Captain/Owner Sleeping at the Helm

[Wotname]

Originally Posted by waterman46

1) .............
3) Torque has zero effect in the direction of boat travel, by itself. You say it may cause some loss of boat speed since it forces the boat off her lines a bit. Could even cause boat to heel less on one tack, more on the other, balancing out to no real effect.
............

Good start! Now consider that it must cause a change of heel rather than it could. Physics is perpetual, not arbitrary.

To maintain the change in heel requires energy constantly. It does not simply balance out. It does not matter if the change of heel is increased or decreased. If increased, then there is less air pressure on the sails but more gravitational potential energy. If decreased, then the opposite applies.

These losses are only part of the total drag of the locked prop, one other major component is the "swirly water" being generated by the stationary blades but that is a discussion for another post.

[Wotname]

Originally Posted by waterman46

1) The point of autorotation was mentioned to show that really significant energy is stored in a prop dragged through the medium of air or water. Enough to nearly stop a descending helicopter with no engine. That energy continues to be constantly created, by dragging a rotating prop. That energy must be dissipated somehow. Not you, but some other posters seem to have missed this point.
.................

While I have responded to section of your post earlier, I forgot to point out the error of thinking that the momentum of the spinning prop has to be constantly created. It doesn't, it only needs to be created once and then it stored and remains stored until there is a change in the system. After that, it is only the frictional losses (heat, noise etc) plus that of the swirly water that requires a constant input of energy.

Momentum is mass times velocity. Neither the mass or the velocity of the spinning prop alters except for that the frictional and swirly water losses which are degrading the velocity. I.E. these two are causing a negative acceleration in the prop and need to be overcome by an input of force (F=m.a).

[Lodesman]

Originally Posted by VChild

MaxProp folding prop.

Not to be too pedantic, but MaxProp is a feathering prop, not a folding prop.

[OloteleMtn]

Some aspiring PhD candidate should use this thread as a thesis for how people can start off talking about a standing watch sleeping morphs into a discussion of mechanical engineering and fluid dynamics.

[gordonhinds]

Originally Posted by Wotname

Interesting opinion but do you have physics knowledge to back up your opinion or any creditable source to support this opinion.

Perhaps a good starting place is to read this http://www.plaisance-pratique.com/IM..._sdarticle.pdf and see if your opinion changes.

Of course you are entitled to keep your original opinion but not entitled to present it as a fact.

Hi, I read the paper and its pretty vague as its based on a tank test not a real boat test. The numbers are all over the place - perhaps 30% less drag with a rotating prop up to 100% less drag. My understanding is more simple - in yacht racing over the past 40 years. If you race the same type of yacht as me and let your prop rotate, I will smoke you - in moderate to light conditions my boat will be about 1 knot faster, which in a three day yacht race equates to a 60 mile advantage. I have never heard of the transmission issue but I'll ask around. But if you think its better to rotate - its your boat to go for it. But if you are racing you will be beaten. Mind you you shouldn't race with a fixed blade prop anyway - folding is faster still (locked off).

[Wotname]

Originally Posted by gordonhinds

Hi, I read the paper and its pretty vague as its based on a tank test not a real boat test. The numbers are all over the place - perhaps 30% less drag with a rotating prop up to 100% less drag. My understanding is more simple - in yacht racing over the past 40 years. If you race the same type of yacht as me and let your prop rotate, I will smoke you - in moderate to light conditions my boat will be about 1 knot faster, which in a three day yacht race equates to a 60 mile advantage. I have never heard of the transmission issue but I'll ask around. But if you think its better to rotate - its your boat to go for it. But if you are racing you will be beaten. Mind you you shouldn't race with a fixed blade prop anyway - folding is faster still (locked off).

Very interesting and thank you for posting about your experience. This is the first time I have heard of any empirical experience of gaining boat speed by locking their prop. Every other instance that I have heard of (or tried myself) has been the opposite.

I am very curious to know more about your setup. E.G. what type of prop, what type of hull, what locking method you employed, what is the hull speed of your boat?

There has to be a reason why your experience is different and if you would be so kind to provide more detail, then it may be possible to know why.

[Uncle Bob]

Originally Posted by gordonhinds

Hi, I read the paper and its pretty vague as its based on a tank test not a real boat test. The numbers are all over the place - perhaps 30% less drag with a rotating prop up to 100% less drag. My understanding is more simple - in yacht racing over the past 40 years. If you race the same type of yacht as me and let your prop rotate, I will smoke you - in moderate to light conditions my boat will be about 1 knot faster, which in a three day yacht race equates to a 60 mile advantage. I have never heard of the transmission issue but I'll ask around. But if you think its better to rotate - its your boat to go for it. But if you are racing you will be beaten. Mind you you shouldn't race with a fixed blade prop anyway - folding is faster still (locked off).

You do realise that particular test is but one of four, or possibly more, that end at the same conclusion don't you? At least two of them are conducted on actual boats in the water, videoed and documented.

[MicHughV]

My first boat had a fixed two blade prop with narrow blades and it was situated relatively close to the trailing edge of the keel. I would always lock the prop with the engine in reverse gear and adjust the shaft by rotating it by hand so the the blades would align vertically behind the keel. I had marks on the coupler to let me know when the shaft had the blades in a vertical location.

Turning the shaft by hand to align the prop vertically, I would feel the prop wanting to spin, but once vertical aligned behind the keel, the pressure on the blades diminished to next to nothing. ok, not quite nothing, but very little. I never once had a problem with the engine transmission. I did not monitor the speed difference, so can offer no comparison, fixed vs. spinning, but the boat always seemed happier once it was locked in this manner.

A 3 blade prop would not work in this situation, and I think there is a substantive difference between a wide 3 blade prop as compared to a narrow 2 blade prop.

I offer this as a " non-scientific" view.

[VChild]

Originally Posted by Lodesman

Not to be too pedantic, but MaxProp is a feathering prop, not a folding prop.

That is correct. The blades do not fold back but rather twist. I wonder if a folding prop would even have sustained damage in this situation. Folding blades probably would have just stayed folded back when the engine started in reverse while moving forward at that speed.

[StuM]

Originally Posted by gordonhinds

If you race the same type of yacht as me and let your prop rotate, I will smoke you - in moderate to light conditions my boat will be about 1 knot faster

More unsubstantiated statements that a lot of us here who have raced many different boats over many years have found to be at complete odds with reality.


IOW, to be blunt, I call BS!

[StuM]

Originally Posted by MicHughV

Turning the shaft by hand to align the prop vertically, I would feel the prop wanting to spin, but once vertical aligned behind the keel, the pressure on the blades diminished to next to nothing. ok, not quite nothing, but very little.

I offer this as a " non-scientific" view.

The difference between your "shielded" prop and a freewheeling prop was minimal, but that little tendency to spin still indicated a little additional drag.

[wingssail]

Originally Posted by gordonhinds

... My understanding is more simple - in yacht racing over the past 40 years. If you race the same type of yacht as me and let your prop rotate, I will smoke you...

So you actually have some experience of similar yachts racing, one with a locked off propeller and one with it free wheeling, and you actually measured the difference?

Amazing.

In my 50 plus years of racing I never saw such a comparison.

[wingssail]

Originally Posted by JPA Cate

As a matter of fact, one of the skippers on whose boat I used to race, had marked the prop shaft -- it had a 2 blade prop, and the idea was to vertically align it with the aft end of the keel -- so that we could all see the mark, when it was properly aligned. It was one of the newbie jobs for me, to check that.

This might have been 45 yrs ago or so.

Ann

We used to do that, with our folding prop, so one blade would not hang down.

It could be viewed but could not be reached with a hand, so we'd "bump" the starter until it hit the right alignment.

Then we got a new engine/transmission and we stopped doing it because I felt that "bumping" the shaft position with the starter was not good for the system.

Probably cost us some races.

[Jim Cate]

We used to do that, with our folding prop, so one blade would not hang down.

Ah, yes, I seem to remember that you have Martec folders... props where the blades are not geared together. Most folders nowadays do have geared blades, and they can't suffer the dread "blade droop". Back in the days when I had a Martec, I did the same as you... in many ways they were good props, quite efficient, but wore badly and did droop.

Long live F-O-F!

Jim

[Dalestr]

Originally Posted by StuM

The difference between your "shielded" prop and a freewheeling prop was minimal, but that little tendency to spin still indicated a little additional drag.

The partially "shielded" narrow two-blade prop could actually have more drag when fixed than when freewheeling as the feel of reduced torque when vertical is a static comparison only.
This discussion has been focused on three blade fixed props in fully-exposed strut mounted propellers and also saildrives. Let's be reminded that a good percentage of boats are still fitted with props in apertures behind full keels or in skegs, often with 2 blade props and little here applies to those scenarios. Largely ignored is how the actual friction on the shaft during "free" rotation will affect the comparative drag. This can vary a lot for different configurations and less common transmissions. A good test would be to find out how much frictional torque on a boat's 'free' shaft might give an equivalent sailing drag. Only on saildrives and outboards do you have true free-wheeling and otherwise you get much noise, wear on the shaft bearing, greater debris snag potential and a spinning folding or feathering prop will wear out much faster so this really should be avoided.
My own boat (Cal 29) has been using a Martec prop (Yanmar 2GM20F) for 5 years and using reverse to lock. The shaft angle is 15deg and the heavy lower blade (15"x 13") will hang down unless the shaft is locked in race position. This rig starts to freewheel at about 5 knots though geared folders might not. If the OP's catamaran actually had folding props, then based on Yanmar's much more detailed Bulletin of 2011 locking the shaft using reverse would be allowed. This matches the guidance of the Volvo manual that we also saw the link to. So, reverse is OK for a folding/feathering prop but not a fixed prop. I presume these engine makers are focused on a strut mounted prop, not on 2 blade props aligned in an aperture. The Volvo example requires brief engine runs every 10 hours of continuous freewheeling for cooling or lubrication. Hurth/ZF allows reverse for locking of the shaft on their mechanical units.