Fit a Flexofold propeller

The old propeller on Triola had dezinced to the point it was so brittle the smallest of knocks would have it crumble away. After reading a the Yachting Monthly article on folding props, I resolved my best option was to go for a two bladed Flexofold prop. The potential for 20% extra boat speed sailing would shave hours off passages and allow us to fly even faster past all the modern light displacement stuff we routinely overtake.

Step 1 – Remove Prop

To start with we needed to remove the old propeller. Its worth doing this before talking to Darglow (Flexofolds UK agent) or directly to Flexofold as you need to know exactly what need to order. Before you begin, its a good idea to clean up any visible thread behind the nut securing the prop to strip off any anti-fouling or remains of sea creatures. 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 the nut off (as modelled by the first mate and galley swab below).

Only back the nut off four or five threads, and work the back and forth to prevent it binding. Do NOT at this point take the nut fully off, the reason for this will become apparent. Next, get out your prop puller. We tried the sort of puller below first:

Prop puller

Prop puller

However we couldn’t get it to fit on the small amount of prop shaft that we had available behind the prop, so 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 as below.

Sealey pullet put to work

Sealey pullet put to work

After a few revolutions of the hydraulic ram, with a bang the prop shot backwards towards the nut (hence we left the nut on earlier to prevent the prop flying off and embedding itself in a member of crew). The nut and prop were then fully removed – we hoped the hardest part of the prop replacement procedure was complete… we were quite wrong.

Proud crew

Proud crew

Step 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 with the shaft with my calliper without configuring it and called Flexofold direct after visiting their website and ordered a 25mm ISO prop. They were very helpful, and delivered the prop in fast time – the trouble was it didn’t fit the taper and didn’t fit the thread!

I’ll pause at this point to suggest another way. Darglow are Flexofolds UK agent, and they are a great company who will provide excellent customer service at every step of the way. They will cost around 20% more (in this single experience I have) for the same prop, however for that you will get a bespoke fitted prop and awesome customer service. In hindsight, I should have paid this uplift – that is not to speak badly of Flexofold in Denmark, they were very helpful and answered numerous questions I had related to the fitting, and, as you will read later on, they were very accommodating also. The process for Darglow is a bit different to going direct, at Darglow, you will send them your old prop and nut and they will make sure the prop and nut they will send you out will fit.

Back to our fitting experience. On discovering the hub did not fit, I properly configured my digital callipers and measured the shaft again to realise it wasn’t 25mm at all, it was 25.4mm – 1 inch!! Be careful and make sure of your shaft diameter! (Unlike us!).

Measuring shaft with callipers

Measuring shaft with callipers

I spoke to Flexofold who advised I send the hub back, along with the old prop nut and the old prop, which we did. On inspecting the old prop they advised us it was indeed a 1 inch IMP shaft, and the nut was 3/4 inch 10 TPI (Threads Per Inch) UNC threaded. They sent this out, and to my relief, the hub fitted, but to my horror the nut would not do up! There are (at least) two other sorts of thread that are 10 TPI, and using thread gauges its almost impossible to tell them apart.

Using thread gauges

Using thread gauges on the mild steel test nut

The others are BSF and BSW – I ordered two cheap mild steel nuts to this specification, the BSF nut didn’t fit, however, the BSW (Whitworth) nut did (you can find a good explanation of these threads here)! I visited an engineering shop, and had them cut the nut to BSW using a tap, went back, tried again, and it STILL didn’t fit. Finally, I ordered a die to run over the prop shaft thread, as well as a BSW 3/4 tap of my own to put through the nut.

Turning the die on the prop shaft thread

Turning the die on the prop shaft thread

Both the tap and the die took material off the thread and the nut. Below you can see the first four or five threads have been cut, note they are sharper than the uncut thread to the left.

First few BSW threads cut

First few BSW threads cut

And finally the prop nut fitted the shaft!

Flexofold prop nut on shaft

Flexofold prop nut on shaft

One final thing I wanted to do before I fitted the hub was to lap it to the shaft. Flexofold reported the old prop only fitted the taper at about 20% of its circumference. This should be at least 70% of its circumference as this connection is all that is stopping the prop sheering the pin and spinning freely. To do this I added some valve rubbing compound to the shaft and rotating the hub on the shaft.

Step 3 – Fit your new shiny prop

This is the easy bit, and Flexofolds instructions are very clear. Slide the hub on without the key and mark the prop shaft with a marker pen and ensure it doesn’t wobble about (this would be a sign 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 ensure that it goes all the way back to where you marked the prop shaft to make sure the hub isn’t ‘riding’ the key.

Next, do up the hub nut – Flexofolds instructions here fall down somewhat as they merely suggest to do it up ‘really tight’ – I work a bit on cars and light to do things by the ‘book’ and like a torque to do things up to. I contacted Flexofold and they suggested 25Nm… on a 3/4 inch 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.

Prop hub fitted

Prop hub fitted

My concern at this point was two fold, 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) Flexofold and they advised this was perfectly normal and the grub screw was supposed to locate onto the body of the nut, not one of the faces.

Flexofold nut on the left

Flexofold nut on the left

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

Fitted Flexofold prop

Fitted Flexofold prop

As with everything messing around with old boats, a simple process can often be fraught with unforeseen problems. Whilst I suggest Darglow might have spotted some of the problems earlier, paying the 20% premium, there still would have been plenty of messing around before we reached a satisfactory solution, and ultimately, even with 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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