Performance Flexofold prop in reverse on 39i

[element]

Looking for some advise.

We have a 39i Performance which is fun, comfortable and fast under sail.

With the 3 bladed flexofold prop (shaft-drive), we are also very pleased with the performance under power ahead.

We do close to 6 knots at about 2000 rpm and can easily hit 8 knots when powering the 39hp Yanmar engine to its max.

I read on this forum and on other sources that most are happy with the 3 bladed  flexofold, and that even reversing is going well, although some more revs are needed.

We are however not happy with the performance going astern. Even with full power in reverse, it takes quite a while before we slow down and go astern. With strong cross-winds, this takes so long that the bow starts to be blown away, which makes turns in narrow water with stronger crosswinds difficult.

We were curious about why our performance astern is less then hoped for and others experience. So I filmed with a go pro the prop.

It appears that the prop does not complete folds out in reverse, which might me the reason for the poorer performance astern. See this short movie (first the gear is set to going forward, and then in reverse).

We were wondering what experiences there are about the performance astern with a 3 bladed flexofold, and if we could do anything to improve our performance.

Looking forward to any thoughts.

[bereboot]

We had the same problem. 
Changed to a max prop.
Problem solved. 
Flexofold for sale. 

[zaphod]

How fast are you going forward when you put it in reverse??

I am very happy with my flexofold. I have no problem stopping the boat when I need to, and propwalk is far less than my old 3 blade fixed prop. When backing out of a slip I can get up to steerage speed very quickly even in the very narrow fairways of my club.

The blades do not necessarily full open in reverse since the forces are trying to push them closed, which is why it takes higher rpms to get thrust. It just takes getting used to.

You can certainly swap it out for a much more expensive feathering prop, but they come with their own problems: higher cost, higher maintenance, and more propwalk.

[NZL50505]

Aug 27, 2021 19:22:23 GMT zaphod said:

You can certainly swap it out for a much more expensive feathering prop, but they come with their own problems: higher cost, higher maintenance, and more propwalk.

Mmmm, not so sure about that last bit. My 3B Maxprop bites hard and immediately in both directions and my propwalk is so negligible that I have literally forgotten which way it is - if I engage astern to leave my berth on a calm day I don’t think I can tell which way it ‘walks’ ie it’s effectively nil. And the bite is so big that I can slip out of my berth quite fast enough by simply engaging slow astern with no extra revs. I would never go back to anything else.

[zaphod]

Don't get me wrong, the Maxprop is a good prop. It was on my short list, but the flexofold won out based on cost vs performance. I have always had folding props so that part didn't concern me.

The reason I say it has more propwalk is simply because independent testing has shown that to be true.

Yachting Monthly did a very interesting review of all of the major props. Their methodology may not have been perfect but it was very good. Granted, the testing was only done on one boat, but there is no reason to expect other boats not to have similar results.

www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807

I have attached some of their results below.

[NZL50505]

Yes I saw that review and agree it looked good. I was going to change the Volvo prop on my 42DS which was superb ahead but non-existent in reverse apart from pushing the stern sideways as if I’d been T-boned by a tug.

From that test I shortlisted both the Flexifold and Maxprop options…. but I decided swapping props on my 42DS was a bit too tricky for me and so I bought a 3B Maxprop (was quite pricey) and it came with a Jeanneau 50 DS attached to it. Much easier. Until my wife noticed. 😊

[zaphod]

Aug 28, 2021 2:58:59 GMT NZL50505 said:

Yes I saw that review and agree it looked good. I was going to change the Volvo prop on my 42DS which was superb ahead but non-existent in reverse apart from pushing the stern sideways as if I’d been T-boned by a tug.

From that test I shortlisted both the Flexifold and Maxprop options…. but I decided swapping props on my 42DS was a bit too tricky for me and so I bought a 3B Maxprop (was quite pricey) and it came with a Jeanneau 50 DS attached to it. Much easier. Until my wife noticed. 😊

That's a pretty good prop that comes with an 8ft boat extension!

I'm sure your wife wasn't complaining too loudly!

[rene460]

Mmm…  I don’t think that prop would work for me, it wouldn’t fit in the pen.  Of course, it would be a 20 ft extension for my boat.

That YW test data is one of the often quoted references and possibly the best around.  Yet even with that, it’s worth reading the full article and understanding the limitations of the testing.  They certainly did an excellent job of sourcing a good variety of test propellors, making it a very comprehensive report.  Understandably, they did not have infinite time or money to be totally exhaustive.  But well worth reading for relative differences especially between different propellor types, and for all the information they were able to provide.

I am thinking of the difference between folding and feathering, for example.  I note that either one is much better for reduced drag when sailing than a fixed prop, but the difference between them is quite small, and may be worthwhile for the generally recognised better performance in reverse of the feathering ones.  Unless you are a really keen racer.  In general, I suspect that small differences may not be over important.

As some feathering types have independent pitch adjustment in forward and reverse, the difference between makes may reduce further with some pitch optimisation.  Also, I believe that some of the newer folding ones have fancy blade shapes that significantly improve reversing performance, but unfortunately I am not sure they were available for the test.

But back to Elements original question, that movie certainly shows the issue very clearly.  While centrifugal force is trying to open the propellor when it is spinning, and in forwards, the thrust forces on the blades help force them fully open.  But in reverse, the thrust forces on the blades are tending to close the blades.  With a feathering propellor the blades are always fully open and rotate to the appropriate pitch for reverse, which is why they generally seem to have better performance in reverse than a folding one.

My only experience is in changing the original 3 blade fixed propellor for a three blade feathering type.  We have nearly no detectable prop walk, but this is likely due to having two rudders so the prop wash does not flow over either rudder.  Reversing out of the pen we have steerage before we are out of the pen without excessive engine revs.  We have to enter the pen with enough speed for steering, and have confidence that reverse will stop us before we reach the jetty, especially with a cross wind, when a bit more speed is required to maintain steering.  It has worked well for many years now, and I am still pleased with the original decision.  The only maintenance so far has been a few shots of grease each time we lift out for clean and anti fouling which we do each year, though with lockdown, we missed last year.  But the prop still works in forward and reverse as of last visit in the gaps between lockdowns.

 If you are keen on the reduced drag when sailing of the folding types, it’s worth investigating some of the more recent three bladed ones, and talking to the manufacturer specifically about the performance in reverse.   But if reverse performance is your priority, it’s worth looking at feathering ones.  And of course, as always, the important thing is to get the best diameter and pitch combination.  Feathering types with pitch adjustments offer a bit more flexibility than those without.  Some say it can be adjusted in the water, but with my lack of diving skills I would leave it until lift out.  That said, I have been happy enough with the manufacturers original recommended settings.

I hope that’s some help.  I am not sure that much can be done about your existing prop, unless something is stopping them from opening fully, which you can check at lift out, or by diving, but worth a call or email to the manufacturers support line.  Just on the off chance that wear and tear is causing a problem.

rene460