mikel@orca.UUCP (Mike Lancaster) (09/19/84)
I am building a 34' sailboat. It has been suggested that I use a rabbit diesel engine. The idea is to use a reduction gear. Further, the suggestion was made to use some copper tubing laid along the keel to cool the engine. The cooling system would be a closed-loop set-up, using fresh-water only. It appears to be a great idea on the surface. No ex- pensive saltwater conversion equipment would be required. The engin is relatively light in weight for the power it produces. There are, however, a couple of questions. What kind of a reduc- tion gear is required? What kind of propellor is desirable? How many feet of what diameter copper tubing is needed to do the job? That already is more than a couple. WE could probably come up with a few more. Do any of you readers have solutions to suggest, or can you suggest ways to answer these questions? Please reply by mail, or if you think of things we believe are of general interest, we get a lively discussion going. Thank you ahead of time. Hopefully yours, Mike Lancaster
jwp@sdchema.UUCP (John Pierce) (09/27/84)
> I am building a 34' sailboat. I started to reply to this directly, but finally decided a public answer might better serve more people. I truly wish this person the very best; I truly hope he gets his boat built, takes her to sea, and that many thousands of miles of ocean pass calmly under her keel. I hope he really already has 40000 miles at sea behind him, and has read Mate' and Hiscock and Street and Jones and Nicholson and Pardey and the rest, and has previously built a smaller craft. But I don't think so. I don't want to be insulting, but the very fact that these questions are here imply that those hopes don't reflect reality. If that's true, if that level of experience isn't there, then for god's sake at least get the books and read them. Find out what the realities of the undertaking are. Lin Pardey estimated that she and Larry had nearly 8000 hours in building their new boat (admittedly, from wood, and I mean from the raw lumber), and this was the second one they built, and they've had 15 years of living on the ocean, working in boat yards, etc, and Larry's one of the finest shipwrights I've ever seen. The original article has been chopped up somewhat to keep related questions together. >It has been suggested that I use a rabbit diesel engine. I don't know what a rabbit diesel costs, nor do I know a whole lot about the details of tuning diesel engines, but I know enough to foresee a whole lot of problems that are likely to offset any possible initial cost advantage. To begin with, there are reasons marine diesels are so damned expensive. Even those that are marinized versions of industrial engines are considerably different from their industrial counterparts. The material of many parts is different to try to inhibit the inevitable corrosion. The power curves are completely different. You aren't going to want to be on that boat with that rabbit diesel screaming along at 5000 rpm. I don't care what kind of (very expensive lead-foam) sound insulation you're willing to put in the engine compartment; it is going to be very unpleasant. >The idea is to use a reduction gear. What kind of a reduction gear is >required? It's unclear what "what kind..." means. If that means "should it be direct drive, or hydraulic, or standard V-drive?", that's pretty much an installation dependent question (though I've never seen an installation a hydraulic drive wouldn't fit it). If it means "what reduction ratios are needed?", that's dependent on myriad things: boat displacement and hull form, engine power curve, propeller type, etc, etc. Anyone who attempts to answer that without knowing those specifics should be immediately distrusted. It also beggars the question (sort of) of where the transmission is going to come from that allows you to have reverse as well as forward. >What kind of propellor is desirable? Besides boat displacement, engine power curve, etc, this also depends on what you want: good power performance, good sailing performance, or both (though they're nearly mutually exclusive). For power performance, you want a standard three-bladed prop. For sailing performance, you want a two-bladed folding prop. The only way you're going to get both is to spend a lot of money for a full-feathering prop (the difference in sailing performance between this and the two-bladed folder is generally only noticeable to racers). In any case, somebody's going to have to do the work to determine what diameter and pitch are needed to match it to the engine and boat; which is made considerably harder if either are unusual. [I can here it now: "I don't care about *performance*, I just want to *cruise*. Downhill all the way, that's me." Now think about this: You're a mile off a lee shore, the wind's at 40 knots and rising, the seas are running 15 feet and building, the engine won't start. Now tell me you don't care about sailing *performance*.] >Further, the suggestion was made to use some copper tubing laid along the >keel to cool the engine. The cooling system would be a closed-loop set-up, >using fresh-water only. No, that's *not* how it's done. The way it's done is to build a jacketed system with a standard marine water-cooled exhaust. The jacket runs seawater; the inner tubing runs engine cooling freshwater. The jacket water cools not only the inner freshwater tubing, but also the exhaust before it passes through the transom. This is too poor a medium for drawing pictures to illustrate it. Go to a marine engine dealer and look at one. It's fairly easy to design and build. The reason for building it yourself is that it allows you to buy a seawater cooled marine engine, and that won't cost you as much as buying one already set up to run freshwater. >How many feet of what diameter copper tubing is needed to do the job? You can actually calculate this from the horsepower output of the engine and estimates of the amount of cooling provided by flowing seawater through the jacket. In practice, look at what the engine manufacturers use, estimate how long that is, and add some just to be sure. You'll probably have to muck around with the theromostat to get the running temperature right. >No expensive saltwater conversion equipment would be required. Saltwater conversion equipment? There isn't any saltwater conversion equipment in an engine cooling system. They either run freshwater (as described above) or they run seawater through the engine. What you really do is go find a decent marine diesel that will do the job. The 15 horse Yanmar works just fine in my Cal 36 (~12000 lbs displacement) with a folding prop. It's more than sufficient for what it's *really* good for - getting out of the slip and charging the batteries. [A real irritation, actually. My new slip is a downwind slip and I really need the engine to get out. The old one was an upwind slip and I could sail in and out with no problem. The $100 a month difference makes up for it, though, I guess.] I'm not pushing the Yanmar in particular, though I've been quite pleased with it; there are other engines that are at least as good (if I really had to recommend something, it would be the small Volvo - I've never heard anything but good about it). Whatever you get, buy it seawater cooled if possible and then build a freshwater cooling system for it; you'll not only increase the life of the engine enormously, but you'll save money, also. If you really feel the need for more power, fine, but think about it carefully. More power means both increased initial cost and increased operating cost. For the difference in price you can buy self-tailing winches instead of standard ones and gain something that's *really* useful. The engine on a sailboat sailboat should never be considered any more than a somewhat unreliable convenience. Putting your self in a position that you have to have the engine to get out of is foolish. The ocean kills fools regularly. John Pierce, Chemistry, UC San Diego {decvax,sdcsvax}!sdchema!jwp