How to stop the water from getting in
QIs there a certain height that the exhaust outlet should be from the waterline? I have heard on the grapevine that it is 250mm or ten inches.
ALAN THORNTON, via email
ATONY REPLIES: This seems an easy question but it is far from that: it depends on the age of the boat; whether it was built to comply with the Recreational Craft Directive (RCD); whether it is a hire or private boat.
I cannot get involved in RCD matters: the paperwork refers to a great stack of ISO standards which are exceptionally expensive to buy. If this is a new build to RCD standards, consult a surveyor. However I suspect the 10in/250mm would be the minimum height.
As far as I can see there are no Boat Safety Scheme demands about hull opening heights for private boats, but for hire boats they must be a minimum of 10in/250mm above the normal laden waterline – so best practice is to stick to the 10in rule for any class of boat.
But things are not quite as clear as specified above. Given that the reason for minimum height is to prevent flooding when the boat heels or gets overloaded, you can reduce it (or even have outlets below the waterline) provided certain other conditions are met, such as a reliable shut-off valve on the skin fitting and suitable pipework. This mainly concerns things such as sink drains, but if the pipework (a metal exhaust pipe, for example) sweeps up to a swan neck and back down to the hull outlet, then as long as the swan neck is 10in above the waterline it meets best practice. Exhaust pipe runs are critical because the exhaust manifold is often below the waterline, and if water gets into the manifold you could be looking at a new engine or costly repairs.