How sailboats sail

[Maine Sail]

Regardless of how you feel on this issue a free rotating prop is causing NEEDLESS wear to the transmissions bearings and seals as well as the cutlass bearing and the stuffing box not to mention any misalignment and the resulting shaft vibration..

I have raced competitively my entire life and have seen it BOTH ways. Some props yield a better turn of speed locked and some better freewheeling. It is not as easy as ALL props will be faster locked or ALL props will be slower freewheeling. This was reinforced by the MIT study which is unfortunately now off line..

Let's also not forget that for those of you with a two blade prop you can lock it vertically in the aperture behind the keel if you mark the shaft and employ a shaft brake.. Plus if you were really concerned about that extra .01 knot you'd be sailing a faster design than an Arlberg..

[Neil Gordon]

Darrell,

>>Sooo.... based on what you're saying above if I understand it right, is that the faster the propeller rotates when freewheeling, the slower the boat must sail (is that right?)<<

Not exactly. If the prop is unlocked and the boat is not moving (and no current), then the prop doesn't spin. As the boat moves fast enough to overcome the friction of the cutlass bearing and whatever, the prop begins to turn. As the boat goes faster, the prop will turn faster, too. (Windmills do this, too... they turn faster in higher wind.) It's just that the boat won't be going as fast as it would if the prop was locked.

>>...and maximum speed (hull speed perhaps) can only be obtained if the prop is locked (right?)<<

Hull speed can be obtained either way but if you lock your prop, you get there sooner and with less wind.

When I stop the boat, it must be going like hell.<<

Actually, it reaches its maximum speed before you start to stop the boat. If you apply forces to stop the boat while the prop is spinning quickly, the spinning prop will, as the boat slows sufficiently, begin to provide thrust rather than drag. The effect will be that the boat won't stop as quickly so you're more likely to over run your mooring.

So we can never unlock our props at anchor (it would be way too noisy)

>>...and you dont believe that if I unlock the prop BEFORE I start sailing, that it will begin at ZERO RPMs and increase RPMs as the boat gains speed, hhmmm?<<

Yes, I do believe that it starts at zero (just like a windmill with no wind) and spins faster as the boat goes faster (just like a windmill with more wind).

[M. R. Bober]

Toss that into the problem set.

Having your prop stolen is a drag but sailing without a stolen prop reduces drag.

Sailing with a stolen prop will increase or decrease drag depending whether you steal a two blade or a three blade prop and whether you lock it or not.

Getting caught with stolen good, props or otherwise, is always a drag.

"You've got to lively up yourself and don't be no drag." Bob Marley

Possible practical locked prop demonstration: rig sails on a twin screw cruiser; lock one let the other freewheel; see which way she heads; report back to this site.


Mitchell Bober recently aka "Milton"
Sunny Lancaster (where we will leave no prop unturned in our search to truth), VA

[Oswego John]

Good morning, yer honor. I plead guilty.

Mitchell, it's another Freudian Slip on my part. They've been coming fast and furious, lately. Sorry.

Consider it an honor for me to err and call you Milton. A very dear friend of mine, who is beyond reproach and is an all around beautiful person and the ultimate gentleman, is named Milton.

I promise to try to remember to call you Mitchell in the future, but because of circumstances on my part, it's not guaranteed.

Well, hey, I got the first letter correct.

O J
PS: I did get my Nova on bagel with a schmear.

[darmoose]

Darrell,

>>Sooo.... based on what you're saying above if I understand it right, is that the faster the propeller rotates when freewheeling, the slower the boat must sail (is that right?)<<

Not exactly. If the prop is unlocked and the boat is not moving (and no current), then the prop doesn't spin. As the boat moves fast enough to overcome the friction of the cutlass bearing and whatever, the prop begins to turn. As the boat goes faster, the prop will turn faster, too. (Windmills do this, too... they turn faster in higher wind.) It's just that the boat won't be going as fast as it would if the prop was locked.

>>...and maximum speed (hull speed perhaps) can only be obtained if the prop is locked (right?)<<

Hull speed can be obtained either way but if you lock your prop, you get there sooner and with less wind.

When I stop the boat, it must be going like hell.<<

Actually, it reaches its maximum speed before you start to stop the boat. If you apply forces to stop the boat while the prop is spinning quickly, the spinning prop will, as the boat slows sufficiently, begin to provide thrust rather than drag. The effect will be that the boat won't stop as quickly so you're more likely to over run your mooring.

So we can never unlock our props at anchor (it would be way too noisy)

>>...and you dont believe that if I unlock the prop BEFORE I start sailing, that it will begin at ZERO RPMs and increase RPMs as the boat gains speed, hhmmm?<<

Yes, I do believe that it starts at zero (just like a windmill with no wind) and spins faster as the boat goes faster (just like a windmill with more wind).

Cause and Effect...........your giving me a headache

I need an aspirin (maybe a whole bottle)

Darrell

[darmoose]

Sooo.... based on what you're saying above if I understand it right, is that the faster the propeller rotates when freewheeling, the slower the boat must sail (is that right?)

>>Not exactly. If the prop is unlocked and the boat is not moving (and no current), then the prop doesn't spin. As the boat moves fast enough to overcome the friction of the cutlass bearing and whatever, the prop begins to turn. As the boat goes faster, the prop will turn faster, too. (Windmills do this, too... they turn faster in higher wind.) It's just that the boat won't be going as fast as it would if the prop was locked. <<

Neil

So we are back to the question i asked you earlier, if i slow the freewheeling prop shaft by hand, does the boat speed up or slow down? You said earlier, it speeds up. Now, it sounds like you wish to reverse yourself??




>>Actually, it reaches its maximum speed before you start to stop
the boat. If you apply forces to stop the boat while the prop is spinning quickly, the spinning prop will, as the boat slows sufficiently, begin to provide thrust rather than drag. The effect will be that the boat won't stop as quickly so you're more likely to over run your mooring.<<

Neil,

What you are saying here is that a boat with a propeller that is allowed to spin wont stop as quickly as a boat whose propeller is locked. I agree with you. There is more drag with a locked propeller. Now you're getting it.





...and you dont believe that if I unlock the prop BEFORE I start sailing, that it will begin at ZERO RPMs and increase RPMs as the boat gains speed, hhmmm..


>>Yes, I do believe that it starts at zero (just like a windmill with no wind) and spins faster as the boat goes faster (just like a windmill with more wind).<<

Neil,

Here you say a freewheeling prop spins faster as the boat goes faster. But, apparently, I cant deduce from that, that if a propeller speeds up, the boat has speeded up?? Can I deduce that if the windmill speeds up that the wind has picked up?(all other things remaining the same)

Darrell

[Neil Gordon]

Darrell,

>>... if i slow the freewheeling prop shaft by hand, does the boat speed up or slow down?<<

Speeds up. The angle of heel also changes a bit.

>>What you are saying here is that a boat with a propeller that is allowed to spin wont stop as quickly as a boat whose propeller is locked.<<

I'm saying that if you remove force that's driving the boat forward, e.g., sails, then various braking forces slow the boat. If the boat slows more than the prop can keep up with the slowing, the spinning prop will provide forward thrust (although not for long). It has nothing to do with the comparative drag between a spinning and locked prop at normal sailing speeds.

>>Here you say a freewheeling prop spins faster as the boat goes faster. But, apparently, I cant deduce from that, that if a propeller speeds up, the boat has speeded up?? <<

It's the cause and effect thing. All else being equal, you can deduce that if the free spinning prop is spinning faster, the boat is moving faster than when the prop was spinning more slowly. You can't conclude that the spinning prop made the boat faster than it would be otherwise, though.

This does seem somewhat different from what happens with sails, though, where you can beat to weather and create more wind the faster you go. (Ice boats do that really well!)

[Maine Sail]

Can someone in warmer climes please build the attached device so we can put this to bed.

it would be simple to build and could be temporarily attached to a the stern of a power boat moving at a known speed through water. The scale would show more weight with more resistance and less weight with less resistance.. We don't need rocket science all we need is a simple test fixture made of 2X4's a fish scale and a spare prop that could spin or not..

P.S. I suck at MS Paint so don't laugh..

[johnny of STORK]

It's called a transom-mounted auxiliary charger. It drags a prop on a pig-tail 15 or 20 feet behind the boat, which spins an alternator on board. It should read out digitally, in volts and amps.
Johnny

[Maine Sail]

But those are expensive and I'm sure no one is willing to put their money where their mouth is for that kind of outlay..

[Matt Cawthorne]

Main Sail,
This is a test of endurance, not a search for the answer. A socratic dialogue without enough data presented to come to an informed decision is entertainment for some. I find it irritating. The sad thing is that some folks will believe that the last person standing had the correct answer. As always with a bulletin board the individual has to pick through the posts to see who and what they believe.


Have a good holiday.

Matt

[John Vigor]

Much simpler to have another boat tow your Cape Dory at, say 5 knots. In the tow line, insert a suitable scale. Put your gear lever in gear. Note scale reading. Take gear lever out of gear. Note scale reading. Observe that fixed prop causes less drag than spinning prop.

John Vigor

http://www.johnvigor.com

[darmoose]

It is unbelievable that experienced sailors don't recognize intuitively that a propeller, otherwise unhindered, having a choice to either remain still or to spin when pulled through the water, will take the path of least resistance (which is to spin), thereby allowing the boat to sail faster. It takes considerable force to hold a propeller still when being dragged thru the water, and that force is working against the forward progress of the boat.

It is like dragging a door in a frame behind the boat and arguing if the door should be open or closed to sail faster

Neil says in my example of a sailing boat freewheeling its prop, that if i artificially slow the prop the boat will speed up. I then ask if I let go of the prop so that it speeds back up, what will the boat do, and he replys it will slow down.

So I ask anyone, how can the prop speed up while the boat is slowing down (just what do you think makes the prop speed up)?

Why dont you help Neil with that, John

Darrell

[Maine Sail]

Darell,

Have you read the MIT study??

Here's a direct quote from the study:

M.I.T. Propeller Study 1994:

"The conclusion from these calculations is that a substantial reduction in drag, and thus gains in boat speed, cab be realized when a fixed pitch propeller is allowed to free-wheel."

I still lock my prop to avoid undue wear and tear..

Here's a link I actually found it after I found it on my hard drive.


P.S. A Max Prop will give you substantially less drag ...
http//:catalpa.amberdell.com/files/propeller.pdf

[Joe CD MS 300]

Darrel, John, Neil,

Unlike Matt who finds this thread irritating, I think it is a riot. I have pictures in my mind of all three of you laying awake at night both frustrated at not being able to convince the others of your position and endlessly trying to figure out a way to do it.

I say keep at it but try to be less polite it will up the entertainment factor. You can not allow the other side to wallow in their ignorance. Remember Winston Churchills words:

You ask, What is our policy? I will say; “It is to wage war, by sea, land and air, with all our might and with all the strength that God can give us: to wage war against a monstrous tyranny, never surpassed in the dark lamentable catalogue of human crime. That is our policy.â€