JACK PLATE

[CedarRiverScooter]

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

 

[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.

 

[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.

 

[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]

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.

 

[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.

 

[RaisedByWolves]

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.

 

[Rookie1719]

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

 

[RaisedByWolves]

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?

 

[clarkbre]

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……

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.

 

[C&K]

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.

 

[clarkbre]

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.

 

[eeshaw]

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?

 

[clarkbre]

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.

 

[Sinkingfast]

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.

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.

 

[C&K]

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.

 

[eeshaw]

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.
20210620_120227.jpg

The front is also supported by the boat's center brace that ties into the floor and transom.
20210620_120249.jpg

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.