Changing from folding prop back to fixed

[charles_reed]

As Tranona has already pointed out - it depends on what you're trying to achieve - in my boat the folding prop disintegrated and the 2-blade fixed, fitted in its place, caused so much noise and slowed the boat down so much that it was changed, after a year for an Autoprop.
The latter has done 22 years, is on its 2nd engine and only now needs new blades to regain its original performance. In hindsight I can't understand anyone having a fixed blade prop on a sailboat and only miserly excess would prevent buying an Autoprop.
Which is, however, the least slippery of all folding or feathering props but easily the most efficient and the most expensive.

 

[tri39]

I'm very happy with my moderately priced Kiwi prop. The forward pitch is adjustable. The reverse pitch is very coarse and may become coarser if the hub is not clean enough. Which means you need plenty of power and a higher ratio in reverse, 20Hp is the absolute minimum. But it will then stop the boat dead! And you can show off in reverse!!

 

[lw395]

What are the real savings. I would be very surprised if anyone is getting 10 to 20% reduction in drag. It seems to be more like 3 to 7% depending on hull and prop size. Is a 0.25 kt speed increase worth the loss of efficiency when motoring.

Data is not easy to find, but racing rating systems give very substantial allowances for a fixed prop, yet everyone still goes for folding.
Try towing a tender with an outboard in gear.
Proper sailing yachts are not about 'efficient motoring'!

 

[Tranona]

The 2 blade folders seem to have quite a limited life. My Radiche has done 700 hours and the blades are now quite loose. It does not seem to be a problem at the start of the season but its starting to get quite noisy with a bit of marine growth. I only have 50mm hull clearance which does not help. Anyone else have any opinions on the Radiche. Can't decide whether to refurbish it go to fixed or spend more money. The Radiche has an unusual blade profile. It seemed to get good reviews but I am not convinced about its efficiency when motoring. I get 5 kts at 2400rpm with 28 HP on a 10m hull with a displacement of 5000Kg. Reverse is not important and not an issue.

Not all folding props are the same. As with many things in life you get what you pay for, and the Radice is at the bottom end of the price range! Much loved by round the cans racers as you get the benefit of low drag for the minimum price and loss of motoring performance is acceptable. Suspect that if you had something like a Flexofold you would not have the same issue of short life and poor motoring performance (if correctly sized you should be getting a higher motoring speed than you quote).

Whether it is worth spending £1000-£1500 on a new prop (that will get you a good 2 blade at the bottom end and a 3 blade folder or featherer at the top end) rather than £3-400 for a fixed does depend on what you value. In general the improvement in speed under sail is in the order of 10-15%, at typical cruising speed, with more at lower speeds where the drag reduction gives greater benefits. If you do a lot of cruising under sail it can have a significant impact on passage times - achieving 5.5 knots average rather than 5 can reduce something like Poole Cherbourg by 1 1/2 hours. You should not with a modern folder or featherer lose any significant motoring performance - usually there is a gain particularly if you go 3 blade rather than 2. BTW your 50mm clearance is not a constraint for the sizes of prop you are looking at and clearance can be greater if you go 3 blade as you can usually reduce the diameter by 1" compared with a 2 blade.

 

[marivor]

I have removed a 2 blade Brunton Auto Prop from my Beneteau 361 due to vibrations and the harshness when selecting gear. I have refitted the original fixed 2 blade prop and now and it has transformed the boat with less vibration and a quiet gear selection. I will not be replacing the Auto Prop but am considering a new fixed 3 blade.

 

[30boat]

The 2 blade folders seem to have quite a limited life. My Radiche has done 700 hours and the blades are now quite loose. It does not seem to be a problem at the start of the season but its starting to get quite noisy with a bit of marine growth. I only have 50mm hull clearance which does not help. Anyone else have any opinions on the Radiche. Can't decide whether to refurbish it go to fixed or spend more money. The Radiche has an unusual blade profile. It seemed to get good reviews but I am not convinced about its efficiency when motoring. I get 5 kts at 2400rpm with 28 HP on a 10m hull with a displacement of 5000Kg. Reverse is not important and not an issue.

I've had a Radice for years now and the blades are not too loose but I haven't used the boat as much as I would have liked.The only good thing about those props ist their reverse and stopping power.In ahead they're terrible.I'm also considering changing to my fixed two bladed but I like the sailing performance...

 

[johnalison]

One of the main improvements with a folding prop, at least in my experience of a single example, is the improvement in flow over the rudder. The feel with the propellor turning is OK, but as soon as I put the engine into gear and lock the prop, the boat is transformed and the helm feels completely smooth.

 

[KAM]

Not all folding props are the same. As with many things in life you get what you pay for, and the Radice is at the bottom end of the price range! Much loved by round the cans racers as you get the benefit of low drag for the minimum price and loss of motoring performance is acceptable. Suspect that if you had something like a Flexofold you would not have the same issue of short life and poor motoring performance (if correctly sized you should be getting a higher motoring speed than you quote).

Whether it is worth spending £1000-£1500 on a new prop (that will get you a good 2 blade at the bottom end and a 3 blade folder or featherer at the top end) rather than £3-400 for a fixed does depend on what you value. In general the improvement in speed under sail is in the order of 10-15%, at typical cruising speed, with more at lower speeds where the drag reduction gives greater benefits. If you do a lot of cruising under sail it can have a significant impact on passage times - achieving 5.5 knots average rather than 5 can reduce something like Poole Cherbourg by 1 1/2 hours. You should not with a modern folder or featherer lose any significant motoring performance - usually there is a gain particularly if you go 3 blade rather than 2. BTW your 50mm clearance is not a constraint for the sizes of prop you are looking at and clearance can be greater if you go 3 blade as you can usually reduce the diameter by 1" compared with a 2 blade.

Why does the drag reduction give greater benefit at lower speed.

 

[charles_reed]

Why does the drag reduction give greater benefit at lower speed.

I'd refer you to page 234 of C A Marchaj "Sailing Theory and Practice". A full explanation and the maths to go with it.

 

[KAM]

In general the improvement in speed under sail is in the order of 10-15%, at typical cruising speed, with more at lower speeds where the drag reduction gives greater benefits.

I presume you mean more hours saved on a slower passage not higher percentage reduction in drag due to the folding prop.

 

[Tranona]

I presume you mean more hours saved on a slower passage not higher percentage reduction in drag due to the folding prop.

Yes. As speed increases other factors affect drag. So the percentage of increased speed is greater at lower speeds. If you motor when speed falls below ,say, 5 knots you do not get this benefit which is why I suggested that it is time saving at cruising speed under sail which is the main benefit. On the other hand if you like to sail as long as possible in light airs you will get the benefit there. The important thing about drag reduction increasing speed is that it is always there with no effort, whereas other ways of increasing speed such as better sails depend on your ability to make use of the potential.

 

[KAM]

Ah think its becoming clearer now. Just to recap sailing at 7kts well heeled there is much more drag from the keel and rudder etc so the benefits of the folder will be reduced compared with sailing more upright at slower speeds. I have been doing some calcs but having difficulty getting real numbers from other boats for comparison. The Yachting Monthly propeller test article seems to have some unrealistically low hull resistance numbers and very high propeller drag numbers quoted.

 

[KAM]

Compared with a 16 inch fixed 2 blader the savings, on drag for my boat work out at 6.3% at 5 kts and 2.3% at 7 kts and 33% at 2.5 kts. This does not include the extra drag from keel and rudder when heeled weed etc so real values will be significantly worse. I assumed zero drag from the folded prop. So the saving is 0.3 kt at cruising speed and very speed dependent, well worth having at low speed but insignificant at high speed. Walking round the boatyard this afternoon I noticed that most of the boats fitted with folders had similar hull shape and displacement to mine and that the wear on all the folding props I looked at, both 2 and 3 blade, were much worse than mine so it looks as if the wear is typical. I think the Radiche is probably just as well made as the more expensive brands. What do others do about sloppy blades. Is it worth shimming the hub to reduce future wear on the pivot pins.

 

[Halo]

Are you asking out of interest or are you thinking of changing back.......if so, why?

My boat came with Yanmar saildrive and fixed 3 blade. Manual says don't lock it in gear when under way. The noise was truly awful. I invested an eye watering £2,500 in flex-o-fold and stripper. After just one season I'm delighted with it. Very responsive in ahead and astern with low speed, imperceptible engagement. Removed it at the end of the season, 10 minutes in diluted brick cleaner and it's as good as new.

+1 3 years in on my flex o fold 3 blader + stripper and I am still amazed how much better she sails and handles in tight moorings. I have a saildrive so that rules out an autoprop. No way I would go back to fixed unless I had absolutely no option