WEIGHT LIFTING.(Brief Article)

In Trailer Boats' January 2000 "Propulsion" column, Technical Editor Jim Barron warned that certain smaller performance boats, such as bassboats (and various other types of high-speed runabouts, I might add) may be incapable of supporting their rated engines due to increases in the weight of new outboards. He also stated his hope that boatbuilders, along with their dealers, would address this issue responsibly with future boat/engine combinations up to the task. What did Barron mean and how can this situation be addressed?

Outboard-powered boats of 20 feet or less are rated for horsepower using a formula largely determined by boat length and beam; the longer and wider the boat, the higher the rating. But the formula gives no consideration to a boat's displacement and whether or not the boat can support the weight of the rated engine. Until recently, such considerations were not needed much. However, considering today's heavier four-stroke powerheads, with more-robust internals and a plethora of added-on components, outboards weigh a lot more than they used to.

When used on current hulls, the results may be reduced passenger-carrying ability, questionable performance and stability, and balance/trim problems due mainly to the added weight at the stern. In fact, some boats might even become dangerous, even though the power ratings are technically correct. What's a boat builder to do?

PUMP UP THE VOLUME

It's simple in theory. Add more volume to the hull below the waterline, and add it where it will do the most good -- at the stern where the extra engine weight is concentrated. In other words, boat displacement can be increased, and the geometrical center of the increased underwater volume (called the center of buoyancy or "CB") can be shifted far enough aft so the boat will not suffer from balance and trim problems at rest or under way when normally loaded.