# Cavitation plate 2 inches below transom, worth raising?



## EddiePA (Jul 19, 2014)

Is it worth raising the outboard? Cavitation plate is exactly 2 inches below the transom. This is on my Sea Nymph 12K with a 15hp Johnson.


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## onthewater102 (Jul 19, 2014)

How hard would it be to do?


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## SumDumGuy (Jul 19, 2014)

throw shims (2x2 or 1x2) under the mount and see if there is performance improvement.
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BTW and this is for everyone!!!

there is no such thing as a cavitation or anti-cavitation plate (contrary to what anyone says on this board or any other). The plate that is talked about on the lower portion of the motor leg is correctly identified as an anti-ventilation plate.
Cavitation is when you boil water on the prop and ventilation is when you suck air from the surface through the prop.
just sayin' ....


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## EddiePA (Jul 19, 2014)

Because of the shape of the top of the transom and mount of the outboard, I don't think I can simply add shims. Adding a shim to the top of the transom would cause the mount to interfere with the rolled top portion of the transom and cause the mount not to sit flush against the transom. A mini jack plate would probably be the solution to raise the outboard if I decided to go that route.






Top speed of the boat with me (165lbs), associated fishing tackle, anchor, 55lb thrust Minn Kota, marine battery and 4 gallons or so of gas is 29mph, so it's not like the boat is hurting for speed. However, I wouldn't mind making it less likely of hitting bottom or other debris in some of the shallower areas I fish. Less fuel usage is also always a plus. If it picked up a MPH or two, I certainly wouldn't complain.


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## lckstckn2smknbrls (Jul 19, 2014)

Yes raise the motor. I'm using a T&H Mini-Jacker.


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## [email protected] (Jul 19, 2014)

Here is one I built for for my motor


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## onthewater102 (Jul 19, 2014)

performance and efficiency will be better - by how much I can't say for sure


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## EddiePA (Jul 19, 2014)

Thanks, guys. I ordered a mini jack plate. I'll report back with results once installed.


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## whitetailhntr (Jul 20, 2014)

29mph!!!! Nice


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## EddiePA (Jul 24, 2014)

Jack plate arrived today. Should I mount it with or without plywood between the plate and the transom? The wood that I have on the transom now is too narrow for the jack plate to mount to it. Need to know if I have to replace it with a wider piece or ditch it all together.


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## JMichael (Jul 24, 2014)

I would replace the wood with a piece big enough to fit the jack plate.The wood provides a big portion of the transoms strength.


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## EddiePA (Jul 24, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=360866#p360866 said:


> JMichael » 24 Jul 2014, 22:12[/url]"]I would replace the wood with a piece big enough to fit the jack plate.The wood provides a big portion of the transoms strength.



My thoughts as well. Thanks.


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## onthewater102 (Jul 24, 2014)

depends on if there is wood on the inside of the transom too - I see in your pics there is a plank outside, but what about the opposite side?

Also - I've seen guys run two strips of 4' 1/4'' thick aluminum angle horizontally on the inside of the transom where the two bolts come through in place of the wood. Makes for a permanent transom stiffener for about $50 delivered from www.onlinemetals.com (buying 8' and cutting it yourself)


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## EddiePA (Jul 26, 2014)

The inside just has a small piece of 1/2" birch at the top of the transom where the outboard clamp mounts.

I hope to stop by Lowe's today to buy more 3/4" birch to redo the transom.


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## EddiePA (Jul 27, 2014)

Finished the install (for the most part). Plate is level with the bottom of the transom in this position. Got it all done in a day, including removing the transom wood and building/installing a new, larger piece. Hope to test her out this week, weather permitting.


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## lckstckn2smknbrls (Jul 27, 2014)

With the 4" set back from the mini-jacker you can raise the motor 1inch to 2 inches higher.


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## EddiePA (Jul 27, 2014)

I have the plate set up to raise the outboard an additional inch or so if needed.


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## JMichael (Jul 28, 2014)

Lets us know what the new top speed is when you have it fully adjusted.


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## whitetailhntr (Jul 28, 2014)

EddiePA said:


> Thanks, guys. I ordered a mini jack plate. I'll report back with results once installed.



What was the coat of ur jack plate?


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## whitetailhntr (Jul 28, 2014)

Cost


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## EddiePA (Jul 28, 2014)

Weather isn't cooperating today. Hope to test the boat out tomorrow after work. The mini jack plate was $100.


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## Siguz (Jul 28, 2014)

EddiePA said:


> Weather isn't cooperating today. Hope to test the boat out tomorrow after work. The mini jack plate was $100.


Where did you buy it/order it from?


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## Charger25 (Jul 28, 2014)

Yall gonna make me buy one before its over lmao


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## whitetailhntr (Jul 28, 2014)

Siguz said:


> EddiePA said:
> 
> 
> > Weather isn't cooperating today. Hope to test the boat out tomorrow after work. The mini jack plate was $100.
> ...



X2


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## EddiePA (Jul 28, 2014)

I ordered the TH Marine mini jack plate thru Amazon. I believe the vendor was called "landmproducts". 

This is a video before the jack plate.

https://www.youtube.com/watch?v=oQNGFUVgIH8


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## EddiePA (Jul 29, 2014)

Went for the first test ride today. With the outboard trimmed in the position that yielded the best MPH without the jack plate (second hole out from full in) and the cavitation plate level with the bottom of the transom, the boat actually lost 1mph and gained a TON of porpoising. Even with the fuel tank placed in front of me as far as the fuel line would allow (didn't have a problem with the tank behind me previously) and the anchor at the bow, the boat was NOT happy. It wanted to porpoise unless I was leaning forward with my hand barely on the tiller. For the hell of it, I went to the third trim hole out and the boat didn't want to plane at all unless I let go of the tiller and leaned way forward. Porpoising was out of control. Also at these settings, the prop wanted to blow out in turns.

It seems the engine being set back that additional 4 inches AND being raised up 2 inches is creating enough leverage that the bow doesn't want to settle. It would probably be ok with a second person in the boat, but I still have a loss of speed, at least with the outboard at this height. I could try raising the outboard an inch via the jack plate, but I think that will worsen the porpoising condition (even more leverage) as well as prop blow out while turning. I tried removing the trim pin to allow the outboard to go full in, but it didn't noticeably help. Lowering it an inch may help, but then I will be adding drag and defeating the purpose of the jack plate.

Also, I noticed quite a bit of water riding up the leg of the outboard. This didn't happen without the jack plate when the cavitation plate was 2 inches below the bottom of the transom.


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## PatinIdaho (Jul 29, 2014)

Here is what I noticed with my setup.
I raised my cav plate to be level with the bottom of my transom and it just did not work well at all no matter what I did.
So I lowered it 1 inch and things improved ALOT.
What I discovered was there is a rib, keel or what ever you call it running down the center of my boat that I believe was the problem.
My cavitation plate is now level with that and its all working together now just fine. Might try what I ended up doing and see if that helps?


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## EddiePA (Jul 29, 2014)

Lowering it an inch below the bottom of the transom (like you did), as I see it, is the only way to go as raising the outboard further, I believe, will worsen my current issues. I gotta say, if that doesn't make a dramatic improvement over what I started with without the jack plate, IMHO, it just isn't worth going through the trouble of buying and installing the thing on a boat this size and it's intended purpose (fishing in smaller lakes). I have a feeling I'm still going to have porpoising issues even after I lower it an inch as it being set back 4 inches is just too much leverage on this little (12ft) boat. Perhaps on a longer boat, say at least 14ft, the set back might help. Not convinced it's an attribute on something this small, however.


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## Boat2fast (Aug 1, 2014)

Thank you for sharing your experience. All your time/trouble/expense may not be completely wasted if it saves others from bad result. 
My experience with similar setups is always to try a 3/4in shim to elevate the engine. If it results in a benefit(usually), then keep it. If it hurts(occasionally) then remove it and there is no loss.
I have a real fast SeaNymph 14R/Johnson 30; the shim hurt. Prop ventilated continually. Without the shim it was fine.
Your 29mph was a great result. It's not often the boats go that well. Yours rode nice and straight too, judging by the video.
The picture is a 1969 Evinrude Sportster 25 on a 10ft SeaNymph. Boat went best with the shim and a 2blade brass prop. 34mph/gps


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## onthewater102 (Aug 1, 2014)

Before you write off the transom jack could you please post a pic of the setup in the two pin configurations you tested it once you installed the jack?


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## EddiePA (Aug 1, 2014)

I pulled the jack off the boat for now, so I can't post a pic of those trim positions. The boat wasn't safe to run with the jack on and I'm using the boat as often as possible (several times a week). When I get more time to test I may try the jack again with the outboard an inch lower than before.


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