How did we get these rules for power on outboard boats? Is there a better way?
Most of the horsepower (or kilowatts in some countries) ratings for monohull outboard boats, used today, were developed back in the 1950's by the BIA and the Yacht Safety Bureau. They ran tests on various boats and came up with the formulas for different types of boats. Primarily the limits on power are due to two factors, one is simply handling of the boat. Putting too much power on the back of a boat can make it handle erratically, spin out, and difficult to steer. The other is weight. Putting a much larger engine on the transom can make the boat sit stern down and make it easy for a following wave to roll over the transom and swamp the boat.
Also different types of hull shapes handle power in different ways. Flat bottom boats with hard chines are much easier to skid sideways in a turn, or to catch a chine and trip the boat, possibly flipping it, than a boat with a vee or rounded bottom. Boats with a shorter transom height sit lower and are easier to swamp than a boat with a full height transom (20 inch). Engines come in standard shaft lengths, generally 15 inch, 20 inch and 25 inch, i.e. short shaft, standard, and long shaft.
But, over the years both boats and engines have changed considerably, so shouldn't the rules have changed? Well, probably, but with everyone required to use the same formulas, at least they present a level playing field. Everyone has to use the same rules. Tests conducted in 2003 indicated that many boats may actually be overpowered using these formulas. More testing needs to be done.
Is there a better way? There may be. Way back when, ABYC developed a test course standard for boats that don't have to meet the US Federal regulations. This course is basically the same one that was originally used to obtain the formulas and measures a boats ability to manuever through a course with turns, without becoming unstable. The Coast Guard used this test course when it developed a separate standard for boats 13 feet or less, with one or two passengers, a 20 inch transom height and less than 40 horsepower. These were popular in the 80's, and called thrill craft, before the advent of Personal Watercraft.
In the 1980's the Coast Guard and Mercury Marine did a lot of testing on different types and sizes of boats instrumented with accelerometers. These instruments measured the acceleration sideways in a turn. The basic theory was, the more power, the greater the sideways, or lateral accelerations. Unfortunately the data revealed that there was no correlation, between power and lateral accelerations in a turn. There was more of a correlation between acceleration and the boat hull type.
The International Standards Organization (ISO) has developed it's own test standard, a collision avoidance test, or barrier test. An imaginary barrier is set up on the water and the boat with rated power, run at full throttle, has to avoid "hitting" (actually crossing over) the barrier by turning at a specific distance which is calculated based on the length of the boat. The test is repeated with larger engines until it can't avoid the barrier. The highest power with which it could complete the turn is the rated power.
On the more technical and scientific side, some naval architects and engineers have done research into resistance on planing hulls and developed formulas based on weight, wetted surface at speed, resistance, angle of the vee and other factors. These formulas are used frequently to determine power for larger inboard powered planing hull boats but aren't really applicable to small (under 20 feet) outboard powered boats.
So where does that leave us? The Coast Guard has been doing some testing comparing the ISO standard to the US standard to see how a boat rates under each standard. Whether this will develop into a new standard is open to speculation.
