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Jack Plates?

Old 09-04-2002, 07:54 PM
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Default Jack Plates?

New to O/B boating, can u explain what the purpose of jack plates and the part they play while haulin --ASS..THANKS
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Old 09-04-2002, 08:18 PM
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s2l a jack plate gives you the abilty to raise the motor to optium hight and it moves the center of gravety back in other words you get more bow lift with out trimming out as far thery for the geare case is not crabbing it is running stright . the rasing of thr motor stright up for less drag and deep for better hole shot
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Old 09-04-2002, 08:21 PM
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Jack plates allow you to adjust your X-dimension, with a hydraulic you can do it on the fly, with a manual you can change it after you test and then retest.
Each hull responds differently to changes in X dimension but usually the higher you raise the props the faster the boat will go-to a point. But some boats will not be able to plane or have other handling problems as you raise the X.

If you want to have lots to play with, get hydraulic plates and then you can trim up, down, Jack up, down, and turn right and left-lots of good buttons to play with
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Old 09-04-2002, 08:36 PM
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The Guys have pretty well answered the Question but I will add A little.

The Jack plate and the two Trim systems gives you a great advantage to over come the Hull Faults Passed down by the Designer and Builder or Molder. See we live in a far from perfect World here.
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Old 09-04-2002, 08:48 PM
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#1) Jack plate moves 400lb motor further aft, moving center of gravity rearward which requires less of a "torque moment" to raise the bow which reduces wetted area which makes it easier for the motor to push the boat which makes the boat have a higher top speed. This change in the center of gravity means that you can now lift the bow higher with the same amount of trim OR you can raise the bow the same amount as before with less trim. Less trim means less propellor thrust is used to push the stern DOWN and more propellor thrust is used to push the boat forwards.

#2) Jack plate moves the propellor further aft which gives the propellor a longer lever arm from which to induce torque to the boat to lift the bow. This longer torque arm means that you can now lift the bow higher with the same amount of trim OR you can raise the bow the same amount as before with the less trim (see above for the ramifications of that).

#3) Jack plate allows you to raise the motor. Raising the motor means LESS gearcase bullet is in the water. The less gearcase you have in the water, the less drag you have from the bullet. Less drag means it is easier to push it through the water. Also, the higher you mount the motor, the less drag the prop blades have on them. With a properly designed surfacing prop, this translates to less drag, more rpm, higher speeds.

Water comes off the hull behind the boat and immediately starts rising towards the surface (actually rises higher than it started which is what causes a wake). If you move a motor back 12", then you will need to raise it some for the water to "hit" the lower unit at the SAME spot that it did before it was moved back. This behavior is more pronounced at lower speeds which is helpful in that it keeps your prop biting better at lower speeds when raised substantially.

Be aware that raising the motor usually translates into less ability to lift the bow. Given the double advantage of more leverage AND more rearward center of gravity, you can see that this issue kind of cancels itself out.

Also, raising a motor will create a more unbalanced load on the propshaft bearings. For this reason, Merc performance gearcases are fitted with a different style of carrier bearing which is designed for surfacing applications. The aftermarket also offers a reasonably priced kit to convert normal (ball) bearing carriers to the more suitable roller bearings.

Also, raising a motor will raise the water pickups. Low water pickup gearcases or aftermarket conversions are available to take care of this issue for those who run extremely high. Medium-high applications can get away with plugging the top water holes and adding a groove from the bullet to the lowest pickup hole with a hand grinder.

** If you currently have a hull that:

- runs with the propshaft set to a height where the centerline of the propshaft is even with a straightedge from the bottom of the hull with the motor at neutral trim.

- reaches max speed with no more than neutral trim.

- is propped to reach max recommended rpm at wide open.

- does not require nor benefit from the application of positive trim.

- runs with no tabs (or tabs full up).

- handles like a dream.


if ALL of the above conditions are not met completely, then you can benefit from a jackplate.

If you choose to install a jackplate, a hydraulic one only costs $350 more than a good non-hydraulic.

For a raceboat, it is easy to find the "optimum" setback and height. For a pleasureboat, you have more chance of varying loads and different load placements fore/aft and want better out-of-the hole performance and better topend performance. For this reason, a hydraulic gives you all of that (once you figure out how to use it).

Find others who have a similar combination and find out what works best for them.

I have seen 20' hulls that like a 15" setback. I've seen 20' hulls that like no more than 6" setback.

Give us some specifics on what you are running and maybe somebody can help.
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Old 04-04-2007, 02:45 AM
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I have a Scarab 34SS that I have mounted three OMC V8's on. What would be the water rise from transom to my engines wich is 35" behind the transom? I have received many answers to this question and is looking for someone with experience. I am NOT looking for reasons to spend $$$ on nosecones, jackplates and broken gearhouses, but rather a solution to run my stock gearhouses deep enough to last. I expect around 70MPH.
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Old 04-04-2007, 06:26 AM
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Rule of thumb I have seen used is the water comes off the boat rising up at an 18 degree angle so it would be about 11 3/8".
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Old 04-04-2007, 08:21 AM
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Here is the deal: Most boat/motor combinations require two entirely different heights for maximum efficiency. Deep for low speed/accelleration, high for top speed. Most boats will not get on plane with the motor at the proper height for top speed. On my last jackplate boat, a 20' Barcone, the difference from 'normal' height to "jacked up" was over 5mph.

How much does the water rise behind the boat? 0 at 100 mph, more at slower speeds (Hmmmmm, maybe I need a jackplate...............). 18 degrees? Get the f**k outta here!
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Old 04-04-2007, 08:31 AM
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Originally Posted by BROWNIE View Post
18 degrees? Get the f**k outta here!

Every boat is different. You have kind of a unique set up so you may be on your own a LOT in trying to set it up. Trial and error is all you can do. Play with the height of the drives and so on. If you are not going to put nose cones on with low water pick ups then you should put on water preasure guages so that you can see if you raise them up to much and start to loose water preasure!!

I love jack plates they really allow you to fine tune the heck out of the boat. You will learn a lot about driving a boat with them as long as you pay attention to the attitude of the boat, tach, and speed.

Put your best foot forward!
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