BoatBanter.com - View Single Post - Props

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