Frameless instant boat scantlings?

Alexandria Seaport has a flat bottomed instant workshop boat called the "Challenge wherry." This boat interests me because it's built with no permanent frames, just a stringer on each side and thwarts attached to the stringers as braces (see below). They don't publish any information on the design, but as far as I can tell it's 17ft long, constructed of 1/4 inch ply and holds 3 adults. In addition to the stringers and chine logs there's also a single keel/shoe on the bottom.

I'm working on a design that is very similar, but larger. On paper it's currently 23ft x 54in x 20in and will probably have 4 thwarts spaced 40 inches apart plus tanks at the ends.

How would I go about developing frameless scantlings for this? Are there goals for strength/stiffness I should be aiming for? Gerr's Elements of boat strength doesn't seem to be useful in this circumstance. Bolger designed more than a few boats that would be suitable to copy, but they all have frames.

 

That's certainly not 1/4" ply.
The boat might be built quickly, but it's not designed to last and is not going to be very torsionally stiff.

That is a gimmick.

The first thing you ought to have is more gunwale reinforcement like a wooden canoe. Second thing is the cross connections ought to be at the gunwale not down low where it allows more twist in the boat.

You need to study engineering to do anything more than just build it and see how it works, trying again as required.

Bigger is even more foolish.

There are real reasons for having frames, its not just useless features.

IMO - and not a humble opinion at that.

Have you ever built a boat before?

 

Welcome to the forum.

The Challenge Wherry is indeed a 1/4" hull. Basicly it's a Gardner bateau with some modifications. This particular design was intended to be assembled quickly (single day) so they use quick set polyurethane adhesives and screw the panels to chines logs, sheer rail and thwart cleats. There's a modest stem and a heavy transom. This is more than enough structure for a fairly stiff boat, that can be raced by day's end. I'm pretty sure the panels are quick set epoxy scarfs and though not terribly durable, certainly up to the tasks for a day build contest and some racing in the end. They're launched raw and I'll bet they are taken home and finished eventually. I think they also make the paddles too.

As to the design of this type of structure, well it's certainly possible, but you do have to have a clue about the engineering involved, not to mention the hydrodynamics. This is especially if you intend to increase it's size by 35%. This types of structures aren't uncommon and can be rewarding to build, though challenging to design (well).

 

Upchurchmr,

I've built a couple of boats, a flat bottomed screw and glue boat from popular mechanics and a Lillistone flint skiff. I'll take screw and glue any day over fiberglass and epoxy.

Seems like the original reason for having frames was to hold the the wooden planks on the side of the boat.

There's no reason a boat like the challenge wherry cannot be built sufficiently strong and stiff. The real question is whether the added weight of "frameless" construction is justified by the reduction in labor.

 

You should know, among other things, how many hours of work equals one kg of wood.

 

PAR,

Thanks for the info on the wherry.

I didn't mean to say the boat is the same, just that it's an open flat bottomed dory with thwarts that can serve as braces.

I've attached what I've been working on. It's intended to be a general purpose rowing skiff for semi protected waters like bays/coastal inlets. Amateur rowing, camping, etc.

The shape and hydrodynamics are dictated in part by the size of a standard plywood panel. I intend to butt the panels with plywood straps or x-mat on both sides.

Right now I'm using 1300 lbs for displacement. 350lb hull + 5x 190lb fat American men. Freeboard should be 13 inches at the low point. I plan to add a skeg and rudder.


Design length : 23.167 ft
Length over all : 23.167 ft
Design beam : 5.357 ft
Beam over all : 5.345 ft
Design draft : 0.572 ft
Midship location : 11.583 ft
Water density : 63.430 lbs/ft^3
Appendage coefficient : 1.0000
Volume properties:
Displaced volume : 21.195 ft^3
Displacement : 0.600 tons
Total length of submerged body : 19.932 ft
Total beam of submerged body : 3.932 ft
Block coefficient : 0.4725
Prismatic coefficient : 0.5491
Vert. prismatic coefficient : 0.7421
Wetted surface area : 59.746 ft^2
Longitudinal center of buoyancy : 11.619 ft
Longitudinal center of buoyancy : 0.154 %
Transverse center of buoyancy : 0.000 ft
Vertical center of buoyancy : 0.339 ft
Midship properties:
Midship section area : 1.937 ft^2
Midship coefficient : 0.8606
Waterplane properties:
Length on waterline : 19.932 ft
Beam on waterline : 3.932 ft
Waterplane area : 49.908 ft^2
Waterplane coefficient : 0.6368
Waterplane center of floatation : 11.613 ft
Y coordinate of DWL area CoG : 0.000 ft
Half entrance angle of DWL : 16.611 degr
Transverse moment of inertia : 43.133 ft^4
Longitudinal moment of inertia : 932.89 ft^4
Initial stability:
Vertical of transverse metacenter : 2.374 ft
Transverse metacentric radius : 2.035 ft
Longitudinal transverse metacenter : 44.355 ft
Longitudinal metacentric radius : 44.015 ft
Lateral plane:
Lateral area : 8.165 ft^2
Longitudinal center of effort : 11.602 ft
Vertical center of effort : 0.350 ft
Hull characteristics above waterline:
Lateral wind area : 42.915 ft^2
Z coordinate of wind area CoG : 1.593 ft
X coordinate of wind area CoG : 11.587 ft
Distance from wind area CoG to DWL : 0.989 ft
Distance from bow (FP) to wind area CoG : 11.612 ft
Minimal board height over DWL : 1.070 ft
Minimal board height over DWL : 4.618 %Lmax
Stability characteristics:
Test stability coefficient : 5.705 if >= 0,8 then OK

 

Please enlighten me.

 

You want to save labor, for which you need to increase the thickness of the wooden boards. Therefore, it is necessary to know how much money is saved in labor and how increase the value of the additional timber should you use.

 

Ok, interesting.

I intend to use hydrotek plywood, which weighs roughly 12kg per 1/4 inch 4x8 sheet and costs $48.40 per sheet locally (including tax at newton moore) Cost is pretty linear for thicker panels.

Per kilogram, Hydrotek is ~$4

I'm not going to put a value on my free time, but the median hourly wage around here is $25.

So at $25/hr, 9.6 minutes of labor equals one Kg of wood. Assuming I did the math right.

 

Okay, it looks like it is cheaper one kg of wood than one hour/man. In addition, it has to deduct the cost of the wood of the frame removed. You have to know now how much would that increase the thickness of the board (total kg of wood added) and to what extent the bottom and side bending would be annoying to passengers.

 

I think some flexing is acceptable as long as it does not make passengers feel unsafe.

As for the increase in thickness, that's one of the things I'd like to figure out.

Aside from Finite element analysis, empirical testing of of scale models or whole boats what else can be done? Can anyone suggest any other successful designs with similar construction?

We could take the Challenge wherry's plating and scale it appropriately (assuming the design is sufficiently strong in the first place). As a wild guess, I'll relate the plating thickness to the cube of the hull's lengths

E.g.

6mm plywood x (23ft^3) / (17ft^3) = 14mm