Folding prop choice

Beiträge: 127
Registriert: Freitag 8. Juli 2011, 17:14

Folding prop choice

Beitrag von BalladExchangeDB »

Dear Balladeers,

We are looking at replacing our fixed 3 bladed 13x9 prop with a two blade folding prop, firstly to go faster under sail, and secondly as the current selection seems underpropped.

We have a Betamarine 20hp with 2:1 reduction on the gearbox. Max engine revs are 3600 in theory, but we can easily reach that with the current prop, and to get to hull speed needs almost full revs.

I have looked at lots of info about folding props and feathering props, and was hoping that someone had some experience to share. I notice that looking through the old MSN system, Aemilia, hull 978, had the same choice but there was no follow up about whether the choice was successful.

We are considering Flexofold (expensive), Gori (early design), Radice (cheap but look a bit primitive) and Varifold (made in UK so less penalty from weak sterling).

There have been a number of articles and test of varying rigorousness, and a two bladed flexofold comes out well.

I have done some preliminary contacts, and most suppliers seem to suggest a 14" diameter. Gori suggested a 14x11.5, but suggested a 14x9.5 for Aemilia with an identical engine and gearbox, and recommended a 16x10 to Josin (although with a higher gearbox reduction ratio of 2.3:1). This concerns me a lot, as they are very expensive for a bit of cast bronze, and I dont want to have to buy two if I make a mistake.

A quick spreadsheet calculation, using the 50% slippage quoted by Josin, suggests that you need at least 11" pitch to get 6.5knots at less than 3000 engine revs/ 1500 shaft revs.

Any comments would be much appreciated.

Cameron, Cracklin Rosie #73
Beiträge: 127
Registriert: Freitag 8. Juli 2011, 17:14

Re: Folding prop choice

Beitrag von BalladExchangeDB »

maccormac wrote on Nov 25, '09
We installed the same Beta 20 engine at the start of the season and have been running with a Flex-o-fold 14x9 two blade folding prop from Darglow Engineering ( 01929 556 512 ask for Bob) I wanted hull speed in flat water at around 2600/2700 revs. We are easily getting this and Darglow were very helpful in giving advice and assistance both before and after purchase. Beta suggest a 13x9 three blade which as you found will rev out to 3600. The grip from the Flex-o-fold is fantastic in both forward and reverse. Yachting monthly tested fairly recently and the Flex-o-fold came out as good if not better than the fixed 3 blade for forward propulsion and scored highly in reverse. It was well worth the extra £100 or so over the cost of the vari-fold alternative.
crosie wrote on Nov 25, '09
many thanks for that. It is interesting to note that the pitch is the same for the 13x9 fixed prop and the 14x9 recommended by Flexofold, but the performance is markedly different, against expectations.

Does this mean that the only real difference is the slippage factor to be applied to each? Am I correct in assuming that the three bladed fixed prop will have more blade area than a two-bladed folding, despite the fact that its diameter is slightly less?

Anyway it is good to hear that the flexofold at that spec works well. They are the front-runner in my mind.
319anne1 wrote on Nov 25, '09
dear cameron,

we have since 4 years our ballad and volvo 18hp 2.4:1, equipted with a radice 15 x 10 folding prop ,and i am highly pleased about it
and there aren't any signs of use.
there is once a year lubricating neccesary ,and further it has the benefits and against like all folding props.

ballad 319
crosie wrote on Nov 25, '09
Dear Anne,

The Radice prop was also a strong contender, as it is muh cheaper than the rest. Do you manage to get near hull speed at reasonable revs? I have done some more research on the pitch calculations, and have found suggestions that slippage is around 30% (less than 50% as I was using before) for shaft speeds up to 1800 revs, ie 3600 engine revs and a 2:1 reduction ratio. However, other apparently well informed sources suggest that maximum efficiency is 60%. Perhaps these two concepts are not exactly the same.

A bit like my sail trimming, this is clearly an imprecise science for the amateur, so I am going to rely on what has worked previously on Ballads.

Kind regards,

sail24hrs wrote on Nov 25, '09
Hello Cameron,

We have installed exactly the same engine in our Aemilia, hulnr 978, as you mentioned, about two years ago. A 13x9 twoblade Radice folding prop has been installed at that time. As I experiance the same issues as you have discribed (underpropped) I have now talked to the guys of Flexofold during the METS in Amsterdam lastweek. They adviced me to install a 14x11 twoblade, which is what I am probably going to do. I always found the Flexofold the nicest looking twoblade folds. The test in Yachting Monthly have conviced me they are probably also the best. They told me the price is 616,- euro's + VAT, which isn't cheap of course (I remember the Radice example was less then half of that) but I think a good prop is well worth it's money.

shanty105 wrote on Nov 25, '09
Shanty has a 13.5hp Beta. When I bought her, she had a 3 blade fixed prop. I replaced this with a 2 bladed Brunton's Autoprop. Expensive, but worth it - 1/2 to 3/4 knot more under power or sail.
jocotton wrote on Nov 28, '09
I’ve now had a season’s experience with a Gori threebladed folding prop, and am more than satisfied.
I’ve half-written an article about the whole process, but finishing it will have to wait until I get home from sailing the east coast of Australia just before Christmas. I’ll send you a copy, with pics.
The gist however is that the 16X10 threeblader performed so much better than the 16x10 twoblader, both Gori.
After much heartsearching and long and useful discussions with a most helpful techie at Gori, we agreed that what was needed was more blade area, the new prop has at least 40% more.
Propbite is vastly superior, manouvering much better, with proper thrust in reverse as a particular bonus. Hull speed plus at full revs. Major improvement in thrust and maintenance of boatspeed when banging into a chop. In short, a very satisfactory choice.
The Gori threeblader also has an ‘overdrive’ function. By keeping the blades in the reverse position then going forward, gives an extra half knot at the same revs. You may have read about it on the Gori website. It works!
The Gori website has a comprehensive calculation sheet for choosing the right propeller. Have you used it!
Other makes may be better than Gori, I cannot say, but I’m more than satisfied with mine.
Good luck with your further deliberations
jocotton wrote on Nov 28, '09
if you send me your email address, to, I'll send you a copy of the unfinished article.
crosie wrote on Mar 8, '10
Dear All,

Just a quick follow up to let you know the results of our research and choice of folding prop.

We eventually decided on a Flex-o-fold two blade 14*9 prop, partly based on a general concensus from all the independent reviews that I could find and also on comments made above. It is unusual to have a confirmation of an identical boat, engine and gearbox, which added a level of confidence. It was however about the most expensive two bladed folder I could find!

We managed to get the prop onto the shaft during the winter layup, with some help from the local shipwright. In theory the fitting would be very straightforward, but in practice it took a lot of muscle work and stress. The main factor was that the shaft thread had been cut a fraction too large (apparently to cope with the machine made nyloc nut which was used in the original installation. This meant that the bronze nut was too tight, despite running the tap through it several times. Finally however, with the aid of cutting grease and pipe wrenches, it went on.

In action it certainly is an improvement in certain aspects, as it has a lot more bite in forward. Close to hull speed was reached at 2500 revs, and got close to 7 knots at 3200 revs. The boat started to squat noticeably and the cockpit fill rapidly with water through the drains so probably this is not an ideal cruising speed. Maximum revs seemed to be about 3400, which is not far off the 3600 max quoted by the manufacturer. In astern it is not so great, and needs a good blast to get any action, but the original wasnt much better. Prop walk in astern seems about the same or slightly better. There does seem to be an increase in vibration which is clearly from the prop rather than the engine, but perhaps this is inevitable when going from a three blade to a two-blade.

Overall I wasnt very impressed by the technical assistance available from the manufacturers and suppliers, particularly in the selection of the right prop. It does seem that flex-o-fold were about right, but I only went that route because of Maccormac's comments above. One supplier told me that my rev counter was wrong because the boat could not have been underpropped with the original 13*9 three blade, as he had supplied it. It didnt make me that keen to believe the rest of his sales talk.

It is clear that there is a huge grey area in the model calculations to take into account blade efficiency or similar concept. This has to be built into any model, and different estimates range from 30% to 70%, which is more than doubling. Whenever I tried to discuss this aspect with the suppliers to make sure that I had made the right choice, and to plug into my simple model of revs, shaft speed, pitch etc, I was either met with annoyance or bemusement.

Perhaps other members could provide info on their revs/shaft speed/prop dimensions/prop make and speed achieved, and I will combine them into a single model which would be a useful empirical guide for the Albin Ballad.


jocotton wrote on Mar 8, '10
Cameron, congratulations on making a choice and being satisfied with the result!.
Two apologies:
1. My original message had a misprint. Prop diameter was 15" not 16". Oh dear! Hope that this did not make your deliberations more complex.
2. I never did get my article finished. The article in the magazine rather smothered it.
The Gori "overdrive" function reduces the engine speed by ca. 200-250 rpm at the same hull cruising speed of 5 to 6 knots, or an increase of 1/2 knot at the same engine rpm. Maybe useful for those who are considering one.