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Default Props - RPM's and Cruising Speed

I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this
speed, I get reasonable fuel economy, because it's not until about
3,200 RPM's that the secondaries join the party and start tapping the
keg, if you will.

Max RPM on the boat is 4,100, at a speed of around 30kts. I very,
very rarely operate at this speed, and when I do, it's usually for
less than a minute.

I'm wet behind the ears when it comes to propping, so a question for
those in the know:

At 20kts, the boat requires slight tabbing, so it's not quite 100% on
plane. If I bump the RPM's up to 3,200-3,300, the boat cruises at
about 24kts, and will plane without tabs. The first comment will
likely be that operating on a full plane is more fuel efficient, and I
would normally concur, but it's not the case with this boat. I've
done the calculations, and it's more efficient for me to keep out of
the 4 barrels.

My current props are 3-blade. Would it be possible to prop the boat
so that the cruise speed at 3,000 RPM's was bumped up a few knots? My
thinking is that if I can get on full plane while keeping out of the
secondaries, I'll enjoy the best of both worlds, higher cruising speed
while maintaining fuel economy (if that's really such a thing on a
boat). 8)

I'm not concerned with losing a couple of hundred RPM's on the top, as
I simply don't operate the boat in that range. Another benefit is
noise. Above 3,100 RPM's, the engines generate significantly more
noise.

Is a 4 blade prop an option? More cupping on my current prop? More
diameter? Other?

Thanks!
  #3   Report Post  
Dave Hall
 
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Default Props - RPM's and Cruising Speed

On Fri, 28 May 2004 13:54:40 GMT, wrote:

I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this
speed, I get reasonable fuel economy, because it's not until about
3,200 RPM's that the secondaries join the party and start tapping the
keg, if you will.


And those guys take more than their share from the keg.


Max RPM on the boat is 4,100, at a speed of around 30kts. I very,
very rarely operate at this speed, and when I do, it's usually for
less than a minute.


4100 is a bit low for WOT.


I'm wet behind the ears when it comes to propping, so a question for
those in the know:

At 20kts, the boat requires slight tabbing, so it's not quite 100% on
plane. If I bump the RPM's up to 3,200-3,300, the boat cruises at
about 24kts, and will plane without tabs. The first comment will
likely be that operating on a full plane is more fuel efficient, and I
would normally concur, but it's not the case with this boat. I've
done the calculations, and it's more efficient for me to keep out of
the 4 barrels.

My current props are 3-blade. Would it be possible to prop the boat
so that the cruise speed at 3,000 RPM's was bumped up a few knots? My
thinking is that if I can get on full plane while keeping out of the
secondaries, I'll enjoy the best of both worlds, higher cruising speed
while maintaining fuel economy (if that's really such a thing on a
boat). 8)

I'm not concerned with losing a couple of hundred RPM's on the top, as
I simply don't operate the boat in that range. Another benefit is
noise. Above 3,100 RPM's, the engines generate significantly more
noise.

Is a 4 blade prop an option? More cupping on my current prop? More
diameter? Other?

Thanks



When selecting a prop the biggest thing you have to consider is load.
More specifically, the load that the prop places on your engine. If
the prop loads the engine too lightly you will be able to over-rev the
engine and you will lose fuel economy and top speed.

If you load the engine too much, it will bog down (lug) and not be
able to develop maximum horsepower, and can cause damage.

Think of it in the same way as driving your car up a steep hill. High
gear will not cut it, so a lower gear will have to be selected which
will allow the engine to produce maximum torque, and thereby the
fastest speed up the hill.

At 4100 RPM, you are already at the lower end of the recommended WOT
RPM range. That might be caused by a dirty bottom or excessive weight.
If that's always been your top RPM, I would suggest dropping pitch 1
or 2 inches, which will give you better hole shot, and better load the
engine. It will do the opposite of what you are seeking though and
will result in a higher RPM for a given speed.

Maybe judicious experimentation with the trim tabs will help keep you
on a better plane at lower RPM, and give you better fuel economy.

Dave
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Wayne.B
 
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Default Props - RPM's and Cruising Speed

On Fri, 28 May 2004 16:47:39 GMT, WaIIy
wrote:

I disagree, the boat weighs about 9500 lbs dry and that is plenty of
power.


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

Depends on your point of view. My 24 ft I/O cuddy with the same
enfine weighs about 5,000 lbs with fuel, gear, etc. At 3,400 RPM it
cruises at 24 to 25 kts which I regard as adequate, but just barely.
I'd really like to be running at 30 kts in flat water.

The fact that the boat in question can plane off and run and 20 kts at
a reasonable RPM means that he has enough power, but barely enough in
my opinion. Most 10,000 lb boats will either be running a single 454
or a pair of 350s, giving them a cruising speed of 25 to 27 kts.

  #5   Report Post  
 
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Default Props - RPM's and Cruising Speed

On Fri, 28 May 2004 14:40:39 -0400, Wayne.B
wrote:

On Fri, 28 May 2004 16:47:39 GMT, WaIIy
wrote:

I disagree, the boat weighs about 9500 lbs dry and that is plenty of
power.


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

Depends on your point of view. My 24 ft I/O cuddy with the same
enfine weighs about 5,000 lbs with fuel, gear, etc. At 3,400 RPM it
cruises at 24 to 25 kts which I regard as adequate, but just barely.
I'd really like to be running at 30 kts in flat water.

The fact that the boat in question can plane off and run and 20 kts at
a reasonable RPM means that he has enough power, but barely enough in
my opinion. Most 10,000 lb boats will either be running a single 454
or a pair of 350s, giving them a cruising speed of 25 to 27 kts.


Just to clarify, the boat does have twin engines..........Also, the
recommended RPM range at WOT is 4K-4.4K..........

I thought too that I should have a higher cruising speed at 3K RPM's,
but 19kt-20kt is it.


  #6   Report Post  
Wayne.B
 
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Default Props - RPM's and Cruising Speed

On Fri, 28 May 2004 21:04:08 GMT, WaIIy
wrote:

There might be a mixup here, Wayne. I would agree that one 5.7 is
underpowered.

He is runnning twin 5.7 straight inboards.


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

One of my neighbors has the same boat with twin 350s and I/Os. He
cruises at 26 to 27 kts but he's pushing his engines to 3600 RPM most
of the time. Conventional wisdom around here is that the I/Os run a
bit faster than straight inboards.

  #7   Report Post  
Bob
 
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Default Props - RPM's and Cruising Speed

I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this
speed, I get reasonable fuel economy, because it's not until about
3,200 RPM's that the secondaries join the party and start tapping the
keg, if you will.

Max RPM on the boat is 4,100, at a speed of around 30kts. I very,
very rarely operate at this speed, and when I do, it's usually for
less than a minute.


There are several things to consider. As others point out, with a 4100
rpm max., there is a distinct risk that there will be detonation
(knock) in the cylinders. High speed detonation is difficult to
impossible to detect, especially when the engines are well insulated
(as they would be in a SeaRay.) Detonation is like hitting the piston
tops with a ball-pein hammer and can eventually punch holes in the
pistons. Also, carburetors try to do the best they can at providing
the right fuel-to-air ratio for combustion. That said, running the
engine just shy of opening the second barrels tends to run the mixture
lean, which will further aggrevate the detonation problem. I have read
in boating mag advice colums NOT to run that way. You will get "save
money" with better fuel economy, but lose in the long run.


As to tabbing, I have a 1989 30ft. Sundancer with flowmeters with GPS
input that shows instantaneous miles/gallon. I have observed several
times that I get slightly better mpg (approx. .05 mpg) by pushing the
bow down slightly with tabs when cruising at 20-25 mph. I would never
have guessed that, 'cause the wetted area goes up (viscous drag), but
it really does happen.
  #8   Report Post  
Woodchuck
 
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Default Props - RPM's and Cruising Speed

Sounds like you need to drop prop pitch a bit to get the WOT RPM where it
should be. Reducing pitch reduces load on the engines at the same RPM and
you might just be able to run a few 100 RPM more before the secondaries
open. Another option may be to have a performance shop change the "rods"
which control when the secondaries open. I presume you have the old Chevy
standby Quadra-jet carbs.

wrote in message
...
I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this
speed, I get reasonable fuel economy, because it's not until about
3,200 RPM's that the secondaries join the party and start tapping the
keg, if you will.

Max RPM on the boat is 4,100, at a speed of around 30kts. I very,
very rarely operate at this speed, and when I do, it's usually for
less than a minute.

I'm wet behind the ears when it comes to propping, so a question for
those in the know:

At 20kts, the boat requires slight tabbing, so it's not quite 100% on
plane. If I bump the RPM's up to 3,200-3,300, the boat cruises at
about 24kts, and will plane without tabs. The first comment will
likely be that operating on a full plane is more fuel efficient, and I
would normally concur, but it's not the case with this boat. I've
done the calculations, and it's more efficient for me to keep out of
the 4 barrels.

My current props are 3-blade. Would it be possible to prop the boat
so that the cruise speed at 3,000 RPM's was bumped up a few knots? My
thinking is that if I can get on full plane while keeping out of the
secondaries, I'll enjoy the best of both worlds, higher cruising speed
while maintaining fuel economy (if that's really such a thing on a
boat). 8)

I'm not concerned with losing a couple of hundred RPM's on the top, as
I simply don't operate the boat in that range. Another benefit is
noise. Above 3,100 RPM's, the engines generate significantly more
noise.

Is a 4 blade prop an option? More cupping on my current prop? More
diameter? Other?

Thanks!



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K. Smith
 
Posts: n/a
Default Props - RPM's and Cruising Speed

wrote:
I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards.
Currently, the boat cruises at about 20kts at 3,100 RPM's. At this
speed, I get reasonable fuel economy, because it's not until about
3,200 RPM's that the secondaries join the party and start tapping the
keg, if you will.

Max RPM on the boat is 4,100, at a speed of around 30kts. I very,
very rarely operate at this speed, and when I do, it's usually for
less than a minute.

I'm wet behind the ears when it comes to propping, so a question for
those in the know:

At 20kts, the boat requires slight tabbing, so it's not quite 100% on
plane. If I bump the RPM's up to 3,200-3,300, the boat cruises at
about 24kts, and will plane without tabs. The first comment will
likely be that operating on a full plane is more fuel efficient, and I
would normally concur, but it's not the case with this boat. I've
done the calculations, and it's more efficient for me to keep out of
the 4 barrels.

My current props are 3-blade. Would it be possible to prop the boat
so that the cruise speed at 3,000 RPM's was bumped up a few knots? My
thinking is that if I can get on full plane while keeping out of the
secondaries, I'll enjoy the best of both worlds, higher cruising speed
while maintaining fuel economy (if that's really such a thing on a
boat). 8)

I'm not concerned with losing a couple of hundred RPM's on the top, as
I simply don't operate the boat in that range. Another benefit is
noise. Above 3,100 RPM's, the engines generate significantly more
noise.

Is a 4 blade prop an option? More cupping on my current prop? More
diameter? Other?

Thanks!


The torque required by a prop as it's revs rise goes up as an ever
steepening curve.

The torque available from most petrol engines is not a rising curve as
the engine's revs rise, mostly max torque occurs around 2/3 of max revs.

You'll notice indeed it seems you may have already that once the boat
is up the speed, clean bottom etc etc the engine is loafing when at
cruise revs, i.e. the prop is consuming much less torque than the engine
is capable of producing at those revs.

Throttle opening (MAP) is an excellent indicator of torque & most
petrol boat engines on a fixed pitch prop, have much less throttle
opening than the revs would indicate. i.e. you cruise around 75% of max
revs but invariably you are only using 50% of available power. The
throttle is only 1/2 open; mark it & check)

Big ships & some small:-) can make great gains by using a controllable
pitch prop, even the same throttle setting can use a different pitch up
or down wind, but the engine can stay at it's most efficient speed. They
can maximise the engines efficiency, say at cruise, the revs would be
much lower so to the fuel consumption & engine/pumps etc etc wear &
tear, by increasing the pitch at that mid power high availabe torque
band to take advantage of the available torque.

You can carry more pitch at cruise revs as you suggest & yes it will
provide a more efficient cruise, fuel noise & most otherwise, however;
without a controllable pitch prop, you will not be able to rev the
engines out to max revs because the extra pitch will prevent it, just
like trying to go up a hill in top gear.

"If" you are careful & aware of the issues there's nothing you stop you
increasing the pitch, just make sure if you do give the engines WOT
(wide open throttle) you certainly don't leave them there for more than
a very short time (say to get up on the plane) & once you settle to your
cruise speed make sure that any increase in thorttle opening "Will"
increase the revs. I good way to drive is to use max throttle, which
will be less revs AND HP than you have now, so make sure it will still
get onto the plane OK??; then reduce throttle till you know the revs
have dropped off a bit & you shouldn't have more throttle setting (map)
than the revs indicate; i.e. 1/2 = 1/2 or whatever.

I have a diesel engine (toyota based) hooked to a controllable pitch
propeller, the pitch is semi automatically adjusted according to the
throttle setting/position, so at very low revs it uses lots of pitch &
you get a very low rev quiet trolling speed. At 1/2 throttle (cruise)
the control makes sure the pitch is reduced so the engine can achieve
1/2 revs, again very quiet & very fuel efficient, then as the throttle
is opened further the pitch is progressively reduced so the engine can
make max revs & max HP (very rarely used as you say).

So if you want, increase the prop pitch further but drive it
accordingly, just as you wouldn't leave your foot on the floor in your
truck up a long hill in top gear, however, we all sometimes give the
engine max throttle for short periods even when it can't rev out. You
will have longer lasting engines because it's as if you have lower HP
engines at WOT.

K

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Lloyd Sumpter
 
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Default Props - RPM's and Cruising Speed

On Fri, 28 May 2004 13:54:40 +0000, marbisiNsP wrote:

I have a 30' Sea Ray Weekender with straight, 260HP, 5.7L inboards. Currently,
the boat cruises at about 20kts at 3,100 RPM's. At this speed, I get reasonable
fuel economy, because it's not until about 3,200 RPM's that the secondaries join
the party and start tapping the keg, if you will.


Hi,
Just thought of something as I was going over the Coquihalla Hy and slipping
in and out of overdrive...

First, I hear you about the noise. I like running my Campion at around 3K
partly because of the noise. Sorry, can't help you there. But...

The secondaries really open because of THROTTLE setting, not RPM, correct? So
how about this: DECREASE your prop pitch. Then you're running at a higher rpm,
where the engine puts out more power at lower throttle setting. I suspect you'll
be able to get that few more knots to get fully on plane at a lower throttle
setting than with the "steeper" prop.

Worth a try...

Lloyd Sumpter
"Far Cove" Catalina 36


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