Prop drag

[pugwash]

I have a 1969 30-ft Holman Sovereign ketch with a 20hp diesel. I don't know the size of the prop but it's on the chunky side. Hiscock says prop drag reduces a boat's speed under sail by 20 percent. Could it really be as much as that? Are there any drawbacks to a folding prop I should know about when considering whether to lash out £500 or so and fit one?

 

[andrewhopkins]

im no expert

but my understanding is that it makes a big difference (hence the handicap effect in racing) on speed.

The downside seems to be slightly poorer overall performance of the blades and slighly more still in reverse. It also can have a dead band when switching from forward to reverse.

Other downside ? one more thing to go wrong ?????

 

[Twister_Ken]

Big difference between folding and feathering

Folding a poor excuse for a prop, feathering an improvement over a conventional prop esp in reverse. Unfortunately, big difference in cost too. There's something in the current Twister Owners Mag about this - I'll post on it when I get home tonight.

 

[pugwash]

Folding/feathering

Maybe I'm getting mixed up here. Do I mean feathering? I was captivated by a prop spinning in a perspex bath at the boat show a year or two ago and it LOOKED very efficient - I think it was a feathering job. Yes, the cost is enormous but 20 percent of speed is worth asking Santa to drop down the chimney.

 

[Guest]

I am going through the same proccess and changing the sail wardrobe at the same time so let me know your findings. have you tried DARGLOW Marine ?

 

[Guest]

Swapping a fixed prop for a folding one also has the advantage that the water flow over the rudder is cleaned up. ie: feels better on the helm. In my experience, feathering blades tend to catch weed. I guess this is why Grand Prix race boats often have a window in the hull just above the prop.

Given the right size and pitch there should be no loss in forward thrust with a folder when compared to a fixed two blade. Where you will lose out is in reverse. You may not be able to shove the boat about in close quarters with the same aplomb, when compared to the fixed one. Nevertheless, one can learn to compensate.

With regard to the sainted Eric, I would take his advice with a pinch of salt. After all he wrote about 40 years ago and things have moved on since.

 

[RobertMartin]

Re: Folding/feathering

Just get up 20% earlier and you'll get the yacht of the dock 20% earlier!!!

Saves cost,
Saves fixing a prop in the atlantic that is not opening quite right,
and we are sailors, we sail, who cares about 20%, get a motorboat if your after speed.

Bobby aka Seawolf..
Sail as if your free

 

[Guest]

A well engineered and designed feathering prop has several advantages over a fixed prop, not the least being the improvement in sailing performance. They function as well and perhaps better in reverse, with a slight lag in grip initially when selecting reverse. They also work exceptionally well at low RPM when motor-sailing. I dont know of the design of your boat but being a Kim Holman design I guess she's a longish keeler, this configuration has shown the best improvement with a feathering prop fitted. I would go for one, the Brunton Autoprop probably.

 

[roger]

I noticed a distinct difference in light air performance when I went from a two bladed folding prop to a 3 bladed fixed one. A typical value I've seen quoted is a loss of 3/4 Kt..
Obviously in strong winds you have enough thrust to push you along at hull speed anyway.
Downsides of funny props:-
high price including spares, anodes etc.
maintenance requirements especially for the very clever ones.
Unreliability - there is a lot to go wrong
Wear of the moving parts and perhaps a shorter life than a fixed prop.
You should at least make sure before buying that your present prop is well matched to your boat - that you can just get the engine speed up to ratedmaximum in smooth water (no wind) and not much more. Mistakes will be expensive to put right.
My feeling is that folding props are less easy to use to the limit in tight manouvering situations so you would be wise to be less bold in the marina in a cross wind for example.

 

[Twister_Ken]

Re: Folding/feathering

A guy has put a feathering 3 blader on his Twister. A quick summary of his points is:

Managed to source a variprop in Germany for approx 60% UK price.
Fitting took 30 minutes.
Pitch can be adjusted forward and astern (separately) using grub screws. If you get the pitch wrong, you've got to get at the prop again (dive, lift or dry) to change it.
Because the leading edge of the blades leads both ahead and astern, astern is more powerful than on a fixed prop.
Stopping distances have decreased allowing him to use more speed (thus control) when entering a berth, knowing that he can slap on the brakes when he needs to (believe me, this is a concern in a Twister!)
After 200 hours motoring, no reliablity problems so far.
Hasn't stopped the infamous Twister trait of being difficult to control when making sternway though!

 

[vyv_cox]

Re: Folding/feathering

I fitted a 2-bladed Brunton Autoprop to my Sadler 34 last year. It has changed the motoring characteristics somewhat and reversing is slightly easier with less kick. Power is delivered at lower revs than before, but it is a bigger prop than its fixed predecessor. I feel that sailing speed is better but I would be hard pressed to claim any objective measurement. No details are given of the claimed three-quarters of a knot improvement. I guess this may be possible with a very slippery hull if changing from a fixed three-blade, but for a cruising type design, especially a long-keel changing from a fixed two-blade, I doubt whether the difference would be detectable

I bought a new prop to match a new engine. Additional cost of the Brunton was more than 100%, 1000 pounds against something like 475. I would be fairly unhappy to have paid this much solely for the advertised prop improvements, but as I was changing anyway I am reasonably satisfied.

Incidentally, the Brunton is much cheaper bought in Holland, but you have to come here to collect it. I now find that the Aquadrive 20000 and 21000 are also cheaper in Holland than in UK and Spain.

 

[gunnarsilins]

Had a 24\" fixed...

...3-blade on my old Moody 42 ketch and went recently for a MaxProp feathering of the same size instead.

Noted benefits:
1: Noticeable better boatspeed, especially in light air
2: " " rudder response under sail, especially in higher speeds.
3: Better thrust in reverse.
4: Noticeable cleaner wake.

Drawbacks:

Apart from the cost, none. (OK, it took some time to assembly it and get everything right)
I have no comments regarding reability yet, had it only since September.

 

[pugwash]

Try this...

Thanks for all the helpful responses. For those still interested, try www.autoprop.com which has lots of useful info plus a stunning testimonial from Tom Cunliffe who ought to know what he's talking about. I'm fully convinced. Father Christmas are you listening?

 

[PeteMcK]

Changed to folding

Over seven seasons I've gone from a fixed two blade (rough both literally and in performance terms!) to a fixed three blade, high blade area Lancing Equipoise (absolutely excellent ahead, LOTS of kick astern), to a Radice two blade folder, on a Moody 33. The Moody has a 38hp engine so it seems stupid to do anything which locks all that power away and there's no doubt the folder does. On the other hand, in racing we were given a 30 seconds in the hour allowance over a folding or feathering prop for the fixed 3-blader, which is nowhere near adequate compensation: the boat was utterly uncompetitive. One of the US prop makers sent me a copy of an article from, I think, Cruising World, comparing all the configurations. From hazy memory, the drag differences at 5 knots were in the ratio 10 / 25 / 100 for folding 2-blade/feathering/fixed 3-blade. The best estimate I could make was that we needed an allowance of about 3 minutes in the hour to be able to compete when a fixed 3-blader is fitted. The boat is well and truly competitive with a folder (3rd in class at the Scottish Series) and the motoring power is just about adequate but without the excess available on tap which she had before. I don't think she'd be much good at motoring into a full gale any more. Fuel consumption is up by a whopping 50%!
My observations:
Fixed 2-blade - Cheapest but worst of all worlds unless you have a long-keeler and can lock it behind the deadwood.

Fixed 3-blade: Cheap (my fancy Lancing prop with twisted blades was under 150 quid five years ago, plain "round" blades are even cheaper), smooth, efficient, simple. The powerful stern kick turned out to be a boon once I got used to it: I imagine it would be less pronounced with plain blades.

Feathering: Can give better astern performance than a fixed prop but at up to 10 times the cost. More vulnerable to corrosion and mechanical damage. If you race, there's still the niggle that it's costing 2 or 3 times the drag of the folder and about 4 times as much to buy. Not for me.

Folding: DON'T. Unless you race enough to justify it and then it's essential. An awful lot of your precious 20 hp would be soaked up in stirring the water. (And does 3 minutes in the hour - or just half a minute according to my handicap committee! - matter if you're cruising? If it it does, offload all that heavy junk you're probably carrying first!)

 

[Guest]

You\'re third most import decision in life

After choosing your mate, boat, the next is without doubt the prop !

Like most things in sailing it normally comes down to a compromise of some kind.

My only advice would be is don't worry about the cost in money terms.

 

[HaraldS]

From all I had read, Iwould have taken an autoprop, then when I talked with my yard they said that they had tried several feathering and folding types. They suggested that I take the danish flex-o-fold which was also half the price. I did agree and haven't regreted it.
I get 9.2 knots forward at 43 ft waterline with a 100HP Yanmar. The engine runs slightly lower than specified at full throttle: 3500 instead of 3600. I usually cruise at 2300 rpm which gives me 7 knots. From max speed I can stop the boat on one and a half length. It works fine backwards with some small initial propwalk.
Despite the fact that it is knd of a folding prop, it develops enormous pull backwards, though it does need some power to make it bite when changing from forward to reverse.
I'd probably takeit again, though I would becurious to try differnt props myself.

 

[pandroid]

Re: Had a 24\" fixed...

I'm still suffering from indecision. I'm looking to change our fixed 3-blader for either a Maxprop or a Volvo folding 3 blade. The Volvo folks say that they aren't keen on the Maxprop as it puts strain on the stern gear when changing pitch (direction) and the Volvo has a special bush which reduces this. However, the Volvo is a folding (albeit a posh one) and other folks seem to speak highly of the Max prop. Maxprop themselves say that HR currently underpitch the existing prop because of the drag.

Any views? What should I do?

 

[alant]

Re: Folding/feathering

Anyone used the Volvo feathering prop

 

[Guest]

Volvo Three Blade Folding Prop

I have changed very recently from a Volvo two-bladed folder to a Volvo three-bladed folder fitted to the Volvo 2020 with saildrive on my HR 31. I changed because, although excellent under sail, the two-blader didn't put much effective power into the water. It achieved a fraction over five knots at maximum economical cruising revs (2500). The three-blader now achieves a little under six and a half knots at the same revs. More importantly, I can maintain a similar speed into moderately significant sea states; previously, a strong headwind and chop in water too narrow to tack brought boat speed almost to zero. Equally importantly, there is no noticeable increase in drag under sail.

 

[gunnarsilins]

Pandroid!

I´m not shure what the Volvo folks mean when they say the Maxprop would put strain on the sterngear.
It might maybe happen if you have a saildrive - the Maxprop is much heavier compared with a fixed one but on a conventional shaft, no I hardly beleive it.
Anyway, the Volvo prop was out of question in my case because it cannot be delivered in a big enpough size, apart from the fact that I don´t trust in folding props when talking reverse thrust.