Freehanding the propeller with a grinder: Excellent!

Okay, I'll be the first one to admit that the title of this post is misleading, if not outright erronous. Propeller blade geometry is an extremely complex area of expertise, and only a select few have the talent and experience to fully understand all of the aspects. I remember this clearly from my time wrenching Formula 2 boats. There are only a very few people in the country considered to have the necessary skill to carry out good repair, and even fewer who can carry out a meaningful adjustment. The pit area was teeming with stories about some failed adjustment that could never be corrected, carried out by the most skilled technicians in the business, which completely destroyed the properties of a match winning propeller. With that in mind, a layman like myself shouldn't even attempt to repair obvious damage, but leave this to an expert, lest I cause more harm than I remedy. However, this principle depends slightly on how bad the starting point is. If, for example, you are looking at a propeller with one blade broken clean off, chances are that you won't make matters worse by welding it back on, regardless of the accuracy of the operation. Of course, there is no guarantee that the end result will be good, or even acceptable, but it should at least be better than when you started. The screw on the Oscilia was such a case.

I first noticed this while undertaking maneuverability tests prior to signing the contract. I had brought a friend along as a deck hand, and he was lounging on the settee in the stern after we departed the harbor. After gently running the machinery up to temperature, I started adding rpm and pitch for a top speed test. My friend immediately came running up into the wheel house, screaming: "Stop! Stop! Something's come loose around the shaft!" As gently as I could over the ear-splitting din, I started explaining that it sounded like tip cavitation to me, but that it was by far the worst I'd ever heard. I was interrupted in the middle of this by loud crashing sounds from the stern. At this I backed off the pitch, and asked him to take the helm while I went back for a look-see. In the galley-cum-lounge I found numerous objects on the floor, including this laptop, and the objects still left on the counter and table were dancing around from the vibration.

After this, we established a safe maximum speed of 4.5 knots, and excitedly awaited the first pull-out so we could have a look at things. I suspected that heavy marine growth would prove to be a partial explanation, but the problem was much, much too bad for that to be all. Indeed, when we pulled her out the growth turned out to be horrendous, even worse than I had anticipated. The whole bottom was covered in ~5" of shells and barnacles, totalling an estimated ton (!). It took a friend of mine well over an hour to simply shovel it off the slipway after we had spent 12 man hours with the high pressure washer to get it off the boat. However, that was not the worst of it. As evidenced by the pictures, the propeller had been severely mangled in a lathe. The Tenfjord CP system had originally been mounted in a large MFV, according to information provided by the seller, and had to be adjusted to fit the Oscilia. As it turned out, the "adjustment" had been carried out by simply turning the complete assembly down to the required size, and installing it on the boat. This procedure resulted in two ugly "corners" being left on each blade, which proved to be perfect starting points for tip vortex cavitation.

The month between the first and the most recent pull-out was partially spent looking up the price of new propeller blades, which turned out to be prohibitively expensive. It also came as no surprise that having the existing blades professionally cut down fell way outside of my budget. Thus I decided to have a go myself, using nothing more advanced than a grinder and my general knowlege of what a propeller should look like. Even though the leading-edge radius should be considerably larger than that of the trailing edge, I chose to apply the same 15 cmd radius to both, as I had no way of knowing what would be correct in either case. I selected a suitable paper disc as a template, held it against the propeller blades and shaded the area to be cut with a permanent marker. I then went to work with the grinder, cutting away as much of the shaded area as practical within the confines of the propeller tunnel. Cutting brass with a grinder is especially nasty work, as it does not produce sparks, but rather warm metal chips which tend to bury themselves in the skin and produce oozing sores afterwards. For this reason I was grateful when the worst of the cutting was done, and I could get to work with a hand file to debur the edges and produce the final finish.

I'm reasonably happy with the looks of the end result. As mentioned, the radiuses cannot possibly be correct for the application, but they are by far preferrable to the pre-existing sharp corners. Also, the propeller tips are now about 1/4" thick, whereas a professionally finished job would have these thinned down to sharp edges. Given the necessary space to work, I would have attempted to fair the leading face into the pushing face, so as to leave the pushing face curvature as close to the factory finish as possible. However, I was afraid of ruining the edge curvature by working with the grinder inside the tunnel, and I had no access to faciliate meaningful material removal with the hand file, so I elected to do this later on the bench, if I cannot locate reasonably priced replacement blades.

After our time on the slipway was up, I cruised out towards our mooring location, excitedly waiting for the engine to reach operating temperature so that I could evaluate the results. I was expecting a significant reduction in propeller noise and vibration, but no significant performance increase. However, once I set the engine to my previous safe cruise setting of 1000 rpm and added four dashes of pitch (on the instrument - I don't know what that equates to in hard numbers), the speed quickly shot up to 6.5 knots, with about the same amount of vibration as earlier. When I decreased rpm by 200 and pitch with one dash, I reached the previous cruising speed of 5.5 knots with nearly no vibration at all. I find this absolutely astounding. I aproached this project with the intent of attaining a small performance increase through minimal effort, knowing that the starting point was so bad that almost anything I did would have a positive effect. The significant performance gains are hugely motivating for me, and I can hardly wait for the 12th of July when I get pulled out again.

There's still plenty to work with, as the vibrations are still worse than any other boat I've tested, though no longer by a factor of two. I will try to apply a larger leading edge radius (maybe 30-40 cm?), and se if I cand find some way of thinning or applying a small radius to the outer edges. Also, I'm increasingly sure that the venturi needs to go. I love the idea of having a competent ice breaker, but at what price? I still haven't made any fuel economy calculations - look forward to an article on fitting a flow meter to the day tank pump - but I suspect that the Oscilia consumes about the same amount per nautical mile as some 100-footers moving at the same speed. I just love the potential for improvement!