Are 1x3 runners under a jon boat worth it?

Discussion in 'Boat Design' started by Raptor88, May 31, 2021.

  1. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    I've read that runners under a flat bottom jon boat:
    1. Helps with tracking straight.
    2. Protects the bottom from rocks.
    3. Helps with securing things inside of the boat.
    4. Has several cons.
    1. I saw one youtube video where the boat builder says that 1x runners are too shallow to anything with tracking straight.

    2. I can't see how three or four 1x3 runners under a flat bottom can protect against rocks. I can see them protecting on a flat concrete boat ramp but rocks are bumpy and would hit the plywood bottom before they hit the 1x3 runners.

    3. I think number-3 is valid since I can see where 1x3 runners under a plywood bottom can help to secure things inside of the boat.

    4. Cons are: Can disrupt water flow to the prop, impedes turning boat on sand, hinders covering bottom with fiberglass.

    Mainly number-1. Do 3/4"H x 2-1/2"W runners under a flat bottom jon boat really help with tracking straight? When not planing, it seems that just the sides would keep the boat tracking straight. Do 3/4" high runners really help tracking while planing?

    I'm wondering if the cons out weigh the "pros".
     
    Last edited: Jun 1, 2021
  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Jeff Spira's 14' Crawdad Jonboat does not have any runners, and the design seems to work well.
    Spira International Inc - Crawdad Jon Boat https://spirainternational.com/hp_craw.php

    Have a look at the samples from the construction drawings here - the 2" x 4" keel is inside the boat, and she has 3/8" thick plywood for the hull shell.
    https://spirainternational.com/study/CrawdadStudy.pdf
    The hull bottom has some deadrise though, so the keel usefully breaks up what would otherwise be a large flat hull bottom into two smaller areas.
    And he does not mention any epoxy glass sheathing - just 2 quarts of epoxy for bonding.

    Maybe the slight 'vee' in the bottom has more effect than longitudinal runners on an otherwise flat bottom?
     
  3. Raptor88
    Joined: Apr 2021
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    Raptor88 Junior Member

    Thanks for that example. One for no bottom runners.
     
  4. Mr Efficiency
    Joined: Oct 2010
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    Mr Efficiency Senior Member

    Good idea if they don't interfere with the prop in-flow.
     
  5. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    Runners might disrupt water flow to the prop.

    If 1x3 runners don't help with tracking straight, I don't see any benefit other than helping to secure things in the boat but even then, screws going through the boat's bottom into runners would seem to "require" that the bottom be fiberglassed "over" the runners. Especially in salt water.

    From a layman's view point, it seems that the cons of 1x3 runners under the bottom have more cons with no real benefits.
     
  6. Ike
    Joined: Apr 2006
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    Ike Senior Member

    They stiffen the bottom longitudinally, especially if the bottom ply is rather thin. They take the place of longitudinals inside the hull. Doesn't matter much at slow speeds but on plane can keep the bottom from flexing. At slow speeds it takes much deeper runners to track straight.
     

  7. Raptor88
    Joined: Apr 2021
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    Location: Hawaii

    Raptor88 Junior Member

    Thanks for your input.

    To minimize the bottom from flexing on plane, I think I'd much rather have internal bottom longitudinals with a clean external bottom to eliminate any possibility of disturbing water flow to the prop with external runners. Plus not having to fiberglass over bottom runners.

    It does seem logical that 2 or 3 inches of draft at slow speeds, 3/4 or 1 inch thick bottom runners would have a negligible affect on tracking compared to the submergence of the sides of the hull. Protection against rocks that is often listed as a benefit for bottom runners, totally doesn't make any sense to me.
     
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