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Photos
& story by Tim Findley
Older
fisher folk may recall a time when this column was a regular
feature in Seafood NZ and before that, Professional Fishing.
Month
after month, year after year I'd scribble away telling
tall tales, mostly with a marine engineering flavour. Because
that's my only claim to fame, small ship propulsion problems.
Anyway, the day came when quota was cut.
In
the meantime, stocks have recovered. Careful harvesting is
called for. Here's the first catch landed.
It's
a new gearbox, it's made by Twin Disc and it's called the
Quickshift.
Here's
how it's different and why it will become the box of choice for
fast craft in the future.
Propeller
calculation breaks into three main areas. One is blade pressure
which determines blade area. Next is diameter which is
determined by propeller tip speed. Lastly, there's pitch and
that is determined by the expected speed of the vessel.
There's
a few variable and other considerations but basically that's it.
Seventh form mathematics allow anyone to solve the sums and
bingo, the result is the prop size.
So
when do we get the gearbox? Soon. First we need to know what the
problem is that new development will solve.
As
more marinas and waterways are created there's much more
emphasis on controlled areas. For instance, everybody knows to
keep below 5 knots in designated channels. Last America's Cup,
Ports of Auckland declared half the Hauraki Gulf a 'go slow'
zone at certain times.
The
Coromandel's Pauanui and Whitianga canal developments impose a 3
knot restriction, as does Gulf Harbour's residential waterway.
There's many other examples. Point is, planing hull boats often
can't go slow enough to comply.
Why
not? Easy. The propeller pitch is so coarse, that even at idle
the boats moves at more than 5 knots.
Can't
slow the engine down any more? No, it will stall. Well, take
some pitch off the prop, that will fix it. Yes it does but the
boat now not only loses idle speed but cruising and top speed as
well.
Besides
which if one has just paid for 600 horsepower it's disappointing
to only be able to use 500hp. In fact one might as well have
bought a 500hp engine because with the lesser pitched prop it'll
do the same job for a lot less capital outlay.
Surely
there's another way around it. Not being able to go slow enough
is absurd. No it's not. It is a recent problem though. Who could
benefit? Aquaculturists have need of a crew boat that will
commute quickly yet manoeuvre with great precision. Quick crayboat operators can empathise with this as can those
spectator chartering for aquatic event like the World's Etchells
and America's Cup.
The
ability to manoeuvre with precision has weekend sailors
undivided attention. Next time there's no Sunday afternoon
footy, try the local marine for an afternoon's amusement.
Parking
modern GRP planing hull launches in a crosswind is problematic
for professionals. Watch the weekend wonders. There's always a
shouting match, a divorce or two, and with any luck, full blown
fisticuffs!
A
cruel but true summation. Chandleries sell expensive bow
thrusters by the dozen. They help but are not the answer. This
new box is. Well, the new box won't park the boat but it will
allow a competent skipper to do so.
Here's
how. Sit in an automatic car. Start up and shift and park to
drive, The car starts to creep forward.
Twin
Disc have extended this feature to their Quickshift. Start the
engine, idle it in neutral. Push the single lever control ahead
and the prop starts to turn.
But
there's no bang, no Niagara Falls of water, the boat doesn't
jerk ahead. The prop is turning so slowly the boat creeps ahead
in the same manner as our automatic car example.
Go
into reverse. Same thing. No jump, no bang, no swirl of water.
Move the controls a little further and the prop speeds up, move
them back and it slows down. Smooth as silk.
Here's
a practical example. Let's look at one of the aluminium Aussie
imports that are all the go these days. 15 metres long, 580hp at
2300 turning a 2 to1 gearbox. The boat achieve 20 knots courtesy
of a 32 Henley Tiger, pitched to 26. Everything's a beaut.
Excellent horsepower per tonne efficiency. Good boat, works
well.
Idle
speed is just on 650 revs which translate to 325 prop shaft
revs. That's 5 ½ turns per second!
The
Tigers huge 26 inch bite equates to moving the boat 3.6 metres
each tick of the clock. Sure, prop slip accounts for some of
this theoretical advance but not much with these highly
efficient swept blade Tiger props.
The
'jump' one gets when engaging gear has been known to pitch
unwary deckies into the tide.
Same
boat, same engine, same prop and crew. Change gearbox to
Quickshift - engage ahead. Prop starts to turn 50 revs per
minute. That's less than a turn a second. The theoretical
advance is only 600 millimeters, less slip. the benefits are
obvious.
Surely
this 'automatic car' technology has been round long enough for
it to have been thought to before? Yes it has, but automatic
cars aren't as fuel efficient as manuals. Why?
Because
in a manual gearbox, the drive is connected directly except when
the clutch is used. Automatic are indirect allowing slippage.
In
a conventional marine gearbox (see cut away photo below) the
drive is connected directly to the propeller. Neutral is
achieved by a multi-plate clutch pack.
To
engage ahead or astern, pressurised oil is directed into the
appropriate pack, energising it and starting the prop in the
blink of an eye.
There's
no 'slipping clutch' effect' - it's either in gear or in
neutral, going or stopped. It's this feature that makes
conventional marine hydraulic boxes fuel-efficient. The only
mechanical loss aside from the gears themselves is that
necessary to power the small oil pump that provides pressure to
close the clutch.
Clutch
closed, the prop shaft is direct coupled as in our manual car
example. To achieve this direct couple effect and yet enjoy the
indirect advantage for low speed, Twin Disc use hydraulics to
slowly spin the prop as ahead or astern is engaged.
Push
the control ahead the prop spins faster. A bit more and faster
it goes until it equates with the engine idle speed and gearbox
reduction. At this point Quickshift performs like a conventional
box and locks up in the fuel saving, direct drive mode.
It's
been a long, long time since there's been any meaningful advance
in small ship transmission. Diesel engines have gone electronic
and are festooned with go-quick of fuel economising gadgets.
It's about time the gearbox guys got on with it.
For
my money Twin Disc's opposition better move fast, or they won't
be.
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