Point better with a folding/feathering prop?

[cmedsailor]

I sometimes read that a boat equipmed with a folding or feathering propeller will point better than with a fixed prop. Can anyone tell me why? What the prop has to do with pointing? Sailing faster, yes I understand this but sailing closer to the wind why?
Thanks

 

[yoda]

I think you are quite right, Overall performance will be better because you sail quicker and that (in my mind) has to be a good thing but I don't think you will sail closer to the wind.

Yoda

 

[dt4134]

It's all about the relationship between hydrodynamic lift (mainly from the keel) and hydrodynamic drag.

If you reduce the drag for the same lift you can sail faster and point higher.

CA Marchaj explains it far better with a few diagrams and a little bit of maths.

 

[grumpy_o_g]

The boat's leeway will reduce but the actual angle of the boat (i.e. it's heading) relative to the true wind won't change as far as I can see. If you draw a simple vector diagram of the force in 2 dimensions parallel to the water surface you could end up with something like one line acting at right angles to the keel and pointing away from the sail (ie to port when the boom's out to starboard) - representing the keels resistance to leeway; one line going to or even passing through the stern, representing the hull's drag; and also one line pointing out more or less at right angles to the boom, perhaps slightly forwards of right angles, representing the force exerted by the sails.

If you change the prop from fixed to folding you are reducing the length of the vector passing through the stern as you are reducing the drag. The boat will therefore increase the forward component of it's speed whilst the others stay the same. e.g. you were going sideways at one knot but forwards at 4 knots. Change the prop and you are still going sideways at one knot but now you're going forwards at 5 knots. Your boat's heading in relation to the wind hasn't changed at all but you boats track compared to the wind heading has improved considerably.

 

[blackbeard]

The boat's leeway will reduce but the actual angle of the boat (i.e. it's heading) relative to the true wind won't change as far as I can see. ....

If another g o g might comment: I think that's right, but would add: if the boat is going faster then the resistance if the keel to sideways movement will increase. So the comparison is 4 knots boat speed and 1 knot leeway, against 5 knots boat speed and less than 1 knot leeway.
Looked another way; if the drag increased for some reason, keeping the sails, angle to wind etc the same, so that speed decreased, then you would expect the boat to make even more leeway. How did the boat go to windward last time you sailed it when you should have been giving it a scrub?

It might seem that the boat is closer winded with a folding prop since you could get away with pinching a bit, but that might not be the best way to sail.

For what its' worth; these things are just about impossible to measure in everyday sailing, and subjective impressions are unreliable, but I fitted a folding prop last year and my impression is that the boat goes to windward more happily and is faster, and handles better under sail, than with the old fixed prop. I have no way of estimating leeway accurately but an assumption of reduced leeway is consistent with observation. No obvious change in angle to wind.

 

[William_H]

Folding prop

The actual speed of the boat is important to this discussion. As the boat approaches hull speed (square root water line length in feet) the drag of the hull pushing through the water making waves becomes huge compared to the other forms of drag including propeller. However most of us I would think beat to windward at somewhat less than hull speed.
Pointing is a bit of a miracle to me. It involves having a little left over force in the right direction to push you along forward. The closer to the wind you point the less forward force you have. But of course more drag means you need more forward force to get movement.
I can really push my little boat to point very close to the wind but the speed drops off dramatically. Any additional drag would reduce the speed even more. So more drag would require me sail away from the wind to get more forward propulsion. So that is how I see it. good luck olewill

 

[Seajet]

There is a lot of B/S talked about this !

The drag from a fixed prop will make a boat slower than a folding prop example, like having a bucket trailed aft having a slingshot effect, so less rig forward power and slower, more vulnerable to external influences such as head wind & currents, and more leeway noticeable.

The benefit of going faster with a folding or no prop is through the rig working better, & apparent windspeed able to produce 'lift' that way - if it is well designed & matched to the hull enough to do it.

Keels come into it very much of course; the deeper / 'grippier' ( below surface drift, on some racing designs involving wings for grip and canting to allow better hull shapes in the water rather than heeling ), & less drag at the likely speeds - through shapes not just depth, the better the progress to windward.

Really radical boats don't use wing keels, just foils to give lift 'steer' to windward and canting keels for righting; they all rely on forward speed for and from the rig, and something like a fixed 3 blade prop' would have the same effect as dragging a concrete building & falling off to leeward proportionately !

 

[Burnham Bob]

quick bit of pedantry and an invite to abuse for nit picking but i thought hull speed was 1.34 x square root of water line in feet which makes my trapper's hull speed just under 7 knots as opposed to just under 5 whoch it would be using the formual above.

on a more serious note, i had to have a three blade prop when i re-engined.......anyone any advice as to what benefits a folding prop might deliver.

i'm a cruiser not a racer (apologies to Steve Miller and the Joker).......so will i really appreciate the difference?

 

[Seajet]

Yes you would benefit, as in faster passage times - handy even 'just' cruising, and speed available under sail may one day get the boat out of trouble, be it clawing off a lee shore or getting into port ahead of bad weather.

May also cut out wear, vibration and noise as there's no freewheeling prop to turn the shaft in neutral, if one is in the habit of doing that.

A folded prop is much less likely to collect weed & flotsam too; of course it's not so efficient in reverse, but I found the 2 blade folder on my Carter 30 ( 13hp saildrive ) perfectly adequate; wouldn't pay to be the type who roars into a berth then grabs a handful of astern power though !

 

[blackbeard]

...on a more serious note, i had to have a three blade prop when i re-engined.......anyone any advice as to what benefits a folding prop might deliver.

i'm a cruiser not a racer (apologies to Steve Miller and the Joker).......so will i really appreciate the difference?

It depends on - well, many factors, but chiefly on how well you boat sails and how important sailing performance is to you. If it's really a motor sailer and you use the engine most of the time - well, not much point. If it sails reasonably well, and you really value the sailing performance and wish to sail as much as possible, the answer to your question is "yes" and you should consider it.

Some other points - folders are good in ahead, but may lack bite in astern, they do vary a bit.
Feathering props are even more expensive than folders but are generally good in astern.
Folders can't be fitted if the propeller is close to the rudder eg in an aperture with a long keeler.
Manufacturers should be really helpful with advice, if one is not, there are other manufacturers.
Check shaft dia, taper, nut thread, engine, gearbox ratio as what the manufacturer may have on file might not be right for your boat.
See innumerable threads on this topic.

 

[Seajet]

As Burnham Bob says he has a Trapper ( not sure which one ), I'm assuming she sails quite well and has room to fit a folding prop.

 

[Skylark]

Within the greater scheme of things surely there are many other factors ahead of the prop as having a positive influence on beating to windward. Rig tension, sail cut and trim being just a few examples.

I'm not a boat designer but it makes sense that reducing drag will increase speed with a slightly positive impact upon apparent wind angle.

I always read the folding prop threads with great interest. Hard facts seem hard to come by and there's strong anecdotal views in both directions. During a typical season, how much do you sail, of which how much under power? Are folding props really less efficient so cost more fuel and/or do they saving sufficent drag to get there sooner.

Second only to the anchor (or colregs) debate

 

[Tranona]

Second only to the anchor (or colregs) debate

Thats because like anchors, peoples' experiences vary and what suits one boat and pattern of use may not suit another. That is also the reason why there are so many different designs on the market, whereas a fixed prop is a fixed prop (almost!).

My theory is that it is as much to do with peoples' attitude to their dispoasable income - run out of things to spend their money on in relation to the boat - and they do look shiny and exciting (even if you never see them except when you haul out). The downsides are pretty low and you can always convince yourself there is an improvement in some way.

This is speaking as a serial folder/featherer (as appropriate) user - in neither case convinced it was money well spent, but can say they do what I want them to.

 

[William_H]

quick bit of pedantry and an invite to abuse for nit picking but i thought hull speed was 1.34 x square root of water line in feet which makes my trapper's hull speed just under 7 knots as opposed to just under 5 whoch it would be using the formual above.

on a more serious note, i had to have a three blade prop when i re-engined.......anyone any advice as to what benefits a folding prop might deliver.

i'm a cruiser not a racer (apologies to Steve Miller and the Joker).......so will i really appreciate the difference?

Yes a pedant being pedantic about something that is very ephemeral. Hull speed is not a precise speed so the formular is not precise. The formular is really to indicate how hull speed changes with water line length.
Hull speed itself is a region on a graph of water speed as related to thrust. (sail or engine) In the region of "hull speed" the thrust (power) needed to increase the boat speed starts to become more non linear so a huge increase in thrust gives not much increase in speed.
Now the rate of change of this effect with speed depends on many factors including length to beam ratio and weight. (displacement for length). Catamarans don't seem to have feel the effect much at all.
Yes most boats can easily exceed their hull speed be it square root of wl length in feet or 1.34 times square root of wl length in feet. My little trailer sailer has been known to double its hull speed but no one said that was easy. (Damn terrifying but exhilarating actually) olewill

PS re folding props. A friend has a nice 28ft sailer with a fixed 3 blade prop. He didn't do very well in racing. Another guy came along with the same kind of boat but with folding prop. He ran rings around the first. Sure the first was OK for cruising but not much good next to a similar boat.