Because the vertical column will not be full of water, it is an engine and
there is a lot of exhaust gas, the vert. rise is not the same as the
backpressure. Measuring the backpressure will be prone to error because you
need to measure static pressure without getting any dynamic pressure. I
owned a boat with the 3GM30 engine. Exhaust hose was 1.5 inches and the
rise was at least 48 inches. No problem with engine was ever noted. Of
course the waterlift muffler has to be large enough to accommodate the water
that will flow back after engine shutdown.
Dave
"John Knox" wrote in message
...

i have read of the importance of insuring that
manufactures specs on back pressure are not
exceeded, but i can find no numbers for my
yanmar 2gm15. does 48" from bottom of water
lift to top of transom gooseneck using 1.5" hose
sound excessive?
john