|
PROPPING UP PERFORMANCE
Any owner interested in improving his vessel's fuel
efficiency and performance instinctively focuses on the prop - but
experts warn it's only part of a much bigger equation.
Auckland-based Henleys Propellers and Marine Ltd
is New Zealand's largest manufacturer of custom-built propellers, and
managing director Mark Power says to many owners are seeking to boost
boat performance fall into the "new-prop-quick fix" trap.
"The prop's only small part of the overall
drive-train and hull equation. A lot of our work involves vessels that
are being re-powered, and it's an area where misconceptions about props
arise most often."
A variety of factors, he says, influence a boat's
performance and efficiency. They include hull shape, displacement,
fore/aft trim, the position of the prop(s) relative to the hull, and the
angle of thrust. "If one or more of these factors is 'out of kilter'
with the overall package, the best prop in the world won't make a blind
bit of difference."
"A vessel's performance is locked into the
vessel's design and final form - a propeller is not a magic wand!"
says Mark Power.
For that reason, Henleys collates and analyses as
much information as possible about a boat design before manufacturing a
new prop. "In the launch repowering situation, I'm always amazed by
owners who are happy to fork out $20,000 plus on a new engine, without
doing any homework about whether it will actually deliver the desired
performance.
"In the worst case scenarios, it's not even clear
if the new engine will fit into the hull and align itself to the
existing shaft. Generally, few consider that they might have to upgrade
the entire drive-train to accomplish the desired performance. That might
include altering engine beds and mounts, larger diameter prop shafts or
material upgrades, a gearbox with a different reduction ratio,
adjustments to stern tubes, moving struts and even rudders."
Remember, he adds, that the optimum distance
between hull and the tip of the prop is 20 percent of the prop diameter.
If a bigger engine demands a larger prop, the prop shaft angle might
have to be increased to meet that requirement of the strut moved aft.
Other adjustment factors to consider are engine room ventilation,
exhaust size, fuel systems and position of fuel tanks - to ensure the
new engine doesn't adversely affect the boat's trim.
Power prefers to tackle prop solutions from a
different perspective. Boat data (for a new design or a re-power
project) is fed into Henleys "hull dynamics" software. The information
includes the designer's or owner's objectives - for example - fuel
efficiency, low revs at cruise speed, or a high top end speed.
Computer analysis quickly highlights any
potential problems. "Only when any issues have been resolved - and they
might include some or all of the 'reconstruction' requirements - can we
consider the appropriate, optimum prop design."
"In our experience over 98 percent of problems
realised on sea trials are the direct result of incorrectly supplied or
installed data, engines, gearboxes, drive shafts, stern gear, and
propellers" - Mark Power.
Outboard props
Henleys doesn't do much work with outboard engine
props, and Power says the outboard brand managers understand their
"craft" pretty well and provide good advice for prop selection. "Our
typical outboard enquiry is when someone has a boat that's a little
heavy for its outboard and the owner wants to pull up two skiers or he
wants to improve the current performance of the standard propeller
supplied - for example with Thunder cats, Surf rescue, race craft. We'll
look at different prop options and standards of tolerance."
He says the easiest, most cost-effective option
for trailer boat owners with different operating requirements (say
fishing as well as skiing) is to have different props, or to fit one of
the composite props with adjustable blades.
That said, if a trailer boat owner suspects his
boat/engine's performance is lacking, there are a number of tell-tale
signs to watch for: "The number one clue that all is not right is if the
engine is revving too high or too low - beyond or below the engine
manufacturer's specifications."
To check the boat's rev range, "the owner should
set the boat up as he would typically have it loaded (say three men,
fishing gear, full fuel tanks, etc.) and take it for a run on a flattish
day. With the throttle(s) all the way home, the revs should be within
the manufacturer's specs listed in the owner's manual. If they're higher
or lower, he probably needs a prop change."
Vibration is another clue that something is wrong.
The prop could be out of balance if a blade's been nicked on a rock -
and will need to be repaired and rebalanced. Out of balance props,
warns Power, quickly affect the engine's lower gear train, and could
result in bearing failure. "And be warned - the very high standard of
outboard engineering has tended to hide the problems associated with
damaged or worn propellers from the end user."
On twin-engine installations is it better to have
the counter-rotating props turning inwards or outwards? "It's not as
important with stern legs and outboards because the prop is behind the
boat," says Power. But on launches we opt for outward-rotating
configurations. The boat exhibits better manoeuvring and tracks better.
Our research shows inward-turning props create "dirty" or aerated water
under the boat, and that can affect boat trim - in most cases boat
resistance goes up, fuel burn goes up and the speed drops."
Does galvanic reaction affect propellers and stern
gear?
"Absolutely. All propellers (including stainless
steel) and stern gear are susceptible. The owner must ensure his bonding
system is adequate for total protection." |
