Montana Riverboats Test Domain Roadkill

Rocker-reducing Background

Rocker-reducing Background

I'll skip the "What is the ideal rocker profile" question. For now let's just say boats can have too much and/or too little rocker. It's important for hull designers to know what the issues are and how to work with them.

The early Oregon driftboats were all designed around 4x8 plywood. To make side panels they scarfed two sheets together to make an (almost) 4x16' foot panel and then cut the side panels from that panel.

The bottom chine edge was usually left as is, with a straight line from end to end. That pattern holds true for most molded fiberglass boats because they all start off as a plywood plug, from which a mold is made.

There are some important historical exceptions to that pattern. Rogue River boats were meant to carry weight down the river so it was important to reduce rocker in the middle of the boat. Rogue River boats have a flatter profile than their McKenzie River "banana boat" cousins. Rogue River boats accomplished that rocker reduction by making a curved rather than straight line chine on the side panels.

A straight chine edge on the side panels creates a lot of rocker. Think of it this way: if you stand a side panel up on its edge, the straight chine is flat and flush to shop floor. If you tilt that panel out sideways (like that side of the boat) and then bend the ends in (they way the ends would bend in order to attach to a stem and transom) then the ends of the side panel rise up from the floor as you bend in the ends of the tilted panel. That's where the rocker comes from.

There are three ways to reduce rocker in dory that I'm aware of:

1) Make the middle of the boat relatively flat, with most of the needed rocker out at the ends rather than in the middle. If the width of the boat has parallel or nearly parallel chines (same side to side bottom width for perhaps the middle 4' feet of the bottom) that profile reduces rocker considerably.

2) Make the chine edge a long curved line instead of stright, with the high point of the curve centered in the middle of the boat, under the point were the oarlocks will go in a later step.

3) You can further reduce rocker at the upstream end of the boat by widening the transom.

Dishing out the chine edge

Dishing-Out-Chine