"Mark" wrote in message
th.net...
I'm trying to figure out why the bottom of the exhaust
bellows is intentionally open. It seems to me this would
allow "air" (exhaust) to get to the prop, obviously not a
good thing.



The holes relieve exhaust back pressure that could "swell up" the exhaust
bellows which would then press against the u-joint bellows which would then
contact the spinning u-joints and wear a hole thru it. The problem is most
pronounced on heavier boats while getting up on plane when the prop is deep
in the water and is not moving fast enough to help "draw" the exhaust out
thru the hub.

MercCruiser had that problem years ago and that is when they went to an
exhuast tube for larger engines, instead of the bellows. A few boats sunk
because of the holes worn thru the bellows.

Bill Grannis
service manager