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Inboard Engine Installation :: Inboard Installation 2 :: Inboard Installation 3

Inboard Engine Installation

The outline below describes engine installation steps common to most of our displacement power boats. The pictures show our TW28. The stringers and bulkheads may look slightly different but the procedure is always the same.

The steps are as follow:
1.Build the engine girders
2.Locate the axis of the prop shaft
3.Drill through skeg (keel)
4.Install stuffing box frame
5.Build and install stern tube (shaft tube)
6.Install and align engine

The stringers are used as engine bed (girders). They must be fiberglassed and tabbed to the hull with no less than the specified layers.

The engine installation, shaft tube included is doe when the hull is completely finished.

We will cover the installation of the engine in these building notes but we recommend investing in a good book: ?Inboard Motor Installations? by Glen L. Witt and Ken Hankinson.

To understand shaft tube installation, we must learn more about the engine.

Drive train components overview.

The picture below shows a typical drive train without the prop. The bulkheads are not shown on the drawing and we show only the port stringer for clarity

Starting from the stern, you see the stern bearing. The long cylinder going through the keel is the shaft tube: a tube through which the shaft runs.
The shaft tube passes through a small floor frame (not shown) and is connected with a rubber hose to a self-aligning packing gland.
The packing gland keeps the water out.
The forward end of the shaft is fitted with the shaft coupling. The shaft coupling is bolted to the engine transmission coupling.

Let?s look at the components more in detail. Starting from the stern, we have the stern bearing.

The stern bearing is bolted to the rear face of the keel. Often, the housing of the bearing will be wider than the keel. This will require some shaping of the keel. The stern bearing is usually fastened with lag bolts (not shown). The lag bolts hold well in epoxy wood flour putty and the stern bearing housing is sealed in 5200.

Here is a picture of a typical stern bearing with its housing:

The serrated black cylinder inside is the bearing itself, hard rubber or sometimes a high-density polymer like Thordon. The inside part is often named cutless bearing.
The bronze housing extend in the keel where it meets the shaft tube. It lines up with the tube or is covered by the shaft tube.

The shaft tube can be purchased or made in house.
There are several ways to build a shaft tube but it is a simple fiberglass tube that will be fiberglassed to the hull.
A PVC tube will make a nice core around which to fiberglass.
The fiberglass walls of the tube should be ?? thick. The inside diameter must be larger than the shaft diameter but not too large. ?? to 3/4? total clearance is fine.

On the engine side, the shaft tube pass through a floor frame:

The tube is fiberglassed to that frame. That seam must be strong. If you use a PVC pipe, beware of the poor bond between PVC and resin. PVC is a good core but the tube should be made of fiberglass around PVC, not PVC only.

The end of the tube should be clean: a rubber hose will be clamped on it.

To seal the shaft tube, we use a packing gland. Here we show what is called a self-aligning packing box.

A rubber hose connects the shaft tube to the packing box. The rubber hose is double clamped on each side. (Clamps not shown)

The packing box is made of two threaded bronze pipes that fit in each other. Between the two pipes, there is packing. Packing is a rope made of greased flax or Teflon. The pipes are tightened to compress the packing and seal the shaft.

There are other types of shaft seals, in particular spring loaded friction seals but they all fit the same way over the same shaft tube.

The shaft coupling fits over the end of the shaft on the engine side.

It is bolted to the engine transmission coupling. In some cases, there is an intermediate coupling that absorbs torque and vibrations.

All those parts maybe a little different than what we show but the principle is always the same.

What is important is to line up all drive train components and the engine with the shaft axis.

The shaft axis will be our reference for the installation of all the drive train parts.

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Or explore the HowTo files at our technical support web site

Inboard Engine Installation :: Inboard Installation 2 :: Inboard Installation 3