Shown are the components of the Stern Jack kit. Some outside labor may be
necessary to have the power trim cylinder extension welded in place. Land &
Sea's variable-pitch Torque Shift prop (right) offers top speeds approaching
the finest stainless fixed-pitch props, and a holeshot that can't be beat.
Land & Sea produces unique products designed to make a boat go faster. Two
of these have some very wide ranging applications. The Torque Shift prop was
the first successful shifting prop on the market. It is one of the fastest
props and still compares in top-end speed with some of the finest fixed-pitch
props available. Nothing comes close to the Torque Shift prop when it comes to
low-end grunt. The Torque Shift prop fits a variety of sportboat applications,
adapting to most V-6 outboards and most MerCruisers, SX Volvo Pentas and OMC
stern-drives.
Less well known but an equally effective performance-enhancing device is
the Stern Jack, an 8-inch extension that goes between the gimbal housing and
upper gearcase on a sterndrive. The effect of this extension is to move the
sterndrive leg back and extend the lever arm of the drive over the trim
range. Trimmed down, the unit sits an inch lower in the water for a better
low-end bite. Trimmed up, the unit not only trims 1 1/2-inches higher at full
trim, but the setback allows the prop to operate in less turbulent water, and
the trim system has additional leverage to lift the hull.
In some high-performance applications, the Stern Jack alone is worth an
additional 3 to 5 mph in top speed and holeshot is also improved. We didn't
get that much with the four-cylinder engine and we didn't expect to. The
performance increase, nonetheless, was still satisfying. The Stern Jack is
available only for MerCruiser Alpha and Bravo sterndrives.
Our purpose here is to show what can be done with a modestly powered boat
to improve both its ski towing ability and top speed. The boat we used is a
Stingray 180 powered with a 3.0L four-cylinder MerCruiser with an Alpha I
drive.
The improvements we made do not come at a low price. The Torque Shift
prop retails for $799 and the Stern Jack for the Alpha drive is $1995. The
Bravo drive Stern Jack is $2495.
This brings us to a very important point. If you need more speed and
power for towing heavy skiers, moving heavy loads or simply because you want
to go faster, the least expensive way to increase performance is to buy a
larger engine when you buy the boat. Adding performance after the fact is
costly.
The Stingray 180 was run with full fuel and two people with test gear
(approximately 400 pounds), and the engine was equipped with power steering.
PROP TESTING: ROUND ONE
Our initial test with the Stingray was with a stock 23-inch-pitch MerCruiser
aluminum prop. This prop produced a top speed of 47.7 mph at 4400 rpm and
acceleration from 0 to 30 mph took 9.9 seconds. Acceleration was definitely
on the sluggish side and the performance of the prop was less than
exceptional. It had a tendency to ventilate easily in turns and release
suddenly when over trimmed. The next prop tried was a 21-inch-pitch Mercury
aluminum. This prop topped out at 47.5 mph, but rpm was over the 4500 rpm
recommended limit by 200 rpm.
Acceleration was much improved with a 0-to-30 mph time of 7.5 seconds.
The 21-inch was a much better prop in its ability to hold in turns and when
trimmed. There was far less ventilation.
PROP TESTING: ROUND TWO
The Torque Shift prop comes in several variations. The first we tried was
Part No.144-126. This prop starts out with a low pitch of 11 inches and its
top pitch range is adjustable from 19 to 26 inches. With the top end pitch
set at approximately 23 inches, we recorded a top speed of 50 mph.
This prop did not do well on hard acceleration. Nailing the throttle
produced excessive slippage (ventilation), and 0-to-30 mph times went up
to 7.5 seconds. Feathering the throttle off the line allowed the prop to
bite better and we whittled the 0-to-30 mph time down to 7.2 seconds, a
noticeably stronger kick than either of the fixed-pitch props.
We then tried Torque Shift prop Part No.144-132. This prop has a low
pitch of 14 inches and the high-pitch position is adjustable from 23 to
32 inches. Adjusted to hold at approximately 23 inches at the high-pitch
position, we recorded an identical top speed of 50 mph at 4600 rpm.
Nailing the throttle with this prop did not cause either the prop to break
loose or excessive ventilation, and our 0-to-30 mph times dropped to 7.0
seconds.
With our initial tests complete, it was back to the shop for the
installation of the Stern Jack. Installation is not difficult, but it does
require some special tools that a good mechanic will have but will probably
not be found in the tool kit of the average do-it-yourselfer. These tools
are the U-joint press and a stud-removing tool. In addition, it will be
necessary to modify the power trim rams with extension kits. This can be
done at your local welding shop, by Land & Sea, or through an exchange
program with Land & Sea, depending on the condition of your rams. The kit
is complete with all necessary parts including studs, seals and gaskets,
drive shaft extension, shift shaft extension and extensions for the power
trim rams. The kit also includes Loctite, Never Seize and a check valve for
the sterndrive oil reservoir. The instructions are very detailed.
With the Stern Jack installed, a process that requires approximately
21/2 hours once the modified components are assembled, it was back into the
water.
PROP TESTING: ROUND THREE
Our first test in the second phase was again with the 23-inch aluminum prop.
This time we recorded a 51.3 mph top speed at 4700 rpm for a 2.6 mph
improvement over our initial run and a gain of 100 rpm on the tachometer.
Acceleration times from 0 to 30 mph went down slightly from 9.9 seconds to
9.8 seconds. Prop ventilation was still a problem and it was obvious that
this prop was not taking maximum advantage of the Stern Jack or the Stingray
hull, which is one of the few 18 footers capable of breaking the 50 mph
barrier with the 3.0L engine.
The 21-inch prop, while not as fast on the top end, showed a greater speed
improvement with a top speed of 49.8 mph with the engine cranking along at
5100 rpm. Acceleration improved slightly with the Stern Jack going from 7.6
seconds to 7.4 seconds. The 21-inch prop was much less prone to ventilate and
a better performing prop overall but with its tendency to let the engine over
rev, it is still not a suitable prop for this outfit.
Back to the 144-126 Torque Shift prop which was still adjusted for
approximately 23 inches in pitch. Top speed went to 51.8 mph at 4650 rpm. With
the 12-inch initial pitch, we expected some of the same ventilation problem we
experienced the first time around. It is amazing what an additional inch of
depth and 8 more inches of setback will do. There was a trace of ventilation
on initial acceleration, but it was considerably less than first experienced
and 0-to-30 acceleration times dropped to 6.7 seconds.
The 144-132 prop, adjusted to 23 inches of maximum pitch, produced a top
speed of 52.0 mph at 4700 rpm at its minimum setting. With the initial pitch
of 14 inches, acceleration from 0 to 30 mph fell off slightly to 6.8 seconds.
Again there were no ventilation problems with this prop on initial
acceleration; at top speed, we could use just a tad more positive trim.
EVALUATING THE GAINS
There is little doubt that the Torque Shift prop and Stern Jack are effective
performance enhancing devices. With the two units, we were able to raise top
speed from 47.7 mph to 52.0 mph. a 4.3 mph gain. At the same time we took our
0-to-30 mph time from a very sluggish 9.9 seconds to a snappy 6.8 seconds. We
could have done slightly better, had we been willing to give up a bit in top
speed. At these performance levels, it is a tradeoff not worth making.
From a performance gain per dollar spent, the Torque Shift prop is the
hands-down winner. The prop alone was responsible for a top-speed gain of 2.3
mph and a reduction of 0-to-30-acceleration times of 2.9 seconds.
The Stern Jack added another 2 mph to the top speed and shaved an additional
0.2 of a second off of the 0-to-30 mph acceleration time. This is respectable
performance for an add on, but the Stem Jack. being a more costly piece of
equipment. was less cost-effective in the over-all project.
It is important to remember that not every boat will respond to these
modifications in exactly the same way. Some will do better and some won't fare
as well. I have yet to experience a boat where there were not at least some
gains with the addition of a Stern Jack. Over the years, Land & Sea has
compiled a large database on the performance gains of a wide variety of boats
equipped with the Stern Jack. Any interested party should contact Land & Sea
for recommendations.
