Stringer and Mast Knee Upgrade

Floor Stringer and Mast Knee Reinforcement

Many Vanguard 15 owners choose to reinforce the floor stringer and mast knee to ensure the boat can handle heavy loads over time. This guide outlines the procedure.

Tools & Materials

  • Epoxy System: Resin and hardener.
  • Fillers: High-density filler (for structural bonding) and microballoons (for fairing if needed).
  • Fiberglass Cloth: 1 square yard (approx).
  • Inspection Port: 5-inch diameter (and installation tools: jigsaw/hole saw, silicone sealant).
  • Prep Tools: 60-grit sandpaper, acetone, rags, vacuum.
  • Application Tools: Small paint brushes, mixing cups, stir sticks.
  • PPE: Gloves, eye protection, respirator.

1. Inspection Port Installation

You need access to the area.

  1. Cut the Hole: Cut a hole for a 5-inch inspection port in the front vertical wall of the cockpit (the tank face).
  2. Protect the Boat: Mask off the area around the hole to prevent epoxy drips.
    • Tip: Don't install the port itself yet. Leave the hole open to give yourself maximum working room. Install the port only after the repair is cured.

2. Mast Knee Repair/Reinforcement

Inspect the mast knee located under the mast step.

  • Condition Check:
    • Intact but poorly bonded: Leave it in place and reinforce.
    • Delaminated/Rotten: Remove it completely. Fabricate a new knee from marine plywood of the same dimensions.

If Replacing the Knee:

  1. Sand the bonding area with 60-grit paper and clean with acetone.
  2. Mix epoxy with high-density filler to a "peanut butter" consistency.
  3. Bond the new knee into place and let cure (5-6 hours).

Reinforcing the Tabbing:

  1. Prep: Lightly sand the bonding area (knee and hull) with 60-grit paper and clean with acetone.
  2. Cut Cloth: Cut 4 strips of fiberglass cloth, 3" x 6" each.
  3. Apply:
    • Paint unthickened epoxy onto the knee and surrounding hull.
    • Wet out a fiberglass strip and place it in a corner between the knee and hull (1.5" on the knee, 1.5" on the hull).
    • Roll out air bubbles.
    • Repeat for all 4 corners.
  4. Cure: Allow to cure for 5-6 hours.

3. Floor Stringer Repair

  1. Prep: Sand the floor stringer area with 60-grit paper, vacuum, and clean with acetone.
  2. Cut Cloth: Cut three pieces of fiberglass cloth in increasing sizes:
    • Layer 1: 20" x 6"
    • Layer 2: 22" x 7"
    • Layer 3: 24" x 8"
  3. Laminate:
    • Wet out the hull stringer area with epoxy.
    • Apply the smallest piece (Layer 1) first. Wet it out.
    • Apply Layer 2 on top. Wet it out.
    • Apply Layer 3 (largest) on top. Wet it out.
    • Note: Stacking smallest to largest creates a tapered edge that is stronger and less prone to peeling.
  4. Finish: Roll out all air bubbles.

4. Final Steps

  1. Allow a full cure (5-6 hours minimum, preferably overnight).
  2. Install the inspection port using silicone sealant.
  3. Go sailing!