JACK PLATE

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C&K
Posts: 24
Joined: 18 Apr 2021, 12:50
Location: Washburn, WI

JACK PLATE

Post by C&K »

Rookie1719 wrote: 27 May 2021, 19:57 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.

Rookie1719
Posts: 41
Joined: 04 Feb 2020, 19:30
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Location: Windsor, Ontario

JACK PLATE

Post by Rookie1719 »

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.
“I have nothing in common with lazy people who blame others for their lack of success. Great things come from hard work and perseverance. No excuses.” Kobe "The Black Mamba" Bryant #8 #24

RIP 1978-2020

C&K
Posts: 24
Joined: 18 Apr 2021, 12:50
Location: Washburn, WI

JACK PLATE

Post by C&K »

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.

Sinkingfast
Posts: 228
Joined: 05 Oct 2014, 13:19
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JACK PLATE

Post by Sinkingfast »

C&K wrote: 28 May 2021, 08:29
Rookie1719 wrote: 27 May 2021, 19:57 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.


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.
2 stroke every time I can
4 stroke only when I have to..I don't like boating with my lawn mower

C&K
Posts: 24
Joined: 18 Apr 2021, 12:50
Location: Washburn, WI

JACK PLATE

Post by C&K »

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.
20210528_233425.jpg
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CedarRiverScooter
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Joined: 13 Nov 2014, 08:01
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JACK PLATE

Post by CedarRiverScooter »

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

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Zum
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Joined: 13 Apr 2008, 11:37
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Location: Nova Scotia

JACK PLATE

Post by Zum »

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.
Image

Rookie1719
Posts: 41
Joined: 04 Feb 2020, 19:30
1
Location: Windsor, Ontario

JACK PLATE

Post by Rookie1719 »

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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“I have nothing in common with lazy people who blame others for their lack of success. Great things come from hard work and perseverance. No excuses.” Kobe "The Black Mamba" Bryant #8 #24

RIP 1978-2020

C&K
Posts: 24
Joined: 18 Apr 2021, 12:50
Location: Washburn, WI

JACK PLATE

Post by C&K »

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?

Rookie1719
Posts: 41
Joined: 04 Feb 2020, 19:30
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Location: Windsor, Ontario

JACK PLATE

Post by Rookie1719 »

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.
“I have nothing in common with lazy people who blame others for their lack of success. Great things come from hard work and perseverance. No excuses.” Kobe "The Black Mamba" Bryant #8 #24

RIP 1978-2020

C&K
Posts: 24
Joined: 18 Apr 2021, 12:50
Location: Washburn, WI

JACK PLATE

Post by C&K »

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
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Location: Bucks county PA.

JACK PLATE

Post by RaisedByWolves »

C&K wrote: 08 Jun 2021, 20:58 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.
Sailbad the Sinner

Rookie1719
Posts: 41
Joined: 04 Feb 2020, 19:30
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Location: Windsor, Ontario

JACK PLATE

Post by Rookie1719 »

C&K wrote: 10 Jun 2021, 08:24 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
“I have nothing in common with lazy people who blame others for their lack of success. Great things come from hard work and perseverance. No excuses.” Kobe "The Black Mamba" Bryant #8 #24

RIP 1978-2020

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RaisedByWolves
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Location: Bucks county PA.

JACK PLATE

Post by RaisedByWolves »

Rookie1719 wrote: 12 Jun 2021, 16:22
C&K wrote: 10 Jun 2021, 08:24 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
How much did you have to raise the motor?
Sailbad the Sinner