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.
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:
1. How to measure for the new engine.
2. How to move the new engine.
3. How to make a perfect alignment.
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.
Often two inches is the magic number. If you got lucky and the new engine mounts sit exactly two inches high then buy a piece of two by four steel extrusion 5mm or thicker. Lay the new rail right over the old engine beds, bolt them down, and set the new engine in place. Sometimes I have to take the extrusion to a shop and have it cut and welded to the special thickness I need. Either way make this particular, all important adapt-a-rail before you lift the old engine.
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. Tip 2-Consider installing a series of lights that illuminate the engine from below. Your new engine is going to be very clean and nothing helps keep an area clean like really good lighting.
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 boat.
The crane lifts the engine onto the yacht's deck. Unless you really trust your crane driver it's often better to move the engine by block and tackle the rest of way to the new engine beds where the movement can be controlled with precision.
Lifting the engine using the boat's rig. Pay particular attention to the gooseneck or connection between the boom and the mainmast. As long as the boom is in center line with the boat the load should be in compression, or pushing into the mast. The problem can occur when we attempt to swing the boom out to the side. The load on the gooseneck turns from one of compression to side load. Inspect the gooseneck carefully and keep the load inline with the mast as much as possible.
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. If I have to lay over the engine to work on it lay some carpet over the engine to help keep comfortable. If you're comfortable you can almost always do a better job.