JACK PLATE

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 ?

They sold you the wrong motor, take it back?

lckstckn2smknbrls said:

They sold you the wrong motor, take it back?

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Not an option at this point. But thanks

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.

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.

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.

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.

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.


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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

Sell it or trade it.

lckstckn2smknbrls said:

Sell it or trade it.

Click to expand...

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.

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.

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?

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.

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

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.

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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.

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.

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.

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.

Click to expand...

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.

Click to expand...

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.

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.