# Jack plates, yes or no?



## beavis

yes, they work awesome


but the most incorrect statement that I see a lot of people make is:

"For every 6 inches of setback, it makes your engine load act like it weighs 50% more."

so wrong


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## Brett

> but the most incorrect statement that I see a lot of people make is:
> 
> "For every 6 inches of setback, it makes your engine load act like it weighs 50% more."
> 
> so wrong


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.


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## beavis

Brett, you didn't need to remove it. I thought those were basically all the notes you were writing about them. Trying to get feedback on everything you were finding. I have just seen that statement too many times when people are arguing against jackplates.


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## Brett

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.


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## Brett

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.


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## marshman

so...??  you are saying that the water doesnt "rise" coming out from "under" the boat??
have you ever looked over the transom in a moving boat??  

bare with me, im not a physicist(see, i cant even spell it, maybe i did, i dont know, lol), but from someone who owned and operated many different types of boats(i grew up in boats...), experimented with various jacks and tunnels and the like...

man, a boat  "displaces" water as it moves forward....correct nautical terms would be "displacement hull" and "planing hull"....i understand we are dealing with planing rather than displacement, but....even though you are on "plane"  you are still displacing water...no boat rides completely up on the top of a water surface...the water parts in the front, goes down the sides, under the bottom, and out the back...

move your hand or a toyboat real  fast in a bathtub or whatever, you will see the "hump"....so...no, the water doesnt actually rise higher than surface of the lake, it just rises to go back to "level", immediately behind the boat....but there is a "hump"....(a deeper running, heavier craft, this hump will be farther behind the boat, hte water has farther to travel to get back to "level"...a smaller, lighter boat, the water pops up right behind)

now, tunnels....different topic...

jacks, i think can help most any craft...required??, nah...personal preference, yeah...a  runabout on a lake, i wouldnt bother...a boat built for any kind of specific purpose that the readers of this would be using, probably can benefit from a jack...again, required?? nah...

now, im going to the bar...im thirsty... ;D ;D ;D ;D ;D


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## HighSide25

dont forget, if you add trim tabs, you can tilt your motor out, jack it all the way up, and trim the tabs all the way down. the tabs will push water to the prop and intake while also keeping the bow down allowing you to get on a plane. this is how you can get in some real skinny stuff, but your speed will deminish by at least 30%


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## Brett

> you are saying that the water doesnt "rise" coming out from "under" the boat??



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.


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## topnative2

One has to remember that you get blow out on turns when using a jackplate to its full potential-----especially manual jacks


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## Tom_C

> One has to remember that you get blow out on turns when using a jackplate to its full potential-----especially manual jacks


 True

Brett, draw this set up:

8" setback
Cav Plate 5.5" above hull
2 degrees from horizontal

With a 3 blade Aluminum prop on my classic with this setup I can run strait or very wide turns. To run at max speed I need to drop the motor down so the Cav Plate is 3.5" above hull and 4 degrees from horizontal.


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## islander1225

i have a bob's cavitation plate and a tsg electric jp and can run the motor all the way up with no blow out, and also i can take full turns with it all the way up with no blow out.




















Alex


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## Brett

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


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## Tom_C

Bertt, Thanks for the drawings.

I need to say I am not a Hydro Dynamic Engineer. This my opinion of why/how jack plates work.

I think Brett has a good point about the hull running angle. The hump theory is an over simplified explanation of JackPlates. Reality is the water will not raise fast enough when you are going faster than 10 or 15 mph. When you add the hull running angle plus water rise into the equation it become clearer how you can raise your motor, decreasing drag, and increasing your speed.
  
I agreed with most hull having a running angle of 4 to 6 degrees from horizontal my hull running angle is less, closer to 2 degrees because of weight distribution. The drawing below show 4 to 6 degrees running angle, but are close enough to show the relationship of hull running angle plus water rise.


I have a Gheenoe Classic 15”6” it has the following setup:

1. 3 blade aluminum prop. When using a jackplate a SS prop with cupping is recommended.  Cupping a prop helps to hold water to the prop decreasing prop slippage. 
.

2. JackPlate with Trim, This unit has 8 inches setback and raises my motor 2 inches at minimum height. If my running angle was 4 to 6 degrees you could add a inch to my numbers.

3. Extented travel TrimTabs, most TrimTabs have 2 inches of travel. My TrimTabs have 4 inches of travel. When my Trim Tabs are up they are out of the water when the boat is on plane.

4. Thrust Director, Cavitation Plate attachment. A Cavitation Plate attachment help to prevent the prop from ventilating, (drawing air down through the water causing the prop to lose grip).


*Without* a Cavitation Plate attachment I can raise my motor so the cavitation plate is 2.5 inches above the hull. You can see below if the water did not rise, my prop would be out of the water. In reality *my prop is about 2 inches below *the surface of the water.  If I try to raise the motor any higher the prop will ventilate, 










By adding a Cavitation Plate attachment I can raise the motor higher. This is possible because the Cavitation Plate attachment helps prevent the prop from ventilating. You can do the same with a SS prop with cupping.










By lowering the Trim Tabs I can force the water higher at the prop . Not only is the water higher at the prop, my stern is higher decreeing the trim of my boat. With this setup I can raise the cavitation plate 5.5 inches above the hull. My motor RPM increases by 400 RPM but I lose 20% of top speed do to the prop slipping and I can forget about tight turns.


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## Flyline

EXCELLENT POST TOM C!

Now if I want a trim tabs with a riserplate (no setback) on my gheenoe LT15 with 25hp 2-stroke and my cavitation plate is 1.5 inches above the bottom of the hull.

If I'm going to add a trim tabs and add a 4 blade heavy cupping S.S prop. then raise my motor 6'' above the bottom of the hull without need a jackplate and more weight in the back.

I'm trying to save weight in the back and run skinny as possible wihtout need a jackplate. I'm just going to see if this works without a jackplate but if not then a electric jackplate on order to run very skinny!


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## Tom_C

whitesnooky,
WARING Do not attempt raising a water cool motor without a water pressure gage!!!


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## Brett

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.


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## Tom_C

Brett, That’s correct. With a cup prop and cavitation plate attachment you may be able to still run with little or no water cooling. Looking back to see if your motor is still peeing is not good enough. When running a 5.5 I lose 25% of water pressure. My intake is still under water, but by using the Trim Tabs to push water to the prop I am creating turbulence in the water flow to the motor water intake. Without the setback of a jackplate at lower speed your motor may have proper water flow to the intake, but as speed increases it is very possible that the water will not have time to rise, before the water intake and it will not take long for your impeller to be destroyed at high speed.


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## marshman

my motor runs "quite" high...i dont have a water pressure gauge....i guess i been "putting my motor in harms way" over these last 4 years by using the "pee gauge"....hmm

my 25--









oh, and this one (not micro)has a water pressure gauge, and maintains 20PSI running that high...the top intake holes are plugged, to prevent sucking air...











***not arguing...just bragging, maybe...


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## Tom_C

> my motor runs "quite" high...i dont have a water pressure gauge....i guess i been "putting my motor in harms way" over these last 4 years by using the "pee gauge"....hmm
> 
> my 25--
> 
> 
> ***not arguing...just bragging, maybe...



marshman,

It looks like your cavitation plate is an inch or two above your tunnel. This is the same as having a boat with the cavitation plate an inch or two above a non tunnel hull. 

My comments were to directed towards those that think just because you mount a jack plate you can raise the motor *4 inches plus *and all is good.


So to the newbie wanting to run a JackPlate to the Extremes the two things I recommend are Water Pressure Gage and a Tack so you can see your RPM.


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## Brett

I've been wanting to determine my hull running angle,
and finally got around to measuring it today.
Bought a semicircular protractor from wally world,
99 cents, cheapest marine gauge I've ever purchased.
and created a hull trim measuring gauge.
Gotta love ******* Tech...

Took some flyline backing and tied it through the radius hole,
and tied a quarter ounce weight to the tag end.
That's my gravity operated analog indicator.
Leveled the hull on the trailer with my 48 inch carpenters level,
and taped the protractor to the side of the hull so that the analog
indicator aligned with the 90 degree mark on the protractor.
White background was taped in place first to ease readability
while running at speed down the channel.











Ran at varying speeds and checked the running angle.
It measured 5 degrees plus or minus a degree depending
on where I stood in the hull.











Pretty good, came up with the same answer I did, when measuring
from pictures of boats running, while viewed on my monitor.

What's your running angle? ;D

I'm so easily entertained...


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## TomFL

Brett you never cease to amaze me. That looks like something I'd rig up while my wife looked at me in disbelier. 

I like the way you think

-T


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## Brett

> I like the way you think
> 
> -T



Be afraid....be very afraid!


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## cantcatchsquat

i was looking for a cheaper jack plate and i found this one that was home made would it work for a 14 foot boston whaler and 15 hp johnson.

thanks pat


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## Brett

It looks like it was designed for an electric trolling motor,
not for the weight and torque of a 15 hp outboard.


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## marshman

you know ..i had to look at that pic for a minute or 2 before i could figure out what i was looking at.....i sure hope nobody would try to hang a 15 on that contraption....


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## cantcatchsquat

does anyone know how to make one(maybe some kind of metal not realy interested in the wooden once).I dont want to spend 150 for a new jack plate.


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## Brett

free plans

http://www.dillon-racing.com/jackplate/index.htm

including photos of build

http://www.dillon-racing.com/jackplate/plans/jack-plate-page-01.htm

and an adjustable trim bracket

http://www.dillon-racing.com/jackplate/plans/jack-plate-page-02.htm


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## cantcatchsquat

thanks


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## cantcatchsquat

i went to home depo and lowes today and couldnt find the aluminum angle that big.Does anyone know where i could find one that size


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## Brett

I purchase my bar and angle stock from a local machine shop.
They even cut to length for a few bucks more.

Check your local yellow pages.


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## anytide

are you looking to make a manual or mechanical version ?? how much hp r u using? i have my own designs for manual plates w 'spring assist works awesome w' my 25 hp on my skiff -easyglide- says it all -anytide-


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## cantcatchsquat

looking to make a manual and im using a 15 hp johnson


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## anytide

what kind of skiff ?? stand-up or seated tiller., any picts ... - maybe can help -anytide-


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## yobata

anytide said:


> are you looking to make a manual or mechanical version ?? how much hp r u using? i have my own designs for manual plates w 'spring assist works awesome w' my 25 hp on my skiff -easyglide- says it all -anytide-


Can you provide more info on your plans? I'm especially interested in the spring assist feature


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## anytide

dam thats old.....
ill see if i can find some of the spec sheets.

shouldnt you be watchung football?


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## yobata

anytide said:


> dam thats old.....
> ill see if i can find some of the spec sheets.
> 
> shouldnt you be watchung football?


Ha! Kickoff is 6:30 I think

Thanks for looking


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## No Bait / Lures Only

marshman said:


> my motor runs "quite" high...i dont have a water pressure gauge....i guess i been "putting my motor in harms way" over these last 4 years by using the "pee gauge"....hmm
> 
> my 25--
> 
> 
> 
> 
> 
> 
> 
> 
> 
> oh, and this one (not micro)has a water pressure gauge, and maintains 20PSI running that high...the top intake holes are plugged, to prevent sucking air...
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> ***not arguing...just bragging, maybe...


My 90 Yamaha raised on jack plate had the top holes plugged and it did well until I trimmed it out toooo far.


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## WhiteDog70810

While I respect Brett's ingenuity, I would like to take this opportunity to point out that iPhone's have a level feature in the compass app; just swipe left. Given how hard it can be to find a vertical longitudinal surface in many boats to hang a gravity operated protractor, placing you phone on the sole might be easier for a lot of folks. I compared it to my dial angle reader and it is the same. I have to admit that 6 degrees looks awfully bow high on dry land.

Nate


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