Your engine is attached to your boat by its mountings and to the water by its propeller. Fitting a new engine must include attention to these components if you are to get the full benefit of a new engine!


This Isuzu 42hp propulsion unit was fitted into the water-management vessel "Eden." These engines are tough, light and compact, and we are distributors and installers of these and other marine propulsion units,  from 7 to 100hp,  manufactured in the UK by HMI.


Below is a Sabre Perkins 300 MTi we recently fitted into a"Lochin 33". The vessel was extensively rebuilt by us, and underwent many modifications.In spite of this being a leisure-rated engine, her top speed is now 22 knots, which isn't bad for a six and a half ton boat!

 

When we re-engine a vessel we will always consider the propeller shaft. The vessel will be out of the water in most cases and more often than not the shaft and support bearing will need attention.
The first consideration is " is it man enough for the new engine" this is calculated giving regard to the power output of the new engine, it's reduction gear selected and pitch and diameter the propeller selected.
If the new replacement propulsion unit (generally this is the correct term for a replacement engine in marine terms as they come as a package with reversing
gear as well) is of the same output, rpm, horse power , rotational direction etc the only consideration is the condition of the shaft and bearings not to forget the shaft gland.
Power boats with out-drives would normally have the out drive re-placed with a new matched unit so this would just leave the propeller to select.


Here's a conventional shaft fitted with new support bearing in the "P" bracket.

Simple points to consider; if your engine has a reversing gear with no reduction it will spin the propeller shaft at the same speed the engine rotates(1:1 the ratio), if the reduction is said to be 2:1 the engine will rotate twice to turn the propeller shaft once and so on. As the reduction increases the speed of rotation decreases, so torque or strength increases. This means for the same engine power output larger propeller shaft diameters are required as the reduction ratio increases.
There are many factors that come into play when selecting propellers, we have found  boats that could never be efficient, reach their full potential or have
allowed the owner to benefit from money spent on new engines because the propeller has not be correctly matched to the boat. I have to laugh at boat jumbles at the heaps off old props offered for sale, you got to be real lucky to find one that's tailored to your boat exactly. It's such an important component and should always be bought with care from a specialist who can help you select it. Brought as blank castings they are normally individually machined to suit your shaft dimensions.           

After this propeller was fitted the owner reported a 2 knot increase in speed. He had owned the Yacht for years and could not believe the difference it made.

The previous propeller was too small for the vessel with engine fitted.

Shaft bearings can be replaced quite easily if you have the right tools, this bearing had shown to have excessive wear and needed to be replaced.

 

 Shaft Bearings (also known as Cutlass Bearings) are designed to be water lubricated. They are installed in the stern tube, or bearing housing and support the propeller shaft.

The outer case is available in two materials:

1/Naval Brass - for traditional installations
2/Phenolic Non Metallic for steel and aluminum vessels

    However, modern materials and advances in cathodic protection have enabled

    boat builders to select the bearing material most suited to the application.
    The inner liner is made of tough synthetic nitrile rubber for best all round performance, it is possible to obtain other materials.

    With out removing the shaft or beating with hammers the worn bearing was replaced quickly with just the propeller removed.

     

    This tool is ideal for the smaller shafts, a similar hydraulic version is used for larger shafts. It should be noted that the shaft is prone to wear and this wear should be checked by careful measurement prior to replacing the bearing.

    Out comes the old one!

    New bearing fitted, not to forget the retaining screw and for good measure we fitted a new shaft anode.