Close

Basic Canoe Design

A project log for A Plywood Canoe

I built this canoe 10 years ago. It was fun but various design flaws has limited it use. This project is about a salvaging the canoe.

agpcooperagp.cooper 02/23/2017 at 07:520 Comments

Basic Canoe Design

Before considering the pontoon and bridge design here is the basic canoe design. I have uploaded a spreadsheet with all the following calculations.

Basic Canoe Measurements

Canoe Basic Metrics
Length Overall (LOA) 3.930 m
Beam Overall (BOA) 0.650 m
Length Water Line (LWL) 3.600 m
Beam Water Line (BWL) 0.550 m Actually the bottom width
Depth Water Line (DWL) 0.075 m
Rocker 0.025 m
Amidships Deadrise 0.000 Degrees Flat bottom
Amidships Freeboard 0.200 m
Forward Freeboard 0.325 m
Rear Freeboard 0.325 m

Note: Bold numbers are inputs.

Design Pressure

Now some basic calculations but most importantly the design pressure:

Basic Calculations
Mid-Section Area 0.041 m^2
Plan Water Line Area 1.320 m^2
Displacement Volume 0.088 m^3
Displacement Weight 90.200 kg
Prismatic Coefficient (cp) 0.593
Displacement Length Ratio (DLR) 42.080
Hull Speed 4.605 kt
Design G Force 3.500 g Light duty
Design Pressure 2.346 KN

To calculate the design pressure I have assumed the canoe is loaded 3.5 time the rated displacement of 90 kg.

Bottom Plywood Thickness

Bottom Plank Design
Design Pressure (P) 2.35 kPa
Span (L) 550 mm BWL
Allowable Stress (S) 14000 kPa F14 Plywood
Plank Thickness (t) 5.03 mm t^2=P*L^2/2/S, ~fixed sides
Design Plank Thickness (t) 6.00 mm
Modulus of Elasticity (E) 8400000 kPa Factored 80% for wet condition
Moment of Inertia (I) 18000 mm^4
Deflection (d) 3.70 mm d=P*L^4/384EI (2% max)
Deflection Ratio 148.74
Maximum Concentrated Load 0.50 kN =S*t*t

Here I calculate 5 mm but the nearest larger plywood thickness is 6 mm. Although this is okay for water pressure it is too thin for an adult to stand on. The middle section of the bottom of the canoe needs to be reinforced.

Bottom Reinforcement

Upon reflection it would have been easier just to double up the plywood in the middle of the canoe or that fibre-glass both sides with 4 oz/sq_yd fibre-glass cloth. The calculations assume a "T" section across the hull (beam-ward) with plywood strips (see image). It is also assumed that two "T" sections share the load as they are quite close together:

Hull Centre Reinforcement
T-Section Modulus
Maximum Rib Spacing (Flange Width) 256 mm Fw<36t+w
Design Rib Spacing 150 mm
Plank Thickness (Flange Thickness) 6 mm
Rib Thickness (Web Height) 6 mm
Rib Width (Web Thickness) 40 mm
A 1776 mm^2
cy 3.8 mm From top of T
cx 0.0 mm From centre line of T
Ixx 10474 mm^4
Iyy 8420608 mm^4
Design Point Load 1.00 kN ~100kg
Span 550 mm
Allowable Stress 14000 kPa F14 Plywood
Bending Moment 69 Nm
Section Modulus 2749 mm^3
Working Stress (per rib) 25013 kN
Distribute Load (over two ribs) 12506 kN
Safety Factor 112%

Here is an image of the reinforcement:

If you look carefully you can see the filled wire holes below the single strip of 4 oz/sq_yd fibre-glass tape.

Side Plank Calculations

If the side plank is the same thickness as the bottom plank then no calculations are necessary but here they are anyway:

Side Plank Design
Design Pressure (P) 2.35 kPa
Span (L) 275 mm
Allowable Stress (S) 14000 kPa F14 Plywood
Plank Thickness (t) 2.5 mm t^2=P*L^2/2/S, ~fixed sides
Design Plank Thickness (t) 6.0 mm Okay

Hog and Sag Calculations

So put 3.5 big guys in the canoe and get two even bigger guys to lift the canoe up by the ends. Is it strong enough (not actually required)?

First the Load Calculation:

Hull Section Modulus Triangular Load & Free Ends
Design Pressure 2.35 kPa
Width 550 mm BWL
Length 3600 mm LWL
Allowable Stress 14000 kPa F14 Plywood
Required Section Modulus 99547 mm^3 SM=PsL^2/12/S

Now the Strength Calculation (assuming a channel section):

Channel Model
Chine Width (web) 550 mm BWL
Side Height (flange) 275 mm
Deck Width (lip) 50 mm
Thickness (t) 6 mm
cy (from bottom) 86 mm
I 75400372 mm^4
Rg 102 mm Radius of gyration
Slenderness Ratio 35
Z 398812 mm^3
Safety Factor 401% Assumes no significant keel or stringers

Yes, a 401% safety factor is okay.

Fibre-Glass Tap Join

You will be lucky to find anything on the Internet calculating the fibre-glass joint strength other than rules of thumb. Here are some actual calculations:

Plywood Fibre Glass Joins
Spw 14000 Pa
b 1 m
h 6.00 mm
M 84000 Nm M=Spw*b*h^2/6
Sfg 120000000 Pa
t 0.12 mm t~((6*M*h/Sfg/b+h^3)^(1/3)-h)/2
F/G Weight 1.87 oz/sq_yd Each side of plywood
F/G Overlap (each side of join) 18 mm =3*h [JS = 15%*(1+L/h)]


Scarf Joins

You will find rule of thumb of beween 8 to 1 all the way up to 12 to 1 for scarf joins:

(source: www.amateurboatbuilding.com/articles/howto/joining_ply/scarf1.gif)

This tells you the fibre-glass tape should span 8 times the plywood thickness:

(source: www.amateurboatbuilding.com/articles/howto/joining_ply/plyjnt5.gif)

However, I found a paper that suggests that 6 times is enough (www.wfdt.teilar.gr/research.files/2005_05.pdf).

AlanX

Discussions