Most boats have to achieve about 2800 rpms to plane off and start moving.
Low hp will still produce this rpm but the time to do it will increase as
you lower the power. Right now you produce a certain amount of power that
allows you to turn a certain prop to 2800 rpms within a certain time. To
maintain 5000 rpms at top end with more power you will need a larger prop.
The catch is you are not producing more power at low rpms so you increase
the time required to achieve 2800 rpms.

Think of it this way. An engine without a turbo in a car will achieve a
zero to 30 mph time of say 3 seconds and a zero to 60 time in 5 seconds.
The same car with a turbo will achieve a zero to 30 mph time of 3 seconds
and a zero to 60 time of 4 seconds (boost kicks in from 30 mph up). Now if
you reduce the gear to keep rpms under redline you just increase the 0 to 30
time to 4 seconds. Just an example.

Given the hole shot problem, heat generated, and trouble to implement hosing
and stuff - I would recommend going ahead and doing the blower. Blower may
use some more fuel but since your cruising with a lower throttle setting,
you should not see much of a change.


Tony
My boats and autos - http://t.thomas.home.mchsi.com



"Hemicuda" wrote in message
om...
So from what I can summarize from the above posts are that the turbo
will probably work great while cruising and at top end but will be a
dog coming out of the hole. This is a dumb question but I thought
that at slower speeds boats require less horsepower to move them
through the water. As the speed increases so does the power
requirement for a certain hull design. Is this true? If so it
appears that the low end lack of boost shouldn't kill me. Or am I
totally of base? The other thing I was thinking of was to put smaller
turbos on the engine to reduce lag but sacrifice top end boost.
Marine engines rarely go over 5,000 RPM so I could probably go with a
smallish turbine housing with a waste gate that will bring on boost
quickly and then dump the excess exhaust gas at higher speeds. I know
the heat will be a problem but I think that is manageable but I don't
want to take forever to get up on plane. Has anyone seen or ridden in
a gasoline turbo'd boat and seen how bad the hole shot was? The
reason I want to stay away from a blower is that although it will
boost power I think it will hurt the fuel efficiency. Also blowers
tend to be expensive and I can score a pair of T3 Turbos from old T
birds which would fit a 5.0 or 5.7L fairly well for cheap at a junk
yard and fabricate a set-up. Thanks for all the help and I
appreciate any more thoughts or info you could pass on!