Engine Replacement

                                                                     return to home page

In my case, that was not a good choice. The original engine in my 43 ft Slocum was a 63 hp Peugeot Lehman diesel. One of the motor mount brackets had cracked off the block. From my steel fabrication experience, I believe that welding cast iron is not a reliable process. Also, I checked with Bob Bain at Jersey Marine, a former employee at Lehman, and discovered that this was a common problem with the engine and a design flaw. For these reasons, I decided that repair was not a good solution for a sailboat being taken into blue water. Thus began my journey into hell.

Sizing the Engine

Since I was essentially starting at square one, the first step was to determine the horsepower which was optimal for the boat. At first glance, you would think that bigger is better and simply select the largest engine that will fit. This is not the case however. Since a sailboat is a displacement hull, it's maximum speed is 1.3 times the square root of the waterline length. Therefore once the  boat reaches displacement speed, adding extra horsepower does not accomplish anything. In fact, excess horsepower causes another problem in that it does not give the diesel enough torque to generate heat and operate efficiently. Ultimately this will result in operating temperatures below the design level and consequently shorten the life of the engine. If you were designing a sailboat from scratch, displacement and horsepower would be the primary variables. Refitting however introduces another factor... the propeller diameter. 

There are two considerations here. First is the maximum RPM's which the propeller can rotate before cavitation begins. Secondly, the propeller must have clearance from the hull of at least 15% of its diameter to operate efficiently. Also the diameter of the propeller and horsepower determine the required diameter of the shaft to handle the torque. Assuming the you have a variable pitch prop such as the Max Prop, you can at least change the pitch of the prop to accommodate  increased horsepower. But even this is limited. When the pitch is increased to the point that it equals the diameter of the propeller, called a blocked wheel, the efficiency of the propeller is reduced. The bottom line is that the original design of the boat quickly becomes a design envelope fixing the horsepower.

Considering the time and expense which you are about to undertake, it is still well worth the effort to do the calculations and determine the optimum horsepower for the boat. 

I found two books on this subject to be very helpful. First, Dashew's Offshore Cruising Encyclopedia gives an excellent overview of the problem. For the nitty gritty theory, Dave Gerr's Propeller Handbook does an excellent job of helping to understand the theory and crunch the numbers. The theory centers around Crouch's Propeller Method which has been reduced to a series of graphs. The first step is to calculate the displacement to waterline length ratio. From here you determine the required horsepower to power the hull at displacement speed. By applying the 80-85% duty cycle for light duty diesel engines typically used in sailboats today, you can come up with the rated horsepower best for your boat.

This is the beginning of the problem, not the end. Now you have to look at design cruising RPM's of the particular engine that you are considering. And remember that if you are thinking of a turbo-charged diesel, such as Yanmar, they sensitive to operating at a high enough RPM for the turbo to kick in. Now you are ready to look at the transmission gear reduction. Using the gear reduction, you can now determine your shaft RPM which in turn determines propeller performance. If you were designing the boat from scratch, you would now simply select the propeller necessary to handle the shaft RPMs. It is likely that the propeller the you have on the boat is the largest diameter that the boat can handle. Remember that there must be at least a 15% clearance between the hull and tip of the propeller. Another important number to remember is that the tip speed of the propeller should not exceed 95 ft per minute to prevent cavitation.

The problem boils down to satisfying three variables..... the horsepower required to move the hull at displacement speed, the diameter of the propeller which may be fixed by the hull clearance, and the required cruising RPM of the engine to create enough torque on the engine to allow it to operate efficiently.

Ultimately calculations need to be checked against experience. The expert that most people refer to is Fredric Lafitte at PYI. I found that he could relate the displacement, propeller size and horsepower to other boats on which he has worked. In addition I relied on Frank Monachello at Marine Pro, the Yanmar distributor in Cocoa, Florida for his experience re-powering sailboats.

Installing the Engine

I have never installed an engine before and hope never to have to do it again. At the very beginning of the project, a mechanic estimated the cost to be $25,000 which I thought was high. After completing the project, my guess is that it could have easily cost $40,000 if the work was done by a yard. I should also explain that I had a major advantage over most others in that the boat was stored behind my steel fabricating business which gave me access to cranes, hoists, tools and welding equipment that most marine mechanics do not have. I also had access to AutoCad for computer drafting, a digital camera to send photos with questions to the Yanmar distributor, an electronic level to precisely measure slope angles and many other tools not even readily available in the typical boat yard.

I selected the Yanmar 75HP 4JH3-DTE turbo diesel engine with a KM4A angle drive... mistake #1. The 75HP engine is very popular with boat builders and was out of stock. So the distributor offered an 88HP 4JH2-DTE at the same price which I decided that the boat could handle. I also checked and double checked dimensions to make sure that the engine would fit thru the companionway. The real mistake was the angle drive as you will see in just a moment.

Before installing the engine however, I set it up in my shop to mount a Balmar 150 amp high output alternator to charge the house batteries. I had this alternator on my Lehman engine and had no end of problems trying to provide a rigid bracket that can handle the 5 hp load that can be generated. Many hours were spent machining a bracket which I attached to the motor mount. I also added another bracket on the opposite side as a spare for future equipment.

My best decision was to build a template of the engine to locate the motor mount bolt pattern as well as the shaft alignment. This saved enormous time by allowing the necessary modification of the engine bed in the hull without dealing with the engine. It also turned out to be incredibly accurate. Again I had the advantage of a steel fabrication shop. I built a steel fixture to support engine and the motor mounts. With the engine on a fixture, I then bolted the propeller shaft coupling to the engine and then attached the coupling to the fixture. Next I removed the engine from the fixture, leaving the motor mounts clamped in place and substituted the template. The template now had the same bolt pattern as the engine and the motor mounts could be attached to the template. By welding a bracket from the coupling to the template and cutting it from the fixture, I also had the exact relationship between the motor mounts and the shaft alignment.

Next I purchased a laser pen used for giving presentations. Using paper targets, I then could find the alignment of the shaft as it theoretically passed through the engine. The template was now ready to install in the boat. Within moments of placing the template in the boat, I realized my mistake. You do not install angle drive transmissions in a hull set up for a straight line transmission. The Yanmar master distributor was less than accommodating... $1000 to swap transmissions and a month lost. Let the buyer beware! At least I was dealing with a template and not the engine itself in the boat. Lesson learned... use a good template.

Error corrected, ready to proceed. I had filled my old cutlass bearing with fiberglass and had a hole machined in the center to allow the laser to shoot a straight line to align the template... it didn't work. The problem was that the beam is invisible unless it hit an object so you could not see the alignment. A string and targets at the stern tube as well as the coupling and front of the fixture worked great. Using the cutlass bearing, the shaft stern tube, the coupling attached to the template and the front target on the template, it was a simple matter to align the engine. I used string to suspend the template and make the fine three dimensional adjustments.

With this done, I found was another problem. The original engine bed was not in a flat plane. In fact it was probably one of the causes which put excessive stress on the Lehman motor mount brackets. In addition to that, the bolt pattern of the old engine and new engine overlapped at a couple holes.

The bed needed to be reworked and elevated several inches to accommodate the new engine. At this point I decided that I needed to use the old engine bolt pattern which had plates imbedded in the hull to securely fasten the new engine. To accomplish this, I used two 1 x 4" 316 stainless steel bars to match the old bolt pattern. I then drilled and tapped for the new engine bolt pattern. In addition, I used several extra pieces of the bar welded together to make up the change in slope and height of the new  engine bed. These assemblies were then machined to match the slope of the engine bed in the boat. Again using the template with the machined bars attached to it, I aligned the bars to their final position. There was still a 2-3" gap in some places between the old bed  and the bars. Using washers and bolts in the old bolt pattern, I was able to set the exact high of the required fiberglass build up. With an electronic level, I sanded and massaged the fiberglass to exactly match up with the bars and template. To fasten the bars to the bed, I used 3M 5100 and bolted to the old pattern.

Finally, it was time to install the engine. The clearance thru the hatch was close, so I built a template of the hatchway in the shop to practice getting the engine in the boat... everything worked. The actual installation involved renting a mobile crane. From crane hook, I hung a chain hoist to allow for fine adjustment. and from the chain hoist, I had built a steel C frame that balanced the engine. The C frame allowed the engine to pass inside the cabin while it was supported by the crane outside. It worked perfect and the engine was in the boat in less than an hour. Once the engine was in the main cabin, an A frame and another chain hoist was used to lower the engine onto the stainless bed bars. 

Foiled again! The bell housing of the engine hit the bed bars! Another little idiosyncrasy of the Yanmar. Now it was time to pull the engine back into the main cabin, cut the bars loose from the 5100 with a sawzall and have a groove cut in the bars for clearance. This particular problem took at least a quart of gin to resolve! But it actually went smoothly and within a day, the engine was back in place.

If you think the job is almost done... think again! Everything must be replaced. Starting with the shaft which now had to be longer. I bought a piece of Aquamet 22 and had a new shaft machined. 

The exhaust however became a major challenge. With the larger horsepower and turbo, the exhaust had to be increased from 2 1/2" to 3" diameter. It may not sound like a big change but the difficulty of bending 3" exhaust hose is a major project. More importantly, a new problem arose.... back pressure on the turbo! Perhaps at this point it may be worth mentioning the Yanmar manuals which I purchased were useless. They contained so little detail that the manuals could have been used to install the engine in a 1955 Chevy as well as a boat. The manuals also contradicted one another regarding the use of a muffler. 

Should a muffler be used? From the viewpoint of a turbo charger...no. It will create back pressure on the turbo which could damage it. Without a muffler, the cooling water left in the exhaust hose could back up into the exhaust manifold creating major damage. The consensus of outside advice was that you need a muffler. Next you need at least a 12" drop from the mixing elbow to the muffler intake for the same reason. That may not sound like much but in a bilge it is hard to achieve. Again back to the custom fabrication of an exhaust riser using laser cut flanges to match up to the triangular exhaust port flange on the Yanmar. But we are not there yet! The exhaust hose needs to be looped to prevent a following sea wave from entering the exhaust outlet and flooding the engine. This of course means that the cooling water must now be forced over this loop thereby creating more back pressure on the turbine. The Yanmar distributor said not to worry.... I was worried. The shake down cruise did not show any abnormally high operating temperatures, so it assumed that everything is OK. One solution for relieving back pressure is a "pisser" valve which essentially bypasses some of the cooling water overboard to reduce flow in the exhaust line.

The moment truth eventually arrived to bolt the shaft coupling to the engine and check for alignment. The engine was less than .006" out of alignment. I could not accept this as being possible and had a mechanic take a look.... it was right on the money. The template had done it's job.

Next new fuel lines and new control wiring harnesses needed to be installed. Guess what? The new instrument panel was larger than the old one and did not fit in the recessed cockpit coaming. Off to the custom fiberglass molder to build a new insert and a acrylic cover.

By now you may sense a little frustration.... but we are not done yet. Gear shift and throttle connections were next. The gearshift on the Yanmar is really stiff and takes 15 lbs of force per their specifications to shift it..... not to worry says the distributor. The throttle turns into the grand finally. The old engine is push to accelerate but the Yanmar is pull to accelerate. So all you have to do is reverse the linkage... right? Wrong. The steering pedestal is set up for only one direction and cannot be reversed. Back to the custom fabrication again. Fortunately, I stumbled onto Pannish Controls (203 -333-7371) which makes the linkage for about a $100.... pocket change in the engine replacement business.

Finally the engine was ready to test! I ran the seawater intake hose into a bucket filled with water. Turned the key and it started immediately. WOW! Unbelievable that everything was hooked up right the first time. So I turned off the key to stop the engine and pushed the red kill button.... but the engine kept running! I could not stop it. Time was running out before I used up all the water in the bucket and damaged the engine. Panic set in.... my first thought was to shut off the fuel, but I realized that there was enough in the filter to keep the engine running quite a while. I went back to the control panel and started pushing buttons. In the same manner in which a chimpanzee can eventually write his name, I discovered that on a Yanmar engine, the key has to be turned on for the stop button to work. Can you imagine the consequences of this arrangement in an emergency  with crew unfamiliar with Yanmar engines? Shame on Yanmar!

In summary, I would think twice before re-powering a boat with a different engine. I would also think twice before selecting a Yanmar...especially a turbo diesel. Back pressure on a turbo is a tricky problem and there are not clear answers.  The Yanmar, especially the turbo, is designed for an OEM installation.

 Also in my opinion little thought has gone into the Yanmar design from a maintenance standpoint. It has been compacted to the point where simple jobs are very difficult. For example, changing the water pump impellor can take 2 hours according the experience of one Yanmar mechanic. You would probably need to pull the engine to change the starter. Drain cocks are everywhere and concealed. It would take one oversight by a mechanic to cause a disaster.

Last but not least, I found that both Yanmar and their master distributor were not helpful. They clearly are interested in the OEM market and not working with an amateur mechanic. If you buy a Yanmar, make sure that your local distributor has substantial experience re-powering with Yanmar engines and is willing to help explain the mysteries that you are about to encounter.

Here's an update.... The sea trials are complete as of August 1, 2001 and the engine is running very nicely. So far in 70 degree Lake Michigan water, there has been no sign of over heating due to back pressure. The optimum cruising speed for the boat seems to be about 7.0 to 7.4 knots without creating a large stern wave. This equates to a about 2200 RPM which may be too low to generate the torque that the engine needs to perform efficiently. I will be watching this closely to see if this is a problem. At full throttle the boat will do 8.0 knots but there is a lot of vibration (cavitation?) and a huge stern wave as the boat reaches it's hull displacement speed.

The stiff gearshift seems to be working with much less force and more smoothly since the the initial break in period.

I resolved the wiring glitch in the Yanmar instrument panels to allow one key in one location to turn on the gauges for both panels. You would think that Yanmar would have built this into the wiring harness.

In November, 2002, I wrapped a crab pot line around the propeller shaft and pulled the shaft out of the engine coupling. Upon closer inspection, the exhaust riser that I had made with a section of flexible metal hose had failed. This allowed exhaust gases to carbon up the air intake cooling manifold. This was corrected with a solid pipe section replacement and a brace which hopefully will allow it to withstand the vibration.

Two different Yanmar dealers have told me that getting at least 3600 RPM's at full throttle is critical to the engine's life. So I have reduced the pitch on the Max Prop one notch.

                                                                    return to home page