Autoprops

[PhatBuoy]

Forgive me if this has been discussed in past threads. As you will see I'm a new user so treat me gently.....
Very interested in hearing about any experience with feathering props verses 3 blade or 2 blade fixed units. Are Brunton's brochures too good to be true? And if they are better performing are they worth the cost??

 

[Twister_Ken]

Supplementary question

Got Bruntons literature in the post today, where it gives some supposedly scientific graphs about drag, speed, revs etc. Problem is you can't actually identify what sort of prop does what very easily, or associate numbers with each.

As far as I can tell, they show two types of feathering prop, as well as folding, and fixed 2 and 3 blade. Autoprop appears to do better than the other feathering prop (and everything else for that matter) which doesn't have the elaborately sculpted blade shape of the Autoprop.

Is this just lies, damned lies and statisitics, or is the Autoprop really better than other featherers? Anybody know?

In response to PhatBuoy, I've seen anecdotal evidence that feathering props are worth about an extra half knot sailing and motoring, so on a x-channel trip - say 60 miles at 5 kts, you'll get in the Cafe du Port about an hour earlier. They are most helpful in light airs. On a coastal trip, it might make the differencce between getting round a headland before the tide turns, so that hour saved could turn into three or four. A folding prop would give the same sort of advantage under sail, but would probably cost speed when motoring, compared with fixed.

 

[HaraldS]

Re: Supplementary question

Think they all make these claims and they are all half true. Or they compare the categories they are best in. Depending on which sales man you talk to, he'll point out a problem with the other. A Gori guy told me that Autoprops develop a huge torque when pulled through the water and on fast boats with smallish engines, like cats with dual engines it might actually start your engine. When I told him that I thought this was great as I had been looking for an emergency starter, he didn't look so happy.
The have been some 'independent' comparison tests in the past.
I ended up with a danish flex-o-fold that I'm very happy with and would put on again if I had to replace it.
Have given details on this prop on a previous thread.

 

[davidwf]

Got Autoprop on a Dufour 32. Not had experience of other types but can say that the autoprop does work well and seems to do all that is claimed for it. In particular its reverse thrust is fantastic for stopping.

At low speeds through the water the prop feathers so on intilally moving off you get very little prop walk (Ahead or astern) so you need to use higher revs to get moving but as your speed through the water increases the prop increases its pitch and you then reduce revs appropriately. The prop then continues to automatically alter its pich to suit the load being placed on it. Net result is that you can use optimum engine revs to drive against a heavy sea as the prop has reduced its pitch slightly due to the increased load. At low loads the prop goes to a greater pitch and you run on reduced revs.

After sales service is second to none. All in all very happy with my purchase.

 

[bedouin]

An old chestnut; as you'll see from a thread below I've been looking into props in great detail. This is a re-hash of what I remember learning from a variety of sources:

As far as I can tell, the Autoprop is probably the most efficient prop that you can put on a sailing boat - in terms of prop performance. Nothing else, fixed feathering or folding, offers the same performance in reverse - but then you have to ask yourself how much time you spend in reverse! The other major benefit is that it is 'auto-pitching' so should be more efficient while motorsailing than anything else - is motorsailing important to you?

The down sides include:

In terms of drag it is fairly inefficient - better than a fixed, but considerably worse than a folding, or the other 'feathering' props (Maxprop or Variprop). It is expensive - three times the cost of a 2 blade folding. I have also heard reports that it tends to turn while under sail - and many owners have fitted shaft-brakes to counter that; this reinforces the point about drag.

So it depends what you want out of your propellor. I have almost decided that a 2-bladed folding prop will give me the best compromise but everyone's criteria are different. The first question to ask is

"What is wrong with your current propellor?" and then decide how best to overcome the problems.

On the down

 

[kidnapped]

Buying a yacht with fixed prop and want to know more about 'folders/featherers' any sources of info would be appreciated.

 

[ultrea]

I fitted a three blade autoprop at the beginning of last season to my Oyster 39. I spend April to September cruisng in the med.
I now get 6.5knots at 1200rpm - a 10% reduction in revs.
I now have control going astern - previously going astern was a nightmare.
Brunton are first class people to deal with.
Downsides are:
Expensive.
You definately need a prop brake if you have a hydraulic gearbox. To say it generates high torque windmilling is nonsense in my experience, at least at normal sailing speeds. I know that Richard mathews had problems with his prop brake, but I gather that was at 16+knots - not a problem for me!

 

[Miker]

I fitted a 2 blade Autoprop on a First 260 and support the above posting. The improved performance under power is as stated by Brunton's. The boat seems to go faster under sail, but that might be wishful thinking. No prop turn at all under sail when engine put in forward gear. Far easier to manoeuvre (is that the correct spelling?) in a marina than the fixed prop.

 

[vyv_cox]

I have a two-blade autoprop on a Sadler 34. I support the statements made elsewhere in the thread - sailing speed up a little, motoring revs down a little, astern manoeuvrability greatly improved. Power delivered to the water is much better, but the prop it replaced was sized for my previous engine, so this may not be a valid observation.

There are subtle differences in the way the prop works, for instance initially higher revs are needed to get moving but as way commences the blade angle changes and speed increases further. Throttle reduction is then needed to maintain a normal cruising speed. Crash stops are greatly improved due to the prop's characteristic of equal efficiency ahead or astern. This is the really big benefit over the folding prop, whose astern efficiency is very low, assuming you can persuade it to open at all!

The two-blade is much cheaper than the three-blade and is available up to 17 inch. I paid almost exactly twice the price for the Brunton that I was quoted for a fixed two-blade of the same size but the three-blade would have been more than three times.

In neutral gear the prop rotates, but without any power. I assume this to be due to the asymmetrical blade shape. I normally sail with the gear in reverse, as the shaft continues to rotate in forward on a Yanmar, but I don't detect any speed change on going into gear.

Interestingly, I visited the marine section of the Science Museum a fortnight ago. A design that looks like the Brunton in all but name was exhibited there, built in the 1920s. There's nothing new.....

 

[Guest]

Taking in account your experience, if you had to specify a 3 blade prop on a new 42 foot cruising yacht, would you choose an autoprop, a maxprop or a gori ?

 

[vyv_cox]

My choice would be the Autoprop. I think there is not much to choose between the three so far as cost is concerned. The Gori has its place on a racing boat for probably the lowest drag available, but I think that even its staunchest supporters would not claim excellent performance astern. I made a careful comparison between the Autoprop and Maxprop but chose the former because I liked its reduced complexity and self-pitching. In practice this has been justified.

The main adverse comment I have after one season's use is that there seems to be something of a mixture of alloys in its construction and consequently the anode was surprisingly wasted at the end of the year. It is possible that I was supplied a magnesium one, as I bought it in Holland, but have yet to check.

In other respects I am quite satisfied. I suspect that the three-blade Autoprop would offer far bigger advantages over a fixed version.

 

[Miker]

Is your Autoprop noisier?

To add to my previous post, it seems to me that my 2 blade Autoprop is a bit noisier (More vibration?) than the old fixed 2 blade prop. The noise is probably the same for similar boat speeds but with the Autoprop at lower revs.

 

[HaraldS]

There is definitely a fourth one to consider. It's the flex-o-fold, also dansih like the Gori, and also functionally closest to the Gori in that it is a folding prop, but the gearing works differnt.
It has a lot to go for it: Lowest price of the three, probably lowest drag or maybe same as the Gori, seems very efficient forward and unlike what people say about folding props it has huge power backwards.
I have no comparisons with other props on this boat, but it is definitely fine in all respects. Originally also had the Autoprop in mind, but a guy at the yard talked me into this one and I don't regret it.
Originally the yard, (Najad), fitted Maxprops as standard, mine was the first flex-o-fold, but seems to have convinced them too as now all bigger boats have it as standard.
Definitely a prop to have on the short list.

 

[Abaker]

Maxprop experience

I have a 17 in. 2-blade Maxprop on a Crealock 34 with a 30 hp Yanmar 3HM34F. The prop is in an aperture in the skeg. When sailing with it feathered in light air I get significantly better speed, compared to an identical yacht with a fixed two-blade, sailed by better crew with better sails. Both forward and reverse thrust are a little better than a fixed two-blade on the same boat. Vibration is about the same. After 2,500 hours, mostly in the Intracoastal Waterway, the Maxprop became too wonky despite annual cleaning, greasing, and zinc replacement during haulout. I had it rebuilt by the US distributor in Seattle. Now with a further 1,000 hours it will soon need rebuilding (if possible) or replacement. No failures so far, but I'll probably replace it with something else.

 

[steffen]

Re: Wonky??

By "wonky" do you mean you hear this rumbling noise at low revs? I have a two-blade Maxprop with quite some play between the central case and the blade axels and at low revs i hear this bonking noise.
What do you mean by rebuild? Expensive?

 

[gunnarsilins]

On my Moody 42....

....I went for MaxProp 3-blade instead of the fixed one.
My choice was between Max and Auto.
A folding was out of question, with 12 tons of displacement I need a lot of reverse thrust when manouvering in tight spots. But when asking around, especially on different forums all over the world, I had some feedback telling that the Auto was a bit more vulnerable when hitting objekts in water, like fishing gear, planks etc.

Anyway, the Max made big difference, noticeable better speed and rudder response under sail in light winds. Better thrust in reverse then before and after fine adjusting of the pitch I can cruise with 200 rpm less and maintaining the same speed.
Honestly speaking, I would think that Maxprop, Autoprop or any other feathering prop would do a good job. The big difference is probably not between them, it´s more a matter of subtilities.
The big difference is between a fixed and a feathering one!

 

[Abaker]

Re: Wonky??

By Wonky I mean the blades and blade axles no longer fit precisely to the central case as when new. The play between blade/blade axle and case was so great that I didn't trust the Maxprop any more. As I understand it, the rebuild involved adding metal and remachining. Cost was US$200. The US distributor apparently lost my prop, then shipped me a different one of the same size, but denied such a thing was possible in their shop. The one returned to me was no longer as wonky, and had some improvements (grease fittings and different method of attaching hub). But some of the pieces were obviously not the ones I had sent. Neither Maxprop had a serial number stamped on it, so ID was arguable. The only time my prop has made a noise detectable above the Yanmar was when motorsailing (in restricted channels with commercial traffic). Then a gust of wind sometimes causes the speed under sail to exceed the speed under power, and the prop complains; I think it starts to feather, then goes back to normal power mode as the gust subsides. A year ago I asked the Canadian distributor about a further rebuild. He has no facilities to do that, must send prop to Italy, and the waiting list in Italy was several months long. I declined.

 

[steffen]

Thats it.

Exactly my problem. In my case it happns primarily with low revs. As soon as i go over 1000rpm it stops because (i assume) the pressure on the prop builds up and pushes the blade axels tio the front of their sitting.
I'll see if i can have it rebuild in Holland.

Thanks