Drag effect of freewheeling fixed prop

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srah1953

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I know this general subject has been well covered.
I am considering changing from fixed prop to a folding or feathering prop and have read through a lot of different posts on the subject. The argument for changing relates to improving sailing performance. I have a Yanmar saildrive and Yanmar requires that the gear be left in neutral when sailing. It spins and quite noisily. I understand that a fixed prop freewheeling has significantly less drag than a fixed one in gear. So, I'm wondering can anyone advise on the improvement that might be expected in these circumstances?

Thanks
 
dylan said:
A test carried out here
Click to expand...

Thanks for this. I had read somewhere that freewheeling had about half the drag of fixed. This test suggests an even greater difference. But I'm still left with my original question, which is, allowing for the fact that a freewheeling prop has considerably less drag than fixed, what might be the potential benefit of changing from fixed and freewheeling to a folding or feathering prop?
 
A folding prop virtually eliminates drag under sail. You will notice the difference at low speeds in light airs but the benefit reduces (in %age terms) the faster you sail. Downsides are mainly to do with motoring performance, particularly reverse, although the latest designs aimed at cruising boats are little different from a fixed prop. Feathering props have slightly more drag, but generally better for motoring. However, much more expensive to buy and maintain. Bruntons Autoprop is a different concept as the variable pitch means the pitch is automatically set to reflect the load placed on it. Main benefit (apart from drag reduction under sail) is in motorsailing, so very popular in boats that motorsail a lot in light airs.

So, when making your choice you need to decide what it is you want to achieve in relation to your type of sailing.
 
Unfortunately the answer (from the work done at Wolfson) is not as simple as one would like - whether a spinning propeller produces more or less drag that a fixed one is down to the speed through the water and, equally, importantly the underwater configuration of the vessel.

Drag will be minimal with a folding propeller, slightly more with feathering props (which have to be locked to work) and most with fixed props (whether freely rotating or locked).

In practice, the choice of a propeller type, purely on the grounds of drag, is over-simplistic and, as pointed out by Tranona, needs to be approached with some caution.
 
A feathering prop will almost certainly offer lower drag than a freewheeling fixed type but don't expect to see massive differences in sailing performance with one. I have had a Brunton's for years and it is excellent, but I cannot say I have noticed much difference when under sail.

As Charles points out, a feathering prop will continue to rotate unless the shaft is locked. With a Yanmar gearbox on shaft drive you need to select reverse to lock it. I cannot say whether the same applies to a saildrive.
 
I have a Brunton's Autoprop. The drag definitely increases if it is allowed to freewheel (because when freewheeling the auto prop doesnt feather) and the boat speed therefore increases if the shaft is locked when sailing.
I think this is different from most other props.
 
A fixed prop gets you a rating allowance for racing, but racers almost always have folding props or Maxprops.
So the drag is probably significant.
I've seen figures of 0.2 to 0.4 knot mentioned.

Also a racing boat would not want a spinning prop churning the water over the rudder when pushing it downwind.
 
see here
For info on your saildrive. Page 6 section 2-1 b & c.
Interesting that Yanmar advise returning to neutral after astern for these props, so in the end it's the same advice as they give for fixed bladed props.
 
Thank you all very much. I hadn't realised feathering props needed to be locked (which I can't do because Yanmar say it will invalidate warranty, so must be doing some damage). Also useful info on Autoprop.
 
dylan said:
No they must be unlocked! It says to lock them to make them fold then unlock them.
See my post#9
Click to expand...

+1

I have had two 2-blade max prop and one 3-blade MP, the shaft needs to be locked momentarily to make them feather, then (meaning a couple of seconds later) it may be put back into neutral if you need to, it will not rotate.

OTOH, do not stop the engine when in reverse as the propeller will remain open, this feature is used by people with MaxProps and shaft alternators for example
 
At last something I know something about!

I fitted a feathering prop this year to replace a two bladed fixed prop.
A significant (to me) improvement - at least .25 of a knot, probably nearer .4. This is about 5% to 10% improvement. Most noticeable when close hauled - the reduced drag is very welcome - even in stronger winds (force 5).

I made the change for sailing performance but have been very pleased with motoring performance as well - particularly into any sort of chop.


PS no need to lock it - I just put into neutral and turn engine off. Occasionally, I've checked that prop isn't rotating (it never has been)
 
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We use a kiwi on yanmar 40hp sdrive from new

It does what says on the tin

Change zinc every year only because of erosion at rope cutter location.

We also have copper cote so anual out of water time is minimal ie lift and hold .
 
I follow Maxprop's instruction for it's propeller to leave the gearbox engaged in forward at low speed, stop the engine and then disengage the gearbox. This feathers the propeller and stops the shaft turning. If I engage neutral and then stop the engine, the shaft keeps turning.
 
nimbusgb said:
Kiwi prop is a feathering type and does not need locking to stop rotating. the blades truly feather in line with the water flow.
Click to expand...

Brunton Autoprop is a feathering type too. I can assure you that if you do not lock the shaft by some means it will continue to rotate. That's not to say that its drag will be huge but it is definitely rotating, maybe half-feathering. I am guessing that this is some function of its self-pitching design, maybe the Kiwiprop is different?
 
srah1953 said:
Thank you all very much. I hadn't realised feathering props needed to be locked (which I can't do because Yanmar say it will invalidate warranty, so must be doing some damage). Also useful info on Autoprop.
Click to expand...

No problem fitting feathering or folding props to Yanmar saildrives. In fact Jeanneau offer flexofold folding props as factory options. as already suggested, once the prop has feathered/folded you can put the gear into neutral.
 
nimbusgb said:
Kiwi prop is a feathering type and does not need locking to stop rotating. the blades truly feather in line with the water flow.
Click to expand...

If you have a shaft drive that is not parallel with the water surface, then I suspect that a good quality feathering prop might even offer less drag than some folding props. A two blade folding prop will have more drag if it folds in the horizontal plane than if it folds in the vertical plane if on an angled shaft. I have a volvo folding prop with curved scimitar like blades and I suspect that it must be quite draggy if it folds in the wrong plane.

For the OP, don't get a Volvo prop. Partly for the reason I mention, but also it has really bad prop-walk. It is also a heavy lump to have on the end of a drive shat. If I had the opportunity to make the choice again, I would get a Kiwi prop, and if I won the lotto I would get a Gori three-bladed - that is really a clever design.
 
vyv_cox said:
Brunton Autoprop is a feathering type too. I can assure you that if you do not lock the shaft by some means it will continue to rotate. That's not to say that its drag will be huge but it is definitely rotating, maybe half-feathering. I am guessing that this is some function of its self-pitching design, maybe the Kiwiprop is different?
Click to expand...

I suspect that if you apply moderate braking force to the shaft, it will pitch itself to provide more torque until the point that it stalls and turns to feathered position. I've always been interested to experiment with a Brunton to see how well it would perform on a hybrid regenerative system. It would be good if it performed as I suspect and so you could let it freewheel to generate electricity, or brake the shaft to feather it for best sailing performance.

A few years ago when I had a fixed prop I tinkered about with a bike computer on my propshaft to count rpm and a variable torque brake (cord around an inline pulley, attached to a spring balance) to get some power curves from my freewheeling prop. It would be interesting to do the same with a Brunton. My folding prop does also spin if I leave it to freewheel and one day I might see how it performs too.
 
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