Jack plates, yes or no?

Good question. I don't know enough about the use of jackplates to
comment, yet. Do they let you run shallower? Do they make you run
faster? Do they improve handling, holeshot? Does the extra load and
weight being pushed back behind the transom affect draft?
How should a motor be set up on one? Procedure for adjusting?
Hydraulic, electric or manual? I'm going to be digging all over the
web pulling up websites and reading all I can find. Those of you that
run them on your boat...your comments would be appreciated.
What I find I'll post.



One use that requires a jack plate is the installation of a jet outboard.
It's the only way to fine tune the height of the leading edge of the
intake to where it won't ventilate. You have to be able to adjust to the
last 1/8th of an inch to get the pump to run it's best, without throwing
spray back over the top of the transom and filling your hull.
For a small jet outboard a minimal setback manual jack plate is all
that's needed. Once set, no further adjustment is necessary.


http://www.sportfishingmag.com/techniques/boatinghow-to/the-skinny-on-jack-plates

I have a problem with that statement. After all the research I've done
on pocket tunnels, the amount of rise of the water off the bottom of
the transom at planing speeds, is not that much. I call BS on that quote.

No BS here, just basic set up info:

http://www.bogerprops.com/Jackplate.html

http://www.tropicalboating.com/powerboating/hydraulic-jack-plates.html

http://www.bobsmachine.com/FAQ.shtml

http://answers.yahoo.com/question/index?qid=20081215002805AAwsXG9

http://www.saltwatersportsman.com/article/Gear-and-Techniques/Jacked-Up

http://www.jack-plate.com/jackplate-information/

http://books.google.com/books?id=1-QONfkgc78C&pg=PA218&lpg=PA218&dq=outboard+jack+plate&source=bl&ots=cBRD1YeZE7&sig=fXm-qUulrY4NJbAiaeX0JhIzLsQ&hl=en&ei=RIKoSezcJKSwmQek27HxDQ&sa=X&oi=book_result&resnum=3&ct=result

http://www.marine-maintenance.com/webpages/noseconehieght.html

http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html

Because a standard template is used to drill the bolt hole pattern
through the transom, a jack plate would allow correcting the height
of the engine installation to the true shaft lengths.

The basics of jack plates as read are these:

Used to adjust the height of the motor to it's highest running position.
The smaller the amount of your lower unit in the water, the less friction/drag is produced.
Less drag means better speed and fuel economy.
Moving the prop further back from the transom allows time for air bubbles to rise up.
That means cleaner water for the prop to bite into.
At idle speeds the prop and skeg are higher up than on a standard transom mount, run shallower at idle speeds.
Does add weight to the transom, which means slightly more draft.
Increases static rotational torque to the transom, if you don't have a strong hull, don't use a jack plate.
The maximum height of lift is limited by the location of the water intake and the type of propeller.
Conclusion: If you're looking for the absolute best performance from your outboard, get a jackplate.
If you just want to go fishing, they're not necessary.

Trim tabs: Any hull large enough to run open water safely,
will benefit from the use of remotely adjustable trim tabs.
Trim tabs allow you to adjust the way the hull runs, fore to aft,
port to starboard, to fit sea conditions and vessel load, while under way.



A very nice setup:

http://www.microskiff.com/cgi-bin/yabb2/YaBB.pl?num=1192143821

 

Agreed, load is load, what should have been said was:
The greater the setback, the greater the static rotational torque.
The basic concept of leverage: Force multiplied by Distance.
Transom needs to be strong enough to resist the increased torque.
The shift in the center of gravity of the hull would not be much.
Incorrect statement removed. Thanks for the heads up Beavis.

 

Yeah, but wrong is wrong. The idea is to have the correct info shown.
So anyone deciding whether or not to install a jackplate,
makes the decision based on the best info available.
I was finding stuff till late last night, and reading that one this morning,
after you pointed it out, was definately mathmatically incorrect.
The sum of the down forces is the hull load.
Rotational forces at the transom only apply to the transom.

Bad info removed.  

 

Verifying what I read through scale drawings,
came up with these images which explain the advantages of a jack plate...

Note: The water level off the bottom of the transom
  is extended past the propeller to allow labeling.
Not to indicate that it remains at that level!!!!



Normal installation of outboard,
cavitation plate 1/4 inch below bottom of hull
when hull setting level on trailer




Normal installation with hull on plane,
cavitation plate is actually further below the surface of the water
because of the hull running angle.




With a jack plate, the motor is installed so that the top of the water intake
is just below the bottom of the hull with the hull sitting level on the trailer.




But with the hull at running angle there is room for more adjustment upwards,
caused by the difference in trimming the motor out to compensate for the angle change.




At best adjustment the tips of the prop and the top of the intake
are just below the surface of the water as it comes off the transom.






At idle speeds, with a hydraulic or electric jack plate, the engine can be elevated to a point
where the intake is just below the natural water level allowing the engine to be run in shallow water.

 

As stated at the beginning of this thread, I don't know enough
about jack plates to be considered an expert. The images were
posted to illustrate the advantages of a jack plate.
Water displaced by a hull, returns to its natural level
after the hull moves on. I don't know the speed at which
water returns back to it's natural level. But looking at
the bottom of the transom, while a hull accelerates forward
shows that it does take a certain amount of time for the
water level to "bounce back". The faster the hull is moving
the further back the return point is. At speeds over 20 mph,
in a small boat, that distance is about the length of the hull
or more. For the purpose of showing the relation of the water
inlet to the level of the water coming off the bottom of the hull,
I extended the lines out past the motor for labeling purposes.
Not to indicate that the water remains below the natural level.
It obviously does return to the natural level.

That make sense?

Note added: The water level off the bottom of the transom
is extended past the propeller to allow labeling.
Not to indicate that it remains at that level!!!!

;D

As I don't use a jack plate, any pics out there that show your setup,
a side view please, at it's best height for running. Show the whole
motor, transom and jack plate. I'd like to see the "sweet spot" as it
relates to a hull. I'm learning as I go.

Install a nosecone with the water pickup on the underside of the
leading edge, and a surface piercing prop, and you can jack the
motor up to where the water level coming off the underside of the
transom intersects the lower unit at the midline of the bullet.
Intake is still under water, prop blades rotate half the time above water level:



But even with this setup, you'd still have to contend with hull draft on plane
and the draft of the lower unit from the midline of the bullet to the bottom of the skeg.
Still need about a foot of water to run.

 

Alex, is that the motor running position or do you trim the motor
out to level the cavitation plate with the direction of travel?

 

You explained it well Tom, thanks.
I see your point about needing a water pressure gauge,
running the jack plate at the level needed for speed and control,
or higher, places the water inlet at the level where just a touch
too high, would allow air to be pulled in. Run at that level,
where air instead of water is being drawn in, you'd overheat the outboard.
With the gauge, as soon as you saw the pressure drop,
you'd know to drop the jack plate down a bump to regain pressure.