Autoprop on long keeled boat ?

[Crowblack]

Having interesting conversations with prop manufacturers about fitting a feathering prop to my longkeeler.

One thing that's come up (I've been advised) is the Autoprop because it's blades independently pitch - if fitted to a longkeeler where the prop is "hidden" behind the keel and is not in open water individual blades will behave differently - therefore affecting (reducing ?) the drive one would expect if it was in clear water. i.e. aft of a fin keel or even on a bilge keeler.

Anyone out there with an Autoprop fitted to a longkeel boat and what's your experience ?

 

[mjcoon]

I have no experience with fancy props at all. But on basic engineering principles, if the blades adjust during each rotation as they go through different water speeds behind the keel they are going to wear out their individual bearings in no time!

So either this individual adjustment is wrong, or it is damped so the blades take up an averaged-out position which is probably very close to that of the other blades.

 

[wilkinsonsails]

We have an Autoprop behind a fairly long keel .This tapers down to 4” wide as it comes towards the prop .
No problem with the prop ,in fact it’s a lot better than the old fixed blade .

 

[Tranona]

Lots of factors to take into account. The big advantage of an Autoprop is if you do a lot of motoring and motorsailing as it adjusts the pitch to maximise the load on the engine usually resulting in lower rpm for a given speed. It is therefore popular on heavier boats with ample potential power. The problems with long keelers if the prop is in an aperture is that the prop is not working in clean water and this can confuse the repitching, particularly in heavy conditions.

The challenge is to get the blades out as far as possible into clear water, but of course this leads to drag of the blades are fixed so a feathering propeller enables you to get the best out of a large fixed blade prop when motoring and minimum drag when sailing. This was the thinking behind choosing the prop for my long keeled Golden Hind where the optimum diameter prop is 17". Fortunately the aperture is big enough vertically for sufficient tip clearance, but the rudder is close. My choice was a Featherstream and the rudder was modified to add some balance but with a cutaway for clearance when the blades rotate. I had this arrangement on an earlier boat to a similar design and it worked very well. The other advantage of a Featherstream is that you can have different pitches forward and reverse which is helpful when some reduction boxes have different ratios forward and reverse, and even if they are the same it can be helpful to have a slightly flatter pitch for less prop walk and more progressive control.

Not used it yet - new engine etc being installed at the moment, but pretty sure from past experience it will work well. If you look around you will find that Featherstream is now probably the most common feathering prop for long keeled boats with props in apertures not least because it was designed specifically to be smaller than others and often the only one that will fit! (less costly than others as well).

 

[newtothis]

Would one of these work?

 

[Crowblack]

Quote - The other advantage of a Featherstream is that you can have different pitches forward and reverse which is helpful when some reduction boxes have different ratios forward and reverse, and even if they are the same it can be helpful to have a slightly flatter pitch for less prop walk and more progressive control.


Tranona, this is interesting, didn't know it was possible to have different pitch for ahead and astern.

My current fixed three blader if dropped into gear astern approaching my berth turning to port - starboard side too - prop kicks to port in astern - - even with the tiller hard over and at slow just in gear prop revs there's a noticeable straightening of the boat - so often have to put back into neutral to bring head round again to get into berth. I've got used to it but would be nice if the prop walk effect could be lessened.

Feathering props by reputation have less prop walk so what I'd like to achieve is to slow the boat on the turn into the berth without the staightening effect and also if I have to give it a sharp burst astern to stop it won't have the vicious kick to port the current prop has.

Does a flatter pitch for astern mean less grip for those times when you need to stop the boat.

 

[IanCC]

+1 for darglow in nicholson 31

 

[Chiara’s slave]

Flatter pitch astern should give you more grip, not less. It certainly did for us. Though our boat is almost as totally unlike yours as is possible, I’m pretty sure that such a basic thing as a prop will behave the same. We changed to a very low pitched prop recently, and the performance in reverse was transformational.

 

[rotrax]

Lots of factors to take into account. The big advantage of an Autoprop is if you do a lot of motoring and motorsailing as it adjusts the pitch to maximise the load on the engine usually resulting in lower rpm for a given speed. It is therefore popular on heavier boats with ample potential power. The problems with long keelers if the prop is in an aperture is that the prop is not working in clean water and this can confuse the repitching, particularly in heavy conditions.

The challenge is to get the blades out as far as possible into clear water, but of course this leads to drag of the blades are fixed so a feathering propeller enables you to get the best out of a large fixed blade prop when motoring and minimum drag when sailing. This was the thinking behind choosing the prop for my long keeled Golden Hind where the optimum diameter prop is 17". Fortunately the aperture is big enough vertically for sufficient tip clearance, but the rudder is close. My choice was a Featherstream and the rudder was modified to add some balance but with a cutaway for clearance when the blades rotate. I had this arrangement on an earlier boat to a similar design and it worked very well. The other advantage of a Featherstream is that you can have different pitches forward and reverse which is helpful when some reduction boxes have different ratios forward and reverse, and even if they are the same it can be helpful to have a slightly flatter pitch for less prop walk and more progressive control.

Not used it yet - new engine etc being installed at the moment, but pretty sure from past experience it will work well. If you look around you will find that Featherstream is now probably the most common feathering prop for long keeled boats with props in apertures not least because it was designed specifically to be smaller than others and often the only one that will fit! (less costly than others as well).

A friend has the same boat as us, a long keel Island Packet SP Cruiser Motorsailer. He had extensive shaft coupling modification and a custom stern tube as his new Autoprop caused severe shaft vibration on his boat.
Our boat is OE bar an Orbitrade shaft seal on an 8 inch exstention tube. We have never had any shaft vibration.
When we have sailed in company we have enquired of each other as to engine RPM at the same speed of each others boats.
The RPM on both vessels is the same at the same speed.
We have the 17X22 - IIRC - fixed three bladed job.
Either his Autoprop is not working or gives no improvement when motoring over the standard prop.
From direct experience, testing done twice off Falmouth and once in Poole Harbour.
The gauges/logs might be missreading, but the vessels were keeping station, not catching or falling behind the leading one.
Very empirical, but a vaugely scientific direct comparison.

 

[Arcady]

A friend has the same boat as us, a long keel Island Packet SP Cruiser Motorsailer. He had extensive shaft coupling modification and a custom stern tube as his new Autoprop caused severe shaft vibration on his boat.

Sounds familiar. I had one some years ago fitted behind a substantial skeg, and despite the best efforts of Brunton, we could never eliminate the considerable vibrations we experienced: bad enough to wear out the cutless bearing in one season. On a couple of occasions when motorsailing in rough weather, the prop aerated and the pitch coarsened suddenly to the point of virtually stalling the engine. All rather dramatic.

That said, there are lots of satisfied customers out there who have enjoyed years of satisfactory service. Just be aware that it’s not a given that it will work perfectly in every application.

 

[NormanS]

I'm one of those who have enjoyed years of satisfactory service. If you ever motorsail, a Bruntons Autoprop is the way to go.

 

[mainsail1]

I have had an Autoprop for 20 years and it has been great.
Replacing it now with another Autoprop.

 

[Tranona]

Quote - The other advantage of a Featherstream is that you can have different pitches forward and reverse which is helpful when some reduction boxes have different ratios forward and reverse, and even if they are the same it can be helpful to have a slightly flatter pitch for less prop walk and more progressive control.


Tranona, this is interesting, didn't know it was possible to have different pitch for ahead and astern.

My current fixed three blader if dropped into gear astern approaching my berth turning to port - starboard side too - prop kicks to port in astern - - even with the tiller hard over and at slow just in gear prop revs there's a noticeable straightening of the boat - so often have to put back into neutral to bring head round again to get into berth. I've got used to it but would be nice if the prop walk effect could be lessened.

Feathering props by reputation have less prop walk so what I'd like to achieve is to slow the boat on the turn into the berth without the staightening effect and also if I have to give it a sharp burst astern to stop it won't have the vicious kick to port the current prop has.

Does a flatter pitch for astern mean less grip for those times when you need to stop the boat.

The thinking behind the flatter pitch is that the thrust at low revs when you drop into reverse is less so to achieve stopping power requires more revs, but results in less initial prop walk. Seemed to work on my old Eventide, but this only had a Yanmar 1GM so power was all a bit marginal.

As it happens with my GH I have ended up with probably too flat pitch because up until Monday I was going to use a TMC40 box with a 2.65:1 in forward but 2.13:1 in reverse. So the prop is 17*11 forward and 7" pitch in reverse. This was chosen because the same pitch forward and reverse (as you get with a fixed prop) means that the engine can be overloaded when engaging reverse and be slow to pick up. I had this problem when I replaced the 1GM in the Eventide with a Nanni 14 and a TMC40 and a fixed prop where the maximum revs I could get in reverse was about 1500 with lots of black smoke!

Over last weekend I was double checking all the calculations and data for the engine (Beta 30) and box and realised that with my setup I was close to the rated limit of the box plus I picked up a number of reports of problems with the TMC40 and 30hp, mostly slipping clutches and slow to engage gear. Phone calls to Beta and Newage (PRM) confirmed that maybe PRM 90 is a much better bet. However the reduction is 2.5:1 both ways so my current prop will be slightly over pitched in forward and under in reverse. Decided to stick with it and see how it goes, but the beauty of the Featherstream is that the pitch can be changed easily with a different "cassette" that contains the stops.

Not sure how much this helps you except to show that there is a lot more to choosing props than meets the eye, particularly when you have a boat that was not designed with motoring efficiency in mind. A fixed 3 blade prop is a crude thing - fine operating in clear water, but stuck behind a long wide keel in a constrained aperture is not good, but an adjustable feathering prop such as a Featherstream or a Maxprop give you more tools to play with to improve motoring performance with the bonus of much less drag when sailing.

 

[Crowblack]

Not sure how much this helps you except to show that there is a lot more to choosing props than meets the eye, particularly when you have a boat that was not designed with motoring efficiency in mind. A fixed 3 blade prop is a crude thing - fine operating in clear water, but stuck behind a long wide keel in a constrained aperture is not good, but an adjustable feathering prop such as a Featherstream or a Maxprop give you more tools to play with to improve motoring performance with the bonus of much less drag when sailing.

It helps a great deal.

I agree my fixed three blade prop is a crude thing but it's interesting how you get used to using particularly the sterns vicious kick to port and bow to starboard to force the bow through the wind in close confines. I've provided fun for my neighbours seesawing out of my berth ! My intention though is to change it for all the benfits a feathering prop you've discussed.

I've experienced an Autoprop on a previous boat and it was superb, the boat did though have the prop in clear water and it seems to make sense that whilst it would provide benefit over a fixed blade prop - - some of it's potential could be lost sitting behind a thick long keel. Also as I'm unwilling to make any change to the rudder to enlarge the aperture - the swept area of the prop may be a problem.

The Featherstream should fit and given advice I might be able to go slightly larger as the aperture's got more top and bottom clearance than fore and aft.

Thank you to everyone for your advice.

 

[Leighb]

We replaced the engine in our Halmatic 30 and needed a new prop, so I asked Autoprop their advice, one of their chaps came to look at the boat, friend of a friend, and said that basically there was insufficient clearance ahead of the prop. The Autoprop needs as much ahead as behind and it would have needed surgery to the after end of the keel to make enough space.
I then decided on a Featherstream from Darlow which was excellent.

 

[robmcg]

You could also consider a Variprop if you are short on space. It has a very small boss so could aid in gaining a bit more clearance. The forward and reverse pitches are also independent and can be adjusted by the user. Just a thought.

 

[Arcady]

You could also consider a Variprop if you are short on space. It has a very small boss so could aid in gaining a bit more clearance. The forward and reverse pitches are also independent and can be adjusted by the user. Just a thought.

My experiences with Variprop have all been good. Expensive, but nicely made and performed well ?

 

[CapPugwash]

I have a kiwi on my Vancouver 32. It came with the boat and seems to do its job fine. I'm no expert and have nothing to compare to though.

 

[Daydream believer]

My Brunton Autoprop on a saildrive on a 31 ft boat has a vicious prop walk. More so than the original fixed prop. It is also a strain when hand steering the tiller for a long period.
As stated above it does cavitate or re pitch in chop meaning that it performs badly in any sort of chop over 1 metre. Excellent for motor sailing though.
I do not understand what difference the gear ratio in reverse has on an auto prop ( or was Tranona referring to a different prop) because one just uses more revs & the pitch of the prop is the same in forward or reverse apart from the fact that it pitches to suit the revs & speed through the water.
The comment in #2 about bearing wearing out ( in my Brunton at least) is not valid. I have had one set of bearings & blade re balancing in 16 years(following a refurb after a bad entanglement so the rubber bush & bearings were changed as part of the package) the blades are constantly adjusting pitch ( particularly in chop)& there is no sign of wear at all. In that time my engine has clocked circa 4000 hours with that prop