for more than 5 years .