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Go Back   SailNet Community > General Interest Forums > Gear & Maintenance
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  #41  
Old 01-11-2010
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Quote:
Originally Posted by dantaden View Post
Valiente, just curious why you decided on a 4 blade feathering and not a 3 or 4 blade folder ?
My boat's a steel motorsailer with a full keel, and I am going to a slightly larger engine (from 52 to 60 HP). The full keel "blankets" the stern somewhat in a way a cutlass bearing aft of a fin keel does not. My shaft angle is near zero degrees.

When I shut off the engine, the four blades will feather to a very small frontal area, and I can rotate the prop shaft to "top center up" to orient the blades for the most minimal of drags. Seems fussy, perhaps, but we are going well offshore and every mile made good will be in part about reducing drag.

The VariProp will allow different pitches in forward and reverse, and I want a relatively shallow pitch for forward that matches the new engine's power curve for economy. In reverse, I want full-on torque for stopping and maneuvering ability. This meant, perhaps ironically, that I wanted a lot of prop blade area for low speeds.

I'll let you know how it all turns out...
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  #42  
Old 01-11-2010
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I would have to check the record book, but I think we put the Autoprop on in the mid nineties. No grease required. We have the boat hauled every couple of years and we check to make sure all three blades are still there . Seriously, it still seems as tight as when new, all the bearings look and feel great and it works just like the day we put it on.
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  #43  
Old 01-12-2010
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Quote:
Originally Posted by MC1 View Post
For cruising, if one is concerned about performance in reverse and/or prop-walk issues, wouldn't a feathering prop be preferred over a folder?
Yes, some feathering props work as well in forward or in reverse.

Quote:
Originally Posted by MC1 View Post
Concerning drag, I don't see why a folder would be much better (if any) than a feathered prop; is there independent data to suggest otherwise? I can't imagine any difference in drag between a good feathering prop and a good folding prop being significant for anything but the most light displacement racing boats being driven at the very top of their performance curve. This doesn't seem relevant to medium / heavy displacement cruisers. Thoughts?
If I remember correctly, the autoprop had made some studies about it. Their
propeller takes way 80% of the drag of a fixed propeller.

Even a folder propeller will ad some drag, so you will probably have a 10 or 15% difference in drag. Not relevant for a cruising boat, but relevant on a racing boat.

Regards

Paulo
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  #44  
Old 01-12-2010
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From a performance perspective here's how I look at it, theoretically.

> 3 blade fixed - 6 knot/hr average in 24 hours = 144nm/day or 1,008nm/wk
> 3 blade featuring - .5 knot in added performance = 12nm/day or 84nm/wk
> 3 blade folder - .75 knot in added performance = 18nm/day or 126nm/wk

A folder over a feather still gives me the advantage of 6nm/day or 48nm/week. By looking at these numbers, on a long passage (w/folder) I can potentially save time on the engine, make up 3/4 day/wk, and less wear and maintenance on the engine.

And on occassion, on the race course, being the spoiler is kinda fun.

+ If I can get an efficient blade while motoring, overdrive or other, there's further savings.

As mentioned earlier I've narrowed the field down to Gori 3 Blade Folder or Autoprop, if they fit. I found I have an additional limitation where the shaft is exposed by only 7 1/2 inches to support both zinc and prop.

Still waiting for sizing and costs.
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  #45  
Old 01-12-2010
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Quote:
Originally Posted by dantaden View Post
From a performance perspective here's how I look at it, theoretically.

> 3 blade fixed - 6 knot/hr average in 24 hours = 144nm/day or 1,008nm/wk
> 3 blade featuring - .5 knot in added performance = 12nm/day or 84nm/wk
> 3 blade folder - .75 knot in added performance = 18nm/day or 126nm/wk

A folder over a feather still gives me the advantage of 6nm/day or 48nm/week. By looking at these numbers, on a long passage (w/folder) I can potentially save time on the engine, make up 3/4 day/wk, and less wear and maintenance on the engine.
I don't think you will have such a difference in speed on an heavy boat.

If 0.75 represents the gain in speed that you obtain reducing in 90% the drag of a fixed blade how do you expect that 10% of the total prop. drag represents 0.25 in speed? It seems to me that the difference will be less than 0,1K.

Regards

Paulo
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  #46  
Old 01-12-2010
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Variprop - Rugged and Reliable!

We're cruisers, not racers. 41 foot full-keel sloop with the prop in an aperature. After doing lots of homework, we chose the VariProp 3-blade feathering. This thing is a German-made piece of fine engineering. Two things that the VariProp does that MaxProp can not: 1) You can adjust Ahead and
Astern pitch easily, with the boat in the water if necessary - the MaxProp must be disassembled to change pitch 2)The VariProp has a built-in brake mechanism that dampens the shift Ahead-Astern or Astern-Ahead. Many props of this type will "clunk" when you shift and they hit the opposite stop - sometimes with a great deal of force. Performance under power Ahead is fabulous, as is reduced drag under sail. Astern is not as good as we had hoped, but much better than the original 2-blade fixed propeller.

Also, regarding maintenance - re-grease EVERY year to displace any seawater that inevitably gets into the hub - cheap insurance!
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As mentioned this is just theory, as I have only had the boat for a year and haven't found another owner with a like boat that can give me the difference.

However, based on my hull speed (8.5 knots) a 5-10% (conservative) performance increase would give pretty close to the additional as above. This is assumption since I don't know the drag coefficient of the current prop in my hull design.

Based on other comments and experiences on this thread I think I'm pretty close but won't really know until I fit it - unless I can get further direction and calcs from the community.
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  #48  
Old 01-12-2010
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Dan

Couple of things having had a Martec folder on my race boat before converting to a 2 blade max prop on that one.

1] delta in speed between the two was less than 0.1 knots in light air which is where the difference in drag will make the most difference. boat was a Frers 36 with a PHRF rating of 90.

2] Since then have installed a 3 blade max prop on the Moody and would never consider another solution. there is a reason everyone compares to the Max Prop. With regard to adjusting pitch without disassembly they do offer a version that allows that [costs more]. We elected not to go with that model and to this day not sure whether that was correct choice or not. We have the prop slightly over pitched on purpose [higher speed/lower RPM] but our engine is oversized for the boat so this is not an issue. It would be nice at times to be able to jump in and either reduce or increase pitch depending on whether we are passage-making or just local sailing where less pitch would give us less prop walk.

There is no perfect solution, however the service / support we have always received from PYI was great.
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  #49  
Old 01-12-2010
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Back when I was Measurer for the C34IA owners association, the folder vs. fixed propeller was a raging controversy in our one-design rule. Fortunately, I had over 20 years of one-design racing results involving hundreds of boats. Our research showed that there was a 30-40 second per mile speed advantage of a 3 bladed folder/feathering prop over a three blade fixed propeller. A much greater advantage for a 2 bladed folding prop. The data didnít show much of a difference between folding and feathering props. Working the numbers, I surmise that the 3 blade folder/featherís offer a 5% advantage which doesnít sound much until you start crunching the 8 and 24 hour numbers. My personal experience is my boat sails much faster in lighter airs and is easier to get to hull speed under sail than motoring which I like as I only have a 25 gal fuel tank.

I have a 3 bladed 15 inch flex-o-fold prop which I am generally pleased with, but I am a little disappointed insomuch it is hard to get the boat up to hull speed (not a problem with the old Michigan prop). I am curious to know the experience of others on this board. Can you tell me your engine displacement, prop diameter and pitch, and, length and type of boat and what kind of motoring economy you are experiencing.
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Old 01-13-2010
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Quote:
Originally Posted by svsirius View Post
1] delta in speed between the two was less than 0.1 knots in light air which is where the difference in drag will make the most difference. boat was a Frers 36 with a PHRF rating of 90.

2] Since then have installed a 3 blade max prop on the Moody and would never consider another solution. there is a reason everyone compares to the Max Prop. With regard to adjusting pitch without disassembly they do offer a version that allows that [costs more]. We elected not to go with that model and to this day not sure whether that was correct choice or not. We have the prop slightly over pitched on purpose [higher speed/lower RPM] but our engine is oversized for the boat so this is not an issue. It would be nice at times to be able to jump in and either reduce or increase pitch depending on whether we are passage-making or just local sailing where less pitch would give us less prop walk.

There is no perfect solution, however the service / support we have always received from PYI was great.
I also have the max prop on my Tayana 37 and have experienced no problems for over 20 years. I haul out every 3 years and repack the hub and recoat the prop. Takes about a full day doing that and while max prop offers a zirc fitting for relubbing the hub, the disassembly allows me to remove all the old grease and repack with new. It's a hassle, but every three years is something I can live with. The major time consumtion is recoating the prop which has held up well with proper underwater primer and hard bottom paint even though using a copper botttom paint is not recommended by max prop. I think the key is proper priming. As far as performance, I don't think there is much difference unless in light air. And that is certainly an advantage. The motoring effenciency is less with a max prop vs a fixed due to the flat vs a curved blade. The max prop also allows setting to a different pitch although once set it remains constant unlike I believe the auto prop changes with speed which is more efficient. I've got the prop set at a pitch of around 14 inches(17 inch diameter) and my WOT setting is less than the Perkins is cabable of achieving in neutral. There has been much discussion on this on other threads. One last comment is that the WOT rpm varies depending on whether the boat is in a current or not, but I've yet to really test that assertion. Passing under a draw bridge with high currents is not the time to be experimenting. Oh...One other thing is that I'm not sure about the best way to "lock" the shaft with the max prop. For years I just shut down the engine and left the tranny in neutral, but now I'm shutting down when in forward and then putting it in reverse thinking that maybe there is more friction on the shaft allowing the max prop to feather better. (borg warner velvet drive) ??
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