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Thread: replacing the bottom in a jonboat?

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    Default replacing the bottom in a jonboat?

    Looking for some opinions on replacing the bottom in my jon boat.
    It's a rivited 1448, 0.060" hull.

    I plan on putting a 4'x8'x0.125" 5052 on the bottom. I'm not sure on the best way to do it. I talked with a local shop and they want to cut the old bottom out. I don't see a reason to do that, other than it might save a bit on labor. I think that cutting the old bottom completly out would cause you to loose alot of structure that is already there.

    Here are some pics of what I was thinking of doing.

    Cut out the runners (represented by the black), grind the outside rivits flush with the bottom, weld the new bottom to the stringer (i think that's what you call it), weld the stringer to the origional bottom just because. Also put a weld from the new bottom to the old in the middle of the runners I removed (forgot to put that one in the picture) The welds are the grey areas.



    On the back of the boat I was thinking of leaving the new bottom sticking out 3/8-1/2" and putting a 4-5" plate on the back of the transom the whole width of the boat.



    I would drill a few holes in the stringers so water can get back to the bilge.

    How would you go about doing this?

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    I'm not exactly sure why your doing this but if I'm following what you are saying, by removing the runners you in effect would be completely changing the structure and its rigidity not to mention how the boat will react on the surface of the water without some kind of strakes or chines (if I got my terminology correct). I'm not a boat builder however I do machine and weld it and if I was going that far I would personally build a new boat but that would have to wait until winter time now Yes I realize that would cost more and aluminum isn't cheap these days.

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    I will put some angle on the bottom for new runners. One of my old boats had the back 4' replaced and never had runners put back on. Man that thing was a slider! I put on 2 peices of T bar and that took care of it.

    I figure I can do it for about 300 in material. The sheet priced out today at 175. I didn't price out any angle. Then wire and gas.

    I have the equipment to do it, and I don't charge myself too much.

    The main reason I'm doing it is so I don't rip a hole in it 100 miles from nowhere. I tend to be a little on the abusive side with my some of my toys.

    I think I'm going to do it. Hardest part will be making that first cut.

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    LOL, a slider is exactly what I was thinking could be an adventure all by itself. Well for $300 I guess I can understand. You might consider leaving 1-1.5" extra at the transom until you decide exactly how you want to set that up. You can always cut off any excess later if it was necessary but you might find it useful to help trim the boat (onstep that is).

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    If you leave the old bottom on and you get water inbetween the new and old bottom when it freezes you will separate the bottoms because of the ice.

    I'm not saying it will, but could it?

    What is 1448 alum? can you weld 1448 to 5052?
    Are you going to TIG or MIG .060 to 0.125?

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    Ya I thought about the water getting in there and freezing. I think there should be enough flex in there where it shouldn't cause any problems. But I've been wrong before.

    1448 is the size of boat, 14' long 48" wide. Don't know what type of aluminum it is made of.

    mig welder

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    Member Erik in AK's Avatar
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    Farmer,
    If wear is a concern then you could save yourself a lot of effort by fastening UHMW to the runners.

    UHMW or Ultra High Molecular Weight is high density teflon plastic--the same stuff used to make snowmachine hyfax.

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    Quote Originally Posted by Erik in AK View Post
    Farmer,
    If wear is a concern then you could save yourself a lot of effort by fastening UHMW to the runners.

    UHMW or Ultra High Molecular Weight is high density teflon plastic--the same stuff used to make snowmachine hyfax.
    What about building a boat out of the stuff. My dad makes snowmachine sleds, and I think making a boat would be awsome. You could just rivet or bolt ribs to the plastic. It might be pretty heavy, but would almost be worth it.

    -Eric

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    UHMW is not very strong, doesn't hold shape terrible well. You'd have to use really thick UHMW to equal the strength of aluminum, and it would be really heavy and expensive.

    Great stuff for wear points, but absolutely not to be used for structural applications.

    To the original question, the runners have two purposes, one for tracking, and two, it provides stiffness to the thin aluminum.

    If the current bottom isn't completely trashed, I'd say it's alot of work for questionable benefit and possible detriment to the hull. I like the idea of just putting UHMW on the bottom of the runners. 1/4" will be plenty, use 5200 bedding compound and rivet them in place.

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    I will be welding new runners/ribs on the outside of the new bottom. Should be plenty of support.

    I don't see how you could form the UHMW to fit over the existing runners. Any tips on that?

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    Moderator Paul H's Avatar
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    I'd use a table saw to rip UHMW to the width of your current runners, and rivet it to the bottom of them. Also I'd run a bead of 5200 on the runner to bed the UHMW in place.

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    Quote Originally Posted by Paul H View Post
    UHMW is not very strong, doesn't hold shape terrible well. You'd have to use really thick UHMW to equal the strength of aluminum, and it would be really heavy and expensive.

    Great stuff for wear points, but absolutely not to be used for structural applications.
    Actually from my experiance it does hold shape fairly well. With stiff ribs, and some bars running the length of the boat, I think it would be sturdy enough. You wouldn't have to use really thick UHMW, 1/2 inch would probably be fine. That would still be a heavy boat, definately heavier than aluminum. So a little more hp would be needed. It would be a fun project if you had money burning a hole in your pocket.

    -Eric

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    Well I'm canning the idea till winter. Decided to fix her up enough to use for the summer. Got tired of looking at the new motor sitting in the corner.

    Thanks for the ideas.

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    Quote Originally Posted by Paul H View Post
    I'd use a table saw to rip UHMW to the width of your current runners, and rivet it to the bottom of them. Also I'd run a bead of 5200 on the runner to bed the UHMW in place.
    I've giving this Idea some thought in the past, but I can't figure how you would mold and attach the UHMW to the rounded keels on a jon boat.

    I also was wondering how to protect your outboard jet unit (besides a tunnel) when log jumping and situations like it when there could be sudden force applied to the unit, and possibly ripping you reverse gate off.
    "When everything seems to be going against you, remember that the airplane takes off against the wind, not with it."
    Henry Ford

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    Moderator Paul H's Avatar
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    Use a shaper or a router in the router table and find a bit with a shape close to the shape of the bottom of the runners. You'd have to go to thicker UHMW to keep a good 1/4" thickness.

    As to UHMW stiffness, I have multiple strips of 1/2" in various thickness for rub rails on my boat, and their laying in the rafters. They look like noodles to me. It has good wear characteristics, but stiff it isn't.

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    so could you mold a piece to fit the whole hull of a jon boat, or would it have to be many small ones?

    I've seen Lab Man's sleds, there good sleds.
    "When everything seems to be going against you, remember that the airplane takes off against the wind, not with it."
    Henry Ford

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    Thumbs up UHMV bottoms - check this out

    If you're interested in UHMV bottoms, check out this site. One of these days I'll e-mail them for a price...

    Also, check out the UHMV intakes!

    http://www.rockproofboats.com/home.html

    http://www.rockproofboats.com/welcome.shtml

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    Member Erik in AK's Avatar
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    BigO thanks for the link...and since its come up has anyone seen the 2007 Jetcraft Extreme Duty? It comes from the factory with a molded 1/2 inch UHMW bottom plus an additional 1/2" keel plate

    Looks like this is the wave of the future for river sleds--now we just need someone to develop custom fit UHMW bottoms for existing boats.

    I'm thinking this winter, when I redeck the floor of my Koffler, I just might invest in some plastic

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    Thanks for the site...To bad they are in PA, looks like a great business. We need one up here. I wonder how that UHMW intake will hold up. I would think that the silt would kind of wreck shop on it.

    I just talked to the guys down at the boat shop about there extreme duty. There is a video on you tube right now of them sliding it across gravel up on the Good Pasture. They said it was about a 200' gravel bar.
    "When everything seems to be going against you, remember that the airplane takes off against the wind, not with it."
    Henry Ford

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