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Short Wave Sportfishing[_2_]

On Fri, 5 Sep 2008 09:04:35 -0700 (PDT),
wrote:

Are they simply to get the engine higher? If so, why not just mount
it higher?

It's a little more complicated than just setting the engine higher (or
lower for that matter) but that is a reason.

Jack plates come in two flavors - fixed mechanical and movable
hydraulic. The fixed mechanical type is used exactly for the reason
you mentioned - to set the engine either higher or lower depending on
needs.

For example, my Ranger bay boat is a 'tweener. The specs call for a
20" shaft, but due to the design of the hull, 20" shaft isn't exactly
correct 22" would be more sufficient. When I had a 20" shaft FICHT on
it, the boat ran fine on mostly smooth to very light chop, but any
thing heavier created a situation where cavitation was a problem. Also
because it was a 'tweener, the boat had zero bow lift when coming up
on plane - it would hit around 3,000 rpm and accelerate to 4,000 in a
blink of an eye - the boat literally leapt out of the water coming up
on plane.

I had thought of using a jack plate on the FICHT, but it didn't bother
me much and as I had two other boats at the time, it wasn't worth
getting into.

When I repowered, I decided to go with a 25" shaft ETEC and just
adjust out the extra two inches by mounting it a bit higher. That
didn't work - now I had the opposite problem - too much shaft in the
water instead of too little. It sounds odd that height placement was
that critical, but on this boat, it is - don't ask me why. So a
mechanical jackplate was installed and I got the engine exactly where
I wanted it to be - end of problems.

One side effect of using a jack plate is engine set back. The axiom is
that for the same horsepower and hull configuration, a longer boat
will also be slightly quicker, a little more efficient and much better
trim control - all of which are true. You can get them in a variety
of setbacks, but the most common are 3 and 6 inches.

Hydraulic jack plates serve the same function as mechanical jack
plates, but are dynamic - you can adjust for differing conditions like
sea state, shallow water running, weather - etc.

With respect to adjusting height by modifying the transom, certainly
you can do that, but it's costly and you can achieve the same effect
cheaper with better results.