# Jackplate Height



## GG34 (May 2, 2014)

Thinking of adding a jackplate. It's a flat bottom boat. How high can I bring the motor up?


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## Smackdaddy53 (Dec 31, 2012)

A lot of factors come into play! That's a loaded question! 
You will need to start in the middle and tweak it as you run it to figure out what she likes.


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## GG34 (May 2, 2014)

Ok the one I was looking at at a 5 1/2 inch setback and could raise 5 inches. Is that enough?


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## Smackdaddy53 (Dec 31, 2012)

That is a loaded question as well. More details please!


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## GG34 (May 2, 2014)

Yeah, I get it. Honestly, I've never used one before so I really don't know what other details to add. I did pull the trigger on one. Now I just have to install it.


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## Vertigo (Jun 3, 2012)

Every boat and motor combination is different. You'll have to experiment to find out what works best on your rig. In general, set the jack plate so that when it's full down, the motor cav plate is even with or about an inch above the keel. If you have a tunnel hull, set the jack plate at full down so that the cav plate is level with the top of the tunnel.


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## devrep (Feb 22, 2009)

4" setback is best for a small boat. One of the factors is how high your motor will pick up water. This can vary by brand and era of motor. I have 2 new tohatsu's (50 and 90 hp) and they seem to pick up water when raised pretty high. They have water drain holes all over the lower unit which may help, not sure.


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## GG34 (May 2, 2014)

Vertigo, so in the full up position, the cav plate will be above the bottom of the hull? in this case a 5 inch JP would bring it 5 inches above the bottom? it's not a tunnel. It's a flat bottom.


Vertigo said:


> Every boat and motor combination is different. You'll have to experiment to find out what works best on your rig. In general, set the jack plate so that when it's full down, the motor cav plate is even with or about an inch above the keel. If you have a tunnel hull, set the jack plate at full down so that the cav plate is level with the top of the tunnel.


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## Vertigo (Jun 3, 2012)

GG34 said:


> Vertigo, so in the full up position, the cav plate will be above the bottom of the hull? in this case a 5 inch JP would bring it 5 inches above the bottom? it's not a tunnel. It's a flat bottom.


Yes, but chances are the only time you'll be able to run with the jack plate all the way up is when idling. No use having the motor lower than necessary, and the cav plate at or slightly above the keel is about as low as you should ever need it. From that point the motor can then be raised to whatever position will work best for the speed and depth of water you're running.


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## WhiteDog70810 (May 6, 2008)

5" lift is more than enough. Most small boats use JPs that have 4" setback and 4" lift. Unless you invest in a lot of other stuff, a JP simply helps you adjust your motor's height for optimum SPEED. The more accurately you mount the JP, the less lift you'll need. As Vertigo said, with 4-5" of setback, just mount it so the motor's cav plate is 1" above the bottom at the lowest setting. 

We also use them to enable a motor to run shallow, but it takes more than just a JP to really see a difference. You'll need tabs, a cupped prop and an anti-ventilation hydrofoil. Some argue the gains are not worth the effort.

One perk of 5" of lift is that you can hypothetically set it for a 15" shaft outboard, but still run a 20" shaft in a pinch. 

Nate


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## GG34 (May 2, 2014)

Vertigo and nate. Sorry for the repeat questions. After reading both posts I see one one recommending the cav plate level with the keel in the up position and the other in the down position. I may be misreading.


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## WhiteDog70810 (May 6, 2008)

GG34 said:


> Vertigo and nate. Sorry for the repeat questions. After reading both posts I see one one recommending the cav plate level with the keel in the up position and the other in the down position. I may be misreading.


Reread Vertigo's first post. We are saying the same thing; cav plate level or 1" above the bottom at the lowest point. You never need the ability to drop it lower than that because the lower unit would then present a larger profile, thereby increasing drag and decreasing speed.

If you want to do the trig to see what we are talking about, measure the distance from bottom of your transom to the front of your cav plate along a line that is continuous with the bottom. Add 4" of jackplate setback to that measurement to get the total distance to the front of the cav plate (let's assume a total of 8" for simplicity). Most boats plane with the bow 6-7 degrees above level. The tangent of 6 degrees is ~.105. 

.105 x 8 = .84"

At .84" above the bottom when level, the cav plate will be level with the bottom of the transom when on plane. We generally can run a bit higher than that, so we round to 1".

Nate


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## GG34 (May 2, 2014)

Got it, thanks guys


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## cougmantx (May 25, 2016)

Okay, admittedly I'm a rookie at this. I've been considering a jack plate for my 16'2" Skull Island with a 30 hp 4 stroke Honda, with a 15-inch shaft. I'm ordering a power tech SRA312P based on power techs recommendation. 

I've been told that a jack plate wouldn't really give me any benefits. What's yall's experience with similar skiffs?


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## Vertigo (Jun 3, 2012)

cougmantx said:


> Okay, admittedly I'm a rookie at this. I've been considering a jack plate for my 16'2" Skull Island with a 30 hp 4 stroke Honda, with a 15-inch shaft. I'm ordering a power tech SRA312P based on power techs recommendation.
> 
> I've been told that a jack plate wouldn't really give me any benefits. What's yall's experience with similar skiffs?


A jack plate may let you run the motor a few inches higher when on plane and maybe four or five inches higher when idling. Unless you really need to run shallow, it probably won't buy much in terms of performance.


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## GG34 (May 2, 2014)

Wanted to revisit this question. The deal I had on the hydraulic JP fell through. Don't want to spend the money so I was considering a manual. I'm not sure if it is worth it or not. My goal is to raise the motor to run shallower. Not necessarily looking for performance. The cav plate is currently level with the hull bottom. would I gain anything from a manual JP? How do I know if I have a fixed 5 inch lift if I will still be able to pickup sufficient water? how hard is it to adjust a manual on the water?


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## WhiteDog70810 (May 6, 2008)

GG34 said:


> The cav plate is currently level with the hull bottom. would I gain anything from a manual JP?


Not much by itself. With 4" of setback, you'll need to raise the cav plate >.84" above a line continuous with the bottom to see any decrease in running draft. You can definitely get higher than .84" in a straight line, but you'll lose speed due to aeration of the water and it will ventilate in corners once you get it up to the high end of the range. I'd bet you only get .5-2" decrease in running draft with the stock prop.


GG34 said:


> How do I know if I have a fixed 5 inch lift if I will still be able to pickup sufficient water?


You will not be able to run the motor with the cav plate 5" high without spending a lot more money. To run a motor that high, you need a cupped prop, a low water pick up, an anti-ventilation plate and probably trim tabs. The forward motion of the boat creates a vacuum behind the stern. Water flows under the hull and is then lifted by this vacuum, creating the hump of water that a jackplate attempts to exploit. The higher it is lifted, the more air that is mixed with the water, i.e. aerated. Clean water has relatively little air in it. Liquids do not compress much under pressure, so clean water gives the prop greater resistance to push against (more thrust). Gases compress under pressure, so aerated water is "fluffy". When a prop tries to push against it, aerated water compresses like a soggy pillow and provides the prop much less resistance to push against (less thrust). Cupping allows a prop to compress aerated water to a greater degree and create more thrust in aerated water than an un-cupped prop. A cupped prop can push the boat forward faster in aerated water, much like a mud tire in mud, but it is slower than an un-cupped prop, much like a mud tire compared to a road tire. As you are aware, outboards over 5 hp are predominately water cooled, not air cooled. Because aerated water has a lot of air in it, it cools the motor less effectively. A low water pickup pulls in less aerated water to more efficiently cool the motor. The tabs channel more water to the prop and the anti-vent plate holds it around the prop.


GG34 said:


> how hard is it to adjust a manual on the water?


For most, you loosen 4 nuts on the sides, then you tighten a nut on a vertical threaded shaft to lift the motor. You then tighten the 4 nuts again. It is very easy, but it takes time. You set it to get the best speed with your prop under normal load and then don't mess with it again unless you change the motor or the prop or your normal load. You do not mess with it every day.

To make this mess work, you need speed. The faster the hull moves, the greater the vacuum behind the stern, the greater the hump of water behind the stern and the greater the distance of the hump behind the stern. A tunnel helps move that hump closer to the stern where it is easier to exploit. This sounds simple, but all this adds weight to the stern or decreases the displacement of the stern. A true microskiff is made for low power and its stern isn't wide enough to support the extra weight of a bigger motor nor can it sacrifice the displacement for a tunnel, so it is best not to expect outrageous decreases in running draft. I have a manual JP and I'll probably get a cupped prop one day, but I'll likely only get around 2-3" of honest decrease in running draft in the end. Vertigo has hulls set up to run really shallow and can more accurately tell you the specifics of the set-up and tuning.

Nate


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