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Old 10-06-2010, 00:11   #1
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Autoprop for an Underpowered Motorsailer ?

Disclaimer - I'm a powerboater, looking to transition into a sailor, and this is my first post.

I'm looking at "motorsailer" type boats, and recently found one that looks promising? But, it was re-powered a couple of years ago with a new engine with only half the horsepower of the original. Now, the original engine was probably a bit over-sized and turned a large, fixed, 3-blade propeller, to allow for motor-cruising using minimal sails? And the smaller replacement probably provides "enough" boost for motor-sailing with a lot the canvas? But, it has to work hard during times when only the motor is pushing the boat.

Would changing over to an Autoprop maximize the limited power available?

Since Autoprops "self-pitch", do they somewhat protect the engine from over-loading/lugging by self-pitching to a finer pitch when the motor can't produce any more power?

Thanx, Dave.
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Old 10-06-2010, 12:27   #2
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Since Autoprops "self-pitch", do they somewhat protect the engine from over-loading/lugging by self-pitching to a finer pitch when the motor can't produce any more power?
There is an Autoprop thread, I have reply to the question in that thread.
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Old 11-06-2010, 10:54   #3
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Autoprop adjusts its pitch in a similar way to an airplane equipped with a mechanical self adjusting prop. At low rpms the Autoprop pitch is very coarse to give a greater bite into water to maximize thrust. As you increase rpm, the pitch becomes finer to reduce strain on the engine; at faster rpm, the prop has less bite into the water, but maintains its thrust by turning faster. The main advantage of the Autoprop is that it adjusts to be efficient at any rpm forward or reverse, and of course when under sail, and with the shaft locked by a brake, it auto feathers to reduce drag.

So yes, it does help prevent overloading or excess strain. The thing to remember is that props have a limit on the rpm applied; if you exceed that limit, you run into high cavitation which will cause both loss of efficiency and wear on the prop blades due to friction from air bubbles being formed and thrown off by the prop tips. If you install an Autoprop, experiment and find the rpm which give you the best balance of speed and economy.(in ships and aircraft this is known as economical cruising speed). Being that you have a comparatively small engine, I would say you will probably have a moderate cruising speed under motor/sail power.

An alternative to Autoprop is MaxProp, which is another feathering prop for sail boats. It feathers when the shaft is locked and deploys the blades when in forward or reverse. Unlike Autoprop, though, it adjusts only to the pitch set by the operator. It does not increase or decrease pitch as rpms go up or down--to change the pitch you have to go over the side and dial it in at the prop (on recent models) or pull the prop and set the pitch out of the water on older ones. With MaxProp, you set the pitch for the rpm range you would be operating in most of the time, so this rather narrows the range of rpms you can work with and be most efficient in without excessive cavitation. The pitch will usually be moderately coarse in order to work well with moderate rpm.
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Old 11-06-2010, 20:41   #4
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I think there is more to Autoprop than the above description.
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Old 11-06-2010, 23:26   #5
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Thanx to everyone who replied or re-posted my questions!

I've read all 18 pages of the other Autoprop tread. It was informative,
but, it often digressed into maintenance and customer-service issues.

Being a former military & current commercial pilot, who has operated mostly
propeller-driven aircraft, I like the aviation/marine-propeller comparisons.
Especially enjoyable, when boat-propellers are said to produce "lift" in the
incompressible medium of water?

All speed is relative, of course. So when we're talking about the difference
in displacement speeds of relatively small vessels, surely every knot counts!

Being a powerboater, I've always solved my speed problems by adding HP.
I look forward to learning how the power of wind & sails can also be applied.

I'm looking for a 50/50 motorsailer as a transitional vehicle, while I learn to sail.
By that I mean, it should have a "strong enough" engine that it can reach hull-speed,
under moderate conditions, motoring alone. Likewise, given moderately "strong" winds,
it should be able to reach hull-speed using sails alone.
If it requires both, to ever attain hull-speed, then I need to look at another boat.

Theoretical formulas indicate that most motorsailers are technically overpowered?
But, lacking real-world experience, I'm asking if this is true?
I want to buy a properly-powered motorsailer, and wondered if the
(expensive) Autoprop might maximize a slightly under-sized engine?
Or should I spend the same funds re-re-powering for more horses?

Thanx, Dave.
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Old 12-06-2010, 00:01   #6
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Not really. Props generally have their best, most efficient performance at fairly low rpm. The advantage of Autoprop over others is that it adjusts itself throughout the rpm range the engine can produce, while others are most efficient within a very narrow rpm range. Autoprop will at least equal the fixed blade props within their most efficient rpm range and will exceed their efficiency in higher and lower rpm ranges. As with other props, the main task will be to mate the proper sized autoprop with the combination of engine and boat. Done properly, and the Autoprop will probably produce more thrust without overtaxing the engine or gearbox.
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Old 10-11-2011, 18:44   #7
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Re: Autoprop for an Underpowered Motorsailer ?

I have an Autoprop. AB Marine had rebuilt the prop in 2007. In May of 2009 and 225 hours later a blade came off. I sent it to AB Marine who sent it Buntons Marine in England. Buntons Marine replaced the blade for $1500. I had the prop lubricated every six months. I am always in deep water. It never touched anything but saltwater. I only go to Catalina and back. Other than the lost blade there was not any evidence of damage or distress.

At the time Steve Armitage of AB Marine told me that losing a blade was a “rare occurrence” and that he did not have an explanation for why the blade fell off. David Sheppard of Brunton's Propellers Ltd, told me losing a blade is an “an exceptional case”. Contrary to their comments the prop has been subject to a recall for lost blades. If you search other support groups and blogs you will find many others that have lost Autoprop blades. It demeans their credibility to say otherwise.

Although I enjoy the performance of my Autoprop my experience with AB Marine and Brunton's Propellers has been extremely disappointing. They are unwilling to stand behind their expensive product. In all their replies I feel like I am hearing from a politician trying to avoid giving a straight answer. I would stay away from anything that might need to be serviced by AB Marine or Buntons Marine (Varifold, Autoprop, Gori).
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Old 10-11-2011, 22:41   #8
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Re: Autoprop for an Underpowered Motorsailer ?

Being the owner of a motorsailer I often wonder how much the boat is overpowered. I have twin 85 hp Perkins. I normally cruise ( when under power) at 6.5 knots on one engine running at 1400RPM. Now I can reach the hull speed of 8 knots with one engine at 2100RPM and way beyond hull speed with both engines running max. The fuel consumption however also doubles. So basically this boat could have been powered with twin engines under 50 hp each without any real adverse performance. Are you sure you need the autoprop?
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Old 11-11-2011, 00:34   #9
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Re: Autoprop for an Underpowered Motorsailer ?

The Autoprop (and I hear Maxprop now make a self-pitching prop as well) has profound advantages for motor-sailing - that is, using both sails and motor at the same time.

I question, however, the OP's plan to "transition" to sail by buying a motorsailer. A motorsailer is a boat for very particular kinds of purposes - the main one, perhaps, being able to run the boat for extended periods from a weatherproof pilothouse. The are not, in general, good boats in which to learn to sail. If you want to sail, just buy a sailboat. You'll learn to sail quickly enough. All cruising sailboats are really "motor sailers" anyway, with decent diesel propulsion systems, and most cruising sailors motor anyway from 10% to over 50% of the time.
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Old 11-11-2011, 09:40   #10
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Re: Autoprop for an Underpowered Motorsailer ?

Quote:
(and I hear Maxprop now make a self-pitching prop as well)
That would be the Ecowind four blade Maxprop.
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Old 11-11-2011, 12:17   #11
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Re: Autoprop for an Underpowered Motorsailer ?

Quote:
Originally Posted by Dockhead View Post
The Autoprop (and I hear Maxprop now make a self-pitching prop as well) has profound advantages for motor-sailing - that is, using both sails and motor at the same time.

I question, however, the OP's plan to "transition" to sail by buying a motorsailer. A motorsailer is a boat for very particular kinds of purposes - the main one, perhaps, being able to run the boat for extended periods from a weatherproof pilothouse. The are not, in general, good boats in which to learn to sail. If you want to sail, just buy a sailboat. You'll learn to sail quickly enough. All cruising sailboats are really "motor sailers" anyway, with decent diesel propulsion systems, and most cruising sailors motor anyway from 10% to over 50% of the time.
I heartily agree with the above advice. If you really want to learn to sail, a motorsailer type vessel is a poor choice for their rigs and hull shapes are compromised for better performance under power. This leads to boats that don't give the kind of feedback that helps one to learn the art of sailing. And Dockhead is also correct in saying that the majority of modern sailing yachts have quite enough power to be used as motorboats under typical conditions.

Cheers,

Jim
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