Are you planning to install or modify an inboard engine yourself? Prepare well, start with the basics, and take measurements. With minimal adjustments, you can replace an engine. For recent engines, you often know the installation dimensions or they are easy to find. Pay special attention to:
- propeller shaft height
- engine mounts
- maximum tilt
- reverse coupling
- rotation direction of the propeller
- power
- propeller shaft and bearing
Make sure to figure out what you need.
Are you still looking for a used engine? Quickly check out our range of used marine engines.
Propeller shaft height
First, check if the engine fits in combination with the propeller shaft height. Is the output of the reverse coupling at the same height as the crankshaft? If the output of the reverse coupling is lower than the existing propeller shaft, the engine will need to be raised.
Engine mounts
On some inboard engines, adjusting the existing engine mounting points is no problem, or the foundation of the steel hull can be modified. In a polyester boat, this becomes more difficult. Therefore, check beforehand if this is possible. Especially with a saildrive, you’re often bound to existing prefab transom fittings.
Maximum tilt
Also check if the inboard engine can handle the maximum tilt while sailing. The maximum tilt is often listed in the manual. A homokinetic drive solves the problem otherwise.
Reverse coupling
The Velvet, PRM reverse coupling, Hurth, ZF, and Twindisc reverse couplings are common reversing couplings. The ratio is often indicated by the engine RPM divided by the output RPM, in a two-to-one (2:1) ratio. Ratio = Two engine revolutions / one propeller shaft revolution (or simply half).
The rotation direction of the output is often indicated as L or R in the 2:1 ratio. L is a two-to-one left-turning reverse coupling for a left-handed propeller.
You may also encounter reverse couplings with an A for angle, for example, in Volvo Penta. These reverse couplings come with a shaft tilted 7 degrees outward. This way, the engine needs to tilt less.
Propeller
Pay attention to the rotation direction of the propeller. There are left-handed and right-handed reverse couplings. The rotation direction of the reverse coupling is indicated when the boat is moving forward and you are looking at the bow.
Be careful with a “Z-drive.” The engine is then reversed, so make sure to check the specifications carefully.
AB Marine Service can perform the power requirement calculation for your boat.
Power
If the power remains in the same direction, you can often reuse the same inlet and outlet connections, such as the exhaust gas outlet, cold water supply, or circulation pipes.
If you are increasing or decreasing the power, check the diameter of the connection on the engine. This is already a good indicator of the required size.
If the exhaust manifold is 45.00 mm, you can use this diameter. Do not go smaller, as it will restrict the engine, leading to potential issues. Going larger is usually not a problem.
Propeller shaft and bearing
Also think about connecting the propeller to the reverse coupling. Some brands are interchangeable, but this is not always the case. You will need an adapter flange. These can be obtained separately upon request or are often supplied ready-made with a homokinetic drive.
If you want to reduce vibration and save time with the connection, consider the advantage of a thrust bearing with a homokinetic drive.
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i AM A COMPLETE DUMMY WITH INSTALLING INBOARD MOTORS. I need a lot of help (Instructions, Pictures etc.) what can you do for me
Sure, Tell me what you want to construct.
I would not say that I am a complete dummy at installing and engine, But I have had two so called marine engineers to fit an engine in my yacht, The last one put the exhaust in back to front
instead of the saltwater returning to the sea it went back into the engine. We all know what happens. This is the start of yet another court case the first so called Marine Engineer I took to Court and won the day. The second so called engineer the is in the process of yet and other court case.
Just to put you in the picture I am time served wooden boatbuilder. and at the age of 84 years.
I would prefer some one else to do the work.
Thank you for explaining your situation so clearly.
We fully understand your position. Unfortunately, we do see cases like this more often, where work that should be done correctly causes serious damage due to incorrect installation. What you describe with the exhaust being fitted back to front is indeed a well-known and very serious issue.
With your background as a time-served wooden boatbuilder, it is clear that you know exactly what you are talking about. Given your experience and the circumstances you’ve been through, it is completely understandable that you would prefer to have the work carried out by someone else.
We are of course happy to advise, support, or help you find a suitable solution where possible. Please feel free to let us know how we can best assist you further.
Kind regards,
AB Marine Service