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 Longtail and prop climbing out of water...DIY cav plate? 
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Joined: Thu Jan 18, 2018 5:14 pm
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Post Longtail and prop climbing out of water...DIY cav plate?
Nice site you guys have here, sorry this is a long post, bear with me. I'm originally from the NOLA area and now retired and living in East TN(yep between the flyways). We have a contingent of resident Canadians, a smattering of greeheads, w/teal and woodies so I decided I wanted a LT type rig. I have two big lakes within 20 min and both get drawn down by the TVA each winter, hence the need for shallow draft. I read and researched and learned that Backwater gets the highest praise, so that is what I looked for. Unfortunately being in a low duck zone it became a needle/haystack thing. Even finding just a hull and trailer to add my engine and frame was frustrating.

I wanted a 1648 flat bottom, but ran across a 1648 MV Polar Kraft DB(duck boat) w/20" transom. On it was a Predator 22hp and a 110" longtail Mudskipper frame. From reading I know that the MV will not give me top speed performance, but the open layout w/gunbox and the price hypnotized me.

So here is where I'm at...the good:

-Engine runs great after I added a aux fuel pump
-Top speed, two 200lb guys and gear is 17mph WOT
-Hull: no leaks

The bad:

-Tremendous amount of "up force" on tiller at WOT needed
-Significant amount of same pressure needed at lower planing speeds

Inside the gun box were spare props, a shaft bushing, two prop nuts, and a Mud Skipper anti cav plate. The plate had obviously been used, but removed and disassembled...."hmmmm" :?:

I put it back together, and literally the thing weighs 5 lbs(it's 3/16th steel plate). The angle was set so the back of the plate would not clear the prop, so I installed it in front of the prop and did a test run. 13mph on plane at half throttle and now a "neutral" tiller handle. Turning at that speed still take quite a bit of effort, but is doable. Remember before the cav plate I was WOT at 17mph and I could taste my spleen while grunting to hold that tiller up.

The river is real clear, and I could see the prop/cav plate running at about 8-10" under the surface, so I'm not happy with that, so back to the shop. Re-bent the plate to extend out over the prop and re-mounted. Down to the lake yesterday evening(last day of Duck season is Sunday) for a quick hunt and a test run.....battery was dead even though it was charged :shock:

So now I am re-thinking this whole Mud Skipper cav plate thing. It runs too deep, It's also just too heavy so as soon as you slow your speed it pulls the prop down, which angles the engine, which makes the low oil sensor shut the engine off. I like the neutral handle at 13mph half throttle, but there is no way I can run the thing that deep.

I have a Millermatic 210, so today I am going to weld up my own lightweight version of a cav plate and with a new battery we are going back out this evening. Another option is to cut the transom to reduce the shaft angle which may or may not help. If any of you have made your own plate, what did you learn? I found one thread on it here and the info was limited. I [think] a lightweight plate can perform the same as the 5lb one yet be adjustable(by bending) and not pull the prop down so deep.

Last option will be to sell the LT frame and by the SWOMP Lite 23 frame 8-) ....but I am not ready to throw the towel in yet.


Fri Jan 26, 2018 8:21 am
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
Original cav plate angle:



Mud skipper plate on left, build on right:



Homemade light-ER plate mounted:



Fri Jan 26, 2018 6:06 pm
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
I started out with a bend/angle that would get the homemade plate above the prop. Took it to the river today and it ran so deep I could not even drive it. Took it off, stepped on it on the deck and bent/changed the angle so it now sat in front of the prop. 10mph max before it would climb up and break the surface. Original angle was 19 degrees, I have read that it should be 15, so I will check it tomorrow. Ran 20 min down river...zero birds.

Can someone correct me if I am wrong, the plate should be set parallel to the bottom of the hull right? Anything other than that will either dive up or down?

One more thing, the prop is running about 1.5 feet behind the "V" created by the hull, so I think that is correct.


Fri Jan 26, 2018 6:17 pm
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Location: West TN
Post Re: Longtail and prop climbing out of water...DIY cav plate?
Goodness. Major props for trying to fab up your own cavitation plate and make things work. Even if you get everything dialed in and running right, that is still a cheap kit and you are seriously limiting yourself in what you can do and where you can go. I'd be looking to make the upgrade to a Backwater SWOMP Lite frame in the near future. Just my humble opinion. Best of luck from the West end of the state!

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Fri Jan 26, 2018 9:41 pm
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
Frank I hear ya, but I just can't give up on it until I have tried everything that my pea brain can come up with.

The longtailers also make a tab that goes on the skeg, this is what I am going to try next. This guy has the same engine and frame as mine along with a 1648 flat bottom and is claiming 27mph using this small tab:

https://youtu.be/Zhc72te6g6w

I can fab this up and weld it on in about 15min....stay tuned


Sat Jan 27, 2018 6:01 am
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
Have that transom cut down to 15"-16" that would help a lot.

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Sat Jan 27, 2018 9:30 am
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
CC I agree and even when I do transition to a Swomp Lite the lower transom would be a bonus since I can get the shorter tail.

Well my 15 min turned into an hour and a half. I grabbed SS plate and it just takes lokger to cut and work it. First plate I did I didn't like, so I started over. I used a section of 5" drill pipe to beat the curve into it. The skeg is some kind of Thai pot metal and even though it was magnetic it didn't like the weld until I preheated it....I think it will hold :geek:

I had no measurements so I had to just compare the placement and angle with a picture I took off the guys video. Here is the semi final product:



Now I know it doesn't look like much, but at this point I might be grasping a little 8-) So it was off to the Holston River to get in a test run and a scout.

Well boys and girls it turns out that ThroazineShuffle75 might be a genius. The tiller remained "neutral" at all speeds except wide open and then I had to lift up just a bit. Darned it I didn't maintain 21mph @ WOT of 3600 rpm. At 2100 rpm I could maintain 17mph. Of course my hunting buddy was at a fishing show in Knox so his 200+ lbs of chewed bubble gum was not in the bow. I did find a nice shallow cove with a hundred or so ducks in it, so it was a good day.

Last day of Duck/goose season tomorrow...headed out at Oh dark thirty. Will update after the hunt..........


Sat Jan 27, 2018 5:53 pm
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
That’s awesome, nice work!


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Sun Jan 28, 2018 9:34 am
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
Well I got skunked, but it was a beautiful day with 100% rain and it never stopped. It took 45 min to idle to the spot in thick fog using google maps on my phone because it was hard to even see the mountains much less the shoreline. Nothing flew when the ceiling was on the deck, when it lifted they were in the stratosphere.



I did learn that the rig does not like 5 gallons of rain water washing up against the transom so I had to do the one handed remove and re-install drain plug thing......a bilge pump is next on the list.

I have an unrelated Q, the floor is a sheet aluminium over 1/2" rigid foam board. It's a Polar Kraft DB model so it's factory installed. I have never owned a Jon with a (semi)permanent floor. I suspect a lot of crap has made it way underneath it, but it's riveted down. Is there anyway to clean that without removing 4700 rivets?


Mon Jan 29, 2018 4:57 am
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Post Re: Longtail and prop climbing out of water...DIY cav plate?
Any chance of that foam board becoming water logged and heavy?

How closed off is it under the floor? Can you prop the boat up on its side and wash the debris out?


Mon Jan 29, 2018 10:51 am
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Post Re: Longtail and prop climbing out of water...DIY cav plate?


Mon Jan 29, 2018 11:55 am
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