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Opinions on folding/feathering propeller


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Fish the other factor is blade area and shape- there are just too many variables to make a definitive decision from a magazine test, prop sizing is part calculation, part guessing and part experience which is why prop manufacturers and suppliers have extensive databases of end use. The best thing is to talk to lots of people, there is a font of knowledge out there  especially other owners of the same boat but even then its not 100% perfect. Different engines have different power curves, there are many gearbox ratios to choose from and is it shaft or saildrive? Then its whether the boat is loaded like a cruiser or is it stripped out and 800kg lighter, does the owner thrash it and even then there are misunderstandings about what that means!

 

I am not a big fan of the kiwi after I saw one on Tucana that had a serious infestation of electric sea mice, they changed it to a bronze folder.

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There is some real devil in the detail with propellers. Referring to the yachting monthly test that Farrari referred to, I see there are two versions on the internet: https://www.yachtingmonthly.com

Kiwiprops reverse pitch is not adjustable. There is too much reverse pitch, and therefore torque required to rotate, for some small engines to overcome at idle...

Lots of great info, I learned again. Actually I have to check the clearance between the shaft end and the rudder. In memory I have plenty of space, but looking this picture (one of the 15 Chargers) no

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I'm pleased as the task to chose the right folding propeller is not the easiest, also I couldn't expect 100% result. Good. 

I realized how difficult to compare those propellers in any circumstances as they are so complex "creatures" and all has good and bad side. Also to get a folding propeller, just like choosing a boat, is a compromise so I accept the fact I will sacrifice a few things to gain a bit of speed, sailing angle and reduce ware on the shaft seal.  So I'm aware of this. Do I have to expect anything else than slightly less top speed forward and less power in reverse?

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That is fairly much the point Native, that there are so many factors. The main point I was wanting to make, is that matching a propeller to a boat, engine and gearbox is as important, or more important, than a number of other factors.

If some guy decides to buy a certain prop cause it gave the highest speed in the boat test, he is likely to be badly disappointed with the speeds he can achieve on his own boat, if he doesn't get across all the other sizing issues.

 

So what are electric sea mice? is that an electrolysis issue? We've effectively got a new kiwiprop (Grandpa's axe) having serviced or replaced almost every component over 15 years. That is another point I don't think people get, any feathering or folding prop will have wear, and wont last forever. Solid bronze props are just that, solid. Folders etc have lots of moving parts wear, flogging etc to monitor over time. A benefit (we see) with the Kiwi, is that is cheaper in the first place, so easier to get our money's worth out of its operating life.

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 So I'm aware of this. Do I have to expect anything else than slightly less top speed forward and less power in reverse?

So you can get substantially more power in reverse, esp with a feathering, but I think you are thinking folding now? which can have less power in reverse.

If you match the prop really well to your boat, or get one with adjustable pitch, you can actually end up with higher motoring speeds over your fixed prop. It all depends how well matched your fixed prop is. With the adjustable pitch, you just tweek it until it is optimised. With a unit where the pitch is fixed, you just have to hope the first guesstimate is close enough, same issue with a fixed prop.

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I have two Kiwiprops.

I get them to service them each year and it costs around $120.00, so $60 each and they are given a full birthday.

 

What I like about kiwiprop is the owner is a bit of a boffin, so went away and looked at how he could get me a little more speed. Actually spent quite a bit of his own time to see what pitch and variables I needed....he came to the conclusion, which he backed up with a shitton on a data, that my boat suffers from terminal slowness disease, and no matter what prop I use, Im going to be going really farken slowly...and he was right.

 

They appear to enjoy their work and want their product to succeed

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So you can get substantially more power in reverse, esp with a feathering, but I think you are thinking folding now? which can have less power in reverse.

If you match the prop really well to your boat, or get one with adjustable pitch, you can actually end up with higher motoring speeds over your fixed prop. It all depends how well matched your fixed prop is. With the adjustable pitch, you just tweek it until it is optimised. With a unit where the pitch is fixed, you just have to hope the first guesstimate is close enough, same issue with a fixed prop.

Yes I'm drifting towards the two blade folding, and actually I hoping a bit more than a "guesstimate" from a manufacturer, whoever it will be. That is the reason why I would prefer to get a purpose made with respect all known factor. Am I expecting too much? 

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I have two Kiwiprops.

I get them to service them each year and it costs around $120.00, so $60 each and they are given a full birthday.

 

What I like about kiwiprop is the owner is a bit of a boffin, so went away and looked at how he could get me a little more speed. Actually spent quite a bit of his own time to see what pitch and variables I needed....he came to the conclusion, which he backed up with a shitton on a data, that my boat suffers from terminal slowness disease, and no matter what prop I use, Im going to be going really farken slowly...and he was right.

 

They appear to enjoy their work and want their product to succeed

Good to have local support, as I'm in Christchurch/Lyttelton I wouldn't expect to get that much support. Do you have them on a cat?

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Am I expecting too much? 

That is a loaded question. It depends on what your expectations are and how fussy you are. And I should say I've never bought a 2 blade folder so I don't have any direct experience with suppliers sizing these to my expectations.

But if you think about what your expectations are, even list each element you want, that would help inform where you want to go.

One thing that is well known is that any folding or feathering prop will substantially improve your sailing, both speed and pointing. There is a marginal (slight) reduction in drag for a folding over a feathering. Unless your into the latest 3Di sails and tweeking etc, I wouldn't rate that as a big factor.

The next question is how important is your motoring performance? If you did 5 knts instead of 5.5 knts at cruise would you care? If you dropped down to 2.5 knts motoring into a nasty chop would this be a major issue? And how much close maneuvering / importance of reversing?

 

Obvious advantage of a 2 blade folding over say a 3 blade folding is they are cheaper (roughly half the price?). There is a perception that they don't give as good motoring performance as a 3 blade (any type of 3 blade). In essence props rely on surface area (Disk Area Ratio, or a bunch of similar measures). Because a 2 blader has less surface area it needs either larger blades (in area, not in diameter). It is a general rule to get the biggest prop that will fit on your boat (in diameter). Once you've got the biggest diameter that will fit, and a fixed blade area, then the next only option is to increase the pitch to get more 'bite' or power into the water. Increasing pitch can reduce efficiency and potentially make it harder to match well with the engine / gb. Short story is a 3 blade is always going to be more efficient than a 2 blade of the same diameter. Does that matter? 2 blades have been used for a long long time, plenty of people are perfectly happy with them. Could you get greater efficiency / better motoring speeds with a 3 blade, most likely, but the prop will cost a fair amount more for a marginal improvement in motoring performance.

 

So if you are happy to enjoy the sailing performance improvement and motor around at what ever, you'll meet your expectations. If you are fussy about getting that last 0.1 or 0.3 of a knt out of the boat while motoring, you might be disappointing.

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What a relief... I'd like to enjoy sailing and the only thing what I want from the propolution to give me a backup in case of emergency, plus short distance motoring like in and out from marina, anchorage like that. I don't think I ever enjoyed motoring anywhere in any distance with a two cylinder diesel so I will try to avoid in the future anyway. 

Obviously on a larger vessel have a bit more importance of the "motoring" capability and I understand it better to do on the most efficient way. So I guess I have one thing left, to pick the brand...

Tough one... Gori, Bri-Ski eventually same price, roughly same lead time... hmmm...

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Have used both Kiwi props and Two blade folders. For a racing yacht I would go for a geared folder.

For mostly cruising I would go with Kiwiprop.

I have always found the Kiwiprop guy very helpful.

We are now on our third keeler with a Kiwiprop. You have to be aware if they get dirty they can really load up in reverse I have found you need to get the revs up quickly soon after engaging reverse gear. For reversing and maneuvering the boat when there is a bit on they are very good.

A two blade folder will often struggle to reverse into a bit of wind and chop.

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