strength and design questions

I'm wanting to build a very shallow drafting craft that will have a bow mounted trolling motor to use when in deep enough water and poled in xtreme shallows in marshes. I want very low flat deck to stay out of the wind as much as possible and thinking along the lines of a large 'stand-up paddle board' design. Only going to support one person a trolling motor and a cooler to house the battery for the trolling motor and light fishing gear.
Probably 12' in length, 4'-5' beam and 6"-7" thickness. Just a long rectangular "box" with flat deck , sides and bottom and bringing the bottom up in a taper to a 2" nose thickness starting the taper about 24" from the bow. I was basing diminsions on the 5' x 12' having 5 cubic feet of floatation for every inch submerged, thus able to float roughly 300lbs. for every inch submerged if weight on boat was evenly spread out. That should be perfect.
I was originally thinking using 2" thick x 6" tall x 12' lengths of foam sandwitched and strengthened with 1/8" marine ply every 4" of foam width.
If i went this route, could i just glass the entire hull or would i need a ply deck to keep from compressing the foam or would glassing be strong enough to begin with? If so, what weight cloth could i use to get away with it being strong enough to walk about on?
If this would take too much glass(ie. heavy) would a stitch and glue built hull be better/lighter?
Never having made a boat before, much less stitch and glue style,,,what thickness ply should i use and how many stringers or bulkheads and how much spacing is needed to support a deck(and what thickness should the deck ply be). And then do you glass and epoxy, or just epoxy the end result? If glass is needed , what weight cloth(s)?
I hope i've described the setup and my concerns properly to give you folks enough to respond.
I'm hoping to build the lightest craft possible and still have a strong enough craft to stand on w/out concerns of buckling.

Thanks in advance for any/all assistance you can provide.

 

I think you need to rewrite this with the correct symbols... " is inches and ' is feet. Preferably use FT and IN instead of " and ' so it is less confusing for everyone. Sounds like you are looking for a Scow...which is eminently easy to build just like you seem to be describing. Filling a cavity with foam then using the foam to support the ply is quite strong...I have done it before. You can use S&G but it would be easier to do it with chine longs, glue and screw.

Steve

 

Thanks for the reply Steve, sorry you found my symbols confusing, but they are used correctly ,,,just as you suggested even( a single quote being symbol for foot and a double quote being one for inches). I'm looking to build this scow with dimensions: 12ft long, 4ft-5ft in beam and 6in-7in tall(thick).
I was thinking of using 6inch widths of 2inch thick foam, 12feet in legth, so that if i stood a section on edge(2inches wide, 6inches tall and 12feet in length i could sandwich 24-30 similar pieces together, placing a thin piece of marine ply (1/8inch ply 6inch wide and 12feet long) between every other piece of foam (every 4 inches)on edge as the foam is for strength.
My concerns were mainly with weight and how to keep it as light as possible....use my method and cover with fg cloth and epoxy, just epoxy, add a deck of ply and then epoxy(and/or fg cloth).......... or forget my method and build a hollow hull with stringers and bulkheads and epoxy that, but if so,,,what would be the thinnest ply i could get away with and how many stringers/bulkheads to use trying to keep things real light...and would i need to use fg cloth using this hollow hull method or would epoxy be enuf.
I'm probably making things more convoluted now...sorry about that. I guess i'll just have to play with it, prbly go with my first idea , use foam and ply sandwich, thin ply deck and fg cloth and epoxy the whole sheebang.
Thanks for your input though.

 

Would be reasonably economic to make it from 4 sheets of 4mm ply. Make sides and two longitudinal bulkheads out of 50mm EPS foam. Place transverse bulkheads in the middle and 600mm either end of midlle. The lateral bulkheads just prevent the longitudinal ones from folding over. The higher you make the deck the more unstable it will be. Giving a slight beamwise crown to the deck and bottom will stiffen up the ply.

A single layer of 320gsm cloth over most of it should suffice. You can overlap on chine and deck anyhow so it will be double in these areas. Two layers on the foam at the side will be enough. Any area that flexes more than you like can have another layer added. Make sure you clean cured surface before placing a second layer of glass.

Using EPS foam means you have to use epoxy.

Round all edges to about 10mm radius before glassing and avoid severe compound curves when laying the glass.

This will end up around 25kg and be quite solid. I class this weight borderline for throwing about. If you have any distance to carry it on foot it could be fitted with dolly wheels. It is the weight combined with the size that makes it awkward for 1 person to handle.

If the boat is used for short periods and has time to dry out then you can use an outdoor ply rather than marine ply. Make sure you have a drain point for draining internally after use. You could use a small sealable hatch in the deck so you can store valuables within the hull (inside a plastic bag in case it gets damp.) THe hatch will also act as a vent to air inside. Leave a couple of drain point in the bulkheads so water can migrate slowly to the drain point and air can also circulate when the boat is stored with the cover off.

Make a cardboard model and provide a picture so you encourage more comment. Only needs to be taped and stapled - quick and dirty but worth the effort.

The idea reminds me of the old style moth hulls:
http://farm3.static.flickr.com/2070/2455607824_366aceae6a.jpg
http://farm3.static.flickr.com/2405/2454780909_08bb4dfebe.jpg

If you want to get fancy you could get closer to these shapes. Might even find a cheap one because they are virtually obsolete in this configuration.

Rick W

 

If you know what draft she'll float at, you have a hydrostatic head. Add a factor of safety and/or and other factors relating to expected service conditions and sea states.

You then have an applied load. Given a panel size, you can calculate the stress of different layups to investigate which is easiest for build or cost or both.

Start with the theory, then derive the numbers from the theory and conclude with your answer from the assumptions. Then compare against what has been built to see whether your assumptions are correct, as a "ball park" figure. Just copying another structure without knowing what it has been designed too, is asking for trouble, and poor structural design practice. If you want to copy, you need to know what the panels sizes are and what pressure was applied to calculate the lay ups.

 

Thank you

Rick,
Thanks! That's exactly what i was needing to hear, you're even suggesting something much lighter than i'd envisioned,,i may add more bulkheads than what you suggest just for good measure since i have plenty already and the foam shouldn't add much more weight at all.

Ad Hoc,,,not so sure about a hydrostatic head , i was just basing my figure on the assumption that one cubic foot of foam should float approximately 60 pounds of dead weight.
Given that my boat will have a flat bottom that is 12ft long by 5ft wide, that gives me 60 square feet of bottom on the water, sink that one inch would mean i now have 5 cubic feet of foam underwater that supposedly will support 300 pounds,,,,if boat weighs 60lbs, me at 180 and if i can keep gear to 60lbs and weight was evenly distributed then boat shouldn't sink any further....i know there are circumstances involved that will affect this but i was generalizing,,,
I'm a simple fly fisherman trying to get to some nice reds and trout. ;-)

I just want a light,shallow drafting , flat deck craft i can pole past some very skinny water to get to some deep ponds I found back in the marshes. The bottom is oyster ridden or knee deep muck so floating over the shallow areas
to avoid slogging through on foot is all i'm trying to do....and a flat deck craft
i can stand up on gives me a much better viewpoint for fishing than getting in there on a kayak sitting down.

 

jtmboat

If you want shallow draft then i strongly recommend that you do a weight estimate of your boat first. Design the hull shape to ensure you have the shallow draft at the given displacement that you calculate. This will give you a datum to work to, without which you'll have no idea what draft she float at until you launch her.

 

Thank you Ad Hoc, will do.

 

The blue EPS foam is around 30kg/Cu.m. You will be surprised how quickly it will add weight. You could even use 25mm thick foam but it tends to be more expensive for a given volume than 50mm.

Unless you have a good method of handling the hull even 25kg is awkward. The good kayaks are under 10kg. I try to keep single person boats under 20kg. Once they get over 20kg you have to devise mechanical aids to handle them as they are are awkward and will act as a sail in any wind.

It pays to do a mock up of these things even to the point of making a test panel. Generally you will get a lot more value out of adding glass than adding foam. If you want a really stiff deck then glass the inside before you you place it on the bulkheads. This makes a sandwich panel with ply core. It will be quite stiff.

If you are intending to car-top the hull then 1.5m gets a bit wide to fit standard roof racks on a sedan. You should find 1.2m will be adequate unless you are very heavy. 1.2m will also result in less wastage.

Rick W

 

To eliminate unnecessary foam, 2" stuff will allow you to have 6" (4"inside) sides and transom (of plywood perhaps, but why not foam?). This would need stringers fore and aft to stiffen the floor. They could be the same foam set on edge 4" tall, all glasses/epoxied in.
This would get your feet deeper when you stand. This will float in an inch or so of water.
Not as cheap as a plywood box, if you use epoxy and glass to sheathe it.
If not of foam:
The cheapest is definitely plywood, maybe 1/4", but that beam you want is extreme (4 ft would be far easier to build). Poling a 4 ft beam craft should be no problem with a flat bottom.
Structurally, design-wise, 4 ft of beam is a snap compared with 6 ft. A flat bottom oilcans easily unless well supported and those supports mean extra work, materials and weight.
In addition, to pole a 6 ft wide craft, your own weight will be to one side, deepening the draft on that side.

 

Alan
What are your thoughts on 320gsm (say 9oz/sq.yd) glass cloth? Is this more than necessary on 4mm ply?

RicK W

 

Yes, except where abrasion is an issue, and at joints. I don't see any reason otherwise to glass plywood. when I wrote, "Not as cheap as a plywood box if you use epoxy and glass to sheathe it." I meant sheathe the foam, not the plywood, but it looks like I meant plywood.
I would glass fir ply to limit surface issues, but really, in a case where there was a choice between 4mm sheathed and 6mm epoxy-coated only, I'd always opt for the thicker plywood--- stronger and less work, and insofar as weight goes, likely a toss-up..