The Offshore Voyaging Reference Site

Feathering Propeller Review

When we took delivery of our new OVNI 435 in 2008, we decided to stick with the standard 3 bladed propeller, partly for reasons of cost (we were running out of cash!).

But on all of my previous boats I’d had either a folding or feathering prop, and fully intended to fit one to Pèlerin when money allowed – none of these units are cheap, but the fixed blade could then act as a spare. Fixed props are fine and have predictable characteristics, but as has been demonstrated in numerous prop tests, a fixed three bladed prop has about the same drag coefficient as a boat’s hull.

When under sail, if the gearbox manufacturer allows, the prop can freewheel, that’s about the equivalent of towing one bucket, but if the prop has to be locked in gear, then make that towing two buckets. That will make a major dent in any boat’s performance, especially in light airs. But the prop drag can be reduced entirely by fitting a folding prop, and by over 90% by fitting a feathering prop, so we knew we wanted to make the change as soon as we could.

What Prop to Choose?

A folding prop was not possible due to space restrictions and memories of poor astern propulsion leading to heart-stopping moments in tight corners. The small aperture around the prop also meant that the interesting Autoprop, which has performed well in a number of trials, couldn’t be fitted. As ready cash was still in short supply, the excellent Variprop we’d had on our previous boat for many years was out, as was the highly recommended Maxprop, too. An interesting development at a good price looked to be the Kiwiprop, with its Zytel blades, so we decided to give it a try.

We were happy enough with the swap, and could soon see the benefits in terms of our daily average runs. Performance under power was good, although the grip astern was less impressive. It was pretty noisy, too, although that wasn’t helped in our boat by natural resonance coming through the flat aluminium plate above the prop. Overall, we were happy enough with it at the price.

Sadly, the prop came to a sticky end up the Owenboy river in Cork, southwest Ireland, when one of the rollers (that stop the blades from traveling too far in astern pitch) came out. A diver carrying out a brief inspection also removed the other two rollers, where both pins were bent. There was nothing for it but to remove the prop, send it off for repair, then re-fit the original fixed-blade prop.

The helpful people at Kiwiprop repaired it and sent it back. Their report suggested that we’d picked up a rope or wire that had caused the damage, and the prop had also been upgraded with the latest titanium pins for the blades. But we never got around to re-fitting it, and when we met someone who was looking for a Kiwiprop of the same dimensions, we decided to let it go.

Try, Try Again

We then put that money towards a new prop, and having sought recommendations decided to go for a new British made Featherstream, a three bladed feathering propeller. The Featherstream uses a bronze hub with stainless steel blades –  stainless allowing thinner, more efficient blades that are more durable and corrosion resistant. The prop seems well engineered, came with an exemplary installation manual, and the price was very competitive.

The blades swivel through 180° to present the same leading edge ahead and astern, the pitch can be adjusted externally (although it’s not easy), and maintenance is claimed to be simply external cleaning and internal greasing annually.

We haven’t got much experience with it yet, but the pitch ahead seems spot-on straight out of the box, and astern is particularly impressive with much improved stopping power. Noise levels are slightly higher than with the fixed prop, but by getting the revs just right it’s possible to reduce the resonance significantly. And now that we know the boat well, we can really feel the difference in sailing performance, particularly in light conditions, which is where we’ve worked hard to make an improvement to our boat.

The Good and the Bad News

Are there drawbacks to installing a feathering prop, cost apart? Well, with most flat blade models, cavitation can occur in certain circumstances. I’ve only ever had this happen once, on our old boat whilst punching out of a harbour into a strong headwind with a really nasty short, steep chop, where fortunately we had enough room to ‘tack’ under power. Fine tuning the pitch to suit the boat can help here, made easier if you can adjust the pitch in situ. And as the prop doesn’t turn when under sail the possibility of running a prop shaft alternator is removed. And there’s no getting away from the fact that they are far more complex than a fixed prop, which will not go down well with adherents to the ‘KISS’ principle. But reliability (in my experience) is good, and if properly maintained they give little trouble.

The big bonus is undoubtedly the improvement in sailing performance. All things being equal, we think feathering props make a significant improvement to a boat’s sailing ability at a cost worth saving up for. Better light airs performance should translate into less fuel consumption and so greater range and endurance – all worthwhile benefits for cruising to remote areas.

Have you had any experience, good or bad, with these or other feathering propellers? Please leave a comment.

Further Reading

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Rikki

Oh no! The dreaded rotating vs. locked prop issue has been raised.
Actually, a locked prop has much less drag than a rotating prop.

A Kiwiprop fits in well with the KISS principle.
Hold on to something before engaging reverse.

John Harries

Hi Rikki

That’s an interesting one, but I’m not sure the answer is that clear. At one point I was on your side and my justification was that a helicopter that has an engine failure and a jammed rotor falls like a rock, but one that has the same failure but can allow the rotor to spin can land, albeit it hard.

On the other hand, when I was a PHRF handicapper years ago, we did some tests and the answer seemed that in most cases the boat was faster with the prop rotating.

I suspect that the answer may lie in the pitch and blade shape of the prop in question. If the locked prop is fully stalled, then lift and therefore drag would be lower. But if flow attaches to the locked blades, then perhaps drag is higher, since, as I understand it, lift and drag are directly linked—more lift, more drag.

Bottom line, I’m just confused!

Chris

When I was still flying, we always feathered the prop on a dead engine. If we couldn’t feather it but could lock it we would. Our range and glide ratio improved. Our understanding was the drag associated with the tip vortices of three stationary tips was less than the very large spiral vortex of vortices associated with a spinning prop.

Colin Speedie

Hi Rikki and John

I’d always believed that a locked prop (with a fixed prop) caused more drag – certainly looking at the turbulence behind our Ovni when under sail with the prop locked (experimentally) in reverse with the three blader it seemed far worse than when the prop was allowed to rotate. But as you suggest, there are reports to the contrary, and I’d love to hear the definitive answer.

Not that we had a choice as our gearbox manufacturer (Hurth) state that the prop should be allowed to rotate with a fixed prop, and only locked in reverse with a folding or feathering prop.

Chris

Our experience is it is better not to put the transmission in reverse even with a feathering prop. When a prop sits in a free flow stream (such as a test tank) it sits there quietly, unmoving. When it is in a natural environment with the swash and eddies that spin off the waves, hull, canoe-body, skeg, strut, etc, they don’t rotate, but they do move. In our first year with our prop (and a Hurth)—after a long sailing leg locked in reverse, we found ourselves completely, irrevocably locked in reverse. We had to start the engine that way. Since then we let the transmission stay in neutral when sailing feathered. Mfg rep said we were not the first to observe this…

Chris

Here’s the skinny from one of my aero-hydrodynamics friends. IT DEPENDS.

Imagine the following:
Water is flowing down a stream and across a straight rock lip. It cascades off and flows into a new stream. From time to time if the lip flow is disturbed, a small bit of super critical flow will cause a stronger dribble, but it won’t last long and the flow will straighten back out. This is form drag caused by the water turning a sharp corner. This is water flowing across the edge of a locked prop. Partially contrary to my earlier remark, vortex formulation is rare with a sailboat locked prop in a free flow environment. (More on that later).

Now think of the pictures you’ve seen of spinning, vortex forming propellers in operation. In the design process, vortex formulation is minimized by shaping the propeller twist and tip curve — for normal cruising prop RPM.

Now consider a free-wheeling prop now spinning in a condition that puts it well below design cruising rpm but well beyond the water flowing over a rock lip mode. In this condition, the prop is at the wrong angle of attack for the water speed compared to its rotation speed so it creates both form and lift drag. Also, the prop twist and tip shape is no longer optimum because the RPM is much lower. Props in this condition become vortex factories [and if you don’t think those create drag, consider all the airliner mods to improve fuel economy by adding tip winglets to shrink the vortexes.] So we now have form, lift and vortex drag all adding up, when form drag was all we had before. But again we are talking about free flow environment.

Non-free flow environments — those modified by hull shape — don’t change the basic physics, but they can change what excites prop behavior. In the end, the biggest factor appears to be vortex formation as a function of propeller rotation speed — which may not smoothly correlate with boat speed due to hull flow effects.

Beyond IT DEPENDS, the general rule we agreed upon was props on struts well away from the hull would be most likely to be draggier when freewheeling than locked. Props in apertures might see no measurable difference. Props under hulls with flat runs aft might also see more rotational drag than hulls with greater deadrise aft. Also if one’s keel sheds significant turbulence at prop level one may see a sinusoidal pattern imposed on the prop rotation speed as fluidic effects arise.

In the end, we concluded sailing in both locked and freewheeling conditions on a day with steady winds, under a lee for an equal distance with a stop-watch would provide the best boat by boat answer — for those conditions

John Harries

Hi Chris,

Thanks for the great comment that explains a complex subject clearly. I think I even understood it…on the third reading.

Enno

Hi John
The maneuver that helicopter pilots fly in case of an emergency is not about drag but about storing rotational energy in the rotor. See here for an explanation:
http://en.wikipedia.org/wiki/Autorotation

Greg

I always thought so, but no longer. See http://www.catamaransite.com/propeller_drag_test.html

John Harries

Hi Greg,
Thanks for the link. Well that settles that, what a relief!

Chris

Colin,
We were fortunate to be able to fit a Max Prop from day one. We found their guidance on preventing rotation when stopping to be more hull-shape dependent than they realized. Experimentation found the answer. Keeping it full of grease and zinc protected has been key to our ability to trust it.

Colin Speedie

Hi Chris

I’m intrigued by your findings re hull shape – I’ve noticed that with all of the feathering props I’ve tried the prop would occasionally rotate slowly, but have always just put the gearbox astern and that’s been it.

Maxprops have a great name for reliability, and although I’ve never had one, our last boat had a German Variprop for seven years of hard work, and was faultless. And we never did more than you mention – just grease and replace the anode once a season.

Chris

Colin,
When shutting down, we:
drop to idle,
shift to reverse,
accelerate to 1200 rpm for a three count,
shift back to idle and shutdown simultaneously.
The prop “flops” from reverse to feathered and stays there.

Colin Speedie

Hi Chris

Thanks for those three thoroughly enlightening comments – real food for thought.

Following your first comment that segues nicely into the third, I’ll change my ways – and there was I thinking it was all so easy. Our boat has a very flat run aft, which may explain why we saw so much turbulence from the old prop when locked.

And thanks to you and your aero/hydrodynamic friend for such a lucid explanation of a previously opaque subject!

Best wishes

Colin

Geir ove

We had a 3 blade Gori on our mono, no problem. Very happy with it. Change out the zinc and rubber dampers every 2 years. On our new Cat we will have 2 Maxprops, and hope to be happy with those? Saildrive this time.

Denis Bone

I installed an Autoprop a few years ago. I had to cut a chunk out of the rudder leading edge to allow enough space for the prop blades to swivel but my Fisher Freeward 30 has a huge rudder and this made no difference to steering. Performance is everything claimed for the propeller both ahead and astern but most impressive was the more than one knot increase in sailing performance. Only complaint, the plastic screws holding the anode are impossible to really tighten because they feel as though they are about to break before they are satisfactorily tight. I also got the impression that the tapped screw holes in the boss were fouled and the plastic screws were not capable of clearing the fouling. The holes must then be cleaned using a plug tap and probably removing metal every time you change the anode. Brunton provided me with stainless steel screws and advised me to coat the areas of the anode around the screw bosses with antifouling to prevent them corroding away, as was happening, resulting in the anode falling off.

Colin Speedie

Hi Denis

We would have liked to try the Autoprop, but lack of space due to its shape ruled that out. On the face of it, they seem to offer so many benefits, and your experience certainly seems to bear that out.

The antifouling tip re the stainless screws is the same one that we use – and saves a lot of time, money and hassle in terms of early replacement of the anodes.

Ben

Ever had any kelp or similar around the Autoprop?

How does it cope? It kills a normal prop, but at least you have enough grunt left to (hopefully) get you out of trouble…

Ben

I am thinking that getting a feathering or folding prop might improve my performance to windward and in light airs by much more than any other way I could spend that money (except by spending it on Diesel…).

Also did it improve the handling under sail, with less turbulence over the rudder?

Anybody heard anything about the 2 bladed CDI feathering propeller, looks like it’s almost within my budget.

I must say I have been impressed with the way a Slipstream with SS blades ate up any chunks of ice that went through it…a crunching sound like an ice machine and that was it, and lots of little ice cubes astern…I’m not sure if a Kiwiprop or CDI with plastic blades would fare so well?

Colin Speedie

Hi Ben

I’d agree that fitting a feathering prop will do just what you suggest – the performance benefits are noticeable throughout the range, so they represent money well spent to me.

Re the turbulence over the rudder and improved handling, certainly the boat feels a little nicer to steer in light airs, but I might be confusing the more sprightly performance with crisper steering – and as we have over tight new rudder bearings it’ll be a few miles before we can say for certain!

Stainless blades should be tough, and it’s good to hear that they cope well with ice, but equally the Zytel blades on the Kiwi should be equally tough. Zytel (I’m told) is extensively used in race cars so must be pretty durable, and it certainly wasn’t a worry for us, although we weren’t in the ice…

Ben

I suppose in the interests of safety I should add that any ice we hit with the prop was pretty small. If we were in heavy ice the engine revs and speed were kept near minimum to stop ice going under the boat. If we hit a bigger piece unexpectedly and it went under the boat (Thud bang bang wince…) we would leap for the throttle and put her into neutral as quickly as we could to try to protect prop, and drive train…