
Outboard jet propulsion
systems
Outboard jets work best on flat-bottomed boats
with a bottom width of 48 inches or more. The shape of
the hull cannot be easily defined.
When it comes to bottom width, the wider the
bottom, the more water it displaces. Because a shallow
draft is important, the wider the boat, the better the
draft. A wider hull sits higher in the water, so it
exhibits less drag. You would conclude wider is
better.
However, is not entirely true, in the sense that
the shape of the hull helps direct the flow of water.
Flat hulls tend to cause more cavitation. A deadrise hull
manoeuvres better and cavitates less than a flat-bottomed
boat.
Many aluminum hull manufacturers have introduced
a whole new line of extra-wide modified john boats that
offer a consistent line of 4 to 6 degrees of deadrise
from bow to transom. It is important that the deadrise
falls all along the hull because the "V" provides a
smoother ride and splits off air-filled water before it
reaches the jet intake.
Eliminating
cavitation
All this information is great if you have not
selected your outboard jet hull yet. But how do those who
already have a jetboat handle cavitation? As outboard jet
standards go, all the hull types cavitate under extreme
conditions. However, cavitation under normal running
conditions should not be tolerated, and should be
eliminated to maintain the life of your
outboard.
If your outboard jet is cavitating, the jet pump
is pulling more air than water. It will not allow the
pump to produce adequate water pressure to bring the boat
on plane. The engine works hard and erratically and this
could cause damage if let go for too long. This type of
cavitation is likely to be a result of improper mounting
height of the outboard or the hull bottom characteristics
causing a poor flow of clean water to the jet intake.
There are several solutions to enhance
performance.
Install intake fins
Some companies offer an intake fin kit as an
accessory designed to reduce cavitation. This kit is easy
to install and offers the advantage of directing a larger
volume of water into the jet intake. The increased volume
of water reduces the amount of ingested air, allowing the
pump to operate efficiently. The added set of fins even
enhances the grip in manoeuvring and assists in reduced
cavitation in tight turns and rough water.
Transom jacks
Transom jacks can help. They come in a variety
of sizes and styles from manual adjustable to powered
hydraulic models. They are easy to install, but you may
have trouble moving the outboard unit off the transom.
Your local marine dealer can install a transom jack for
you with very little effort and take any concern of
managing the 200-pound weight of the outboard.
Beyond the ability to adjust the height on the
fly, if needed, the transom jack extends the intake foot
about six to eight inches farther back from the transom.
The water flow there is more likely to be clean,
providing better intake efficiency. Many of today's
fiberglass bass boat hulls are designed with a step
transom that places the engine farther out to take
advantage of the power increase that results in faster
lift.
On the negative side, the transom now acts more
as a fulcrum because the outboard weight is shifted
farther back. This might require minor adjustments to
your payload placement. This mounting placement also adds
more torque to the transom.
Even though modern hull transoms offer multiple
knee braces for additional support, be sure to check with
the hull manufacturer first, before adding a transom jack
mount. A transom saver, which is not often associated
with an outboard jet, may also be a smart choice if you
travel over uneven ground on land.
The jet intake requires a clean flow of water
free of air bubbles to allow the pump to operate at peak
efficiency. Flat-bottom john boats tend to carry these
air bubbles straight back into the intake. Keels can also
aid in increasing this effect because they introduce even
more air into the intake. The center keel can be removed
two to three feet from the transom forward. The goal is
to produce an apron of clean, air-free water just wider
than the intake.
Doing this requires some special equipment and
no doubt renders most, if not all, hull warranties null
and void.
Reducing excessive
spray
Spray is a problem that can plague outboard
jet-powered hulls. Water deflects off the transom and the
intake foot below the bell housing, and sprays into the
back of the boat. While annoying, it can also cause some
cavitation, and should be resolved.
Even though for some the idea of adding a spray
deflection plate to the transom is not appealing, I have
seen some splash plates mounted higher and directly to
the outboard. This keeps the water from spraying into the
boat, but it doesn't do much to direct water or prevent
any cavitation. However, it is an option. It is also
worth mentioning that some hulls made exclusively for jet
outboards even have a spray deflection plate welded to
the bottom of the transom.
The stock impeller on all outboard jets is a
lightweight three-blade design that is easy to machine
because it is made of aluminum. Because it's aluminum, it
is soft and prone to dulling and nicks. After time, this
wear can cause significant loss in performance. Proper
maintenance requires that the impeller be removed,
sharpened, and have a shim(s) removed to ensure that the
proper clearance between the impeller blades and intake
housing is met.
A newer design is a stainless steel impeller
that offers four thinner but stronger blades. This design
feature enhances payload capabilities in two ways. First,
the area is increased because of the extra
blade.
This added area increases the traction and
improves the water flow to the pump. Second, the blades
are thinner, so they cut through the water with less
resistance, even though there is more blade area. A
stainless steel impeller also features better
tolerances.

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