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Old 23-03-2010, 07:57   #1
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Varifold vs Max-Prop: Ideas ?

I just launched my boat after bottom job. One of my planned changes had to be postponed until the next haul out: installing a feathering prop.

I asked a company in the UK to quote for a Autoprop, which I thought would be great, but they refused to sell me one, saying the aperture of my boat is relatively small and there would be problems with cavitations with that prop/engine/gearox combination. (Thought this was jolly honest of them). As I did not have enough time to research the subject thoroughly I kept my old fixed blade one for the time being.

So here are the questions for you lot that know far more than I do:

Would it be better to wait until the time comes to re-engine and change the whole combination (engine, gearbox ratio, prop), than just take one prop from the lots on offer? Not something I could afford this week...

I looked into Maxprops and Varifold 4 blade units. Max seem to be relatively inefficient but simple, I have found precious little about Varifold ones. Does anyone have any knowledge and/or experience that would help me with the decision?

My boat is a motorsailor so I would not want to lose too much performance on power, but I get tired of dragging the fixed prop while under sail....

Any opinion welcome. Cheers.
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Old 23-03-2010, 08:04   #2
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I'm pondering the same subject. In your case, as a question, does it make sense to put a folding/feathering prop on a boat with an aperture? Is it done? I thinking the whole aperture arrangement is somewhat inefficient and that the prop drag may be inconsequential.

But perhaps since you are a motorsailor the prop is quite large and folding would pay?

Something I learned today is the folding will catch fewer stray lines than the feathering ... while sailing.
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Old 23-03-2010, 08:16   #3
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I simply do not know the answer to that question daddle. I think the only way to find out would be by experimenting. With the full keel I do not seem to have problems picking up lines or nets as I go over them. Only my own painter when it gets dropped at the wrong time....
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Old 24-03-2010, 00:15   #4
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Here is a picture of the aperture. It only took me a day to figure out how to post pictures...
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Old 01-04-2010, 02:15   #5
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Quote:
Originally Posted by Capn Morgan View Post
I just launched my boat after bottom job. One of my planned changes had to be postponed until the next haul out: installing a feathering prop.

I asked a company in the UK to quote for a Autoprop, which I thought would be great, but they refused to sell me one, saying the aperture of my boat is relatively small and there would be problems with cavitations with that prop/engine/gearox combination. (Thought this was jolly honest of them). As I did not have enough time to research the subject thoroughly I kept my old fixed blade one for the time being.

So here are the questions for you lot that know far more than I do:

Would it be better to wait until the time comes to re-engine and change the whole combination (engine, gearbox ratio, prop), than just take one prop from the lots on offer? Not something I could afford this week...

I looked into Maxprops and Varifold 4 blade units. Max seem to be relatively inefficient but simple, I have found precious little about Varifold ones. Does anyone have any knowledge and/or experience that would help me with the decision?

My boat is a motorsailor so I would not want to lose too much performance on power, but I get tired of dragging the fixed prop while under sail....

Any opinion welcome. Cheers.
You should dig up the comprehensive test Yachting Monthly did a while ago. Here's a lead for you: The Ultimate Propeller Test - video | YM Plus | Yachting Monthly

It can be found somewhere or another online in PDF form.

For a motorsailer I guess the Brunton Autoprop is the best choice (if you can make it fit). Much better motoring performance than anything else, including fixed props. For motorsailing, the automatic gearing is really a key feature -- the prop will gear itself to the load so you can cut the revs down, when you're getting thrust from sails at the same time.

The cost of that is lots of money, and also much more drag when sailing than other folders or feathering props. So this is not a good choice for a racing boat or a high performance sail cruiser. But for a motor sailer or a heavy cruising boat (which is really a motor sailer too, even if no one admits it), it's just the thing, in my opinion.
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Old 01-04-2010, 06:24   #6
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I can see why Autoprop are not keen - in the ahead position the blades would be very close to the skeg. Similarly you may be space constrained by your rudder for a folding prop, altho you can soon measure for that.
There is one other option that is often forgotten, the Luke Feathering Prop
www.peluke.com
the beauty of this design is that it uses twisted blades so performance will be the same as a fixed prop. Its drag would probably be similar to an Autoprop, more than a Max or folding. It would be best used with a shaft brake whilst sailing
It is old technology, to vary the pitch you have to carefully grind (or build up) cast stops on the hub. It is a shame they do not convert to the geared design like all the other featherers as it would probably open up their market
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Old 01-04-2010, 09:10   #7
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I find the performance of a 3-blade Max prop very good, both when in use and when feathered. It would work very good for you.

I don't like auto-props at all. Every boat that puts hours on it has to replace all the blade bearings every haul-out: expensive, lots of work and in many places parts not available so FedEx costs too.

cheers,
Nick.
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Old 01-04-2010, 11:20   #8
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We have been using Brunton Autoprops for about 12 years, I guess, and put massive motoring hours on the one on the old boat (I guess around 5,000 hours of motoring on the old one) without ever changing any bearings, just grease them up good every year or two. We've never had the slightest problem (other than I just burned up the one on the new boat with electrolysis, but that's not the prop's fault). Some other people have had various kinds of trouble so I can only tell about my own experience and you have to make up your mind. We have not found them particularly demanding of maintenance or cleaning. Maybe we've been lucky. I do have the impression that the the Autoprop has to be very exactly and very successfully designed for your particular boat, and maybe some kinds of drive shafts and underbodies are better and some are less suitable for the peculiarities of the Autoprop. But they worked flawlessly in both our old boat (semi-long keel) and new boat (high performance bulb keel), despite very different design. Butter-smooth, silent, no vibration at all, and that great feeling which I can only describe as saying -- you always feel like you're in just the "right gear".

In any case, I think it's pretty clear that motoring performance of the Autoprops is superior to anything else, and I personally find the "automatic transmission" like variable pitch, which is very different from all other props, is really extremely useful, optimizing thrust and revs under all conditions, saving you beaucoup fuel, noise, wear & tear. Some people don't quite understand that the design prevents lugging the engine, so you can just loaf along at a relaxed RPM with every bit of power of the engine being tranformed into motion. We love them. But maybe we've been lucky. Your results may vary.

Oh, and sailing performance is just about the worst of all the moving-blade propellors. Quite a lot of drag, compared to folding props, although it's still a fraction of that of fixed props.
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Old 01-04-2010, 21:07   #9
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The Autoprop had a well-deserved reputation for throwing blades in years past. Brunton's recalled the suspect models and allegedly redesigned the prop to eliminate the problem. I still don't like 'em and am glad they are not big sellers here in the States.
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Old 02-04-2010, 10:36   #10
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Yes, what Dockhead describes is so far from what I have seen that we must be talking about different manufacturers or generations of auto-props. It is impossible that he had no maintenance on one for 12 years while the ones I saw had worn out bearings after one single Atlantic crossing. I didn't even know about throwing blades.

If they changed the design to correct these problems they deserve another look at them.

The ones I saw looked like this:

Parts that went bad are no. 8 and 9. Probably failure of the lip seal led to failure/corrosion of the thrust race. Other models listed on their website don't have this model thrust race anymore, leading me to think the difference might be in the different models they offer.

cheers,
Nick.
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