Running Gear

Have a seat.  This will be a controversial post.

So, can you successfully cut a propeller down to size on your own? Would you dare even try? I’ve done it twice now, and it worked great, but there are limitations. . .

First, why would I, or anyone for that matter, do this? Because my goal in sailing is to make it as reasonably affordable as possible, and I’m convinced that a lot of work done by “professionals” in the marine industry can be done just as well, and much less expensively by me.

So a couple of years ago my prop and shaft fell victim to a vicious crab pot line attack. Neither survived. They now reside somewhere on the bottom of Herring Bay. All through the winter that followed, I searched for a used prop that would match what I needed. No joy. I finally bit the bullet and purchased a new 3-bladed prop. My old prop was a 2-blade, and I’d always wanted to try the 3-blade.

Turns out, I hated the 3-blade. It needed to be reduced in pitch, and that would have improved it’s performance under power, but nothing would improve it’s performance under sail. It was like dragging a bucket behind. For a boat that’s as heavy and slow as a Watkins 27, that’s just not going to work.

I began the search again this winter, and again found nothing in the used offerings that would suit. I finally bought a prop that was too long, with a pitch that was one inch too shallow. I would cut it down myself, and it would be a grand experiment! After all, I only paid $95 for it instead of $350+ that a new one would cost.

Why did I think I could get away with this? I’m not a propeller technician.

Well, you see, I’ve done it once before, and it was crazy successful. When I repowered Cay of Sea 13 years ago, I needed a new prop then too. The old original prop spun up too far, by 500 RPM, and it had lost too much zinc to have the pitch adjusted (this makes them brittle). When I went looking for props I found out how expensive they are, so I turned to the used market. I happened to find one that was slightly too long – about .5″ at the corners. I cut off the corners so that it wouldn’t hit the bottom of the hull on every half revolution. It turned out to be the perfect match. I could develop all the RPM in the specs (slightly too much, actually) and nearly achieve hull speed.

So this time I was able to find a 15″x12″ right hand prop with one inch shaft, and I thought I would try again. After all, I needed a 13″x13″ rh 1″. So, if I cut an inch off of either side. . . perhaps the pitch would allow it to spin up too far, but I could always get it repitched next winter when I haul again. On the other hand, because the dimensions of the prop were larger over all (larger hub, broader blades, and longer blades) I might get away with only trimming the ends, having the broader blade size compensate in area for the slimmer pitch.

This is the old prop that I trimmed.

New three blade prop I hated.

New prop. Sorry for the bad photo – I forgot to get a good pic before launch.

What follows is an illustration of what I did to trim down to size.

Not the actual prop – I found this image on line. Note the pink color – this indicates loss of zinc. This prop would be pretty brittle.

This is a very crude photo, but it might illustrate what I did – in part. I carefully measured from the middle of the hub to the end of the blade on each side and marked it, then double checked by measuring back from the end of the blades. Making sure I was removing the same amount of material from each side, I used a cut-off wheel with my Dremel and trimmed the ends of the blades. I used my stationary sander to round over the corners and relieve the edges (thin them down to match the rest of the prop edges).

After making sure everything was smooth and even, and rechecking for balance, I installed the prop and crossed my fingers.

So how did it work?

I was really wasn’t confident that I had got it right – I fully expected the prop to need repitching, but hoped to get away with it for this season. However, I my hopes were fulfilled beyond my most optimistic expectation. The prop spins up to exactly 3600 rpm – specified in the engine manual as developing max horsepower – and achieved what I was led to expect from the prop size calculator on regarding boat speed. It actually performs better under power than the prop I lost – that one allowed the engine to over-rev slightly (to 3800 rpm).

So, the limitations are, as alluded to above, that 1) you cannot change the pitch of the prop at home. This takes specialized equipment (torches, measuring tools, bending jigs) that the average diy guy doesn’t have at his disposal. I would think that the balancing of the prop would also be a critical component of changing pitch. 2) you cannot easily change the inside diameter for receiving the shaft. In fact, I don’t know if you can actually go smaller that original, but I do know that the shaft opening can be resized larger.  Again, specialized equipment, etc.  Add to that, the shaft hole is tapered with a key slot cut into it – who can do that at home without a machine shop?

Finally, selection of your not-quite-perfect prop is critical. I think more that two inches oversized wouldn’t work well. I think you would lose too much blade surface area, and be left with too much hub. Also, I would think you would be limited to props that were designed with symmetrical blades – where each blade has the same shape top to bottom. Some blades are elliptical, and I think that shape would be difficult to replicate in a shorter profile.

So there you go.  Tell me what you think? Would you be brave or foolish enough to do this yourself, like me?

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