The old, three-bladed, fixed boat propeller on my Albin Ballad Triola had de-zinced to the point where it was so brittle that the smallest of knocks would have caused it to crumble away.

After doing some research, I decided that my best option was to go for a two-bladed Flexofold folding propeller.

The first mate applies torque to the nut, while the galley swab supports the storm leg brace. Credit: Mark Ryan

The potential for 20% extra boat speed under sail was particularly enticing as it would shave hours off passages and allow us to fly even
faster past all the modern light displacement stuff we routinely overtake!

However, the process wasn’t quite as simple as I’d expected: here’s how I did it.

1: Removing the existing propeller

To start with, we needed to remove the old propeller. It’s worth doing this before ordering a new folding propeller so you know what you need to order.

Before you begin, it’s a good idea to clean up any visible thread behind the nut securing the prop to strip off any antifouling or the remains of sea denizens.

The first prop puller we tried didn’t fit. Credit: Mark Ryan

Once this is done, brace the prop with a long bit of timber (we ‘borrowed’ a storm leg from the yard), and use a suitably long breaker bar to back off the nut.

Only back the nut off four or five threads, and work it back and forth to prevent it binding.

Do not, at this point, take the nut fully off: the reason for this will become apparent.

Keep the ‘key’ safe as you will reuse this when fitting the new prop. Credit: Mark Ryan

Next, get out your prop puller: the first one that we tried out wouldn’t fit on the small amount of propshaft that we had available behind the prop, so it was on to plan B.

I got out my big Sealey PS982 bearing puller set and, after a bit of trial and error, got it set up.

After a few revolutions of the 10-tonne hydraulic ram, the prop shot backwards towards the nut with a bang. (This is why we left the nut on earlier, to prevent the prop flying off and embedding itself in a member of the crew).

The Sealey puller is put to work removing the old propeller. Credit: Mark Ryan

The nut and prop were then fully removed: make sure you keep the ‘key’ safe as you will reuse this when fitting the prop.

At this juncture, we hoped the hardest part of the prop replacement procedure was complete… but we were very wrong.

2. Figure out exactly what you need, and order it

This is where we came a cropper. I had previously purchased a 25mm shaft anode, and it ‘fitted’ (albeit tightly).

I lazily measured the propshaft with my digital calliper without configuring it, then called Flexofold directly after visiting their website and ordered a 25mm ISO folding propeller.

Flexofold was very helpful, and delivered the folding propeller quickly – but the trouble was that it didn’t fit the taper and didn’t fit the thread.

I’ll pause at this point to suggest another approach – which is to order the prop via Darglow, Flexofold’s UK agent.

Measuring the shaft with callipers: make sure you calibrate your calipers, otherwise you’ll be in for big trouble. Credit: Mark Ryan

It will cost you around 20% more for the same prop, but for that, you will get a bespoke, machined, fitted prop and excellent customer service.

The process is a bit different to going direct: with Darglow, you send them your old prop and nut and they will make sure the prop and nut sent out to you will fit.

Back to our fitting experience. On discovering that the hub did not fit, I properly configured my digital callipers and measured the shaft again – and discovered that it wasn’t 25mm at all, it was 25.4mm (1in)!

The moral is, to be careful and make sure of your shaft diameter.

Turning the BSW die to cut a new thread on the propshaft. Credit: Mark Ryan

I spoke to Flexofold, who advised that I send the hub back, along with the old prop nut and the old prop, which we did.

On inspecting the old prop a member of staff advised us it was indeed a 1inIMP shaft, and the nut was 3⁄4in 10 TPI (threads per inch), UNC-threaded.

This was sent out and, to my relief, the hub fitted: but to my horror, the nut would not do up!

The first four or five BSW threads have been cut. Credit: Mark Ryan

There are at least two other sorts of thread that are 10 TPI, and using thread gauges it’s almost impossible to tell them apart.

The others are BSF and BSW –so I ordered two cheap mild steel nuts to this specification. The BSF nut didn’t fit, but the BSW (Whitworth) nut did!

I visited a local engineering shop and had them cut the nut to BSW using a tap: I went back, tried again, and it still didn’t fit.

Finally, I ordered a die to run over the propshaft thread, as well as a BSW 3⁄4 tap of my own to put through the nut if necessary.

Both the tap and the die took material off the thread and the nut.

In the photo above, you can see that the first four or five threads have been cut: note the treads are sharper than the uncut thread to the left.

This is the Flexofold prop nut on the shaft. Credit: Mark Ryan

Always use a good cutting oil when doing this sort of work.

Finally, the prop nut fitted onto the shaft!

One final thing I wanted to do before fitting the hub was lapping it to the shaft. (Flexofold reported that the old prop only fitted the taper at about 20% of its circumference: there should be contact on at least 70% of its circumference, as this connection is all that stops the prop from shearing the pin and spinning freely.)

To do this, I added some valve rubbing compound to the shaft and rotated the hub on the shaft.

3. Fitting your shiny new folding prop

This is the easy bit, and Flexofold’s instructions are very clear.

Slide the hub on without the key and mark the propshaft with a marker pen, ensuring it doesn’t wobble about: this would be a sign that the taper/hub is wrong and doesn’t fit.

Remove the hub and put the key in the keyway, slide the prop hub back on and make sure that it goes all the way back to where you marked the propshaft to make sure the hub isn’t ‘riding’ the key.

The prop hub fitted: make sure the prop hub isn’t ‘riding’ the key. Credit: Mark Ryan

Next, do up the hub nut – Flexofold merely suggest doing it up ‘really tight’, but I occasionally work on cars and like to do things by the book with a torque setting.

I contacted Flexofold, and they suggested 25Nm. On a 3⁄4in thread, this is not tight at all as far as I’m concerned, so after some research into what other prop manufacturers suggest, I settled on doubling this to 50Nm.

The Flexofold nut is on the left. Credit: Mark Ryan

I had two final concerns at this point.

The prop nut locking grub screw, which I assumed would locate onto a flat face of the prop nut, was locating onto part of the circular body of the nut.

Additionally, there was a bit of unused thread on the nut. I contacted the long-suffering (but still very helpful) staff at Flexofold, and they observed that this was perfectly normal and that the grub screw was supposed to locate onto the body of the nut, not onto one of the faces.

Finally, the blades were secured in place using the locating pins and locking grub screws, and the prop.

The fitted Flexofold folding propeller – all shiny and new! Credit: Mark Ryan

As with everything when messing around with old boats, a simple process can often be fraught with unforeseen problems.

Darglow might have spotted some of the problems earlier if I had gone that route, but there still would have been plenty of messing around before we reached a satisfactory solution: and, ultimately, even after purchasing the die and tap and all the other paraphernalia,

I still ended up saving money by going directly to Flexofold in Denmark. Caveat emptor, I suppose!

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