Foam Core Proa Hull

Foam Core Proa Hull - Optimized Dimensions

Adjusted from Aluminum Design for Foam Construction

Material: 50mm (2”) XPS foam core + 6oz fiberglass skin
Total Weight Target: 16 kg (35 lbs) Manufacturing: CNC rotary mill shaped foam blank

Foam Design Specifications

Foam design: Lightweight core, can be larger/lighter

Adjustments due to foam:

  1. Increased depth - Foam floats higher, need more depth for ballast
  2. Slightly wider beam - Foam needs more volume for same displacement
  3. Thinner walls - Fiberglass skin is structural, foam is just core
  4. Smoother curves - No hard chines (foam can do compound curves)

MAIN HULL DIMENSIONS (All measurements in mm)

Overall Specifications

Dimension Foam Design Change Reason
Length (LOA) 4876mm (16’0”) None Perfect length
Beam (max) 660mm (26”) +50mm More volume for buoyancy
Depth (max) 430mm (17”) +49mm Better ballast capacity
Draft 127mm (5”) -25mm Foam floats higher
Weight 16 kg (35 lbs) -50% Foam + glass vs aluminum

Cross-Section Shape

Foam: Round-bilge (smooth compound curves) Why: CNC rotary mill can shape smooth curves easily in foam

STATION-BY-STATION DIMENSIONS

Station Spacing: 609mm (24”) intervals along 4876mm length

9 stations total:

Station Distance from Bow Width (mm) Depth (mm) Shape Notes
0 (Bow) 0mm 203mm (8”) 965mm (38”) Sharp entry, V-shape
1 609mm 356mm (14”) 660mm (26”) Transitioning
2 1219mm 508mm (20”) 508mm (20”) Forward shoulder
3 1829mm 610mm (24”) 457mm (18”) Approaching max beam
4 (Mid) 2438mm 660mm (26”) 430mm (17”) Maximum beam
5 3048mm 635mm (25”) 432mm (17”) Holding beam
6 3658mm 559mm (22”) 457mm (18”) Aft shoulder
7 4267mm 406mm (16”) 610mm (24”) Narrowing stern
8 (Stern) 4876mm 305mm (12”) 864mm (34”) Narrow exit

CROSS-SECTION PROFILES

Station 0 (Bow) - Sharp Entry

Width: 203mm (8")
Depth: 965mm (38")
Shape: Deep V, 45° deadrise angle
Rocker: 762mm (30") rise above baseline

Profile points (mm from centerline):
- Keel: (0, -965)
- Port chine: (-102, -635)
- Port gunwale: (-102, 0)
- Starboard: Mirror symmetry

Station 4 (Midship) - Maximum Beam

Width: 660mm (26")
Depth: 430mm (17")
Shape: Rounded bilge, 35° deadrise
Rocker: 0mm (baseline, lowest point)

Profile points (mm from centerline):
- Keel: (0, -430)
- Port turn of bilge: (-254, -279)
- Port gunwale: (-330, 0)
- Starboard: Mirror symmetry

Station 8 (Stern) - Narrow Exit

Width: 305mm (12")
Depth: 864mm (34")
Shape: Rounded U, gentle exit
Rocker: 508mm (20") rise above baseline

Profile points (mm from centerline):
- Keel: (0, -864)
- Port side: (-152, -533)
- Port gunwale: (-152, 0)
- Starboard: Mirror symmetry

FOAM BLANK REQUIREMENTS

Cutting from 4’ × 8’ Sheets

Hull requires:

  • 3× sheets XPS foam, 1220mm × 2440mm × 50mm (4’ × 8’ × 2”)
  • Cost: 3 × $35 = $105 for foam blank

Cutting strategy:

  1. Rough-cut foam to 5000mm × 700mm × 500mm rectangular blank
  2. Mount on CNC rotary axis (between centers)
  3. Mill to final shape in 2-3 passes

Foam waste: ~40% (unavoidable for boat hull shape)

AMA (OUTRIGGER) DIMENSIONS

Scale: 70% of main hull (smaller, lighter)

Dimension Main Hull Ama Notes
Length 4876mm 3413mm (11’2”) 70% scale
Beam 660mm 462mm (18”) 70% scale
Depth 430mm 301mm (12”) 70% scale
Weight 16 kg 6 kg (13 lbs) 70% volume

Foam required: 1 sheet (4’ × 8’ × 2”)

FOAM CORE THICKNESS

Fiberglass Layup Schedule

Exterior:

  • 6oz fiberglass cloth (2 layers)
  • Epoxy resin saturation
  • Total thickness: 1.5mm

Interior:

  • 4oz fiberglass cloth (1 layer)
  • Epoxy resin
  • Total thickness: 0.8mm

Total wall: 50mm foam + 2.3mm glass = 52.3mm

Structural strength:

  • Compressive: 172 kPa (25 PSI) - foam core
  • Tensile: 68 MPa (10,000 PSI) - fiberglass skin
  • Combined: Sandwich composite (very strong for weight)

BULKHEAD POSITIONS

4 bulkheads required:

Station Position (mm from bow) Purpose Material Thickness
2 1219mm Forward structural 12mm plywood Full-height
4 2438mm Midship, crossbeam mount 12mm plywood Full-height
6 3658mm Aft, crossbeam mount 12mm plywood Full-height
7.5 4013mm Stern structural 12mm plywood Partial-height

Bulkhead attachment:

  • Glassed into hull with 2 layers 6oz tape
  • Epoxy filleted joints
  • Not load-bearing (foam hull is monocoque)

ROCKER (LONGITUDINAL CURVE)

Keel line rises at bow and stern for wave performance

Position Height above baseline
Station 0 (bow) +762mm (30”)
Station 1 +381mm (15”)
Station 2 +127mm (5”)
Station 4 (mid) 0mm (baseline)
Station 6 +76mm (3”)
Station 7 +254mm (10”)
Station 8 (stern) +508mm (20”)

Smooth curve connects stations (not straight lines)

BUOYANCY CALCULATIONS

Displacement

Hull volume: 0.42 m³ (420 liters)

  • Foam core: 16 kg
  • Fiberglass skin: 8 kg
  • Fittings/hardware: 6 kg
  • Empty weight: 30 kg (66 lbs)

Water displacement at 430mm depth:

  • 0.42 m³ × 1000 kg/m³ = 420 kg capacity
  • 420 kg - 30 kg = 390 kg payload (860 lbs)

With ama:

  • Ama displacement: 0.12 m³ = 120 kg
  • Ama weight: 6 kg
  • Total capacity: 500 kg (1100 lbs) with outrigger

SHEER LINE (TOP EDGE)

Graceful curve from bow to stern

Station Sheer height above waterline
0 (bow) 762mm (30”) - High bow
2 508mm (20”)
4 (mid) 430mm (17”) - Lowest sheer
6 483mm (19”)
8 (stern) 610mm (24”) - Raised stern

Smooth fair curve - no hard corners

Key aspects of foam design

  • Overall length (4876mm)
  • Basic proa configuration
  • Three-use system (fishing/sailing/expedition)
  • Mounting point types (A/B/C/D)
  • ✅ Wider beam (610mm → 660mm) - More buoyancy
  • ✅ Deeper hull (381mm → 430mm) - Better stability
  • ✅ Rounded bilge - Foam can do compound curves
  • ✅ Lighter weight (31kg → 16kg) - 50% reduction
  • ✅ Higher sheer - More freeboard for waves
  • ✅ Smoother curves - No hard chines needed

FUSION 360 PARAMETERS

For Parametric Model

User Parameters to create:

LOA = 4876 mm (Length Overall)
BeamMax = 660 mm (Maximum Beam)
DepthMax = 430 mm (Maximum Depth)
Stations = 9 (Number of cross-sections)
StationSpacing = 609 mm (24 inches)
FoamThickness = 50 mm (2 inch core)
FiberglassThickness = 2.3 mm (layup schedule)

Adjustable in design timeline - change one parameter, whole hull updates

MATERIAL REQUIREMENTS

Foam Core

Item Specification Quantity Unit Cost Total Supplier
Main hull foam XPS, 1220×2440×50mm sheets 3 $35 $105 Home Depot
Ama foam XPS, 1220×2440×50mm sheet 1 $35 $35 Home Depot
Bulkhead ply 12mm marine plywood, 1220×2440 1 $85 $85 Lumber yard

Foam Subtotal: $225

Fiberglass & Epoxy

Item Specification Quantity Unit Cost Total Supplier
Fiberglass cloth 6oz plain weave, 50” wide 18 m $8/m $144 Fiberglass Supply
Epoxy resin 635 epoxy, gallon kit 2 $95 $190 US Composites
Cabosil thickener Fumed silica, 1 lb 1 $18 $18 US Composites
Roller covers 4” foam, solvent-proof 12 $2 $24 Hardware store
Squeegees Plastic, various sizes 3 $5 $15 Hardware store

Fiberglass Subtotal: $391

TOTAL MATERIALS (hull + ama): $616

WEIGHT BREAKDOWN

Complete Boat

Component Weight
Main hull (foam + glass) 16 kg (35 lbs)
Ama outrigger 6 kg (13 lbs)
Bulkheads (4× plywood) 4 kg (9 lbs)
Crossbeams (2× aluminum) 3 kg (7 lbs)
Mounting hardware 2 kg (4 lbs)
BARE BOAT TOTAL 31 kg (68 lbs)

With sailing rig:

  • Mast + boom: 8 kg (18 lbs)
  • Sail: 3 kg (7 lbs)
  • Running rigging: 2 kg (4 lbs)
  • SAILING CONFIG: 44 kg (97 lbs)

Still cartop-able solo!

CRITICAL DIMENSIONS FOR CNC

Foam Blank Preparation

Before CNC milling:

  1. Stack and glue 3 sheets XPS foam (50mm each)
  2. Creates 150mm thick blank (overkill, but easier to clamp)
  3. Rough-cut with bandsaw to 5000mm × 700mm × 500mm
  4. Mark centerline and station positions

CNC setup:

  1. Mount blank between rotary axis centers
  2. Align centerline to rotation axis
  3. Zero Z-axis to blank surface
  4. Run surfacing operation to true up blank
Tags:
Categories: