Chris Newport wrote in message news:3340094.BAxqyOrQ7F@callisto...
On Friday 24 September 2004 2:47 pm in rec.boats ZanderU wrote:

You often see the 'cruise speed' mentioned in boat descriptions and
brochures. But what is this speed and how is it determined?

The number is determined mainly by marketing issues.

In general it is the speed at which the boat was designed
to operate in a continuous cruise with reasonable comfort
and fuel economy. The tank range is usually specified at
this speed. The word "reasonable" here is rather subjective
and will be interpreted by the brochure writers according
to the market they are targeting and/or competitor figures.
...

This sounds right since (for displacement hulls) I consistently
see cruise speed as just under hull speed. I would be much more
interested in the speed that would maximize range. That would
be quite a bit lower than cruise speed then!

Harry Krause wrote:

My recollection is that BOATING magazine's "cruise speed" for planing
boats is the throttle setting that produces the best fuel economy while
the boat is on plane. Typically, it is somewhere around 70-75% of the
engine's top RPM on a boat with a gas engine.


and Gould 0738 commented:

Actually that would not be correct. The greatest fuel economy with a planing
hull boat (eliminating the "dead idle" option) is achieved just after the boat
reaches plane.



I think you are both correct but some tweaking of Gould's comment is
required. Engine load and fuel flow rate won't be minimized simply by
getting the boat on plane. Once on plane, proper trim of the boat via
load distribution or engine, outdrive or tab trim will often result in
an increase in RPM and speed for a given throttle setting. Properly
trimmed and on plane, the ideal cruise speed (with best fuel economy)
may be several knots or mph higher than the boats natural planing speed.

Diesel engines are easy. The manufacturer's recommended cruising RPM on
my boat is 200 below maximum RPM. Simple.

Eisboch

Properly
trimmed and on plane, the ideal cruise speed (with best fuel economy)
may be several knots or mph higher than the boats natural planing speed.

Doesn't that contradict the law of physics?

"On plane" can be a question of degree, but once the drag from the surface has
reached its minimally attainable amount, adding additional speed, especially
through a single speed transmission, will burn fuel *less* efficiently.

I'll go along with "the ideal cruise speed" may be several knots faster than
the speed at which a vessel achieves plane, but I can't buy into a theory that
states fuel economy improves with increased engine rpm (after the vessel is on
plane, properly trimmed).

On Saturday 25 September 2004 9:56 am in rec.boats ZanderU wrote:


The number is determined mainly by marketing issues.

In general it is the speed at which the boat was designed
to operate in a continuous cruise with reasonable comfort
and fuel economy. The tank range is usually specified at
this speed. The word "reasonable" here is rather subjective
and will be interpreted by the brochure writers according
to the market they are targeting and/or competitor figures.
...

This sounds right since (for displacement hulls) I consistently
see cruise speed as just under hull speed. I would be much more
interested in the speed that would maximize range. That would
be quite a bit lower than cruise speed then!

This is where you have to do some simple sea trials.
Assume that you always want to arrive with 20% fuel
in reserve for contingencies. If you want a more
conservative contingency adjust the numbers below.

Take the maximum design load (which you will usually need
when leaving on a long crossing), with the weight of
full fuel tanks. Fuel up to 20% and add ballast to get to
the weight at which you would arrive (max - 80%fuel).
Now do trials at various speeds and plot the fuel
consumption.
Repeat with 60% and 100% fuel, to get halfway and
departure consumptions. With any luck you now have 3
lines on the graph which will give you the optimum
consumption speed at 3 loads. These should be about
the same but slightly worse at full load.
If you have things like aircon and stabilisers you
need to make sure that you include these in the test,
this may mean adding in the generator consumption if
your generator needs to be running.

Keep these graphs and compare them with actual daily
fuel usage - this will tell you when something is going
wrong such as fouling or wear. In practice you will
find that heavy seas and weather will also have an effect.



--
My real address is crn (at) netunix (dot) com
WARNING all messages containing attachments or html will be silently
deleted. Send only plain text.

Eisboch wrote:
Gould 0738 wrote:
Properly
trimmed and on plane, the ideal cruise speed (with best fuel economy)
may be several knots or mph higher than the boats natural planing speed.


Doesn't that contradict the law of physics?

"On plane" can be a question of degree, but once the drag from the surface has
reached its minimally attainable amount, adding additional speed, especially
through a single speed transmission, will burn fuel *less* efficiently.

I'll go along with "the ideal cruise speed" may be several knots faster than
the speed at which a vessel achieves plane, but I can't buy into a theory that
states fuel economy improves with increased engine rpm (after the vessel is on
plane, properly trimmed).



I think my original statement included "for a given throttle position".
The increase in RPM and speed at a given throttle position is a result
of further reduction of drag and load on the engine due to proper
trimming. No increase in throttle is required.

Eisboch


BOATING magazine's tests show that for most real planing boats, the most
economical cruise speed while the boat is on plane is quite a bit higher
than "several" knots faster than the speed on which a boat achieves plane.






--
We today have a president of the United States who looks like he is the
son of Howdy Doody or Alfred E. Newman, who isn't smarter than either of
them, who is arrogant about his ignorance, who is reckless and
incompetent, and whose backers are turning the United States into a pariah.

What, me worry?

Gould 0738 wrote:
Properly
trimmed and on plane, the ideal cruise speed (with best fuel economy)
may be several knots or mph higher than the boats natural planing speed.


Doesn't that contradict the law of physics?

"On plane" can be a question of degree, but once the drag from the surface has
reached its minimally attainable amount, adding additional speed, especially
through a single speed transmission, will burn fuel *less* efficiently.

I'll go along with "the ideal cruise speed" may be several knots faster than
the speed at which a vessel achieves plane, but I can't buy into a theory that
states fuel economy improves with increased engine rpm (after the vessel is on
plane, properly trimmed).



I think my original statement included "for a given throttle position".
The increase in RPM and speed at a given throttle position is a result
of further reduction of drag and load on the engine due to proper
trimming. No increase in throttle is required.

Eisboch

The fuel flow test results shown in boating mags shows that with many boats,
Eisboch is correct. .


"Gould 0738" wrote in message
...
Properly
trimmed and on plane, the ideal cruise speed (with best fuel economy)
may be several knots or mph higher than the boats natural planing speed.

Doesn't that contradict the law of physics?

"On plane" can be a question of degree, but once the drag from the surface
has
reached its minimally attainable amount, adding additional speed,
especially
through a single speed transmission, will burn fuel *less* efficiently.

I'll go along with "the ideal cruise speed" may be several knots faster
than
the speed at which a vessel achieves plane, but I can't buy into a theory
that
states fuel economy improves with increased engine rpm (after the vessel
is on
plane, properly trimmed).

Eisboch wrote in message ...
Harry Krause wrote:

My recollection is that BOATING magazine's "cruise speed" for planing
boats is the throttle setting that produces the best fuel economy while
the boat is on plane. Typically, it is somewhere around 70-75% of the
engine's top RPM on a boat with a gas engine.


and Gould 0738 commented:

Actually that would not be correct. The greatest fuel economy with a planing
hull boat (eliminating the "dead idle" option) is achieved just after the boat
reaches plane.



I think you are both correct but some tweaking of Gould's comment is
required. Engine load and fuel flow rate won't be minimized simply by
getting the boat on plane. Once on plane, proper trim of the boat via
load distribution or engine, outdrive or tab trim will often result in
an increase in RPM and speed for a given throttle setting. Properly
trimmed and on plane, the ideal cruise speed (with best fuel economy)
may be several knots or mph higher than the boats natural planing speed.

Diesel engines are easy. The manufacturer's recommended cruising RPM on
my boat is 200 below maximum RPM. Simple.

Eisboch

unfortuantely for me testing as many boats as i have has taken the
shine from boating. cruising speed is a mix of comfort AND mecahnical
load in my mind. as some have touched on it is around 75% of wot.
there are a lot of factors involved for planing hulls but a correctly
powered correctly propped boat is what we are discussing. i have found
that cruising speed rpm for sterndrives is slightly higher, usually
around 80% of wot.
engine load is definately lowered in a planing boat, if ever im
checking for ignition,preignition,detonation problems in a rig i
always use the half plane position as here is the most load. remember
a prop is designed for maximum efficiency at wot, so at takeoff the
engine is working the hardest, there are ways to lessen this of course
such as vents behind the prop blades, helping a boat achieve plane
much faster. bottom line, cruise for comfort and safety.

The fuel flow test results shown in boating mags shows that with many boats,
Eisboch is correct. .

About what?

About a properly trimmed boat getting better fuel economy once on plane and at
a given throttle setting? Yes, indeed.

About the easy misread of his post implying that boats get progressivley better
fuel economy at higher RPM's, once properly trimmed and on plane? I don't think
that's what Eisboch meant.

There is a period of time when the boat is technically over the bow wave and
could be said to be "planing" where there is still more resistance than there
will be when the boat achieves optimum plane. In this window, the "giving it
more throttle improves the fuel economy" model might apply......but the boat
hasn't actually met the condition of being truly on plane during that period.