Fixed vs. Free Wheeling Prop - Updated !

[Maine Sail]

Love to see MaineSails resulting tests before I release some extensive tests that I have found. But I trust MaineSail to still run the tests in a methodical way and show the results that he gets.

But here it is - Freewheeling has much less drag ! (but its usually not good for many transmissions and bearings with all the extra wera and tear)

(cut to the chase - go to pages 37 and 40 for figures 6 and 8 here -> http://strathprints.strath.ac.uk/5670/1/strathprints005670.pdf )

or another one but I think you'll need to purchse study for full result

http://www.sciencedirect.com/scienc...serid=10&md5=503e86665aa47e781a6ce7ea5b02a4bf

Tom,

Those are both the same study. You really don't need to buy the second one as it basically says the same thing as the accepted manuscript does. This was one of the studies we discussed last time, as well as the MIT study (I have copies of both). Some here made arguments that seemed to have some merit, specifically tank wall interference, and in the case of the MIT paper, the vortexes & turbulence caused by the internal piping.

So with all that disagreement, even after two Universities published their findings, I am doing this very simple, non-tank but more real world test, to finally get a more precise answer.

I plan to video this one so there is no confusion..

 

[Ross]

Strut produces upwash and turbulence .... dont need to complicate the question nor the answer as then you will never be able to isolate nor reconcile the 'truth'.
KISS

The strut is a component of the propulsion system as such must be included.

 

[Tom S]

Re: Fixed vs. Free Wheeling Prop - Coming Soon !

Some other very important info I found for people to remember if they want to freewheel - not the best for your tranny - beware

"there was a letter in the 01/15 issue of Practical Sailor which I think pretty much reflects the conventional wisdom on the subject. Here it is: Freewheeling Prop Re: the recent correspondence on prop drag and freewheeling (November 1, 2000). Surely the answer is straightforward. If you have a pressure-lubricated gear box (I believe Borg Warner for instance) under no circumstances freewheel as there will be no oil flow to the box. Hurth, one of the commonest boxes fitted to sailing boats and above all to Perkins/Westerbeke, definitely advises against freewheeling and goes as far as to suggest always engaging the reverse gear to hold the shaft. Reason: Lack of proper splash lubrication to the plates, which wear quickly if badly lubricated with ATF fluid. Unpressurized splash lubricated bevel gear boxes can probably freewheel safely"

BUT from Gordon Torresen (who knows a thing or two on marine deisels ;o) )

The reversing gear on the 2QM15 can freewheel if you choose. The splash lubrication is working well even if no gears are turning. The clutch cage spins with the output shaft (now being spun by the propeller), is in the lube oil, and looks and functions like a paddle wheel. The letter from Practical Sailor is an uninformed opinion. Just because they print it doesn't make it right. Although there are some reversing gears that will be harmed by free wheeling, the Hurth style is not among them. As I recall the results of the drag studies, there wasn't a great deal of difference, either way. I think I would just pop it into reverse, eliminating the noise and saving wear on the oil seals.

 

[Tom S]

MaineSail - yep you are correct - it is the same study - no need to buy one )

I must have missed the first discussion (damn - work will do that at times - makes me miss trolling the sailboat Bulletin Board threads)

It will be interesting to see your findings -- if you have little to no shaft friction or negative torque I think the drag will be less on the freewheeling prop, especially for 3 blade props and at higher speeds

 

[Maine Sail]

Neutral or Reverse

Some other very important info I found for people to remember if they want to freewheel - not the best for your tranny - beware

"there was a letter in the 01/15 issue of Practical Sailor which I think pretty much reflects the conventional wisdom on the subject. Here it is: Freewheeling Prop Re: the recent correspondence on prop drag and freewheeling (November 1, 2000). Surely the answer is straightforward. If you have a pressure-lubricated gear box (I believe Borg Warner for instance) under no circumstances freewheel as there will be no oil flow to the box. Hurth, one of the commonest boxes fitted to sailing boats and above all to Perkins/Westerbeke, definitely advises against freewheeling and goes as far as to suggest always engaging the reverse gear to hold the shaft. Reason: Lack of proper splash lubrication to the plates, which wear quickly if badly lubricated with ATF fluid. Unpressurized splash lubricated bevel gear boxes can probably freewheel safely"

Tom,

As you mentioned you can't always believe what you read in Practical Sailor. When it applies to many of, if not most of, the ZF/Hurth gearboxes installed on Westerbke & Universal engines that statement is technically not accurate as you know, at least according to the Universal & Westerbeke manuals for ZF/Hurth gears.

With many sailboat gear boxes like Kanzaki's (Yanmar) they should only be left in neutral but not reverse. ZF/Hurth or JS can be left in both neutral or reverse but most all say not forward when moving forward. Some others can not be left in neutral so it is always best to consult your individual manual for your gear box.

Practical Sailor really needs to bone up on their fact checking before hitting "print"..

From Universal:

UNIVERSAL DIESEL WITH HURTH TRANSMISSION

Model HBW-50 (2:1)
Used on Models 12, M2-12, M-18, M3-20, M4-30, M25 and M-25XP

Model HBW-100 (1.8:1)
Used on Models 30, 35, and 40

Model HBW-150 (1.9:1)
Used on Model 50

Model HBW-150 V-Drive (2.13:1)
Used on All of our V-Drive Models

CAUTION
DO NOT LEAVE GEAR IN FORWARD WHEN SAILING. GEAR MUST BE IN NEUTRAL FOR FREE WHEELING OR SHIFTED INTO REVERSE TO LOCK PROPELLER WHILE SAILING.

From the C-34 site:

 

[RichH]

The strut is a component of the propulsion system as such must be included.

But a strut is usually of good hydrodynamic foil shape which has minimal point of 'stagnation', a round pipe has a much larger stagnation point.

 

[Don12364]

Re: Fixed vs. Free Wheeling Prop - Coming Soon !

Be real nice to get a simple answer without having to wade through reams of equations - all of which have constants and variables we cannot replicate on our own boats.
We will all be much indebted to you Main.

For info the UK magazine "Yachting Monthly" - current edition (May) did a similar test on 15 fixed, folding and feathering props all sized about 16 x 11. They tested bollard pull in forward and astern and maximum speed and stopping time using a 32' fin keeled yacht. Also a drag test by having the prop on an outboard motor leg mounted over the stern of a rowing skiff and towed this at speeds of up to 7 knots whilst measuring the DIFFERENCE in the towing force of the skiff between individual props whilst both fixed and free to rotate.
As the prop was completely free to rotate IMHO it invalidated the test for other than for saildrive installations. I think Main's test will be more representative.
They also measured the propwalk for each type with some surprisingly large results.
For those interested, the 10 page report (incl photos) is titled "The Ultimate Propeller Test" and you should look at http://www.yachtingmonthly.com/ym/any_questions/20090316104916ymanyqs.html for information.
Also click on the gallery of images for 7 pages of views of props in pieces etc. They sell reprints of articles via .pdf files if anyone is interested.
Their conclusion that a whole knot of speed is lost with a fixed prop not rotating is also probably overstated because it doesn't account for the water dragged along by the hull and reducing the water speed over the prop.

BTW Main, I notice the pivot point on your rig is well forward of the prop. Does it all balance at an angle equal to the shaft inclination on a typical yacht or will you need to move it aft a little?

 

[Ross]

But a strut is usually of good hydrodynamic foil shape which has minimal point of 'stagnation', a round pipe has a much larger stagnation point.

The goal here is to determine qualitive differences not absolute quantitative differences. A streamlining shape could be used to cover the pipe but the lower level of turbulence would have the same influence on the prop whether fixed or free turning so the differences in drag would probably be the same ratio. Granted the vortices shed by the cylinder alternate left and right but the frequency is high enough that the mass of the system and the gross sensitivity of the instrumentation will average the results.

 

[Maine Sail]

The goal here is to determine qualitive differences not absolute quantitative differences. A streamlining shape could be used to cover the pipe but the lower level of turbulence would have the same influence on the prop whether fixed or free turning so the differences in drag would probably be the same ratio. Granted the vortices shed by the cylinder alternate left and right but the frequency is high enough that the mass of the system and the gross sensitivity of the instrumentation will average the results.

I will simply flip it around to satisfy both of you. Push and pull.. The height of the string will change but that is about it other than moving the friction bearing from front to back, which is easy.

I am not looking for exact number just an answer to fixed vs. freewheel causing more drag. I doubt my 3/4" dia black pipe will make a huge difference but we'll soon find out.

Saturday the tide is high at 7:00 am so I may be able to get to this then other wise it will be sometime next week when the tide is right in the evening or early morning. I need the tide so I can run parallel to the beach where I know there to be minimal to no current. I also need to patch the skeg, which I wore through, on my Walker Bay before then too..

 

[Ross]

Re: Fixed vs. Free Wheeling Prop - Coming Soon !

For several years I worked in R&D and discussions like this were frequent and always civil. There are no "right" answers here and there are no "wrong " answers. And when it is all done there may be debate as to the interrpretation of the results.

I expect that the drag of fixed props will be several percentage points higher than the drag of a free prop. I expect the drag on a pushed prop will be nearly the same as the drag on a pulled prop.

 

[kloudie1]

Re: Fixed vs. Free Wheeling Prop - Coming Soon !

OUTSTANDING, Main!! I am looking forward to your results. I think your test rig design is fine for giving qualitative numbers. There are things that affect the absolute numbers, but in the general scheme, this rig will answer the question. Your simulation of the mechanical drag on the prop is critical for this comparison. Going to be interesting to see if when you change the mechanical drag, does the hydro drag shift considerably? I agree with Ross about the pipe support, but a two blade, with one blade lined up with the pipe should show less drag than if it were not lined up.. Going to be interesting !!!
Transmission …general note on the Kanzaki and Hurst NON HYDRAULIC transmission. The engaging cones on the transmission shafts have grooves like screw threads that screw themselves together when engine driven. When the shaft is being turned with the engine stopped, as in forward gear with the engine shut down and boat sailing, the threads will tend to unscrew and the cones will slide over each other, resulting in lots of wear to the cones which will make the threads disappear and result in the transmission slipping in forward gear. If the transmission is positively in neutral, no cones are touching and the only wear is to the bearings and gear faces, which are lubed by moving through the oil ( As Tom quotes from Torresen). The loads on the bearings and gears are extremely low (less than one horsepower) compared to being engine driven, so wear is so low as to be negligible (wear can be a cube function of load). This assumes a proper oil level and alignment. In reverse gear, the threads screw themselves together and lock up without slipping so there is not even negligible wear. More transmission wear when freewheeling?, yes, but negligible compared to motoring. Hydraulic transmissions are very different and the manufacturer's recommendations should be followed .(translation: I don't know anything about specific hydraulic transmissions)

 

[sailormanbigd]

Interesting Test

Hi MS, forgive me if i missed it but how do you simulate the friction of the bearings and tranni coupled with the weight of the shaft/flanges/zincs, etc on a boat -with your ingenious setup? I mean that a boat mounted prop with the friction of these things would cause the prop to not exactly "freewheel" right? I would think that you would have to put a torque wrench on the prop nut and see how many pounds of force it takes to get a boat mounted shaft/prop to spin and create the same conditions on your set-up. A boat mounted prop does'nt spin until you get to a certain amount of boat speed or current flow right? Very cool post, and you got way too much time on your hands-don't you have a "honey do" list?

 

[Tom S]

Re: Fixed vs. Free Wheeling Prop - Coming Soon !

What would be really cool is if he could lock and disengage the prop from spinning while the boat was moving and if he did see a jump or lowering in drag pull --

(Knowing Mainesail he'll probably rig something up and then video tape it )

 

[Maine Sail]

It's compared to

Hi MS, forgive me if i missed it but how do you simulate the friction of the bearings and tranni coupled with the weight of the shaft/flanges/zincs, etc on a boat -with your ingenious setup? I mean that a boat mounted prop with the friction of these things would cause the prop to not exactly "freewheel" right? I would think that you would have to put a torque wrench on the prop nut and see how many pounds of force it takes to get a boat mounted shaft/prop to spin and create the same conditions on your set-up. A boat mounted prop does'nt spin until you get to a certain amount of boat speed or current flow right?

With my boat and PSS seal my prop will spin at under 1 knot. I compared the jig to my own boat using the digital scale. I pulled my prop with the digital scale and Gorilla tape from the outer tip of the blade and then compressed the friction bearings on the jig, with pipe clamps, until I matched the pressure it required to spin my prop while on the boat. Every boat has different friction depending upon transmission, cutlass condition and stuffing box. This is measured against my own boat as the baseline.

Again this is NOT for exact numbers, though I have tried to replicate my boat as closely as I can including the props depth below the surface of the water, I did this because I know someone from which ever side does not win will probably enter into a debate about water density at a certain depth.

This is to determine if a free wheeling prop caused more or less drag than a locked one.

Very cool post, and you got way too much time on your hands-don't you have a "honey do" list?

Yes and it's longer than the "boat do" list. As for time on my hands I have less than you'd think. It took me three months of 10-15 minutes here and 20 minute there to get to this point.. Stuff like this definitely takes a back seat that's why I'm working today and not testing.

I am very pumped to see how this turns out and I'm sure I'll be kicking myself for locking my prop all these years, and sailing slower, if I find out freewheeling is better..

 

[Maine Sail]

What would be really cool is if he could lock and disengage the prop from spinning while the boat was moving and if he did see a jump or lowering in drag pull --

(Knowing Mainesail he'll probably rig something up and then video tape it )

I am going to attempt to pull the nail that locks the shaft with a string but I don't know if it will work or not..

 

[48dodge]

We'll have this thing nitpicked to the point where you'll regret doing it, Mainesail. I don't think it matters what you come back with, I'm sure some people will think it's wrong because of some aspect. You may have better luck with pulling a cotter pin instead of a nail. Thanks for doing the test.

 

[sailormanbigd]

Re: Fixed vs. Free Wheeling Prop - Coming Soon !

thanks for the explaination, i agree that someone is going to excitedly debate the results of your findings, sounds like you have thought it out long and hard, and of coarse ther' can always be variables overlooked but me thinks you are gonna get some interesting outcomes. Not so sure that prop lined up with strut will cause less drag. Could be a laminar flow thing caused by strut and increase drag? we're gonna find out pretty soon. Another dumb question...

 

[Hermit Scott]

Re: Fixed vs. Free Wheeling Prop - Coming Soon !

You should have built the set up out of aquamet 22. I don't think it will hold up in the transatlantic test. Seriously though, I like the prototype. Did you build that plywood bench set up just for the picture? lol
I can't wait for the results.

 

[48dodge]

I do have ONE complaint about the whole setup. Being that MaineSail built the rig, I can't believe it's not made out of teak and marine grade plywood, all varnished to the hilt.

 

[sailaway78]

Too Much Time

First let me say that I really enjoy your posts and photos, and this looks like another good one. Will be very interesting to see the final results. I have seen studies that state without question that fixed is better, and that free wheeling is better. Second, I have observed that you have way too much time on your hands. You probably need a real, full time job. And the third and final comment, while the results may be interesting, many of us have no choice, as the tranny shouldn't free wheel according to the mfg. specs.