50hp 40hp sailboat folding/feathering prop wanted

Check with Bacon

http://baconsails.biz/aupr.html

 

Most people tell me that you mark the drive shaft and put it in position so no blade hangs down and that is good enough for the non geared. This is only an issue in lighter winds. Price is an issue here though so unless I can find a used max prop or similar I cant afford it. I am looking on the used market.

I don't need the worlds best reverse. I have a large engine usually just over idle speed gets all the reverse power I need so I am willing to loose out here. As long as I can get used to it I will be fine.

 

Thanks. I will look around that site. I really need a RH 1-1/4 18x15 or similar.

 

"So I think even if I dont get 1K the gains will be still be decent.


Most sailboats will motor at cheap speeds , hardly ever at hull speed or top speed.

At SL 1 (or so) the usual LRC the biggest difference in efficiency will be in fuel burn.

On a 20,000 lb boat so few HP is required ( probably 12 -14hp,, 3/4 GPH ) that large blade area only creates more drag for the thrust produced.

Since the boat will sometimes be sailed , and sometimes motor sailed , the pitch can be slightly higher than "optimum" if the diameter is sufficient.

This higher pitch presents less area to water flow while under sail.

So the basic question still remains , is 1 or 2 seconds per mile worth an extra $2000 to you?

 

On a long range cruiser ,a fixed two blade prop can be worthwhile. A freewheeling prop, while sailing and fit with a standard alternator,, can generate substantial electricity for recharging batteries or it can power a small hydraulic pump to supply oil presssure to an autopilot.

Ive sailed several boats like this from the Dutch shipyard Jongert. Very effective and robust . The added resistance of the fixed prop was only noticeable in light wind.. at full power sailing the loss is insignificant.

 

This post is a little better than your first, at least it contains information.

So are you saying that large props actually will increase fuel consumption compared to smaller props when used at low power levels? Can you please provide reference for this claim?

This is of interest to me as I cruise at around 5k (top speed 8k) I use less than 0.5gph. However this goes against what I thought I knew. I had always thought large props turning slower were more efficient due to reduce slip and when I try to research the subject this is still what I find.

http://www.psychosnail.com/boatingarticles/boatpropellers/propellerdiameter

A few sec a mile is not worth $2000. I am thinking minutes in a mile and DAYS on a large crossing like the one I done last year (California to the Marquesas). I need a new prop anyway as mine is toast. The difference for a standard 2 blade and cheap folder is more like $400 not $2000.

I am currently trying to find out the pitch of a well priced used geared folder which may be suitable. Its only 17" and my standard prop is 18".

 

From the SSCA flea market board,

http://forum.ssca.org/viewtopic.php?f=11&t=14543

"large props actually will increase fuel consumption compared to smaller props when used at low power levels?"

NO props with far too much surface area ,increase induced drag.

Larger diameter and lower RPM almost always are better , IF you can swing the diameter.

The problem is the usual 2 blade may have far wider blades than are needed to create the correct blade area.

Hence the suggestion for a "sailing" (skinney blades) prop.

To increase speed 1 K thrust must be increased , which usually is more fuel flow.

A really poor prop will harm performance , but for most cruisers there are a dozen prop pitch/ diameter/ RPM combinations that will be too close to measure realistically.

To spend huge $$$$ on a prop to go from .5gph to .490 would be efficient , but hardly cost efficient.

 

Amel yachts come with this. Its not something I will retrofit on my boat though. I hate the noise of the freewheeling prop and its under our bed.

Most sailing is done in light winds. Hull speed is hull speed, so prop drag wont matter off the wind when the wind is fresh at you are hitting hull speed.

However we need all the efficiency we can get when going upwind. My boat has a shallow wing keel and the large rudder presents as much lateral area as the keel to compensate. Right now it has massively disrupted flow going over it due to the prop slashing its efficiency. You can feel the prop wash at the helm.

I think reducing this prob alone will be worth a folder.

 

Nice link. That prop seems pretty good for my boat. Pricey at $1500 but much cheaper than new. I can get a new 2 blade folder for less than that.

 

I hate the noise of the freewheeling prop and its under our bed.

Freewheeling is the cause for boat speed loss.

A locked prop only drags the hub and blades thru the water , with a 2 blade sail prop , not much at all, as the blades are stalled .

With a freewheeling prop the projected area of the spinning blades along with the induced drag from making the prop spin actually do work (spin the shaft ) is many many times the drag.

Folks with high blade area 3 blade props will even notice acceleration on locking the shaft , just not as much as many blade areas come close to 85% of the projected prop area.

On aircraft the prop is feathered to stop its rotation , not to reduce the area the stopped blade is exposed to.

The difference between feathered and freewheeling may change the L/D from a nice 12-1 to a brick like 3-1.

Try it , you may gain that K you are looking for under sail for almost free.

 

Thats normally how you handle a fixed prop. You sail locked and only freewheel when you need to generate electricity.

For me a freewheeling prop generating electricity is much quieter and more natural than a diesel engine

 

Fred. I have heard the various theories on what causes more drag, freewheeling VS locked.

Seems to me that this is highly dependant on the situation/prop etc etc, sometimes freewheeling is better sometimes worse. I always have mine locked. Although yanmar now says I should freewheel it or it may damage the gearbox. I have used my GPS average speed function to test this and I there is no measurable difference in speed with my 2 blade prop.

The prop test I linked to in the first post found a their locked 3 blade prop to cause double the drag compared when it was allowed to freewheel. However I am sure in some situations this may be reversed.

 

There are plenty of tests made by sailing magazines and propeller laboratories. All of them show that any fixed propeller used in a sailboat has clearly less drag while freewheeling. Typically the drag is about 50% less while freewheeling.

Most of these measurements are in a propeller test unit, which is quite similar to a sail drive. It may be that for a badly inclined shaft installation freewheeling does not reduce drag.

Also Volvo Penta recommends freewheeling for reduced drag.

Here are some measurements:
http://strathprints.strath.ac.uk/5670/1/strathprints005670.pdf
http://www.multibank.de/henning/sf/manuals/faltpropeller-test.pdf
This is a bad translation of the one above, note that "held" and "free rotating" are mixed in the table.
http://www.propelspecialisten.dk/download/segeln_magazine_test_2008.pdf

There are many other test I have read as well, but not available in the web.

Here is another measurement, which did not measure freewheeling, but gave some thoughts about it:
http://www.catamaransite.com/files/propeller.pdf

And another:
http://www.plaisance-pratique.com/IMG/pdf/Drag-Thesis.pdf

 

Very good Joakim. I was always skeptical because it takes almost no power to turn a shaft in neutral, as can be seen when you turn it by hand. Now add a generating system to the shaft and things may work out differently.

 

OK guys.

I just purchased a used Flexofold prop which came out on top in the prop test of 15 props. I got an amazing deal (provided its as good condition as they say and fits!)

Would anyone like to guess how this will perform compared to my old prop as its not the same pitch and diameter?

Flexofold 2B prop: 17" - 17P (possibly 17.5P as they had to measure it)
Standard 2B prop 18" - 15"

I think that the extra pitch will make up for the diameter loss (with some efficiency loss at high speeds). Possibly this will also me more efficient when motor sailing? Either way the price was too good to refuse. $450.

Thoughts?