Outboard motor conundrum

[spark]

I don't have a small outboard motor to hand to study so maybe someone can settle an argument.

From memory, the steering axis of most small outboards (up to c. 4hp) appears to be concentric with the axis of the drive shaft. If this is the case what stops the body of the machine being turned in the opposite direction to the rotation of the engine? Prop walk? Or is the drive shaft offset from the central axis of the shaft casing? Or am I missing something more obvious?

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[BarryH]

Nothing really apart from the steering friction screw fitted to most of the small ones.

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[webcraft]

Spark looks gorgeous.

Could even change my thinking re. owning a wooden boat . . .

When do you plan to complete her, and will she be a one-off or are you hoping to build more?

- Nick


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[duncan]

surely the shaft itself only transfers its force through the bevel gaers to the prop shaft / thrust bearing / prop which then , through it;'s interation with the water finally exerts a force.

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[LORDNELSON]

If you hold the handle to steer this should stop the engine from rotating

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[steve_cronin]

Answer:- Nothing - it does...

...try to rotate but the friction clamp stops it.

I had a Mercury 2.5 up to three years ago and I left it running in tickover on the transom of the inflatable whilst I went ashore. It gradually rotated around and the water outlet deposited a good half inch of water in the bottom of the boat

Steve Cronin

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[spark]

This was the argument that I favoured but the opposition was arguing hard against it (equal and opposite forces on the vertical shaft).

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[spark]

Re: Answer:- Nothing - it does...

This was the opposition's position but I'm not convinced. Surely the torque, even on a 2hp motor, is greater than two fingers it takes to push the tiller in either direction. If the friction clamp was countering the engine rotation the tiller would be easy to push one way and hard the other. Your example of the motor rotating sounds like it could have been prop walk.

As a test, can somebody, next time they're using their flubba, slacken the friction screw completely and report back what happens?

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[oldharry]

If the friction screw is slacked off with the engine in gear, it will turn itself round. This is (partly) why outboards are fitted with friction adjusters.

Thats why you saw Old Harry going round in tight circles the other week after his sandwiches fell off the seat into the bilges.... (that was before we reached the Pub!)

The amount of thrust from prop walk is relatively small, and the mechanical advantage provided by the length of the steering arm reduces the thrust at the handgrip to a level where it is barely noticeable.

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