Ill confess to being very surprised at the number of people who have read my article on gas versus diesel engines and applied it to sail boat auxiliaries. I thought that it was clear in the article that it only applied to power boats. In any case, most of the information contained that article does NOT apply to sail boats, except the repair cost factor, which is significantly higher than a comparable gas engine.
As to whether gas or diesel is preferable in sail boats, as far as I know, there are no longer any gas inboard engines available for sail boat auxiliaries. I could be wrong about that, but I havent seen any, and a net search hasnt turned up any. So, as near as I can tell, its a moot point.
Since Ive been asked this question innumerable times, Ill give my viewpoint here, though its strictly a matter of personal opinion.
If I were primarily a day sailor, Id be quite happy with the Atomic Four, as it is very quiet and smooth running. With its old valve-in-block design, it hasnt proved particularly reliable, and doing a valve job on it can be quite expensive. Of course, there is a major carbon monoxide problem with gas engines in sailboats so that you must keep the exhaust system in tip top shape at all times or risk killing someone.
But I dont think Id want that engine were I involved in extensive long-range cruising. It has a very weak cooling system that is prone to overheating because of the incredibly small size of the water pump. It doesnt take much to bring the whole system down.
Auxiliary diesels are often a real head-scratcher to me. Frankly, I dont know how boat owners can tolerate the horrific noise and vibration these engines make. Here we have sailors, most of whom profess to detest the internal combustion engine, and yet have the noisiest engines of all in their boats. Part of the reason for this is that sail boat builders have never put much time or thought into engine installation. Small diesels, by virtue of their few cylinders are poorly balanced engines. That means that they have to have even better mounting systems, which they usually don't. The simple rule is that the more cylinders there are in an engine, the more those cylinders balance out the natural vibration. Thus, the 4 cylinder engines run quite a bit smoother than 2 or 3 cylinder engines. So, if there is an option for 4 cylinders versus two, I'd opt for four, even if it is more power than I need.
A two cylinder engine has so much vibration that its almost impossible to dampen it except by the most extreme and costly mounting methods. Basically, that is cost prohibitive to most people.
You may be aware that Yanmar once supplied an engine mount with their engines that was very good at reducing vibration, but, unfortunately, the mount was so flexible that extreme engine movement caused serious engine alignment problems and were ultimately abandoned, many of which had to be refitted with other mounts.
There are three ways to minimize the vibration problems of sail boat diesels. The first, if youre buying new, is to opt for at least a four cylinder engine, regardless of whether you need that much power. The second is to upgrade the engine mounts. The Bushings, Inc. Mounts supplied with most diesel installations are just not adequate. There is a new mount out, with which Im very impressed with the design, and is supplied by Caterpillar on all their engines. Unfortunately, I dont recall the name of the manufacturer, but you can contact any Cat dealer to find out.
The new Caterpillar mount. Note that this engine is not properly aligned and is distorting the mount. The top plate below the second nut should be level but is cocked at an angle.
Naturally, they are not cheap, but if you cant stand to live with that horrible vibration, its really a small price to pay.
I find many sail boats fitted with all manner of fancy and expensive shaft couplings. For the most part, these are of little value since, if your shafting is set up right, they are not needed. The vibration does not come from the shaft or propeller (unless those items are out of whack) but the engine itself. And the way to deal with that is in the engine mounting system. Unfortunately, most sail boat mounting systems are poorly designed. Often, theyre little more than a small raised foundation glassed to the hull. Unlike power boat stringers, which run the length of the hull and distribute the vibration over a wider area, these engine mounting pads concentrate the vibration at a single point. Thats not helpful.
Transmissions Considering the low power of the engines, you should never experience transmission failures. That they do occur is usually the result of two problems. Excessively loose, weak or worn out engine mounts allow the engine to move. This throws the shaft out of alignment and puts strain on the output shaft bearings. Plus, sailors are prone to ignoring the condition of shaft cutless bearings. When these wear out, the shaft then jumps around, also causing strain on the gear box. Beware that if the bearing is installed in the keel, it is going to wear at a higher rate than one on a free-standing strut due to less water lubrication available.
Engine movement should not be perceptible. If you can see the engine moving or shaking from vibration (and many of them do), then you've got an engine mount problem, and damages of one sort or another will surely occur. If the top of the engine is rocking back and forth, so will be the bottom part, where the shaft connects to the gearbox.
The other common problem is an oversize prop, either in diameter, pitch or both. To get a handle on this, you need to consult a propellers load chart and see whether the load delivered by the prop exceeds the torque rating of the transmission. If it does, you needn't look any further for why you gear box failed. You can get the box torque ratings from dealers or the manufacturer.
Factors That Affect Diesel Auxiliary Engine Life
Under ideal circumstances, sail boat diesel auxiliary engines should last the life of the boat. That they rarely do is usually the result of two things: (1) Failure to maintain properly and, (2) faulty exhaust system installation.
As with power boats, exhaust riser failure is the number one cause of major engine damage. Second is excessive exhaust system back pressure caused by creating risers that are too high. Sail boats often have two risers, a riser being any upward direction of the exhaust piping intended to prevent water backflow through the exhaust system. First there is the riser at the engine, usually a cast iron or stainless steel part. After this there may be a secondary riser consisting of the exhaust hose -- usually after the muffler -- that makes a high upward sweep before turning downward and out through the hull. In larger sailing yachts, Ill often find these to be in excess of four or five feet high.
What we need to realize here is that the exhaust pressure has to push large amounts of cooling water UP that four or five foot height. This causes pressure to build up in the engine, with the result that the engine runs far less efficiently, burns more fuel and runs hotter, often causing overheating and premature valve wear or even value burning problems.
The illustration above shows the right and wrong way to design an exhaust system. The top illustration shows how the dry riser eliminates the problem of exhaust pressure having to move a lot of water. In the lower illustration, not only does exhaust pressure have to move water through the muffler, but nearly straight up through a water cooled pipe or hose. In addition, there is often a a section of hose that lays along the bottom of the hull that also fills up with water, so that exhaust pressure has to move that water, too. And since water weights 64 lbs per cubic foot . . . . well, you get the picture. Remember the school kid trick of stuffing the potato up the school bus exhaust pipe? Pow! Instant artillery piece. Ohmygod, Johnny was shot with a potato. But nowadays the kiddies don't mess around; they use real guns.
Getting the right balance between minimal exhaust pressure and reducing the risk of water entering the engine takes a bit of expertise of the sort that comes with years of experience in simply knowing what works and what doesnt. There are no nice design books that anyone can go to for a quickie solution, since exhaust systems tend to be designed around available space to put them. That, of course, is having the horse push the cart: a good, reliable system should be designed first, regardless of space requirements, but that is not the way things are done these days. Precious interior design always comes first. After all, warranties will long run out before you, the owner, are called upon to pay the price of these design faux pas.
When it comes to riser design, its best that the riser section not be water cooled, but dry insulated to the point where water is injected into the cooling system on the down side of the rise. This completely eliminates the potential for riser failure wrecking the engine, but it also poses the hazard of setting the boat on fire if the insulation is not maintained. Due to vibration, insulation does not last forever, but begins degrading on day one. Eventually, if not replaced when it is degraded, it will likely start a fire. The cost of insulation is not high, but many boat owners overlook this hazard.
If you have picked up on the fact that dry insulated risers will eliminate the exhaust back pressure problems too, hooray for you, youre right, it does. If youve got one of those exhaust systems that pushes water up three feet before pushing out the exit port, you should seriously consider having your exhaust system redesigned, for the ultimate benefit of increased engine life will likely be well worth the cost.
Another factor that gives rise to reduced engine life is the fact that most of these engines are poorly ventilated, if at all. Many people fail to understand that much of the engine cooling derives from the air around it, so that if the engine is closed up in a little box without an adequate air flow, the ambient temperature around the engine rises dramatically.
Most of the engine manufacturer installation spec sheets I've seen say that ambient engine room temperature should be no more than 125 degrees as an absolute maximum. Even at that, reduced performance and cooling is occurring. In most sail boats that I've seen, the temperatures run well over that, so it's no surprise to me that, combined with high exhaust back pressure, that so many boat engines don't run for the mythical thousands of hours before overhauls, often not even reaching 1000 hours.
If this describes your engine installation, I'd suggest improving the air flow to the engine. This is best done with a forced air blower, bearing in mind that blowers should always pull air out, not in.. Why not? Well, because if you're pushing air in, those stinky engine smells get spread throughout the boat. Adding a forced air blower is most recommended for those that will be doing long-range cruising and a lot of motoring, particularly in the hotter climates like the southeast US.
The trick to force air blowers is finding a way to bring in air that is not wet and full of salt crystals, otherwise this ends up going through the engine. Unfortunately, auxiliary diesels are rarely ever fitted with air intake filters, so the better way to deal with this is to fit the blower inlet duct system itself with a filter. That will prevent salty air from rusting up everything within the engine compartment.
Usually the best place to install an intake duct is within the box formed by the sheet winch mount foundation. Or, less good, would be a scupper on the aft deck, bearing in mind that you'll be able to hear the blower from the cockpit when installed under the winch pod. But, if you create a good filter box with a foam filter, this will cut down the noise considerably. Of course, there are good blowers and the noisy cheap ones. The ones that operate smoothly and quietly are never cheap. Plus, you want a blower that is rated for continuous duty operation, which the usual kind found at the local marine store are not. If you're looking for a good, quiet but powerful blower, check with the industrial supply houses like Granger or manufacturers like Hartzel or Dayton.
What, the engine has no air intake filter? Have you checked yours, because it probably doesn't. Guess what happens when you have a rubber drive belt that is chaffing and fraying. You know how all that black stuff goes all over the engine compartment? Of course, the engine isn't sucking that stuff in. Is it? This gives new meaning to the term "burning rubber," and why all engines should have air intake filters.
So there you have yet another reason why I say that sail boat engines are usually poorly installed and why they often don't last as long as they should.