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 stirling engine 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.