Mounting an outboard motor correctly
Here is a brief outline on mounting an outboard motor correctly.
The first two things that need to be established before mounting an outboard motor on the transom of your boat are pretty obvious:
1: The centre of the boat stern – hopefully this will also be dead centre of the transom and vertically above the centre of the keel.
2: The transom height needs to suit the outboard and ensure that the propeller will be in the correct depth of water in relation to the bottom of the boat’s keel.
- The height of the transom needs to let the cavitation plate, just above the propeller, sit below the bottom of the keel. About 25 mm or so below the keel is usually about right but you might need to seek advice on this as the shape of your boat’s hull will have some effect on this.
- If you are mounting two outboards on the transom…
You will have to decide how far apart to place the outboards and come up with a centre to centre distance. Then you will need to determine that each motor will be mounted at a height appropriate for its shaft length. This will be below the bottom of the hull and not necessarily the bottom of the keel. Obviously, if the motors are identical and hull shape is the same each side of the centerline, then this will not be a difficult calculation. There may be some occasions when the transom height will need to be modified but many outboards have height adjustment built in.
Boat performance will be affected by the setup (Propeller depth in water and outboard trim), as well as propeller size and pitch. Boat size, weight and placement of passengers and gear as well as hull shape will also affect performance so take that into account also.
If you get the outboard mounted correctly on the transom and are not satisfied with the performance, look at your boat as someone else drives it passed you.
- Does the outboard look to be the correct height? How much spray is the prop throwing up?
- Is the boat trimmed correctly?
Then check revs and boat speed against the manufacturer’s recommendations if any. Too big a prop will prevent the motor reaching full revs and will increase fuel consumption and too small and the motor will rev easily, possibly beyond the recommended maximum rpm and it may lack torque to push a heavy load.
If the boat is running pretty well you might decide it’s OK as is otherwise you may have to take advice on prop size and pitch. Then start spending money by trying a couple of alternative props. See the page “Boat Propellors” here.