New to forums with Jon Boat Questions

[jnwaco]

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.

[messabout]

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.

[jnwaco]

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.

[Lt. Holden]

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.

[jnwaco]

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.

[jnwaco]

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.

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.

[Lt. Holden]

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.

[jnwaco]

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.

[alan white]

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.

[Ad Hoc]

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.

[jnwaco]

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!

[jnwaco]

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

[Ad Hoc]

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 ...

[jnwaco]

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

[messabout]

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.

[jnwaco]

Thanks Messabout -

I really like the grating. I'm still looking for some that's about on par with aluminum as far as price goes. I thought about hooks getting caught up in the grating, or dropping weights or other items through the openings. That's not so bothersome with a set of long needle nose fishing pliars.

Do either of you have a name brand or company for grating that you've seen or heard of being used in boating applications? The colors do make it pretty attractive and it definitely seems like it will last a long time if it's UV resistant.

I'm also still trying to find out the weight rating on this boat. The USCG tag is long gone and Alumacraft did not return an e-mail. I'm going to give them a call, but they may not know. I tend to be a little weight conscious given the unknown capcity of the boat, the desire to fish shallow, and the liklihood of fishing four people from the boat occassionally. As far as I can tell, the aluminum and plywood would be within 10 pounds or so from each other, depending on how much epoxy or coating you put on the ply. The grating looks to be quite a bit heavier, even though much of it is open.

Oh, and based on the residue line on the boat from the initial outing, the boat seems to draft about 3-4". That's before i took out the existing plywood and carpeting. I could probably back into the capcity if I dug up the calculations for displacement and measured the angles of the sides of the boat, but....

[jnwaco]

Here are a couple of pictures of the boat. And actually, the first five ribs are broken. This is something I could not have examined when I bought the boat because of the plywood flooring, so I need to deal with it. I did get a bargain on the boat, at $2,000. Carburetor kits, impeller, seals, and lower unit fluid will set me back another $120 or so. I had budgeted about $5,000 to get a boat up and running with trolling motor, new decking, etc. Need new controls for the motor as well. Those will set me back a bit.

But first, I need to deal with the ribs. Then the flooring and motor. Then....

You'll might notice that there are four coats of paint on the boat. The original Alumacraft gray, a light blue and a glossier dark blue, and then the newest layer, black, on the outside.

[Lt. Holden]

For the grating try customcompositesok.com

Didn't see the photos before I posted, I gotta wonder how that happened? On the bottom side of the hull, is there any wrinkling or other signs of stress? Looking at the photo, I would remove the added wooden stringers and try to find C-channel Aluminum that would fit on top of the frames to "sister" over them and then weld them in place.

It's hard to believe a quality, mass-produced aluminum boat like this would have such damage. As to the grating, I think your search finds that Aluminum is the material of choice, especially since you can weld it across your newly 'sistered'; frames and add a great deal of rigidity in doing so. To alleviate the 'lost sinker syndrome' get marine carpeting and cut it to fit, then have the edges bound (at a carpet binding service, shouldn't cost much) and simply lay it in place. Any time it gets really wet simply remove the carpeting to dry. I have been down the plywood (endlessly sealed in epoxy) and carpet route before and will never go there again.

[Ad Hoc]

From those pic's it basically looks like the ribs are under sized for the "normal duty" or the boat was run in conditions it wasn't really designed to, or just run aground once, damaged and then somone tried to repair, and did a poor job.

Also, the repair weld is just on the top exposed surface (the weld bead looks too large for the thickness of plate, perhaps gap filling too). But the underside cannot be welded, where there is a gap to the long't channel section shape, one assumes to provide some kind of strength, but obviously hasn't worked. This small recess will allow sea water to sit and slowly eat away at anything it can that has been left down there. Sounds like you had all sorts of crap, from cleaning it out.

You also need to be careful with bare wood on ally. The fact that you have a wooden sole doesn't help matter from the pictures. The wood by the looks of it wasn't protected from the ally, this lead to poultice corrosion. You only need to get a little bit of this and then your structure becomes weaker.

Either add a keelson along the boat the stiffen up the centreline and hence half the span of the transverses, or stiffen up the transverse ribs. Since the existing structure is now weaker after welding it. (you have lost approx 50% of the unwelded strength used by riveting). The fact it has cracked again is evidence of this.

[jnwaco]

Thanks for the suggestions Lt. Holden & Ad Hoc. Very much appreciated. And the best part is I'm learning the proper terms for the parts of the boat.

I bet the welder can find some sort of aluminum channel to fit over the existing ribs and weld them in place. The ribs are definitely smaller than a similar sized boat made today. I don't think the skin is thick aluminum, maybe 0.063" or 0.1", so the welder will have to be careful.

I imagine that the 29 year old boat was used on the gulf or big lakes and was "rode hard and put up wet", so to speak. Much of the dirt and grime you see in the photos would not vacuum up and will have to be brushed off, washed off with pressure washer, or just left there. I suspect it is a mix of fine treated lumber and dirt, but it's stuck to the aluminum.

The bow of the boat appears to have struck something, but the underside of the hull is in good shape, no dents or creases.

[Ad Hoc]

wow, very thin plate. That may explain the over sized weld beads. Probably MIG'd rather than TIG'd...or just gap filling anyway.

You now need to decide which way to go....
1)Stiffen up long'tly, ie add a keelson
or
2) Increase the frame (rib) sizes.

No point doing both. Then decide what kind of section to add and importantly, the grade and filler wire. Since have you any idea what grade of ally the boat is?

It may also be worth putting on a rubbing strake underneath too, for future 'rubbing' which can be sacrificial. In your case perhaps a 4~5mm FB...ops 3/16". This to be on the underside channel that runs along the length.

[jnwaco]

Ad Hoc - I do not have an idea of the aluminum grade. Is there a way to tell from looking at it? I'm guessing 5052, but not sure.

If I go with a keelson, will it have to be one continuous piece that sits on top of the ribs or can it be sections that fit in between the ribs? I'm liking this idea because it might require less fabrication than the sister ribs.

I'm new to boat terminology, but when you mention a "rubbing strake" you're talking about a protrusion from the hull? And this would go down the centerline of the boat? There are already some runners on the bottom of the boat (not sure what they're called - kilgores?) that would rub before a 4mm strake would. You kind of lost me there since I'm a complete beginner with the boat terminology - but I enjoy learning! Thanks for taking the time to help out!

[Ad Hoc]

Well, considering it looks 'shaped' or extruded, i would say 6000 series. Most 6000 in the US is 6061 or 6063. If you're buying plate, again most 5000 series plate in the US seems to be 5081/5454 or similar. Again, these grades only seem to be generally used in the US.

If it is the 5454 type, then a wire of 5056A will be needed. If the 5081 type then 5356/5556 type. But either wont have a major affect which ever you use. But if you are buying plate from 6000 series, you will need 4043A type wire.

The keelson to be one continuous structure from bow to stern. The height depth of the web to be at least twice that of the ribs/frames, with cut outs to allow the ribs to pass through. With a suitable rider/flange to it. It is not 100% necessary for the web of the keelson to be welded to each of the transverse ribs, since the ribs are just being 'supported' by the keelson. But it is best if you can, as a load path for the shear load. Just weld on one side of the cutout, no need for a tight fit. You can make the eight to suit a new sole or any other "eqpt" you wish to add. It makes a suitable seating for it.

Rubbing strake. It is just a bit of metal that is used for rubbing against, like a bumper on a car. When you boat grounds on rocks or sand, the rubbing strake is rubbed and eroded away, not the main structure, hence it being sacrificial and replaced when needed.

So a FB of say 2"x3/16" welded along the length of the bottom, just on the centreline. The width 2" (just nominal) is just to suit the width of the channel that is already there, since i don't know what are the dimensions.

[jnwaco]

Thanks Ad Hoc. You are definitely a big help!