folding props

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zefender

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Volvo's latest ad talks about its new 4 blade folding prop. In the copy it claims speed increases of 'up to' 1.5 kts. Has anyone any experience of speed increases via the 3 blade folding version it succeeds? COmments about the new 4 blade? Are they horrendously expensive? Are the speed claims valid? Is there a price to pay in performance in reverse?


I don't race but 15% or so extra speed under sail would be welcome!

Any comments?

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are they claiming extra speed under engine (compared with what?) or are they talking about reduced drag under sail which any 2, 3 or 4-bladed prop will achieve?

it would be interesting to do a test.

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They are claiming the speed increase for sail but also talk about less vibration etc so presumably a benefit for either.

My main question though is about less drag when sailing. Are these kinds of speed benefits achieveable? The 4 blade seems to be designed for boats 40'+. I'm not sure whether the 1.5 kts increase applies to a 70' monster, maybe normally sailing at 12kts or my 40', bumbling along at 6.5.

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yes, you could get up to 1.5 knots improvement by changing from a 4-blade fixed to a folder. less difference if you start with a narrow 2-blade fixed.

any folder will give this level of improvement. note that the volvo 3 & 4 blade folders don't give a very tidy profile when folded and will have more drag than a 2-blade like flex-o-fold.

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Not in my experience

I replaced a fixed two-blade with an Autoprop. There may be a slight speed increase but it's nowhere near 1.5 knots. The total cross-sectional area of a prop is small in relation to the CSA of other underwater surfaces and the reduction obtained by making it fold cannot be more than a couple of percent. Even taking the area of the fixed prop as the total swept diameter, which I understand to be the correct method, the change is probably well under 10%.

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Re: Not in my experience

Vyv, was that speed increase under sail or power? I'm asking because i am seriously looking at the autoprop for my boat, and would like your views on it, the good and the bad, the prop has got rave reviews from a lot of people, but what's your opinion on it.............................keith

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We went from a 3 bladed fixed to a 3 bladed folder (MaxProp). I'd say the 1.5 difference is quite achievable - but it may not always be at the top of your speed range. When the boat is fully powered up there's probably power to spaer to drag a fixed prop along.

The big difference is in the marginal conditions, when you DONT have excess power. Consider drifting along in F2 - and the difference a towed bucket might make. Our 'normal' maximum speed went up by about a knot - our marginal speed went up by more than that. Because the Maxprop self pitches (and I dont know that the Volvo does r doesnt) you also get a tremendous benfit when motor sailing, as the engine revs can be dropped, the prop will set itself to a really coarse pitch, and you still get a useful lift with the engine ticking over.

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Thanks for that. Yes, its really in moderate conditions that I'm more interested. The kind of time when maybe you might think about using a bit of engine to motorsail back to the berth when time is getting on!

What kind of cost do these things cost - very roughly. Is it a mortgage or just horribly expensive? Thanks for comments.

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rough costs: for a 16inch prop, brunton quoted around £1500 against half that for the flex-o-fold i settled for.

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Re: Not in my experience

We're very happy with it.

Sailing speed - maybe 0.5 knot. This is not to be sniffed at, half a knot is a worthwhile advantage. But 1.5 knots, no way.

Under engine a big improvement, better manoevrability and better torque at low revs. We definitely have more power lower down. Less kick, particularly astern. Motor sailing is pretty much as the reviews say, 1000 rpm plus sailing in light winds makes a considerable difference to speed, is quiet and very fuel efficient.

Very good going astern, this is the big advantage over a folder. Emergency stops highly effective.

First season we consumed its own anode but last winter replaced the shaft anode. Last week we hauled out and its own anode was untouched, shaft anode a little corroded.

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I guess it depends on your perspective ... in our case I seem to remember a bill of more than £2k - but we aso fitted cutters around that time, and I cant off hand remember what that bill included. We also needed to be lifted two or three times to fine tune the pitch limits to ensure best performance, and that cost about £100 a time.

The only significant negative is the lack of 'bite' at zero speed - the pitch is very fine at low speeds, and as a result you dont get huge initial grip. It is very manageable - but look out for it to start with. Against that, if you go astern with way on, then the grip is immense, and the boat stops very fast, which cant be said of all folding props.

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Re: folding props: stopping power

our 2-blade folders will stop the boat from 11 knots in less than her own length!

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Racing handicap allowance

Typically, the handicapping systems give you an allowance of only about 1% on the time correction factor if you have a fixed three blade instead of a folder. In my own experience racing with three different props fitted (and appropriate handicaps for the setup), it's not quite enough but it's not far away. I'd say the difference is in the 2-3% range and nowhere near the grand results claimed by some prop manufacturers.
Note, however, my folder, a Gori, consumes twice the fuel my Lancing 3-blade used.

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Re: Racing handicap allowance

Is that Gori a 2 or 3 bladed folding prop?

I've just fitted a Gori 3 blade, and I wasn't anticipating increased fuel consumption. I shall have to watch it carefully.

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Re: Not in my experience

Agree with this. We have a 3 bladed MaxProp and I sometimes forget to feather it (put it astern) until I hear the tell-tale rumble of the prop shaft rotating. It generally gives about 0.5 knots, perhaps a bit more in light breezes but no way have I seen anything approaching 1.5 knots.

It's power astern is awesome!

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