# JACK PLATE



## Rookie1719 (May 16, 2021)

Hey everyone,

Can anyone help when Jack plates, I read some topics here but a lot were DIY. 

I bought a new outboard the dealer told me I needed a long shaft outboard for my transom. The other day I got flooooooded with water - I was told it’s because the outboard is too low ... the manual does say to be 1” from
Bottom - well mine is 6 inches. It’s a 16” transom. 

Anyways instead of building my owner I’m too nervous to lose this outboard. I was told to get a Jack plate ... can anyone help with what I should look for ? And will a Jack plate actually fix the issue ?


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## lckstckn2smknbrls (May 16, 2021)

They sold you the wrong motor, take it back?


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## Rookie1719 (May 16, 2021)

lckstckn2smknbrls said:


> They sold you the wrong motor, take it back?



Not an option at this point. But thanks


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## MrGiggles (May 16, 2021)

That plate is pretty overkill for a short shaft motor. The TH Marine Mini-jacker is probably what you want.

You want the cavitation plate on the outboard to be roughly level with the bottom of the hull. If yours is way below, that's a problem.


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## C&amp;K (May 16, 2021)

While I am a firm believer in jack plates to trim the boat out, unfortunately in your case, 6" is too much. It will place the point of thrust on the transom too high. A jack plate is a lever.

The purpose of a jack plate is to change the height of the thrust point (the prop) without changing the thrust angle (trim). It allows setting the anti-ventilation plate on the outboard to the correct height so the plate is slightly above the pad to ventilate the prop on plane to get peak horsepower and speed (and throw that nice 60ft rooster tail). It also allows getting the outboard further away from the transom to get out of the turbulence where the water leaves the keel and lifting strakes and shifts the CG of the boat aft. Since the pad starts to rise as soon as it leaves the hull, placing the outboard further aft with a jack plate allows getting more of the hull out of the water and still keep the prop in the water where it has good "bite" (minimum slippage) and control the slippage with how much you ventilate the prop. On most bass boats, for instance, running at 90-100 mph with a jack plate, the top third of the prop blades are actually out of the water so it can wrap out to 6,000 rpm with a big pitch prop. And this also requires a special low-water pickup for the water pump.

A jack plate is a performance accessory, not a device to correct for having the wrong shaft length for the height of the transom.

Installation of the PT-130 on our Pro 17 allows automatically raising the outboard (20" shaft on a 20" transom) by 1.75" and increased the top speed of the boat by 7 mph GPS. But correcting for 6" because of the wrong shaft length is too much and would be considered a very badly rigged boat.


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## newyota (May 16, 2021)

What kinda boat?.On aluminum boats here on the river I have seen people get risers made to bolt to transom and seen people even make them out of wood which I think is sketchy.


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## 14altra (May 18, 2021)

If returning it is absolutely not an option you may be able to "fix" your outboard depending on the motor. It may require a new drive shaft but some of the the "long leg" motors simply have an extension on the housing and a longer drive shaft. In other words replacing the vertical driveshaft for the 20+" with one for the standard length "leg" and removing the extension. Again depending on the motor you may be able to find a used shaft or at worst a new one should only be a couple hundred dollars at most.


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## ktoelke54 (May 22, 2021)

I think a riser is a good option. That TH mini jacker will work also, it’ll get you an extra three inches if you keep it level with top of the transom. You can squeeze 5” inches if you fudge a bit. But if you need 6”, a riser would be a better option. 


Sent from my iPad using Tapatalk


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## JL8Jeff (May 22, 2021)

You could try the cmc riser plates. I see dealers use these on boats all the time. I personally don't like raising the motor that much without reinforcing the transom, but that's me.

https://www.amazon.com/dp/B003S8XX5A/ref=as_sl_pc_tf_til?tag=tinb0ab-20&linkCode=w00&linkId=&creativeASIN=B003S8XX5A


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## lckstckn2smknbrls (May 25, 2021)

Sell it or trade it.


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## C&amp;K (May 25, 2021)

lckstckn2smknbrls said:


> Sell it or trade it.



I would concur with that one. Buying a new leg, driveshaft, shift shaft, then having to disassemble the lower unit to change the driveshaft, just doesn't make sense unless you can find used parts really cheap. Correcting with a jack plate or riser for a long shaft engine on a 16" transom is too much. Long shaft motors are common as dirt, they are not too hard to sell. Then buy a new one with the correct shaft length.


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## turbotodd (May 26, 2021)

one of the many "local" boat builders will modify the transom either way, to run a long on a short boat, or to run a short on a 21". It's not too expensive either.

A jack plate isn't going to be the best option here. Plus, it'll look kinda weird and place the tiller at a much higher level making a longer run uncomfortable.


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## eeshaw (May 26, 2021)

If it was my boat I'd either sell the motor or modify the transom so I could use it. I don't think that the jack plate is a viable option. Just out of curiosity, what kind of boat do you have it mounted on, is it a john boat?


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## C&amp;K (May 26, 2021)

I'd like to point out that modifying the transom is no better option than a jackplate or riser. With the wrong outboard for that hull, the thrust angle will never be right without using an extreme trim position. Whether you modify the transom or use a jackplate, the hull was never designed for that at an extreme of using the wrong length shaft on your outboard. There is a reason the boat came with a 16" transom.

Again, I am a firm believer in jackplates that allow adjusting the thrust height without changing the thrust angle. But this is a performance adjustment to get a few more mph out of the boat that usually involves changing the height no more than 1 or 2" to ventilate the prop and get the engine to peak hp with a big pitch prop. Or adjusting the height for shallow water operation without changing the trim angle. This is not the application for that.


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## Rookie1719 (May 27, 2021)

Thanks everyone for your thoughts and input. 
Unfortunately we think we got taken advantage of at the dealer. I’m not too happy with what happened, but we are over it. We told them the size of the transom and they sold us a long shaft. Because of Covid we bought it early this year and picked it up in April - and the exchange and return time limit has expired. I have 0 interest in converting it. 

A friend had the Jack plate from another boat and gave it to me. We did install it. The motor now sits 1.5” under the bottom which is closer than 6. I was concerned about the over kill but we countered the weight by building a compartment in the front with weights and battery. 

We haven’t had an issue since with water flooding us and getting up on plane. As far as the force on the transom I don’t see how it would be more if it’s 1” under as the manual says. 

So fingers crossed no other issues other than getting skunked on the most populated walleye lakes in NA. 
CHEERS


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## C&amp;K (May 28, 2021)

Rookie1719 said:


> As far as the force on the transom I don’t see how it would be more if it’s 1” under as the manual says.



That's too bad that a dealer would do that.

It's not the height of the prop, it's where the force is transferred to the transom. Jack plates increase the twisting forces on the transom. It's a lever. To use an example that's easier to understand; let's say you're behind the boat and pushing on it with your hand to make it go. You push on the top of the transom and the forces on the transom are withing what it was designed for. Now, take it to an extreme. Put a 6 ft tall attachment of some sort on it and push on that with the same force at the same angle. The torque, or twisting force transferred to where it actually pushes the boat forward, is the length of your lever times the force.

If the amount of twisting developed is within the design limits of the transom, no damage will occur. If the outboard is is the maximum hp that the boat is placarded for, it runs a good chance of exceeding the forces the transom was designed for. It will eventually break it. Most times this requires extra bracing in the transom.


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## Rookie1719 (May 28, 2021)

Thanks for the response 

The motor is only a 15HP - personally I wanted to rebuild the transom and add the height that way. But the Jack plate was available and easy to install. 

The max HP is 25 HP for this boat - so I think I’ll be okay. We do mostly just trolling and creek fishing. But this definitely has made me go purchase a bilge pump! 

Quick back story ... the reason the dealer sold me that length was because my previous motor was also long. Which is odd because this was a package sale bought and it came with a 8 HP long shaft. I tried trading the outboard in but it was laughable at the trade value - brand new 2011 4 stroke merc they offfed me $500 ... sold it for $1,800.00. 

But I’m new to all this boating stuff - so I appreciate the input.


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## C&amp;K (May 28, 2021)

With a 15 on a boat placarded for 25, it probably will be fine. We're used to dealing with much higher horsepower engines with jack plates. Most typically bass racers who have a boat that goes 71 mph, but without getting the top 1/3 of the prop blades out of the water with a 25P to wrap it out to 6,000 rpm can't get it to 73 mph. One of their buddies has one that will go 72 mph. So they'll spend $2,000 to get another 2 mph out of it, including a jack plate and a new 27P stainless prop.


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## Sinkingfast (May 28, 2021)

C&K said:


> Rookie1719 said:
> 
> 
> > As far as the force on the transom I don’t see how it would be more if it’s 1” under as the manual says.
> ...





Remember the force is being applied from the propeller to the transom. The moment or couple comes from force times the lever arm length. There are at least 2 forces causing a couple. One is center of gravity times distance to the transom. That won't change with height. The other is the distance from the prop to the transom. As the prop approaches the transom..as in raising the prop using a plate or other..the moment or couple is lessened at the transom. If the prop were directly accross from the transom the force would be only compressive at the transom...except for the weight of the motor causing a couple. 


The couple or moment forces on the transom are less with the plate and more with a 5 or 6 inch motor extension hanging below the boat.


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## C&amp;K (May 29, 2021)

An outboard motor, due to its design, is constantly trying to flip itself over on it's back and provides lift to the bow. The higher you raise it on the transom, the more bow lift it produces up until the prop starts ventilating. Due to moving the weight of the engine back (usually 5" and raising its height, the twisting forces on the transom become quite extreme.

Notice the .250" 6061 T6 reinforcement plate used with a jack plate to spread the load over a wider area of the transom. Without that, I've seen them literally crush a wood-core transom and the jack plate comes loose.


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## CedarRiverScooter (May 29, 2021)

Be sure to support motor when trailering. The highway bounce will also stress the jack plate mounting bolts.


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## Zum (May 29, 2021)

Glad its working for you, i think it will be fine.
I thought you might build up your transom just to keep the water from coming over. There do make or you can fabricate "tabs" to help with the water coming over the top, when coming off plane or reverse.


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## Rookie1719 (Jun 8, 2021)

Any thoughts on this guys… the manual says the plate needs to be 1” under the boat. Mine is exactly 1” after adding the Jack plate. I took it out and it got to plane relatively quickly as soon as I give it … it levels out. 

But the rooster tail off the back is ridiculous, it’s not coming in the boat or anything but it shoot ups. I read that’s created when the motor is too low ,.. but I followed what the manual says. 

Any suggestions on what to do about it? Should I be concerned ? I also added more support for the transom to it doesn’t flex - I don’t think I needed too considering it’s rated for 25 hp. But it makes me feel better.


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## C&amp;K (Jun 8, 2021)

A rooster tail is normally caused by trimming up so the prop shaft is not parallel with the water flow. The tips of the blades throw the water and create the rooster tail. I can't see that being the case with a 15hp motor.

It sounds more like you got something merely plowing the water and directing it straight up as it comes off the hull. I don't know what that would be - a transom-mounted transducer maybe?


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## Rookie1719 (Jun 9, 2021)

I’ll try to get a video and post it on YouTube next time I’m out. I’m def looking into swapping it for a short shaft… the dealer I bought from said it was my fault. Which isn’t fair because I’m so new to this hobby… I told him the transom length and they told me the long shaft. Got an extra $600 compared to the short. 

But Maybe the angle of the outboard is wrong - maybe adjusting the position might help idk. I have a transducer mounted too but that’s never given me this issue. Even though my last motor was 8 hp - never seen this before.


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## C&amp;K (Jun 10, 2021)

Well, 8hp probably wasn't enough to get the boat on plane or get any speed. 15hp isn't enough to have the prop throw any significant rooster tail even if it was trimmed way up and ventilating the prop. So what I was thinking was, now that you're going faster and maybe getting the transducer up on the pad it's simply catching the water and making it spray upward. You should be able to look and see what's making the rooster tail.


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## RaisedByWolves (Jun 10, 2021)

C&K said:


> A rooster tail is normally caused by trimming up so the prop shaft is not parallel with the water flow. The tips of the blades throw the water and create the rooster tail. I can't see that being the case with a 15hp motor.
> 
> It sounds more like you got something merely plowing the water and directing it straight up as it comes off the hull. I don't know what that would be - a transom-mounted transducer maybe?



It’s his lower unit/mid section, I have the same issue.


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## Rookie1719 (Jun 12, 2021)

C&K said:


> Well, 8hp probably wasn't enough to get the boat on plane or get any speed. 15hp isn't enough to have the prop throw any significant rooster tail even if it was trimmed way up and ventilating the prop. So what I was thinking was, now that you're going faster and maybe getting the transducer up on the pad it's simply catching the water and making it spray upward. You should be able to look and see what's making the rooster tail.




I played around within the trim and engine height. Last night had no significant tail like the other day - minus the transducer but it was small. 
Working smooth


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## RaisedByWolves (Jun 12, 2021)

Rookie1719 said:


> C&K said:
> 
> 
> > Well, 8hp probably wasn't enough to get the boat on plane or get any speed. 15hp isn't enough to have the prop throw any significant rooster tail even if it was trimmed way up and ventilating the prop. So what I was thinking was, now that you're going faster and maybe getting the transducer up on the pad it's simply catching the water and making it spray upward. You should be able to look and see what's making the rooster tail.
> ...



How much did you have to raise the motor?


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## clarkbre (Jun 20, 2021)

C&K said:


> While I am a firm believer in jack plates to trim the boat out, unfortunately in your case, 6" is too much. It will place the point of thrust on the transom too high. A jack plate is a lever……





C&K said:


> I'd like to point out that modifying the transom is no better option than a jackplate or riser. With the wrong outboard for that hull, the thrust angle will never be right without using an extreme trim position. Whether you modify the transom or use a jackplate, the hull was never designed for that at an extreme of using the wrong length shaft on your outboard. There is a reason the boat came with a 16" transom.
> Again, I am a firm believer in jackplates that allow adjusting the thrust height without changing the thrust angle. But this is a performance adjustment to get a few more mph out of the boat that usually involves changing the height no more than 1 or 2" to ventilate the prop and get the engine to peak hp with a big pitch prop. Or adjusting the height for shallow water operation without changing the trim angle. This is not the application for that.



You guys are actually 180 degrees off and giving bad advice. 

In the OP’s case where a 6” taller jack plate will make the “thrust angle” too high isn’t wrong. The transom and motor are not levers. They are stationary objects. Due to how the boat sits in the water and where the propulsion of the motor is (at the propeller….not where it mounts on the transom) the OP would benefit from bringing the cavitation plate closer to the bottom of the boat.

The real pivot point is where the bottom of the boat and the transom meet in the water. With the cavitation plate even or within 1” below that, the propeller sits near the bottom of the boat and provides forward force. Being that it’s near the bottom of the boat, the force applied does not “lever” anything other than pushing the boat forward.

Where the cavitation plate sits 6” below the boat and the propeller even lower than that, the force is pushing lower in the water. Where the transom & bottom of the boat meet in the water is the leverage point. The propeller, sitting lower (tall vs short shaft on a short transom) in the water applying force, causes more thrust and the front of the boat raises up. It really comes down to where the propeller is in the water (height) in comparison to the bottom of the boat…not the transom height.

I do have to ask C&K, my boat is a Lund WC16 that comes in both a tall 20” and short 15” transom. Using identical motors but having the correct length for the transom, what performance gains will there be on either transom? There won’t be any because the propeller is at the correct height for the boat. But, if you have that long shaft motor on the short transom boat, the performance is crap. There’s an extra 6” of motor dragging in the water and the thrust low in the water will cause the bow to rise severely and the boat will take a long time to settle down.

*To the OP:* I was in your position 14 years ago. Bought my 16’ boat, trailer and 15hp motor as a package for a screamin’ deal. The downside was that it was a short transom and long shaft motor. After lots of adjusting, it never worked right. I designed a jack plate and a local welding shop built it for me. Once mounted and water tested there were absolutely no downsides to the modification. It was like a totally new boat. The hole shot was much better, it got on plain fast, the boat handled way smoother and the top speed increased by about 2 mph. The only adjustment was raising the cavitation plate up 6” to even with the bottom of the boat. It helped tremendously with no ill effects.

I will also add that I've trailered that same boat and motor on the jack plate for thousands of miles and after all these years, there is no idication of stress to the jack plate, motor or transom.


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## C&amp;K (Jun 20, 2021)

clarkbre said:


> I do have to ask C&K, my boat is a Lund WC16 that comes in both a tall 20” and short 15” transom.



Jackplates are a simple problem of mathematical vectors and lever forces. The thrust is developed at the prop. But the force is transferred to the boat at the point of attachment of the outboard. The longer that lever, the higher the twisting forces are on the transom. While you can get by with this as shown in your photos with 15 and 25hp outboards, try it with a 150, 200 or 250 on a bass boat. You'll break the transom.


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## clarkbre (Jun 20, 2021)

Well, you're saying I'm wrong and applying your logic to motors and boats with 10x what the OP is asking about and I have done with success. 

I do agree that using a jack plate on a 150-200hp engine is too much. But, why amplify the OP's question and lie about the results. The reality is that if he set it up similar to mine, his results would be positive. 

What was your experience doing this on a smaller boat with a 10-25hp engine? It doesn't appear you have any as you're speaking in theory instead of actually doing it.


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## eeshaw (Jun 20, 2021)

clarkbre, on that riser you had constructed, how thick are those pieces that straddle the transom on each side? Also, how big are the bolts that attach it to the transom?


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## clarkbre (Jun 20, 2021)

eeshaw said:


> clarkbre, on that riser you had constructed, how thick are those pieces that straddle the transom on each side? Also, how big are the bolts that attach it to the transom?



The front and rear plates are 1/8" thick and sandwich 4 pieces of 1 1/2" tube aluminum stacked and welded.

To secure it to the transom, the 1/8" plates are through-bolted with (4) 3/8" bolts that are 5" down from the top of the original transom.

The motor that mounts to it is a 15hp Suzuki 2 stroke that has a dry weight of approximately 90 lbs. Mounted on the riser, there is absolutely no flex or wiggle. It is a very stout piece.

I think the key to the strength is in the low through bolts. I looked at the pre-made jack plates but didn't like that they only bolted to the rear of the transom. I designed mine to carry the load to both the front and rear as to not peel off the rear. 

The through bolts 5" down from the top of the transom.



The front is also supported by the boat's center brace that ties into the floor and transom.


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## Sinkingfast (Jun 20, 2021)

C&K said:


> clarkbre said:
> 
> 
> > I do have to ask C&K, my boat is a Lund WC16 that comes in both a tall 20” and short 15” transom.
> ...




Given a 15" and a...say 65"....if both mounted on transon at same prop location...65" using clamp extension or plate...the transom sees no difference except for weight. Simple beam calc. Anything outside of the applied forces is not a function. Anything below the prop and above the transom is a non-factor as far as the transom is concerned.


A crap load of hi hp bass boats use movable jack plates.


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## C&amp;K (Jun 20, 2021)

Sinkingfast said:


> A crap load of hi hp bass boats use movable jack plates.



Yes, I've rigged so many of them over the years at our marina here I've lost track of how many I've done. Our bass boat has one on it. But never to adapt the wrong length outboard to a transom. That is bad rigging and moves your status of marine technician to master cobbler. There's a right way and wrong way to rig a boat, with outboard propulsion the first step is selecting the right outboard for the boat.

I've also seen and repaired or replaced my share of failed transoms caused by jackplates.

We do, however, use risers on 9.9 - 15 hp kickers to correct for hull deadrise on some boats.


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## eeshaw (Jun 21, 2021)

clarkbre said:


> eeshaw said:
> 
> 
> > clarkbre, on that riser you had constructed, how thick are those pieces that straddle the transom on each side? Also, how big are the bolts that attach it to the transom?
> ...


That's what I figured you'd had done. That should be fine for a small engine. I think the only thing I'd have done differently than what you did was to put a radius in the two corners of the cutout that straddles the knee. Thanks for the reply to the question.


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