Do you sail with your transmission in gear?

[gamayun]

Originally Posted by mstrebe

Take your boat out on the water, get under sail on a good consistent reach, and record your speed for ten minutes with the prop locked. On the same reach, put your prop in neutral, and record your speed again. Do this a few times to take out the effects of variable winds and waves.

You'll see a .25 to .5 knot improvement in average speed when the prop is freewheeling.

This is what I have determined, too. Don't know the transmission, but the engine is a Yanmar 3GM30 with 2-blade fixed prop. I plan to get a folding prop soon so I'll never be tempted to look at these spin v. no spin threads again or have to listen to that prop whine when we started moving faster than 4 knots

[Jim Cate]

Originally Posted by gamayun

This is what I have determined, too. Don't know the transmission, but the engine is a Yanmar 3GM30 with 2-blade fixed prop. I plan to get a folding prop soon so I'll never be tempted to look at these spin v. no spin threads again or have to listen to that prop whine when we started moving faster than 4 knots

Good one! You will likely find that you do that >4 knots thing more often, too!

Jim

[Dockhead]

Originally Posted by StuM

Not according to the practical tests that have been reported and discussed here on numerous occasions.

Check out http://www.cruisersforum.com/forums/...tml#post324198

Indeed. MaineSail, who is CF's own Mythbuster, has in my opinion conclusively laid to rest the myth that a locked prop has less drag.

But as someone else suggested -- you can test it yourself quite easily.

[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.

[jmschmidt]

I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.

[mstrebe]

Originally Posted by jmschmidt

I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.

Again, test this yourself. It's easy.


Sent from my iPad using Cruisers Sailing Forum

[transmitterdan]

Originally Posted by jmschmidt

I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.

For most transmissions there is so little heat generated by the free wheeling prop that it does not cause much drag. When the prop is turning it is in effect "getting out of the way" of the water. When locked it causes a lot more turbulence in the water and more drag. If the shaft were producing energy when turning then you would be right. There have been many studies where the extra drag of a locked prop was measured. What possible reason could someone have for lying about their results?

[kas_1611]

Originally Posted by jmschmidt

I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.

As has been shown in controlled tests and through individual experience a free spinning prop produces LESS drag than a locked fixed blade prop.

A folding or feathering prop will produce minimal drag, no one is denying that, but if you have a fixed blade one then it is better to let it spin than lock it out (where transmission allows) to reduce the drag to as low as reasonably practicable.

[Maine Sail]

Originally Posted by jmschmidt

I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.

There are some out there who still believe the world is flat too...

Both MIT and the University of Strathclyde's Ocean Engineering Department studied this and both found freewheeling sailboat props = less drag. Yachting Monthly also did a test, after I built my own test jig back in 2005, and physically dragged a number of props through the water.

I repeated my test in 2010 with a Maritime Skiff and more powerful 40HP outboard and used 4 different fixed props. Not once did freewheeling = more drag than fixed. I have not yet been able to measure a locked/stalled prop cause less drag freewheeling and in most cases the drag is significantly more when locked, up to double & more.. My actual "towed through a real ocean" test is directly in-line with both MIT's findings and The University of Strathclyde's findings, as well as that of Yachting Monthly and in-line with Dave Gerr too.

From The Strathclyde White Paper:

"The experimental results confirm that a locked propeller produces greater drag than does a freewheeling screw (up to 100% more drag was observed, this being at higher speeds). Furthermore, for the freewheeling case, the magnitude of the hydrodynamic resistance is significantly affected by the amount of frictional torque on the shaft, low torque being accompanied by low drag."

And of course what would Dave Gerr, author of the Propeller Handbook and Director of the Westlawn Institute for Marine Technology, know..

In regards to a locked fixed prop...

"It will generate the least drag when it is free to rotate."


https://youtu.be/jI-UG9RSlJo


.

[Wotname]

Originally Posted by jmschmidt

I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.

I dunno about counterintuitive but the main reason why nobody wants to believe that a freewheeling prop produces more drag than stopped one is simple - because it isn't true!

As many other have posted, look at the empirical evidence already referenced in this thread and others of the same ilk.

However, if you need a rational and bullet proof theoretical explanation, please read on. By the way, you won't need to know any fancy maths or understand complicated formulas, just a basic understanding of first principles of physics.

First, let's get some basic physics dealt with.

Energy can't be created or destroyed, just changed from one form to another.
All the forces on a sailboat can be resolved into 6 vectors (2 for each axis).
When the vectors are equal and opposite, velocity remains constant, when they are not, acceleration (positive or negative) occurs.

OK, next look at our situation; we only have two conditions to consider

1. The prop is allowed to rotate as freely as it can be given the confines of the gearbox being placed into neutral - we will call this freewheeling for the purposes of this discussion.
2 The pro is held stationary by either a shaft lock of by placing the gearbox into gear (ahead or astern) and thus having the engine compression preventing it from rotating - we will call this locked.
These are the only two conditions available to us. Note, we are not considering feathering or multi pitch props.

Now comes the explanation…

We only need to consider two of the six vectors: thrust and drag. Any forces that cause the the boat to move forward can be resolved into one vector - thrust. Any forces that hinder this forward motion can be summed together and becomes drag. When the thrust and drag are equal, the velocity of the boat remains constant. However, if some force acts on the boat to increase the thrust vector, the boat will accelerate until the drag increases and balances out the new thrust vector. It will then remain at this new velocity until something else changes. Of course, the reverse is also true.

So in our simple explanation, we have a sailing boat complete with a fixed bladed prop and and prop shaft. However, instead of a gearbox and engine, just consider the inside end of the shaft is fitted with a simple crank than can be turned by hand. I assure you that this arrangement acts in a similar manner as a gearbox and engine as far as the forces in question is considered.

Provided nothing prevents the crank from turning, the prop, shaft and crank will start to rotate as soon as we get some boat speed happening. For our purposes, we will say this occurs around 3 kts STW.

So assume we are sailing along at 5 kts with steady breeze, the prop will be rotating - for our example, let's say it is doing 30 RPM in a clockwise direction.
It is freewheeling in essence. If our velocity is constant (5 kts), then the thrust and drag vectors must be equal and opposite.

Now put you hand on the crank and by pure muscle power, speed up the rotation to say 60 RPM in the same clockwise direction. Clearly this is going to take some effort. So what happens to this extra energy that has been introduced into our steady system. Just as clearly, the prop is now going to be providing some additional force on the boat - in fact, it will increase the thrust. So now the thrust is greater than the drag and the boat will accelerate to a new and increased velocity. It will continue to accelerate until the drag caused by the extra speed balances out the the new thrust vector. At this point, the velocity will remain constant again but will now be be say 6 kts. While you keep turning the crank with the same vigour, the boat will now be doing 6 kts.

When you think about it, this what occurs when you start the engine while motor sailing.

OK you get tired and let the crank go, the boat will slow down back to 5 kts and the prop and crank will be freewheeling again.

For any one still reading, this is all quite intuitive and surely non-controversial.

Now lets say, you put your hand back on the crank and apply some effort in the opposite direction and slow the crank down to say 15 RPM. Again, this will take some effort but in the opposite direction as before. So what happens to this energy that has now been placed into the system. It is not hard to understand that it must be adding to the drag. Remember that the effort to speed up the crank added to the thrust, the effort to slow the crank is opposite and therefore must be added to the drag vector. Really, this is intuitive once you think about it. So if the drag vector is now increased compared to the thrust vector, the boat must slow down until the vectors equalise. This occurs as the slower motion decreases the other drag effects until the total drag vector equals that of the existing thrust. For this argument, let's say the new velocity is 4 kts. Once again, if you let go the handle, the energy you were applying to slow down the prop will be removed from the system and the prop RPM will increase back to 30 RPM as will the boat speed increase back to 5 kts.

Almost done

This demonstrates that forcing the prop to slow down from it's natural freewheeling speed cause the boat to slow down. It doesn't take much imagination to see that applying even more force to slow it down to a standstill MUST decrease the boat speed even further. Likewise, releasing it from a locked position and allowing it rotate freely, must increase the boat speed.

Remember, it is all about the total energy in the system. Adding energy in one direction will cause an increase in the thrust vector and adding the same amount of energy in a opposite diction will increase the drag vector.

Thus when compared to a freewheeling prop, starting the engine (adding energy) will make the boat go faster while adding energy in the other direction direction (locking the prop) will slow the boat.

Of course, chucking the prop and shaft and all that engine gear into the briny depths will get rid of more drag and the boat will go faster.

And oh, I almost forgot, drag is always proportional to speed; it isn't a fixed quantity for any particular boat.

[StuM]

Originally Posted by Wotname

However, if you need a rational and bullet proof theoretical explanation, please read on. By the way, you won't need to know any fancy maths or understand complicated formulas, just a basic understanding of first principles of physics.
...

Brilliant!
Best illustration of the forces involved that I have ever seen.

[charliehows]

isnt it marvelous! even when intuition AND empirical science match up - there are still plenty of people out there who are adamant that the opposite of both is true. In this instance, fortunately, it doesnt really matter unless you're designing a volvo americas cup boat - ask those guys if they think a locked fixed prop is a good idea. Otherwise - what percent of difference does it make on a leisurely cruise up the coast? if it takes a bit longer - meh! - horrible prospect - slightly more time lazing at the helm - disaster...
unconvinced? - for a small fee i can offer my design for a nugon driven prop braking system that converts the extra energy derived into gold - yes, SOLID GOLD!!!

[Wotname]

Originally Posted by jmschmidt

I guess nobody wants to believe that a free-wheeling prop produces more total drag, both parasite and induced, than a stopped one or a feathered one, but it's true. What do these people think makes the prop free-wheel if not the energy it is extracting from the forward inertia of the boat under sail thereby slowing the boat for an given condition? Counterintuitive maybe, but true nevertheless.

To be fair, jmschmidt is half right. The freewheeling prop is of course extracting energy from the forward direction of the boat thus slowing it down. This true but somewhat irrelevant because in essence it is comparing the freewheeling prop to no prop. Where as we need to compare it to a locked prop.

What you have failed to consider it the energy required to keep the prop locked. This energy is also slowing the boat down and it is greater than that of just letting the prop spin freely.

It is easy to over look the energy required to lock the prop because IMO, nothing APPEARS to be moving when the prop is locked. But of course this isn't true. Consider the following.

If the prop wants to turn CW, then to stop it you must try to turn it CCW. So to stop it, you either use a shaft lock or use the engine compression to stop it. In either case, the hull is used to resist the turning motion. In effect you are trying to turn the hull CCW. In fact the hull WILL heel over very very slightly when you lock the prop when sailing. Again this can be proved by looking at the vectors but unless you really can't see this, I will leave the explanation for another day .

So the energy required to hold the boat slightly heeled over also extracts from the forward motion.

[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.

[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?