Paul Fritz wrote:
Doesn't seem unusual to me at all. At around 2500 rpms is about where
mine drops off plane and essentiallly plows water.....much of the energy
produced by the engines is being used to push water out of the way, not move
the boat forward. At rpms around 1500, more energy is moving the boat
forward than pushing water,(less wake for example) and the same is true at
3000 with the hull on plane.

A bit of physics here... with the hull on plane, the energy required to
lift the boat is coming from the engine.

It's possible that gas mileage for light planing hulls has improved in
recent years. But the efficiency curve still isn't going to go backwards
to any significant degree, and adding any load to the boat is going to
make the curve steeper (ie higher penalty on mileage at higher speeds).

Motorboaters deny it to their dying breath, but it's like arguing
against gravity. Speed costs. Sorry to be the bearer of bad news.

Fair Skies
Doug King

Gould,
Since you boat with a trawler I think you have forgotten what happens when a
planning hull is no longer planning but is "muscling" it's way through the
water. Most boats drop off of planning at 2200 rpm to 2800 rpm.
The numbers do look reasonable for a "planning hull".


"Gould 0738" wrote in message
...
Mileage at 1500 rpm (6mph): approximately 6.2mpg
Mileage at 2500 rpm(9mph): approximately 3.5 mpg
Mileage at 3000 rpm(25mph): approximately 6.5mpg


TILT!

(occurs to me you kids don't fully appreciate that term).......

The performance curve seems out of whack.

The boat travels 6 mph at 1500 rpm.
Seems really slow, but let's use that.
It ought to do 6mph at a fast idle, one would think.

2500 rpm is only 9 mph? Still seems really, really, really slow, but let's
use
that.

If 2500 rpm is propelling this vessel at 9mph, it stretches the
imagination to
consider that adding 500 rpm will increase the speed by a factor of 2 1/2
times, or an additional 16mph.

Not saying it isn't so, just that it seems very unusual.

Sure the 2500 rpm isn't 19 mph, instead of
9?

On Thu, 03 Jun 2004 15:03:10 GMT, "John Smith"
wrote:
Gould,
Since you boat with a trawler I think you have forgotten what happens when a
planning hull is no longer planning but is "muscling" it's way through the
water. Most boats drop off of planning at 2200 rpm to 2800 rpm.
The numbers do look reasonable for a "planning hull".


"Gould 0738" wrote in message
...
Mileage at 1500 rpm (6mph): approximately 6.2mpg
Mileage at 2500 rpm(9mph): approximately 3.5 mpg
Mileage at 3000 rpm(25mph): approximately 6.5mpg

==================================================

The numbers are reasonable for a planing hull, problem is that they
didn't test at lower than 1500 RPM. At 1500 the boat is already
exceeding its hull speed by a considerable margin and leaving a big
wake. If they had tested at 1000 RPM the results would have been as
expected and shown the best fuel economy. No one wants to travel at
that speed with a planing hull of course, but that is where the best
economy is.

Gould,
Since you boat with a trawler I think you have forgotten what happens when a
planning hull is no longer planning but is "muscling" it's way through the
water. Most boats drop off of planning at 2200 rpm to 2800 rpm.
The numbers do look reasonable for a "planning hull".

Once a month I do test runs on a wide variety of different boats. A number of
them are planing hulls. I cannot remember a boat where an increase of 500 rpm
would have brought the boat from 9mph to 25mph. As I said, I'm not in a
position to comment on that specific boat, but the numbers seem strange to me.

Once a boat is over the bow wake and on plane, (which should be occuring
somewhere in the low to mid teens speed wise), in most cases fuel consumption
goes up exponentially to obtain additional speed. It would be interesting to
view a speed/fuel chart for the boat NOYB is referring to.

A boat with the operating characteristics NOYB described would be frustrating
as hell. A difference of 500 RPM increases the speed by 2 1/2 times? Unless you
wanted to run (and the conditions permitted) 25mph, you'd have a dickens of a
time setting the throttle for a speed somewhere between 9mph and 25mph and
keeping the boat there. Everytime a mosquito landed on the throttle lever you'd
pick up or lose a couple of miles an hour. :-)

Doesn't seem unusual to me at all. At around 2500 rpms is about where
mine drops off plane and essentiallly plows water....

Ok, let's say your boat drops off plane at 2500 RPM. (Are we talking OB motor?)

Let's say that just off plane in your boat is
9 mph.

At 2600 rpm we'll say you're back on plane at 10-11 mph. Adding 400 rpm to that
will bring you to 25 mph?

Something is fracturing my paradigm. Very light boats, outboard motors, or
something else I don't typically experience.

Best economy is at 1000RPM (4.3mph): 10.33mpg

At 1500rpm (5.7mph) : 6.7mpg

At 4000rpm (27.2mph): 7.22mpg

Now that seems more reasonable.
A 2500 rpm increase taking the boat from
6 mph to 27 is more believable (to me) than a 500 rpm increase taking the boat
from 9mph to 25.

"Gould 0738" wrote in message
...
Everytime a mosquito landed on the throttle lever you'd
pick up or lose a couple of miles an hour. :-)


Well it seems like the answer is to keep mosquitoes off the throttle lever.
: - )

"Gould 0738" wrote in message
...
Doesn't seem unusual to me at all. At around 2500 rpms is about where
mine drops off plane and essentiallly plows water....

Ok, let's say your boat drops off plane at 2500 RPM. (Are we talking OB
motor?)

Let's say that just off plane in your boat is
9 mph.

At 2600 rpm we'll say you're back on plane at 10-11 mph.

My 17' Whaler can stay on plane at about 13mph. Very few v-bottom boats can
plane at that slow a speed...so I doubt the Triumph is back on plane at 2600
rpm and a speed of 10-11mph.

Adding 400 rpm to that
will bring you to 25 mph?

An engine needs to run at a higher RPM to initially jump onto plane than it
does to maintain planing speed. However, I think you're incorrectly
assuming that the boat started planing at 10-11mph.




Something is fracturing my paradigm. Very light boats, outboard motors, or
something else I don't typically experience.


Light boats, yes. Outboards? Probably no different from an
I/O...definitely different from an inboard.

I think what you're missing is that there is less drag on a boat when it is
running fast and high out of the water, then when it's running at a speed
off-plane or just on-plane.

It's the same reason a boat runs faster with the engine trimmed out. Less
drag.

"Gould 0738" wrote in message
...
Best economy is at 1000RPM (4.3mph): 10.33mpg

At 1500rpm (5.7mph) : 6.7mpg

At 4000rpm (27.2mph): 7.22mpg

Now that seems more reasonable.
A 2500 rpm increase taking the boat from
6 mph to 27 is more believable (to me) than a 500 rpm increase taking the
boat
from 9mph to 25.

The reason the first one didn't make sense to you is that you're forgetting
that the point of maximum drag is at the speed you're running right before
you jump on plane. At 9 mph, that boat is pushing tons of water. At 25
mph, that boat is displacing much less water.

"Wayne.B" wrote in message
...
On Thu, 03 Jun 2004 15:03:10 GMT, "John Smith"
wrote:
Gould,
Since you boat with a trawler I think you have forgotten what happens
when a
planning hull is no longer planning but is "muscling" it's way through
the
water. Most boats drop off of planning at 2200 rpm to 2800 rpm.
The numbers do look reasonable for a "planning hull".


"Gould 0738" wrote in message
...
Mileage at 1500 rpm (6mph): approximately 6.2mpg
Mileage at 2500 rpm(9mph): approximately 3.5 mpg
Mileage at 3000 rpm(25mph): approximately 6.5mpg

==================================================

The numbers are reasonable for a planing hull, problem is that they
didn't test at lower than 1500 RPM. At 1500 the boat is already
exceeding its hull speed by a considerable margin and leaving a big
wake. If they had tested at 1000 RPM the results would have been as
expected and shown the best fuel economy. No one wants to travel at
that speed with a planing hull of course, but that is where the best
economy is.


Exactly!