© By Ellis Simon


            Many cruising skippers dream of replacing their gas engines with diesels so they can have greater range, greater reliability, lower cost, or whatever.  Most give up on the idea and either buy a diesel boat or stay within limits.  We’ve had two diesel boats and two gasoline boats, from 34 to 40 feet in length.  We bought a ten-year-old 40-foot double cabin cruiser a year or two ago.  The boat is perfect in many ways for my wife and me, and we intend to use her for extensive cruising.  But she had aging 454 cid Crusader gasoline engines.  We’ve made the conversion to diesel, at a cost of $46,000 plus 900 hours of labor.  This article is intended to provide information for others who have contemplated the same move. 


            The first step was to select the engines.  The Crusaders are advertised to produce 320 horsepower at 4400 rpm.  With the 2.91 to 1 reduction, the 24 x 26 propellers were turning 1512 rpm.  The boat did 25 knots at full throttle.  There are several unknown factors in the horsepower and rpm numbers.  First, unless they are carefully calibrated, low cost tachometers can be off by 10 percent.  Second, these numbers may have been different when the engine was manufactured, ten years ago.  Third, the engines had been rebuilt, and who knows what was changed.  Fourth, the exhaust systems appeared to be undersized. 


            I researched four or five engine manufacturers and narrowed the choice down to two prospective engines.  The 285-hp, V8, Detroit Diesel 500TI and the 315-hp, 6-cylinder Yanmar 6LY-UTE.  I had also considered the Detroit 6V53TI and the 6-71TI.  These two engines were eliminated on the basis of a high weight penalty and a nearly insurmountable interference between the exhaust and the fuel tanks.  The Yanmar had a lot going for it, especially size and weight, but the price quoted was $ 21,600 per engine, while the DD 500TI was quoted at $ 16,000 per engine.  $ 11,000 is a mighty persuasive deciding factor.  There is a weight penalty of 350 pounds per engine and perhaps 150 pounds for the generator.  But that is acceptable.  The DD 500TIs were supposed to fit the same space box as the Crusaders, and they do, with exceptions.  More on that later.


            I also had some brand loyalty, in that I had owned a boat with 4-53s for seven years, and there had been little trouble with the engines.  Another argument for the Detroit Diesels was the fact that we plan to do quite a bit of traveling outside the US.  On previous trips to Canada, I found that most all of the power boating is commercial fishing, and most of them use Detroit Diesel.  This means that parts might be easier to come by, and that the mechanics might know something about the engines. 


            The DD 500TI puts out 285 hp at 3200 rpm.  With a 2 to 1 reduction, the projected propeller rpm is 1600.  Just a little above what the Crusaders turn.  The sea trial showed that the diesels peak out at 2850.  It will be necessary to reduce pitch on the propellers in order to achieve maximum Rpms.


            Once the engines were ordered, we turned to the generator.  Here, not much science went into the selection.  My yard is a Kohler dealer and a Yanmar dealer.  Since he had lost out on the main engines, I thought it fair to give him the generator.  As it turned out, Kohler had a boat show special and I was able to get a 9-kw unit, powered by a Yanmar 3-cylinder engine for $ 8,000 with sound shield.  When you’re anchored out, no generator can be quiet enough, so we went for the sound shield. 


            Next came the issue of the exhaust system.  The boat had a simple, straight 4-inch exhaust out the side of the hull.  The trouble is that it is just 43 inches from the engine exhaust elbow to the aft engine room bulkhead.  There was no way I could fit a stock muffler in the space and make the 90-degree turn.  I went to Centek, which has recently acquired the Vernatone muffler line from Vernay.  Mr. John Ford, who is a consulting engineer for them, worked with me to develop a special version of their Super Vernatone muffler.  It cost $ 750 for a muffler that sells at discount for $285, but it fits nicely and is large enough to accommodate the 275 horsepower.  Those six-inch holes in the side of the hull are quite a sight, though.  It was worth the cost and effort, as the engines can hardly be heard on the flybridge and yet, the backpressure will meet the Detroit Diesel requirements at all speeds.  (More on that later.)


            Regardless of what it is, if you are changing from gas to diesel, you can’t use what you have.  It has to be bigger.  Bigger water supply.  Bigger fuel supply.  Bigger electrical cables.  Bigger exhaust.  The stock boat came with small gasoline filters, which would not be usable for the diesels.  I ordered two RACOR filters, which turned out to be the one item that no catalog supply house seemed able to deliver.  They were ordered in March, and they didn’t arrive until mid July. 


            I had planned to use the existing fuel tanks, and had devised a scheme to flush them.  When I was pulling out the old exhaust system, I discovered that the port fuel tank was leaking gasoline.  Some debris had gotten between the tank and the batten and just enough water had entered the debris to cause corrosion. 


            So this meant replacing both tanks.  Replacements were ordered from the tank manufacturer who happens to be located in Toms River just north of where we have the boat.  I probably jumped the gun, as I went with an exact replacement, and didn’t consider other materials such as Monel.  With the engines and the tanks out of the boat, I cut slots to access the engine mounting bolts.  In the process, I found that the shelf each fuel tank sits on was not fastened at the outboard side.  So the tank was banded to a piece of plywood that was held in place by 8 self-tapping screws on one side. 


            To further aggravate the situation, three of the four 2x4 uprights, which are supposed to anchor the tanks at the top with a half-inch bolt, were bolted to the tanks but not to anything else.  There wasn’t even a hole for a bolt.  So this meant those tanks, each weighing 1200 pounds when full, were free to roll around down there.  Apparently someone else had noted the problem, as there was an attempt to fiberglass in supports on the outboard side.  Unfortunately, this did not work, as there was too much space between hull and shelf.  So, painting and securing the shelves was a time consuming activity. 


            To assure that the tanks themselves stay in good shape, I put them through the Awlgrip process.  They have a couple of hundred bucks worth of paint on them, and it is the same system that you’d use on an aluminum boat hull.  I applied 2-inch squares of 1/8-inch neoprene rubber to the bottoms of the tanks with contact cement.  The idea is to provide plenty of water drainage and air circulation for drying when some water does get in.  The tank battens have been provided with the same sort of channels under them. 


            We started the project on April 1, working the two weekend days and an occasional evening at home.  Replacing the fuel tanks was an effort that nearly equaled the engines in time consumption.  As a result, we had the dealer hold the engines until we were ready for them.  We also changed our plans for the summer at that point.  In May, I expanded my weekends to 3 days.  We’d planned a 4-week cruise in August, but it was beginning to look like a significant part of that time would have to go to the project. 


            The new engines arrived late in June and we started the installation right away.  Both shafts were out of line, and lay hard against the stuffing box wall when uncoupled.  I had to get that corrected before aligning the engines.  The gas engine mounts were a little on the light side for diesels, so I went to a steel supplier and had him cut eight sections of 5-inch angle iron, each 9 inches long.  When I got the engine positioned in line with the shaft, these were fitted and mounted. 


            Getting the machinery in and out of the boat was another challenge.  Everything had to go through the salon window on the starboard side.  It looks like the marine architect planned it that way, for the window is exactly the size of the gas engines with everything stripped down to the valve covers.  The sequence was to have the yard reach through the window with a crane and lift the engine up out of the bilge to the cabin.  It was set on the cabin floor, which had been shored up from below.  Then I set up a special rack, which I had constructed of 4 x 4s.  This rack had two angle iron rails suspended by four 5-foot long 1/2-inch jackscrews.  The jackscrews lifted the engine up from the deck to the overhead.  I drove each jackscrew with a 3/8-inch electric drill, a couple of inches at a time. 


            The yard then came in with a large boat hauling type forklift.  The forks were extended through the starboard window and the engine was set down on rollers on the forks.  The engines were rolled through the window by hand, to eliminate the danger of damage from the movement of the large machinery.  On removal of the Crusaders, the clearance was so tight that the second Crusader had paint scraped off one valve cover as it passed through the window.  Once the engine was outside the boat, the forklift just took it away.  Installation was the reverse process.  But when it came time to bring the new diesels through the window, my luck ran out.  The stripped engine was just an inch too tall.  It was a matter of cutting notches in the window frame or taking the oil pan off each engine.  I cut the window frame. 


            The generator was removed by the same process, clearing the way for removal of the fuel tanks.  The new tanks presented another problem.  While they only weigh 80 pounds empty, they are roughly 2 feet by 2 feet by 8 feet.  They’re rather bulky and sensitive to scratches (the new paint job).  I found that the top of a Ford van is roughly even with the salon deck.  Backing the van up to the boat gave us a nice walkway to walk a tank into or out of the boat.  Once the tanks were in the engine space, large pieces of carpeting were used to protect them as they were slid into place. 


            We disassembled the first diesel engine and found that taking all the plumbing, the intercooler and the turbo off the engine takes more time than planned.  In fact, we worked on that task half of Friday, all day Saturday, and were just completed at seven, Saturday evening.  To add to my misery, it was cold and rainy throughout the job.  Sunday, the yard was closed, so we could not do any lifts. 


            We went down again on the next Wednesday and set up for the forklift move.  Positioning the engine on the forks went well and by 11:00 we were ready to try the lift.  But things fell apart from there on.  The rains had softened the yard surface, and the forklift started to mire in the earth.  We had to abort the lift, and nearly dropped the engine in the process.  The yard fixed the ground problem and finally, at about 3:00, we made the lift.  It was here that I found that the engine was still too tall to fit through the window.  There was nothing to do but to cut a half-inch out of the aluminum window frame.  The engine then rolled in the window and was positioned for the special rig.  But the rig interfered with the starter motor, so I had to take a break to remove the starter motor.  This would be simple, but one of the bolts is trapped so that it has to be turned as the motor, which weighs about 75 pounds, is removed.  Finally, the rig was positioned and the forklift was withdrawn.


            We lowered each engine onto temporary beams across the stringers.  Then, with the heavy lifting equipment clear, we picked the engines up for the final fitting process.  Jackscrews, supported by timbers across the deck, were used for this task.  Much to my disappointment, I found that there was an interference with the engine stringers.  On the starboard side, each engine had rather large oil fittings, which projected into the stringer.  On the port side, the alternator projected into the stringer.  There was no choice but to cut notches in the stringers to accommodate these pieces. 


            With stuffing boxes removed, and the intermediate strut pulled back, the shaft was loosely bolted up to the engine and the whole assembly was positioned for proper shaft to boat and shaft to engine alignment.  The new engine mounts were then clamped in place.  They served as templates to mark the bolthole patterns on the stringers. 


            The engine was then hoisted up out of the way.  (Thirteen turns per inch on each jackscrew.)  Holes were drilled in the stringer and the mounts were bolted in place.  It’s a good idea to bolt the mount into place with the first bolt and use the mount as a drill guide for the rest of the holes.  Otherwise, it’s a struggle to get the bolts in.  Then it was back down onto the mounts with the engine.  Recouple the shaft and realign everything.  The intermediate shaft strut was then positioned and fastened in place. 


            With the engine in its permanent position, it was time to reassemble all the plumbing and hardware.  This took about two days per engine.  Next came the installation of the exhaust system, the water system, the fuel system, and the controls.  There was nothing particularly difficult about any of these systems, but it takes a long time to do the work.  By this time we were well into our planned vacation time, and working 12-hour days. 


            When the boat was launched, the dealer sent a mechanic down to start up the engines and put them through sea trials.  All went well, except that the engines would not come up to the full 3200 rpms.  The port engine turned 2850 and the starboard engine turned 2900.  As a result of the excessive loading, the exhaust backpressure ran a little higher than permitted by the manufacturer.  My guessing game on the shaft speed was a little off.  Not to worry, a little arithmetic showed that 2 inches off the pitch on the propellers should set things right.  So we hauled the boat and had the wheels repitched.  (This alone cost $ 600, not included in the bottom line below.)  With 24 x 24 inch propellers, the port engine turns 2950 and the starboard turns 3000.  This game is getting expensive in a hurry.  I decided to ask Michigan Wheel to recommend a solution.  The solution came back promptly.  They recommended 22 x 22 inch four bladed cupped propellers.  This will add another $1800, not included in the bottom line below.  I’ve decided to save the cost of another haulout and have the new wheels installed during the spring outfitting. 


So, what’s the bottom line?                   Engines             $ 32,000

                                                            Generator                          8,000

                                                            Fuel Tanks                        1,250

                                                            Mufflers                             1,500

                                                            Air Sep Filters       1,500

                                                            Yard Work                          500

                                                            Miscellaneous                    2,000


                                                            N J Sales Tax                    2,805


                                                TOTAL PURCHASES            $ 49,555


Deduct sale of gasoline machinery                                                  3,500


                                                TOTAL COST                        $ 46,055


            These are the outlay costs.  I happen to have a well-equipped shop and I am an engineer, so virtually all of the labor for design, procurement, and construction was my own.  At present, we have about 900 hours invested in the project.  That would add fifty thousand dollars to the yard bill if it were all done by a yard. The miscellaneous parts were ordered from industrial supply houses and marine supply houses at discount prices.  The project officially commenced with the removal of the first exhaust hose on Saturday, April 1, 1995.  It could be considered complete on August 19, 1995, approximately 2 months behind the original 3-month schedule.  It was also 500 hours over the original 400-hour estimate.  If there is any message we want to get across, it is that a project like this takes more time and money than you expect. 


            To be realistic, it would be necessary to compare the above with the cost of simply replacing the old gas engines with new gas engines.  The bottom line estimate for this alternative is as follows:


                                                            Engines             $ 12,000

                                                            Generator                                   0

                                                            Fuel Tanks                          1,250

                                                            Mufflers                                  300

                                                            Air Sep Filters                0

                                                            Yard Work                            500

                                                            Miscellaneous                      1,000


                                                            N J Sales Tax                         903


                                                            TOTAL                         $ 15,953


            Accepting the fact that the engines and the tanks needed replacing, the additional cost of the diesel option is around 30 thousand dollars.  In the cruising rpm range, the savings in advertised fuel consumption is over 20 gallons per hour for two engines.  So the diesels will pay for themselves in fuel savings in about 1,200 to 2,400 hours, depending on per gallon costs and speed of operation.  Since we’ll be cruising extensively, we will put several hundred hours on the engines in a single year.  So diesel is the way to go.  And this doesn’t take into account the new, larger and quieter generator, or the increased range.


            If you contemplate a project like this, you should also take care not to let the combined cost of engines and boat exceed the book value of the boat.  If your costs are in excess of the book value, you will have trouble securing adequate insurance to cover your investment.  You’d also have trouble recouping your investment if you were to sell.