# How much can I raise this jack plate?



## rjackh

Just bought a tunnel southfork with a 5" jack plate this afternoon. I held a square flush with the tunnel and it looks like I can bring the jack plate up another 2-2.5" and the cavitation plate would be level with the tunnel plane. Is this right? I figured I would bring it up like this then go for a test run tomorrow and adjust as needed. This is my first time running a tunnel and messing with a jack plate so any advice or instructions is appreciated. Thanks.


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## mchildress

I would put the prop shaft in the middle of the tunnel and fine tune from there.


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## Sgrem

you arent testing until you have a water pressure gauge. When gauge drops pressure below acceptable then that is your upper range.

Mine is mounted where the bottom of skeg is at the bottom of my hull when trimmed out just a touch.....way way way higher than you have yours but i have tested like crazy to make sure it will do that and at what rpm etc while maintaining adequate water pressure.

When i am running 2/3's of my prop is out of the water.

Get a water pressure gauge first...likely you will be able to mount it where the center of your nose cone is about 1" or so below the top of your tunnel.


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## rjackh

I just talked to a local mercury service dealer and he personally didn't recommend a water pressure gauge because they can be inaccurate and because my motor probably doesn't run very high pressure. He said there is an alarm that would sound if the motor started to get too hot anyways. He suggested setting the cav plate level with the tunnel and fine tune from there to get as high as a want without getting the motor hot. Granted, he also said they don't have a lot of experience with tunnels. What do yall think?


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## aguaflaca

rjackh said:


> I just talked to a local mercury service dealer and he personally didn't recommend a water pressure gauge because they can be inaccurate and because my motor probably doesn't run very high pressure. He said there is an alarm that would sound if the motor started to get too hot anyways. He suggested setting the cav plate level with the tunnel and fine tune from there to get as high as a want without getting the motor hot. Granted, he also said they don't have a lot of experience with tunnels. What do yall think?


I personally wouldn't run a tunnel hull boat with a jack plate without a water pressure and temperature gauge.
I had both on my Merc 75 on a 16' Flatlander.
And yes it was low pressure. Usually showed 10-15 on a 30# gauge, but you could tell when it dropped and knew you better hit deeper water ASAP.
I wanted to know my pressure had dropped, BEFORE the motor got hot. I might also find a new service tech.


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## Rancher86

Fishing and running super skinny, water pressure gauge is a MUST! Let's say you're on a mile or so flat of 6" water, no holes to set down in in sight, and you got it jacked all the way up, it might run fine for a quarter mile or so, but if you don't know what kind of pressure your getting though your motor, you don't know if it's cooling correctly. So halfway through that mile long flat, your heat alarms goes off and your screwed because you have no choice but to kill the engine so as not to ruin your powerhead. Then your grounded, with no deep water in sight. Screwed basically. 
So, IMO, water PSI gauge is a must and will tell you accurately the limitations of your boat jacked up to run skinny. If your not getting decent pressure jacked up and running skinny, then you can only run over short sand bars jacked up, not long hauls in back lakes where there is sometimes no deeper water for miles. It all depends on where and what type of fishing you do though. But I would definitely raise that motor at least some, it looks way low. Good luck and find a mechanic who has experience with tunnel hulls. Like pretty much any mechanic on the Texas Gulf Coast where 80% of the boats they deal with are tunnel hulls. They can give you a much better evaluation.


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## Sgrem

um...dont ask him anything anymore....

My Tohatsu (they are very low pressure high volume) never....ever...sees more than 3psi.....ever. I had to get a 0-10psi gauge just to see it move at all. Most people freak when they see that. As long is the gauge is not on zero my buggy will run all day. But imagine trying to fine tune from 2psi down to 1psi for max shallow water performance. He has already told you it is low pressure so the changes will be very slight and hard to manage...small adjustments in trim/jack plate height can shut you down. Sometimes water is just a spit spray out of the pee hole like a windex spray bottle. The pee hole is just a tattle tale and doesnt accurately show what the motor is getting. Yes the alarm will sound but then it is too late and it will shut you down on the bottom in the middle of somewhere you will have to push for the rest of the day. Get a gauge and go find your max heights. My cavitation plate is in the middle of my transom way above the tunnel. I have a low water pickup as well. BTW....that is not an adjustable jack plate....it stays that high always.


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## cva34

The cav plat is the one below where your SQ is..Prop about center of tunnel ..But if tunnel is only 5" deep maybe only even with bottomm...Start out low and work your way up..A real helpful is come up with a way to measure how high/lo is with marks (fingernail polish) piece of alu ruler ..And a pointer so you know where your at from Drivers seat. and you can always go back to that optimum point.


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## aguaflaca

Didn't notice that was not a hydraulic jackplate. 
My Merc would run 3-5# on idle and 10-12# running hard with motor all the way up. 
I had a Bobs low water pickup also.


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## JimD

I have a Bob's JP on the BC. Fishing south I have mine set to go as high as it will go which is about an inch more than the water pressure "peeing" will allow. 

Think my tohatsu had 4 motor mounting holes and uses the middle ones for most bays.

You can run it any where in between as long as you have the stream of water peeing out of the motor.


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## rjackh

Its a manual jack plate. Should I still get a water pressure gauge? I'll see if I can find a better service guy in Conroe or Houston that is more helpful.


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## aguaflaca

rjackh said:


> Its a manual jack plate. Should I still get a water pressure gauge? I'll see if I can find a better service guy in Conroe or Houston that is more helpful.


I would run a pressure gauge on anything. just my 2 cents. 
What HP is that motor? 
installation was very simple on my 1998 75 HP.


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## rjackh

Motor is an '02 merc 40 horse 3 cyl 2 stroke. I talked to Quality Marine Service in Conroe and he advised installing the gauge, its supposedly pretty cheap too. I don't have any experience working on outboards so I will probably have them do it instead of try myself.

Once I get the gauge, if I see the needle bouncing or running very low while running the boat, I need to stop and lower the plate? Meaning I need to keep the tools needed to do so in the boat at all times?


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## aguaflaca

Not sure about the 40, but on my 75 all I had to do was take out a plug, install a NPT to hose fitting and run hose to gauge in console.
But if you don't feel comfortable doing it get somebody else to. 
I would carry the tools at least until you know you have the jackplate set at right height and are getting acceptable pressure while running.


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## ATX 4x4

http://www.texallaluminumtrailers.com

These folks do fantastic service in Montgomery.


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## Sgrem

rjackh said:


> Motor is an '02 merc 40 horse 3 cyl 2 stroke. I talked to Quality Marine Service in Conroe and he advised installing the gauge, its supposedly pretty cheap too. I don't have any experience working on outboards so I will probably have them do it instead of try myself.
> 
> Once I get the gauge, if I see the needle bouncing or running very low while running the boat, I need to stop and lower the plate? Meaning I need to keep the tools needed to do so in the boat at all times?


No you dont need the tools with you. I understood that what you are asking is how high can you run your motor....

Install a gauge.
Run boat.
Check water pressure.
Raise motor.
Check water pressure again.
Rinse and raise and repeat until your pressure is too low for motor to cool itself.
Lower motor a touch. 
Put tools back in garage.
Fish and drink beer and high five that you are running shallow as you can now.


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## RubiconAg

Also monitor your steering. My Dargel will loose "steerability" or cav out during a turn or hole shot long before I loose pressure while on plane. I hate when people ask on the forums "what number do you run your jackplate on." This is a ridiculous question as each boat is different and mounting to hole position so vary. The depth and width of a tunnel as well as size and weight of a boat all factor in.

I can run mine on plane at max, 6" up with great pressure but I have 0 turning and absolutely no hole shot, I can't get on plane anything over 3.5". Each boat is different. You have to get out and run, move it and test some more. Yes a pressure gauge is first but that's only the first step and not a tell tale sign of where it can run.


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## paragod

Correct u will need tools to move the plate. Only 2 gauges u need a tach and a H2O psi.


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## rjackh

sgrem said:


> No you dont need the tools with you. I understood that what you are asking is how high can you run your motor....
> 
> Install a gauge.
> Run boat.
> Check water pressure.
> Raise motor.
> Check water pressure again.
> Rinse and raise and repeat until your pressure is too low for motor to cool itself.
> Lower motor a touch.
> Put tools back in garage.
> Fish and drink beer and high five that you are running shallow as you can now.


Makes perfect sense, thank you.


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## TBird1610

I personally would run from any mechanic that advised against running a water pressure gauge on a tunnel boat! As RubiconAg said the gauge is only your first step. Steering and cavitation come next. Won't do any good to find your high spot if your prop blows out or you end up on the bank at every turn


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## aguaflaca

TBird1610 said:


> I personally would run from any mechanic that advised against running a water pressure gauge on a tunnel boat! As RubiconAg said the gauge is only your first step. Steering and cavitation come next. Won't do any good to find your high spot if your prop blows out or you end up on the bank at every turn


this X1000


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## Rancher86

Sorry bud, but I didn't notice where he said he was running a Tohatsu, b/c he didn't. He did mention the low pressure, which I overlooked. Either way, if you don't have a pee stream, something's wrong. High pressure or low. And you can get low pressure guages (0-10) for motors like Tohatsu's. Find a new mechanic for sure. You don't want to rely on a guy who doesn't ever deal with tunnel hulls, it's a whole other ballgame from conventional hulls. So just get a low water pressure gauge, easy to install if you know a little about engines, or get a low water pickup mounted on the transom and run the hose to your intakes (like "Mr. Egram" has done w/ his Tohatsu). That's the easiest way to ensure you always have good pressure, coupled with a gauge. Low pressure or not, your motor still needs cooling water... Even 3 PSI on a Tohatsu can be good enough, but it's always nice to know so you can mount your engine after test runs to make sure your set up at it's optimum skinny running level.


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