by Capt. Mike Holmes
Switching from Gas to Diesel Engines
Until about three years ago, I was an avid outboard engine enthusiast. The big V-6's I was running on my 24 foot boat had given me performance and reliability unheard of in boats that size a few years before. I was convinced my future vessels would be outboard powered, fast and seaworthy.
Several things have happened though that could have changed my mind, including environmental concerns, the development of larger four cycle outboards, and the increasing price of behind the transom power. What actually changed my focus from outboard power was falling in love. Yes, I fell head over heels for a 31 Bertram of 1962 vintage - an old, gasoline inboard powered boat! I fell for the traditional lines, rough water performance, and fishing history behind the Bertram.
Like most owners of older inboard boats, however, I soon became confronted with the harsh reality of repowering in the near future. Since I had long held gas inboards in a less than favorable light, I have been planning, on a diesel retrofit almost since the beginning. The advantages of diesel, on the surface, at least, seem obvious; better economy, longer range, longer engine life, and increased dependability. The problem with the old 31 and many other boats in the same size class - is finding a suitable diesel to install. In the way of older engines, those most used in 31's were 453 Detroits, six cylinder Perkins and Mitsubishies, Cummins 504 and 555 V-8's, and 3208 Cats.
Newer diesels are now on the market that are much more suitable. The 4BTA and 6BTA Cummins engines are good choices, as are the four and six cylinder Yanmars. Cat 3116's are very powerfull, but still a bit large and heavy for this boat. Any of these engines can provide better economy than older gas engines while at least equaling their top speed.
So the solution is simple, right? Just drop a set of modern, high-tech diesels in the old boat and bask in the glory of high cruising and top end speeds with low fuel bills, and cruise to wherever I want! Well, it turns out to be not quite as simple as us old outboard guys might think. In my case, my old express model was originally built for a Houston man who kept it in Seabrook, and it came with Chrysler big blocks and 1:1 ratio transmissions. These direct drive gears cause the propellers to turn the same speed as the engines, therefore, rather small props must be used. With a light load, the boat is fairly quick, but with a full load of fuel, ice, bait, and charter customers, the little props can't push enough water to make decent speed.
To change to diesels would require much bigger props, but my struts aren't long enough to accommodate them. This means changing struts, shafts, and props - with possible glass work to the hull to change the mounting angle of the engines. AND, the transmissions must be exchanged for reduction gear units to turn those larger props at a slower speed, allowing the engines to rev to a workable range.
Then there are the other changes. The four cylinder Yanmars will drop in with no change in exhausts or water inlets, but going to six cylinders will require six inch exhausts - with new mufflers and transom exits - larger water intake valves, and the big 8D batteries. Of course, electrical and instrument changes will need to be made, and the fuel tanks cleaned and fitted with return lines.
In my case, it would be easier to buy a diesel boat than convert mine. Because I bought the boat for a very low price, I could probably recoup the investment in diesels if I sold it later on, but to figure a payout on fuel savings over the expected life of the engines would be very optimistic.
After determining that a diesel swap, while not out of the question, would not be the smartest thing I've ever done, I thought briefly of switching to outboards. I really believe a pair of V-6 EFI two cycles would push the Bertram well, and I wouldn't have to worry about transmissions and struts and shafts all having to work together - and also work with the engines. Unfortunately, the list price of a pair of outboards is about the same as a pair of six cylinder diesels with gears!
So what is there left? To be "stuck" with gas inboards? It could be a lot worse!
When I bought this boat, it had a brand new EFI 454 on the port side, and older carburetor 454 on the starboard. There has always been a very noticeable difference between the two powerplants. This new breed of gas engine from several marinizers utilizes multi-port fuel injection, sophisticated cooling systems, computerized engine management systems, better mounting systems, more corrosion resistant materials, and more shaft horsepower than in the past. The EFI engines are more reliable and fuel efficient than gas engines of the past, and should last longer. With the high horsepower and torque ratings of the gas big blocks, speed is there for those who want it. In fact, some new boat builders are offering their "tournament" models with 502 big blocks as standard equipment, and Mercruiser has just released a very interesting 454 rated at 370 shaft horsepower.
List pricing for newer gas engines is sometimes as high as mid-range diesels, but installation costs are much less, and actual purchase price may be as much as half the cost of a new diesel if one does some shopping. In conversions, however, be aware that counter-rotating engines are almost a thing of the past - opposite rotation these days is accomplished in the transmission. In my case, I'll need to change gears to get proper performance anyway, but those with a set of otherwise good Borg-Warner 1.5:1 or 2.1:1 ratios will not be able to use them, since these older gears do not reverse (except for the BW 1.91:1). Borg Warner and Hurth both make updated transmissions that will reverse the rotation, however.
By going with another EFI 454, I will only need to buy one engine and two transmissions, plus a larger set of props. Because I can fit a 17 inch wheel with my original struts, I won't have to change struts or shafts. Performance at cruise and top end should be vastly improved with a load. Bottom line, new power at a fraction of the cost of a new diesel conversion. I'll have to carry extra fuel for a tuna or billfish trip, but as my boat mentor, Chick Sharp of Freeport, is fond of saying, "You can buy a lot of gasoline for what those diesels would cost!"