New to forums with Jon Boat Questions

Discussion in 'Boat Design' started by jnwaco, May 26, 2009.

  1. jnwaco
    Joined: May 2009
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    Location: Waco, Texas

    jnwaco Junior Member

    Hi,

    I've been reading and have a design question that I did not see addressed previously.

    I bought a 1980 Alumacraft 18' aluminum flatbottom boat. It's riveted. It has a 65 horse mercury outboard on it. The layout is below. I intend to use this boat for fishing on local lakes and rivers, freshwater.

    The boat came with carpeted plywood sole/floor that I've pulled out because of the onset of rot.

    I want to replace the sole with aluminum sheet over the ribs and build a deck on the front of the boat on top of the gunwale. These parts I've shaded in gray.

    The first four ribs have cracked in the center. Apparently, they had been rewelded only to crack again. I imagine being a flatbottom that the previous owners tried to run some really rough water, or that this is just 20 years of use.

    Where the gas tank is located there was not a seat there previously. There is aluminum framing welded in place and the gas tank sits in that framing. It was sheathed in plywood and will sheath this in aluminum as well.

    My questions are:

    1) Is there a proper way to repair the ribs at the front of the boat without replacing them?

    2) The deck at the bow of the boat will need support underneath to support people standing on it. Should the deck be braced vertically with supports that are attached to the floor of the boat (on the ribs)? I've seen others with just horizontal frames underneath the decks. Would adding vertical supports create any unwanted affects? I would think that it might help stiffen up the front of the boat.
     

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  2. messabout
    Joined: Jan 2006
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    Location: Lakeland Fl USA

    messabout Senior Member

    1. Consider sistering the broken ribs. That means to add a partial rib of a dimension similar to the original rib. Weld it in place right next to or on top of the old rib. The height if the rib is the dominant dimension when calculating stiffness in this application.

    2. Vertical struts from deck beam to stringers will stiffen the bottom a little. If you use struts that land on the stringers, or the rib, use a short aluminum section at right angles to the strut where it lands on the rib or stringer. The idea is to spread the load over a larger area of the part that the strut lands on.

    Sole; replace the rotted ply with ply of similar dimension. Do not use aluminum plate for the sole. Please do not even think of using carpet in your boat again. It is the evil contrivance of the devil himself. Carpet is a wonderful rot promoter, stink holder, weight adder, and other miserable things. Paint the new ply with some decent oil paint top and bottom. Let dry. Paint the top again and while it is wet, sift a light coat of fine sand or pumice into the wet paint. Let the paint dry. Vacuum off the loose sand, put another coat of paint over the remaining sand. Now you have a non skid surface. Be conservarive with the granular stuff it does not take much to do the job. You can use ordinary house paint for this job if , and only if, you are willing to let the house paint cure for at least ten or twelve days before getting it wet or walking on it. The house paint is very durable if you use it right. It is also easy to repair if needed. Cheap too.
     
  3. jnwaco
    Joined: May 2009
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    jnwaco Junior Member

    Thanks for the reply!

    I had imagined that stringers would need a decent footing and not just attach directly to the rib, because of the lack of weight distribution.

    Why would you not use aluminum for the sole? And don't worry, I have no intention of putting carpet back into this boat. I just went out and vacuumed no less than 10 pounds of dirt, rotten wood, old candy bar wrappers, old screws and bolts, etc. out of the bottom of the boat. The rotting wood was actually blocking some of the water from draining from the front to the back of the boat, definitely a bad thing.

    I had considered aluminum - 1/8" 5052 for the sole for ease in washing out the boat after a particularly slimy trip (I like to catfish and bowfish). I had anticipated an aluminum floor screwed down so it can be removed if necessary, coated with a light colored line-x prior to installation.

    Price is not a huge factor - I don't mind paying for the aluminum and the deck will be aluminum anyway - which will probably run about 3x the price of ply prior to installation costs. I just thought I might get a bit more mileage out of the aluminum.
     
  4. Lt. Holden
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    Lt. Holden Senior Member

    I have mentioned this several times in reply to similiar posts (but no one has dared); have you considered (in addition to your framing repair needs) extruded or pultruded fiberglass grating or Aluminum grating like they use on equipment (and marine) catwalks and platforms. Google these subjects and there are several prominent suppliers. If you wanted to, you could install carpet with snaps or Velcro on top of this and in the worst case pull it out to dry.
     
  5. jnwaco
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    jnwaco Junior Member

    I've not considered that, Lt. Holden, but I'm not opposed and will do some research. I just want something that's not going to rot and/or crack after a few years under the Texas sun.
     
  6. jnwaco
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    jnwaco Junior Member

    From what I've seen after a quick review - the fiberglass grating is about 2.5x the price of the aluminum. I realize I said price wasn't a huge factor, but it is a consideration. Fiberglass grating looks to run about $10-12 per square foot where the aluminum is about $5 per sq ft that I've priced.
     
  7. Lt. Holden
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    Lt. Holden Senior Member

    I had not priced it recently, yes that is steep, the alternative is to paint as suggested and perhaps "DriDek" open mesh rubber snap-together tiles over so all the water drains off and it can breathe.
     
  8. jnwaco
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    jnwaco Junior Member

    There's not a chance of plywood going back into the boat. I've had to replace too many plywood soles to be attracted to that idea. We had to literally remove livewells, steering console, seating, etc., just to replace the rotted ply in my dad's boat. Mine is a far simpler boat layout, but still, I just see plywood as a sponge - eventually it rots.

    Sure, aluminum can crack and fatigue, but the aluminum in this 29 year old boat has outlasted no telling how many plywood interiors.

    I really did like the protruded fiberglass grating, and perhaps if there's a source that's less expensive, I'd still be willing to go that route. I went to about 10 web sites last night selling the stuff, and it was all rated at far more weight than I'll be subjecting it to. Seems like there would be a slightly less robust grating that might be more mildly priced. The stuff I was looking at was rated for loads in excess of 1,000 pounds for a 12" span (approximate spacing of boat ribs). And that was their light duty grating.

    I like the open structure of the grating which would make running wires and cleaning out the boat a snap. Was there a brand name or company that you remember as a supplier of the grating? Most of the web sites I visited were industrial type sites with no pricing. I got pricing from a couple of sites and ebay.
     
  9. alan white
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    alan white Senior Member

    Speaking of gratings, I would build a removable sectioned cedar sole. I would cleat pieces together, something like 2 1/2" x 3/4" strips, 1/4" spaces between, with pressure treated cleats under to create panels (dimensions of cleats dependant on spans).
    Such a sole would last a very long time if treated with a Deks olje type product every so often.
    Real wood is not only very inexpensive in this case, but also a pleasure to walk on, especially in bare feet. It's quieter too, and also looks much better than the other options mentioned.
    As far as labor goes, the wood deck would probably be the fastest to build as well.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    janwaco

    "..The first four ribs have cracked in the center..."
    Centre of what, the span between frames, or centre of the web, of centre of the weld bead?

    "..Apparently, they had been rewelded only to crack again..."

    If they have then you have no idea how many times it has been rewelded nor if the job was done properly. Constant rewelding of aluminium is asking for serious trouble.

    What do the ribs look like, can you post a picture and/or drawing?

    If you don't mind paying for the aluminium, then worth investing in marine grade, if you're close to or using it on sea water.
     
  11. jnwaco
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    jnwaco Junior Member

    Yes, I will take some pictures tomorrow and post them up. They have been repaired previously and cracked at the repair weld. I like the idea of a sister rib next to the cracked ribs.

    I just finished pulling the rotten plywood out today and vacuumed out the debris, so the pictures tomorrow will show the cracks and failed repairs.


    I found a few more sources of fiberglass grating tonight and sent off for a quote or two. The other issue with this grating is it's weight - some as much as 3.5 pounds per square foot. I estimate about 80 square feet of decking, so that would be a good bit of weight to add to a boat that I want to draft very shallow. I even considered just putting the front deck on and putting up with walking over the ribs.

    Thanks for everyone's input!
     
  12. jnwaco
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    jnwaco Junior Member

    Ad Hoc - the ribs cracked down the centerline of the boat (running from bow to stern).
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    That sounds pretty systemic, seems like she was either dropped/grounded hard or poorly designed/poorly built.

    Would be better once we can see some pictures ...
     
  14. jnwaco
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    jnwaco Junior Member

    Thanks Ad - will get them up after work today. I'm thinking it may have been used in the gulf some.
     

  15. messabout
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    messabout Senior Member

    I favored ply for the fact that it is stiffer. Half inch ply has more rigidity than 1/8 aluminum sheet. Ply will deflect less over a given span than the aluminum sheet. If screwed to the ribs or stringers as you suggest, it will add signifigant rigidity to the bottom of the boat. It will also allow the boat to run with less noise. If it is carefully painted, maybe even saturated as in WEST system, it will last for years. You can still remove it for cleaning if desired. As Alan says, wood is much more friendly to walk or stand on. His grate suggestion would do as well as the ply, for the stiffness factor, but be considerably more labor intensive while building. Grates are pretty and functional, however they have a way of swallowing small items such as sinkers, reel pawls, and other things that you might drop. I am not being hard headed here (ply vs. Alum. vs. grate), only exploring the nuances of material selection.
     
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