In a build, the mould or frame is where the boat's geometry is fixed. A dimensional error here carries into every later stage and cannot be corrected after launch. It is where supervision is cheapest and most effective.
By material
- GRP
- A female mould is taken from a plug; the boat's outside is the mould's inside. Fairness, surface quality and release agent application decide.
- Steel and aluminium
- Frames and longitudinals are cut to the drawings and set up on a strongback; plate is laid over this frame.
- Wood and one-off composite
- Built over a temporary mould or frame, removed once the hull is complete.
Check points
- Against the drawings. Length, beam, frame spacing and critical dimensions.
- Symmetry. Centreline and port-starboard measurements.
- Fairness. Waves or hollows in the mould surface or frames.
- Hard points. Location of reinforcement for keel, engine beds and deck hardware.
- Record. Measurements documented with photos; the stage payment depends on this sign-off.
Continue with this topic
- Next stage: hull lamination: Ply-by-ply supervision and evidence.
The mould on a GRP boat
In GRP production the hull is laminated inside a female mould. The mould surface directly sets the quality of the boat's outer surface; a wave or mark in the mould repeats on every boat. So at the mould stage, surface fairness, symmetry and dimensions are checked. For a one-off mould, how long it will last and whether it may be used for other boats relate to the mould rights in the contract.
The frame on a metal boat
Steel and aluminium builds start with a frame, set up inverted or upright. Frames and longitudinals are placed at the stations on the design drawings; many yards now prepare these parts by numerically controlled cutting. The frame's accuracy and fairness decide how well the plating will sit.
What is checked at this stage
- Principal dimensions and station measurements against the drawings.
- Symmetry and centreline of the frame or mould.
- Surface fairness; checking for waves and hollows with a batten.
- Preparation of the mould surface and application of release agent.
Why check early?
An error in the mould or frame carries into every later stage. A dimensional difference found now is fixed with a small correction; once the skin is laminated or plated, correcting it is very hard. So mould and frame are the build's first check point, and the first stage payment is tied to it.
Records
Measurements and photos form the first pages of the build file.
The owner's part
This is also the stage where layout decisions are cheapest to change. Any change to bulkhead positions or openings is best settled before the structure goes in.
The owner or their representative should plan a yard visit at this stage to see the layout in place; decisions on bulkheads, hatches, tanks and machinery settled now avoid later cost and delay. It is also the moment to confirm access for future maintenance of tanks and machinery.
A layout that makes maintenance access hard means extra work and cost at every service for the boat's whole life; a small correction now pays back for years.
After the visit, layout decisions are sent to the yard in writing and confirmed; a decision left verbal becomes an argument later. Each confirmed decision gets a number and date in the build file.
The decision list is kept current throughout the build.