Hi Dave,

Thanks for setting me straight on the rpm / torque settings of marine
engines. I thought that if you cam a marine engine for maximum torque
at low rpms, you could prop bigger and by a combination of the two
changes run your boat at lower rpms at a lowered cruising speed for
much improved fuel economy. I even assumed that with more torque at
lower rpms you could use that bigger prop to get the boat to plane at a
lower water speed, enabling low speed planing fuel economy.

But those were a heapload of interweaved assumptions, which might be
any which way? Please tell me, why do boats have engines which are
cammed for high rpm torque? Is it because there is greater fuel
efficiency when running at 80% of Wide Open Throttle? This would be
true because that is where the torque peak is situated in terms of
rpms. But in an engine cammed for lower rpm torque, wouldn't it be true
that you would get the best fuel economy at the lower rpms where the
torque is found?

I'm not concerned about engine weight because 2 engines will be
powering a 10,000 lb boat, with 250 to 300 hp and 200kw per engine,
this isn't a tough task, and they could well lose a bunch of horses at
high rpm if it meant improved GPH or more usefully improved MPG.

Wouldn't it be worthwhile to sacrifice an unused 35 knot max speed for
a new economical 15 knot cruising speed? If it will plane slower and
use less fuel at lower rpms I'll be a happy camper, with enough
gumption to speed up occasionally if really needed once in a while.

TIA.

Rich

Hi Dave,

Thanks for setting me straight on the rpm / torque settings of marine
engines. I thought that if you cam a marine engine for maximum torque
at low rpms, you could prop bigger and by a combination of the two
changes run your boat at lower rpms at a lowered cruising speed for
much improved fuel economy. I even assumed that with more torque at
lower rpms you could use that bigger prop to get the boat to plane at a
lower water speed, enabling low speed planing fuel economy.

But those were a heapload of interweaved assumptions, which might be
any which way? Please tell me, why do boats have engines which are
cammed for high rpm torque? Is it because there is greater fuel
efficiency when running at 80% of Wide Open Throttle? This would be
true because that is where the torque peak is situated in terms of
rpms. But in an engine cammed for lower rpm torque, wouldn't it be true
that you would get the best fuel economy at the lower rpms where the
torque is found?

I'm not concerned about engine weight because 2 engines will be
powering a 10,000 lb boat, with 250 to 300 hp and 200kw per engine,
this isn't a tough task, and they could well lose a bunch of horses at
high rpm if it meant improved GPH or more usefully improved MPG.

Wouldn't it be worthwhile to sacrifice an unused 35 knot max speed for
a new economical 15 knot cruising speed? If it will plane slower and
use less fuel at lower rpms I'll be a happy camper, with enough
gumption to speed up occasionally if really needed once in a while.

TIA.

Rich

You have to remember that your boat does not have a transmission that allows
you to change gears.
If I set a boat up to run at 3000 rpms w/ a 20 pitch prop at 1/2 throttle
and that is where max power is created, then you would probably only hit
about 4000 rpms at wide open as you would start falling off on power. This
would give you a very poor top end speed also.

So, they setup the engine to turn 5000 rpms at wide open with a 20 pitch
prop to achieve better top speed and the 3000 rpm throttle setting may be a
little more than 1/2 throttle.

Most people after a period of time find that their boat is too slow. There
is the exception but this is the normal rule.

You of course can do a lot of things w/ a cam to change the profile but you
will be affecting other things as well.

--
Tony
my boats and cars at http://t.thomas.home.mchsi.com
wrote in message
oups.com...
Hi Dave,

Thanks for setting me straight on the rpm / torque settings of marine
engines. I thought that if you cam a marine engine for maximum torque
at low rpms, you could prop bigger and by a combination of the two
changes run your boat at lower rpms at a lowered cruising speed for
much improved fuel economy. I even assumed that with more torque at
lower rpms you could use that bigger prop to get the boat to plane at a
lower water speed, enabling low speed planing fuel economy.

But those were a heapload of interweaved assumptions, which might be
any which way? Please tell me, why do boats have engines which are
cammed for high rpm torque? Is it because there is greater fuel
efficiency when running at 80% of Wide Open Throttle? This would be
true because that is where the torque peak is situated in terms of
rpms. But in an engine cammed for lower rpm torque, wouldn't it be true
that you would get the best fuel economy at the lower rpms where the
torque is found?

I'm not concerned about engine weight because 2 engines will be
powering a 10,000 lb boat, with 250 to 300 hp and 200kw per engine,
this isn't a tough task, and they could well lose a bunch of horses at
high rpm if it meant improved GPH or more usefully improved MPG.

Wouldn't it be worthwhile to sacrifice an unused 35 knot max speed for
a new economical 15 knot cruising speed? If it will plane slower and
use less fuel at lower rpms I'll be a happy camper, with enough
gumption to speed up occasionally if really needed once in a while.

TIA.

Rich