Prop rotation when sailing

[Chrusty 1]

I don't think they have even proved that, because they have not been very systematic about measuring the shaft torque and replicating it. But it's probably true all the same.
But it's a bit like proving that square wheels are better than triangular.

Measuring the drag on a prop makes it very obvious to me that folding props are worth the cost and maintenance, IMHO.

I wish some bright spark would come up with a folding prop for outboards! Even though they don't seem to exert as much drag as on an inboard setup.

 

[Tranona]

I wish some bright spark would come up with a folding prop for outboards! Even though they don't seem to exert as much drag as on an inboard setup.

Never be a market. Looked at that 30 years ago and the cost of the prop would rival the cost of the whole outboard. Cheapest folding prop is c£800, not much less than a complete new 5hp outboard.

One of the positive features of an outboard when used as an auxiliary on a small yacht is that it can be tilted or lifted to get the prop out of the water. Appreciate that this is not always easy or practical, but that is usually the fault of the boat's designer, not the outboard.

 

[Chrusty 1]

Never be a market. Looked at that 30 years ago and the cost of the prop would rival the cost of the whole outboard. Cheapest folding prop is c£800, not much less than a complete new 5hp outboard.

One of the positive features of an outboard when used as an auxiliary on a small yacht is that it can be tilted or lifted to get the prop out of the water. Appreciate that this is not always easy or practical, but that is usually the fault of the boat's designer, not the outboard.

I do take your point, No, I can't tilt my outboard, it lives in a well, but from what I gather from the Seal forum, it shouldn't be much of a problem, as long as it doesn't make too much din while the prop is free wheelin'

 

[Tranona]

I do take your point, No, I can't tilt my outboard, it lives in a well, but from what I gather from the Seal forum, it shouldn't be much of a problem, as long as it doesn't make too much din while the prop is free wheelin'

Sort of proves the point - designer not taking advantage of the properties of the outboard but trying to capture some of the benefits of an inboard. Others have achieved both with a well - Achilles and Anderson (as Seajet will tell you) or some Hunters as examples, but you might then lose something else, typically convenience or loss of cockpit/locker space to allow the motor to tilt as in the Hunters.

No such thing as a free lunch.

 

[VO5]

Unfortunately, for such a discussion as this, available published evidence supports both theories, fixed prop = less drag, rotating prop = less drag.
Papers on the aerodynamic behaviour of airscrews are of doubtful validity, the shapes of blades and the behaviour of the element being sufficiently dissimilar to make any but approximate parallels misleading.
The work done at Wolfson suggests that fixed props do produce less drag at low speeds, probably to the point at which laminar drag becomes a smaller component in the overall drag, to be replaced by turbulence.
A further complication is that provided by feathering and folding props.
By practical experiment, I've found that an unlocked prop starts rotating @ 2.5-3.0 knots. Being an Autoprop mine won't feather unless braked and both drag and noise increase geometrically as speed increases.
As my boat surfs quite easily, I've never experimented at high speeds, but fear that the drag of an unbraked propeller would prevent that happening and the vibration from the rotating assembly would shake it to pieces.
So I for one am quite happy with the hypothesis that an unbraked propeller increases drag.

I confirm to have had the same experience with my 3 blader Bruntons Autoprop. It would start to rotate at slower speeds. The makers offered the reason that one of the blades if not instantly aligned would be enough to get the prop to spin.
They suggested locking the gearbox in astern while sailing.

I don't like this idea at all. Gerboxes are known to lock if this is done.
Also some gearboxes are not lubricated while the engine is not running. Also it is pointless to have it spinning intermittently as that defeats the object.

So I fitted a shaft lok assembly in the engine room with a push pull control from the cockpit to lock and unlock as needed.

I am no longer concerned with the arguments that rage about the effect on speed of fixed props whether spinning or locked.

What I do know is I am guaranteed silent, vibration free sailing and an increasre of speed of minimum 1 1/2 knots...plus...whereas before she would not move below a top force 2 / bottom 3, nowadays she will ghost along on only 4 knots of wind. In a tide free environment I got her to ghost (only main and Genoa) in 2 1/2 - 3 knots of wind.

That to me is worth the investment I have made by radically changing the propulsion model and its management.