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The Smoother The Ride, The Better
by
Steve Horsfall
Ask The Expert The Smoother The Ride, The
Better I would like to know if motor height adjustment would help
with reducing porpoising. I am running a Honda 70 hp on a 1860 Crestliner
jon boat. I have the pin closest to the boat and while at high speeds
I have to keep the motor trimmed all the way down. Any upward adjustment
and the boat starts to bounce. I thought maybe I would just need
to place some sand bags under the front deck to solve this.
Right now, the motor is mounted in the third
hole from the top. Can I improve performance by re-mounting my motor
up or down? This is a great rig for hunting and fishing. Speed was
never my intent, but a smooth ride is.
The boat is equipped with adjustable 1-1/2-inch
trim tabs at the bottom of the transom and I am testing their effectiveness,
but so far, I am not able to improve the ride.
M. Ruhlenhak
e-mail
Moving your engine down may help the bouncing problem
somewhat, but I suspect the real problem is more of a built-in feature
of the jon boat. Most jon boats are designed to plane off quickly
with smaller engines and are not really designed for high speed.
They are more of a work-type boat designed to carry heavy loads
in shallow water. Trim tabs will help, but they will need to be
larger than 1-1/2 inches to be really effective. Extra weight in
the bow and/or less weight in the stern are the simplest solutions.
PROP DECISION
I have a 1995 320 Baja with twin 454 MerCruisers that deliver
365 hp per engine. The boat includes a sink, stove, fridge, TV and
weighs approximately 7200 pounds - less fuel. I am currently running
23-inch pitch Mirage props which the boat originally came with.
I am turning out 5100 to 5200 rpm and topping out at 68 to 72 mph,
depending on the chop. I added Drew steering stabilizers and wanted
to add four-bladed props to improve steering and stability. However,
I was told I will lose some top end with four-bladed props, which
is okay because I want stability over speed.
I was looking into the Mercury Bravo One, 4 blade
props but was told to go to a 22-inch pitch. One of my buddies mentioned
not to go with the Bravos, but go with an Intimidator four-bladed
cleaver because that is what he runs on his single screw. What do
you suggest?
S. Dodol
e-mail
In my opinion, I would go with the Bravo One, 4
blade props. However, one thing you should be aware of is they are
difficult to shed weeds from. You won't have to worry about this
at high speeds, but if you cruise in a weedy area, you may find
it difficult to clear the weeds before you take off. Otherwise,
go for it. (Top)
DETERMINING GEAR RATIOS
I need to determine gear ratios on a couple of MerCruiser sterndrive
upper gear sets. One is of a 1MR upper drive. The drive gear has
17 teeth and the pinion gear has 19 teeth. The other is a 1987 Alpha
One and the drive gear has 20 teeth and the pinion gear has 24 teeth.
G. Patterson
e-mail
The 17-19 upper gear set is used in the 1.84 ratio
drive and the 20-24 upper gear set is used in the 1.98 ratio drive.
ENGINE PLACEMENT FOR INCREASED PERFORMANCE
I have an 18-foot Lowe tunnel boat with a 48 hp SPL Evinrude
outboard. I purchased the boat used and noticed near top end speed,
or even during initial throttle-up on rough water, the prop can't
achieve a good bite. I am using a new factory aluminum prop and
the motor is at its lowest possible point without a jack plate.
Where should the motor be positioned (cavitation
plate), in relation to the bottom of the tunnel?
G. Carty
e-mail
Engine height and propeller selection for a tunnel
boat is always a bit tricky. Racing tunnel boats (using racing propellers)
usually run with the prop shaft at center-line, even with the bottom
of the tunnel sponsons. The propellers are designed to run with
about half of the prop out of the water during high speeds. Short-shaft
engines are often used to obtain this high propeller placement.
In pleasure craft tunnel hulls, the cavitation
plate should be about even with the bottom of the sponsons or slightly
higher. This will help cut down on cavitation with standard propellers.
However, a cleaver-style stainless steel prop, along with raising
the engine height, is the best thing you can do to make your tunnel
boat really perform. Bear in mind, this will not be good for water
skiing, etc.
(Send
your questions to: stephen.horsfall@sympatico.ca) (Top)
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