On 03 Jun 2004 16:12:17 GMT, (Gould 0738) wrote:
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.

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

Actually, that's just about how my 5,000 lb, 24 ft I/O performs. From
1000 RPM up to 2500 the boat is just pushing water and making waves.
Speed climbs from around 5.5 kts to about 8. Starting at 2500 the
boat will come up on a slow, labored plane at about 15 kts which is
useful only for running in a nasty chop with the bow up. Starting at
2800 it comes up onto a semi-comfortable plane with a speed of 18 to
19 kts which can be sustained with minor throttle tweaking. At 3000
RPM and 20 kts the bow comes down, the boat trims out nicely, and it
gains about 1 knot for every 100 RPM thereafter. Anything under 3000
RPM is not really fun for very long.

Obviously these characteristics vary a great deal depending on the
length and weight of the boat, and the type of motor.. My 12 foot
inflatable for example will come up onto a nice stable plane at about
12 knots because of its light weight and short length.

Wayne.B wrote:

On 03 Jun 2004 16:12:17 GMT, (Gould 0738) wrote:

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.


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

Actually, that's just about how my 5,000 lb, 24 ft I/O performs. From
1000 RPM up to 2500 the boat is just pushing water and making waves.
Speed climbs from around 5.5 kts to about 8. Starting at 2500 the
boat will come up on a slow, labored plane at about 15 kts which is
useful only for running in a nasty chop with the bow up. Starting at
2800 it comes up onto a semi-comfortable plane with a speed of 18 to
19 kts which can be sustained with minor throttle tweaking. At 3000
RPM and 20 kts the bow comes down, the boat trims out nicely, and it
gains about 1 knot for every 100 RPM thereafter. Anything under 3000
RPM is not really fun for very long.

Obviously these characteristics vary a great deal depending on the
length and weight of the boat, and the type of motor.. My 12 foot
inflatable for example will come up onto a nice stable plane at about
12 knots because of its light weight and short length.



My 25-footer does 60 mph at 2000 rpm. Attached to its trailer and being
towed by an SUV...

"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?)

I/O on a 28ft Wellcraft Monte Carlo. :-)


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?

I don't stay on plane at 10-11, it pops out of the hole somewhere around 20.
At about 3000 rpm, I run at about 25 mph, at full throttle on flat water I
max at about 38.






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

"Gould 0738" wrote in message
...
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. :-)

Naw. You might only have to jump the engine speed to 2550 to get up on
plane...and your speed might go up 5-7 mph. Once on plane, every 100rpm
might increase your speed 2-2.5 mph until you hit 25mph.

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.

I'm not forgetting anything. Even though I gravitate toward semi and full
displacement hulls, I have been on plane in many dozens of boats and the
numbers don't make sense.

Let's apply some logic.

If wew agree for purposes of discussion
that the boat is not on plane at 9mph, (and it most likely would not be), and
that we have only 500 rpm to "spend" to get the boat from 9 mph to 25, observe
what has to happen:

We will need to spend at least 100-200 RPM (maybe more) to climb the bow wave
and maintain a minimal plane. For purposes of discussion, let's say when we
throttle up enough to climb the bow wave we achieve 14 mph. The unspent 300 RPM
are going to take this boat from 14 mph to 25? Almost doubling the speed? No,
they are not.

The other example, a 2500 RPM difference between 6 mph and 27 mph is more
believable than a 500 rpm difference between 9 mph and 25.

I had a similar experience. My 1994 290 Searay would go about 12mph
at 2000rpm and slowly ramp up to about 16mph at 2900, then stay on plane
at 27mph and 3200 rpm.

That's not all that similar. You were using
1200 rpm to go from 12mph to 27......not 500 rpm to go from 9mph to 25.

Naw. You might only have to jump the engine speed to 2550 to get up on
plane...and your speed might go up 5-7 mph. Once on plane, every 100rpm
might increase your speed 2-2.5 mph until you hit 25mph.


Reasonable statement, but not consistent with the original example.

"Gould 0738" wrote in message
...
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.

I'm not forgetting anything. Even though I gravitate toward semi and full
displacement hulls, I have been on plane in many dozens of boats and the
numbers don't make sense.

Let's apply some logic.

If wew agree for purposes of discussion
that the boat is not on plane at 9mph, (and it most likely would not be),
and
that we have only 500 rpm to "spend" to get the boat from 9 mph to 25,
observe
what has to happen:

We will need to spend at least 100-200 RPM (maybe more) to climb the bow
wave
and maintain a minimal plane. For purposes of discussion, let's say when
we
throttle up enough to climb the bow wave we achieve 14 mph. The unspent
300 RPM
are going to take this boat from 14 mph to 25? Almost doubling the speed?
No,
they are not.

Chuck,
I apologize for confusing you. You're absolutely correct.

Numbers should read:
1500 rpm (approx. 6mph): 6.2mpg
2000 rpm (approx. 8mph): 4.5mpg
2500 rpm (approx. 9mph): 3.5mpg
3000 rpm (approx. 16mph): 6.5mpg
3500 rpm (approx. 25mph): 6.2mpg

From the graph, it appears that the boat comes on plane at a little bit over
2500rpm. The 500 rpm increase from 1500 to 2000 rpm gives a speed increase
of only 2mph. The 500rpm increase from 2000 to 2500 gives only a 1mph speed
increase. The 500rpm increase from 2500 to 3000 rpm gives a speed increase
of 7mph. The 500 rpm increase from 3000 to 3500 gives a speed increase of 9
mph.

As I was stating earlier, the worst efficiency is at the point right before
the boat jumps on plane...and then there's a very rapid rise in the boat
speed per 100 rpm increase after that.

As Wayne B. said earlier, the best mileage would come at 1000 rpm or less
(but the boat would be going less than 5 mph). The second best mileage
comes at an engine speed on 3000rpm...and a boat speed of 16 mph.

"Gould 0738" wrote in message
...
Naw. You might only have to jump the engine speed to 2550 to get up on
plane...and your speed might go up 5-7 mph. Once on plane, every 100rpm
might increase your speed 2-2.5 mph until you hit 25mph.


Reasonable statement, but not consistent with the original example.

You're right. See my post with the corrected numbers above.
Here they are again:
3000rpm is 16mph and 6.5mpg.
3500rpm is 25mph and 6.2mpg

Chuck,
I apologize for confusing you. You're absolutely correct.

Good lord!

That statement, (to me from NOYB), is a collector's item. I think I'll take it
out and have it bronzed. :-)

The new numbers make sense.