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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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