sailboats , engines, and waves

[barret bonden]

In certain wave patterns my inboard sailboat engine occasionally takes on
salt water in the aft cylinders; does my explanation hold to physical
principles ?



From the engine exhaust gas is sent out , then up six inches, back (aft)
six inches , then down a foot and then horizontally out a 10 foot pipe to
the sea. As the pipe drops the foot, it is joined by sea water that has
passed through the engine as coolant. (this "riser" is meant to stop this
sea water from getting back into the engine)



My "theory" :



Boat pitches forward on a wave and water runs forward the 10 feet towards
engine, pooling at the base of the riser.

Boat then drops on the wave; water now at base of riser stays in place
(inertia) and thus "rises" up pipe. (is this sensible ?)

Boat is moved aft on a wave and thus water is sent forward back along upper
small horizontal portion of exhaust pipe back into the engine .





Picture a wave moving from left to right. Any point on this wave is in
clockwise motion as it rises and falls . Thus a boat is moved backward (and
water in it sent forward) at the bottom of a wave when running downwind (
following sea) or at the top of the wave when running into the sea



My full scale experiments suggest this is possible. I have no recent
physics however and am worried I'm making lots of reasoning errors. ( I
should add I've been all over the engine; it's not a gasket or manifold
problem)



I'm in the process of re-engineering the exhaust system; any ideas
greatly appreciated. Losing the engine approaching a harbor
can be a serious matter, and is never any fun ..

[Herman Family]

Post by barret bonden
In certain wave patterns my inboard sailboat engine
occasionally takes on salt water in the aft cylinders;
does my explanation hold to physical principles ?
From the engine exhaust gas is sent out , then up six
inches, back (aft) six inches , then down a foot and then
horizontally out a 10 foot pipe to the sea. As the pipe
drops the foot, it is joined by sea water that has passed
through the engine as coolant. (this "riser" is meant to
stop this sea water from getting back into the engine)
Boat pitches forward on a wave and water runs forward
the 10 feet towards engine, pooling at the base of the
riser.
Boat then drops on the wave; water now at base of riser
stays in place (inertia) and thus "rises" up pipe. (is
this sensible ?)
Boat is moved aft on a wave and thus water is sent forward
back along upper small horizontal portion of exhaust pipe
back into the engine .
Picture a wave moving from left to right. Any point on
this wave is in clockwise motion as it rises and falls .
Thus a boat is moved backward (and water in it sent
forward) at the bottom of a wave when running downwind
( following sea) or at the top of the wave when running
into the sea
My full scale experiments suggest this is possible. I have
no recent physics however and am worried I'm making lots
of reasoning errors. ( I should add I've been all over the
engine; it's not a gasket or manifold problem)
I'm in the process of re-engineering the exhaust system;
any ideas greatly appreciated. Losing the
engine approaching a harbor can be a serious matter, and
is never any fun ..

How about:
1. Your exhaust line occasionally gets filled with water.
If the boat is pitched forward, water will run up the 10'
horizontal.
2. If a wave then hits the exhaust line head on, it will
give a little pressure pulse that can push the water up the
6" into the cylinder.

Solutions.

1. Exhaust to the atmosphere, rather than into the water
2. Install a check valve to prevent sea water from getting
into the exhaust line.
3. change the direction of the exhaust outlet from
horizontal to downward. Note that this will change your
back pressure.

Also note that if the horizontal pipe is below sea level
that it is at least partially filled with water. A 1' wave
will easily push that water up 6" simply from its momentum.

Michael