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One Week Engine Change
By: Scott Fratcher - Marine Engineer/Captain
Changing the main engine of a boat sounds like a huge project. Cranes, plans, alignment, choosing the right engine, it can all seems overwhelming to the yachtsman.
In reality for the DIY changing a main engine in a yacht is relatively straightforward. You don’t have to learn all the special skills of a master mechanic as you never rebuild or even work on the diesel engine.
It’s a simple matter of removing the old engine, rebuilding the engine mounting rails, and then bolting the new engine in place. It might sound complicated, but it’s really rather simple.
Do it yourselfers often take over three months to get their engines installed. I have devised a system outlined below where we can change a main engine in just one week. The one-week engine change means the boat, and often the family living area is only torn up for seven days.
Step 1-Buy New, DON’T REBUILD!
This is a very important point. Boat owners will often look at their worn out, leaking, engines wanting a rebuild of the old trusted “friend”.
Unfortunately the numbers don’t add up. Consider that a rebuild job will normally cost about half the price of a new engine. Most rebuilds only come with a short term and local guarantee. In other words head to the islands and if complications arise the local mechanic that rebuilt the engine will want the engine returned to his shop or marina for service.
I come across engines all the time on the cruising circuit that have five hundred hours after a new rebuild. They are blowing and burning oil leaving the owner back to square one.
Besides, the price estimate I just gave is only the cost of rebuilding the block. The old heat exchanger, oil cooler, gear box, alternator, and high priced injection pump may still have five thousand original hours on them. Rebuild all of the extra components and your well on your way to the cost of a new engine.
Measure before you pull the old engine
Ask many “do it yourself” yacht owner about an engine change
and the sticking points will be:
Lets start with how to measure. This semi-simple process takes about two hours and is outlined below.
The measuring technique is the heart of an engine change. The first thing you will need to do is find the existing height of the crankshaft center to the bottom of the existing flexible engine mounts. To do this take a flat edge and lay it across the engine beds. Use this straight line to measure down from the crankshaft center to the engine bed line.
This is your first important figure. This is the line of the prop shaft progressed forward. If the gearbox has an offset, or a drop, then you must add or subtract this figure to your final measurement.
Now look at the drawings for the new engine and find the distance from the center of the crank to the engine beds. Add in the gearbox offset if any and you should now have two separate measurements. The difference between them is the thickness you will have to make up, or cut down from the excising mounts to make the new engine fit.
Simple Now that you know
Strangely after many engine changes with this system I have not come across an engine bed that needs to be lowered. For some reason they all have to be raised 10mm to 50mm.
The difference between the old engine and new is the all-important measurement. If you got this right then you can have the “adapt a rail” pre-made before you remove the old engine.
Lift the old engine, bolt down a pre-made piece of steel rail (to make
up the height difference), and set the new engine in place.
Tip-While the old engine is out it is the perfect time to paint the
engine area white, and maybe service the bilge hoses or anything else
that runs under the engine.
Often the DIY can save a bit of money by measuring, and doing the heavy lifting themselves. Once this is complete call your mechanic to do the alignment and inspection. This can be a win/win for all. The local mechanic gets some of the work and catches the basic mistakes, while the owner pays to have the difficult part of the work completed and yet does the easy work himself.
The Rest of the Fit
You will have to measure width, height, and depth of the new engine along with the motor mounts, but most modern engines are much smaller than the twenty year old diesel you will be removing.
The only time size has been a problem has is when we remove a small engine and replace it with a much larger model. The popular Perkins 4-108 is now replaced by a Yanmar that is about 2/3rds the size of the original.
The 56hp Yanmar is about the same size as Perkins 4-108 giving almost half again the horsepower.
Lifting the Old Engine
The lifting, and removal of the old engine is the next step of what some do it yourselfers may find overwhelming.
I almost always lift from the main boom supported by halyards. I attach a block and tackle to the lifting point and then run the bitter end to a winch.
I use a land crane to set the engine on deck then take over myself
with a boom lift. I always use two lifting points on every aspect of
the lift. If any single line were to break the engine would not fall.
The lifting lines are tailed to the two largest sheet winches on the
A common challenge with a new engine is a reverse throttle action on the injection pump. At the helm push might become pull, or however you move your throttle control now may be reversed. We can choose to get used to his new throttle linkage, or we can build a throttle reversing mechanism.
This reversing mechanism should not be taken lightly. Engine control systems must be 100% dependable. If the throttle linkage were to fail during docking it could cause an expensive accident. For this reason take your time and over build all shifting/throttle linkages. Notice in the photo we used 4mm plate on the reversing system and red Locktight on all bolts.
Occasionally it may pay to haul the vessel for the engine change, but this is only if other work needs to be performed, or if the prop and maybe shaft are to be changed. Otherwise it’s almost always easier to do the engine change while the boat is at the dock.
Tip: If you’re lifting the engine on it’s end then consider
slipping a plastic garbage bag right over the bottom to catch any oil
Tip2: Cut up a piece of plywood to cover the floor around the work
site. Often times the new engine arrives in a plywood box that can be
cut up to use as material to protect the floors.
The final engine alignment is often the last tricky item on the DIY’s mind. Will all the measurements be correct? Will the shaft simply slip onto the new gearbox?
By learning a few engine alignment tricks you can save a big chunk of money and time by preventing the rebuild of gearboxes, broken shafts, or worn couplings.
Remember, if your on a new engine installation we should align the engine before we make the holes to bolt the mounts in place. Only once the engine is perfectly aligned should the first mount be bolted down. Then the alignment is checked again. If all is still correct then bolt another mount in place and so on till all the mounts are securely bolted in place.
The first step to aliening an engine is to locate the prop shaft in the middle of the stern tube. I like to start with the shaft just over center (at the 3/5ths mark) so as the flexible engine mounts sag over time the shaft will remain in the center of the stern tube.
You may have to block up the shaft to keep it in place. Remember the engine aligns to the shaft. Once the shaft is in place the engine should follow, not the other way around.
Clean both flanges using a wire brush. Push the prop shaft flange against the gearbox flange. Ensure the centering rings set in place. Give the flanges a twist back and forth to make sure they are seated well together. Slide a feeler gauge between the flange faces. Measure and mark the area of greatest gap.
Before you proceed rotate the prop shaft one hundred and eighty degrees or a half turn. Check the feeler gauge readings. They should remain constant. If the readings changed from our first measurement then the shaft is bent or the flange is bad. If you find a bent shaft and it exceeds the four thousands tolerance of the alignment then fix that problem before you continue with the alignment.
Often times we suspect the shaft is bent or some other aspect of the drive system is out of true. This simple shaft rotation test confirms the problem or sets our mind at ease.
Adjust the engine mounts a bit at a time to close the gap in the flange faces. Keep all four mounts as even as possible. Keep checking alignment till you are within .004”. At that time tighten all the mounting bolts and check alignment again. Wiggle and shake the engine as much as you can.
On larger engines you may have to get two people pushing and pulling to get the engine to move around. The idea is to make the engine settle on her mounts. Check the alignment again and adjust your mounts to fix any gap that reopened. Keep repeating this procedure until the engine is sitting in perfect alignment.
Expect to take about six hours to do a perfect engine alignment. A good experienced technician can normally complete this process in about an hour. Of course this means all the bolts are loose, clean and access is easy.
Tip- Resist the urge to move a single mount that builds up a little
pressure on the opposing mount to make a small adjustment. When the
rubber in the engine mounts get hot it will soften and fall out of alignment
causing the engine to begin vibrating.
Tip- If you find you have to lay on top of the engine to reach the
flange to measure your weight will almost assuredly change the compression
of the mounts and thus your alignment. If this is the case then you
will have to find some clever way to keep your weight off the engine
while you test the flange.
The next step in the engine change is hooking up the exhaust system. Although this seems straight forward some thought should be put into the part of the project. If your new engine has a higher horse power than the original engine its possible the exhaust diameter will need to be enlarged. Consider this project carefully. It can cost more in labor to enlarge the exhaust system than to change the engine.
Tip-Often the exhaust manifold outlet on a new engine will be on the opposite side as the original. If this is the case a simple solution to connect to make an S curve from exhaust hose and two 45 degree elbows that allows an easy, inexpensive connection to original system.
Most new engines come with a “plug and play” wiring harnesses. In other words the wire loom installation is simply a matter of pressing together the supplied connections. Still, the wire loom should be secured to the engine with wire ties to prevent vibration. Where the wire loom passes from the vibrating engine to the rigid hull a loop of extra cable should be left to take up the vibration. The wire loom should be secured every eight inches along the complete rout to the engine panel. This may seem like a lot of securing points, but in years to come all new wires from the panel to the engine will eventually follow the wire loom possibly adding up to a large amount of wires.
Preventing water backing up into the engine
The most common cause of damage to a marine engine is salt water intrusion due to poor installation of the water cooling hoses.
In order to reduce the chances of filling a new engine with salt water remember:
Tip-Resist the urge to tee into the sink drain as a place to “trickle” the anti siphon water. More than one new engine has been ruined when the sink was plugged and filled with water to wash the dishes thus sealing the anti siphon loop.
Remember, just because the engine did not flood for the first year does not mean the water system installation is correct. It may simply mean the rubber impeller in the salt water pump is blocking the flow of seawater at rest. The day may come when the impeller wears and allows water to creep pass filling the exhaust system and the engine with destructive seawater.
Move the new engine aboard with a crane or the boats rig.
We use a crane here, but only because were in dry dock. Normally we would change an engine at the dock and thus not need a crane.
Here we have switched to the boat's lifting gear
The new Yanmar engine just fits
Here we show the new Yanmar
This is the end of day two. I'm being silly laying next to the new Yanmar, but this is a lot of progress for two days work.
Here the engine is in place and were begriming to hook up the control.
The old Perkins 4-108 had a two inch exhausted while the new 75hp Yanmar needs a three inch exhaust.
Here is the shut off valve inside the boat. We normally would not use such a large gate valve, but the insurance required it.
Notice how thick the top of the old auqa lift type muffler was
We installed a new Max three blade 22" prop.
Here is the new exhaust system in place
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