I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide open
throttle (WOT). Many of you are probably familiar with a sweet spot. Cars
and trucks have been known to have them. Seems like going to WOT can eke
out a little more speed, but things are straining, coming back down a little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is my
fuel consumption more efficient at the sweet spot than wide open? Sounds
like it would be -- a little bit---as you come back on the throttle to get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious

Yes, it is more fuel efficient with the throttle backed off slightly. Best
fuel economy is usually found at 3200 to 3500 rpms as a general rule. This
assumes your on plan at these rpms. Anything over about 4000 rpms starts to
drink fuel. 5000+ is really bad fuel economy in general.
If you have to run the boat at WOT all the time then your underpowered.

--
Tony
my boats at http://t.thomas.home.mchsi.com



"RB" wrote in message
...
I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide open
throttle (WOT). Many of you are probably familiar with a sweet spot.
Cars
and trucks have been known to have them. Seems like going to WOT can eke
out a little more speed, but things are straining, coming back down a
little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is my
fuel consumption more efficient at the sweet spot than wide open? Sounds
like it would be -- a little bit---as you come back on the throttle to get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious

Based on the tests I have read in boating magazines where they plot fuel
consumption vs. rpm, an outboard powered planing hull usually get best
mileage at 500 - 800 rpm above planing speed. This works out to 3800 - 4300
rpm on the boats I have owned.



"RB" wrote in message
...
I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide open
throttle (WOT). Many of you are probably familiar with a sweet spot.
Cars
and trucks have been known to have them. Seems like going to WOT can eke
out a little more speed, but things are straining, coming back down a
little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is my
fuel consumption more efficient at the sweet spot than wide open? Sounds
like it would be -- a little bit---as you come back on the throttle to get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious

Unlikely WOT ever gives best economy unless it is severely underpowered and
required WOT to get the boat running right. Anyway it is not advisible to
run at WOT continuously, the engine's not gonna last. The Mercruiser
recommendation is not to run at WOT for more than 2 minutes.

Best economy is speed dependent (assuming engine has enough hp) than engine
or (this assumes planing hulls are on plane and displacement hulls are not
trying to plane) and is affected by the hull/load combination. Most modern
boat hulls and engines are already at their best according to the intended
design applcation, so there's not much the user can do here.

Drag, however, is the largest horsepower consumer (read as fuel), in the
fluid flow equation, drag increases as a square of speed. Fortunately, and
the boater can control this. If you have a fuel flow gauge, its the easiest
way to find the best speed but you can rely on your ears, the sweet spot is
unmistakable.

As an example of speed vs fuel burn. I have a 23ft planing hull with a 5.7
mercruiser i/o, following are numbers read off a flow guage.

1. 22 kts 48 litres/hr
2. 29 kts 118 litres/hr

The 7 kts increase is only one third of 22kts, but the fuel burn is 1.5
times more. Speed costs!

Tan PS


"Tony Thomas" wrote in message
news:NL0dc.217444$po.1077466@attbi_s52...
Yes, it is more fuel efficient with the throttle backed off slightly.
Best
fuel economy is usually found at 3200 to 3500 rpms as a general rule.
This
assumes your on plan at these rpms. Anything over about 4000 rpms starts
to
drink fuel. 5000+ is really bad fuel economy in general.
If you have to run the boat at WOT all the time then your underpowered.

--
Tony
my boats at http://t.thomas.home.mchsi.com



"RB" wrote in message
...
I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide
open
throttle (WOT). Many of you are probably familiar with a sweet spot.
Cars
and trucks have been known to have them. Seems like going to WOT can
eke
out a little more speed, but things are straining, coming back down a
little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is
my
fuel consumption more efficient at the sweet spot than wide open?
Sounds
like it would be -- a little bit---as you come back on the throttle to
get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious

"RB" wrote in message
...
I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide open
throttle (WOT). Many of you are probably familiar with a sweet spot.
Cars
and trucks have been known to have them. Seems like going to WOT can eke
out a little more speed, but things are straining, coming back down a
little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is my
fuel consumption more efficient at the sweet spot than wide open? Sounds
like it would be -- a little bit---as you come back on the throttle to get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious



Is this not basically "cruising speed"?

db

"RB" wrote in message
...
I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide open
throttle (WOT). Many of you are probably familiar with a sweet spot.
Cars
and trucks have been known to have them.

In an automobile, slower is almost always going to give you better fuel
economy, assuming that you remain in the same gear. There is a direct
connection between the pavement and the engine such that the engine will
rotate the same number of times to cover a mile regardless of the speed. If
the wider open the throttle is the more gas it will consume to cover that
mile.

On a boat things are not so easy. On a displacement hull there is a certain
"hull speed" that can be achieved with minimal power and best economy.
Attempting to go any faster will result in burning much more fuel with
little or no increase in speed.

On a planing hull there is a tremendous increase in economy once the boat is
up on plane. Depending on the hull design, there can be further reductions
in drag with increased speed as the hull lifts and less hull is in contact
with the water. At some point the increase in "lift" is offset by the
increase in water drag and wind resistance and going any faster will
decrease your economy.

Add on top of all this the performance of the engine. At lower RPMs the
engine is making best use of the fuel it is taking. As the RPMs increase
there isn't sufficient time for the fuel to completely burn and create the
maximum amount of torque. The result is a lot of waste heat out the exhaust
and a loss in efficiency.

Put it all together and it becomes impossible to come up with a generic
answer. It would be rare (and certainly not in a recreational boat) that
WOT was the sweet spot, but it is possible. I would expect that the most
likely sweet spot is the speed right after the boat has come up on plane and
the bow has settled into a more even keel.

Rod

On Wed, 7 Apr 2004 18:15:28 -0500, "RB"
wrote:

I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide open
throttle (WOT). Many of you are probably familiar with a sweet spot. Cars
and trucks have been known to have them. Seems like going to WOT can eke
out a little more speed, but things are straining, coming back down a little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is my
fuel consumption more efficient at the sweet spot than wide open? Sounds
like it would be -- a little bit---as you come back on the throttle to get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious



The "sweet spot" you're probably referring to is the point just before
the secondaries open on a 4-barrel carburetor. Not sure about o/b's
though. With fuel injection, it's a bit harder to tell what optimum
cruising speed is by feel, but just keep it in the manufacturer's
reccomended crusing RPM range and that'll be about right. If you
really want to know, get a fuel flow meter or sometimes you can plug
into your fuel injected engine's diagnostic port and get this info
with the right equipment.

"Slambram" wrote
If you
really want to know, get a fuel flow meter or sometimes you can plug
into your fuel injected engine's diagnostic port and get this info
with the right equipment.

They also make fuel flow meters that can link with output from common GPS
units for miles per gallon read-out . I can't remember what that link is
called - all I remember is that my (fairly old - maybe 10 years) GPS doesn't
have it. Of course, this stuff isn't cheap, but for a gadget freak, it
would be worth it, I'd guess.

G

On Thu, 8 Apr 2004 14:50:24 -0800, "Griss" wrote:

"Slambram" wrote
If you
really want to know, get a fuel flow meter or sometimes you can plug
into your fuel injected engine's diagnostic port and get this info
with the right equipment.

They also make fuel flow meters that can link with output from common GPS
units for miles per gallon read-out . I can't remember what that link is
called - all I remember is that my (fairly old - maybe 10 years) GPS doesn't
have it. Of course, this stuff isn't cheap, but for a gadget freak, it
would be worth it, I'd guess.

NavMan makes a unit like that. It's actually not all that expensive,
considering the price of fuel these days--and considering the difference
throttle setting may make on actual fuel economy.

I have the fuel meter alone, not the one that talks to the GPS. IF I ever get
around to installing it, I'll let you know how it works!

http://www.navman.com/marine/products/fuel/

Joe Parsons

In my opinion.... The short reply to your specific question is "Idle
Speed". We all know that WOT is not economical because of prop slip, etc.
The "sweet spot" is the point where the boat is planing at a moderate speed
(as compared to WOT) the boat is trimmed out well with minimum yaw and has
no list. Then you have reached the point where you can run an enjoyable
speed, throw a nominal wake and not dump gas through the engine(s)
excessively. Inboards, outboards, I/O's, water jet, the same rule applies
to all of them.

Almost no prop slip occurs at idle speed, you are pushing a minimal amount
of water out of the way of the bow and not wasting energy on throwing a
plowing wake. Most of us don't buy power boats to idle the though.

Don Dando




"RB" wrote in message
...
I've had several boats with big single and twin o/bs in the past. I
remember most as having a "sweet spot", which was just down from wide open
throttle (WOT). Many of you are probably familiar with a sweet spot.
Cars
and trucks have been known to have them. Seems like going to WOT can eke
out a little more speed, but things are straining, coming back down a
little
seems to let everything harmonize and get in the groove, and run more
smoothly.

The question I have is: assuming a boat with o/b has a sweet spot, is my
fuel consumption more efficient at the sweet spot than wide open? Sounds
like it would be -- a little bit---as you come back on the throttle to get
to the sweet spot. Or, is it that there's no real reason to run at the
sweet spot, other than it "feels right", and good?

Just curious