Transom Height

[quinag]

I'm considering buying a new Honda 5 HP 4-stroke standard shaft to replace an aging (unknown vintage) Mercury 4 HP 2-stroke. The engine should fit (just) in the cockpit well in my Swift 18, but the attachment point is a lip which is only about six inches above the water level, which means that the anticavitation plate will be well below the recommended minimum 4 inches below the water. The Mercury seems push the boat along OK apart from vibrating and smoking badly even after having it serviced. (The mixture was correct at 100-1). Has anyone any experience of this problem or will the engine work OK? Presumably the boat's designer was quite happy, as no-one seems to manufacture an ultra short shaft outboard.

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[duncan]

you shouldn't experience any major problems but will obviously draw more than you might (unless you have a fixed keel - excuse ignorance of Swift).
Additionally you will create a greater angular moment to drive the bow up rather than along but unless the power of the engine has the ability to send it **** over *** thats no big issue either.
Cavitation plate and it's relationship to hull are critical to planning hulls but not really in displacement mode.

ps If you are wanting to get rid of your old o/b I have a son who would love to learn more about engines and gain something to drive the tender - and 2 hp/2stroke sounds perfect!
Good luck

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[Gunfleet]

If the minimum is four inches below the water and your plate will be below that what are you worrying about? Presumably the maximum is when the motor is immersed!

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[andy_wilson]

If the leg is so far immersed the exhaust will tend to vent through the relief valve rather than the exhaust outlet below the water (except possibly at full power).

The result of the extra back pressure, and the fact that it is probably breathing some of its own fumes are a cause of power loss and smoking.

Many an outboard-in-a-well owner have modified the engine to vent the exhaust nearer the waterline to reduce back pressure, and away from the well to prevent choking the engine.

Bet the owners association (if there is one) would have something to say about this.

There is an upside of an engine so deeply immersed. In rougher seas when the stern lifts it reduces the chance of the prop getting close enough to the surface to suck air (sometimes incorrectly termed cavitation - which is something else entirely).

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