Certainly you can look at the literature from any boatbuilder and quickly establish what maximum-horsepower rating any particular model carries. But how does a builder determine that figure? Can all boats run on a single engine or twin or more engines? And must you select the maximum horsepower for your boat for it to perform optimally?
Boatbuilders and designers have two basic ways to determine what horsepower to use: one very technical, the other not so. The technical method requires tank testing of a scale model or full-size hull to determine its resistance (drag). Or, they can use various computer programs to predict this information. They multiply the resistance by the quasi-propulsive coefficient. The type and style of the hull, the speed, the range and the engine configuration (single/twin/triple) also figure into the total resistance equation. Knowing total resistance allows you to dictate the horsepower required to push the boat at a given speed. Then you must decide how to break up that horsepower: For instance, 1,200 hp could be four 300 hp outboards or five 250 hp outboards. Each change of engine requires a new calculation of the propulsive coefficient. Thank God for computers!
The easier way looks at similar hulls that have gone before and mimics those horsepower requirements. Nobody ever claimed that computerized fluid dynamics was child's play.
So, what decisions do you - the buyer - have to consider?
Single or Twin Engines
As you'd expect, both choices have advantages and disadvantages. A single engine will be lighter on the transom than twins and obviously half as expensive. If you choose a two-stroke, you'll need only half the oil of twins too. On the other hand, many people purchase twins as an insurance policy: If one engine goes south, you always have the other as a fallback. At the same time, however, many boats are weight-critical and can only handle a single 300 instead of twin 150s.
Another question many people ask today is - twins or triples? Had we not done the testing ourselves, we probably wouldn't have believed it, but three engines don't use all that much more fuel than two. Why? It appears that three engines don't have to work as hard, so all three use less fuel at a given speed than you'd expect. Since twins have to work harder at that same speed, each engine uses more fuel than those in a triple configuration.
Every boat comes with a plaque stating the maximum-horsepower rating for that vessel. Legally, the U.S. Coast Guard rating plate only applies to boats under 20 feet in length. For boats over that size, the government leaves rating to the builder. The means by which builders determine maximum horsepower for this category of boats is: L x W x 2 - 90 = hp.
Mounting more power than the rating plate states usually voids the warranty as well as your insurance. And yet, some "Smokin' Joes" do it anyway. Go figure.
Karl Sandstrom, product manager for BRP's Johnson and Evinrude, feels a more critical question is: "How fast do you want to travel at a particular rpm?" He allows that most outboards run most efficiently between 3,500 and 4,500 rpm. So you need to determine what speed you want to be going at that rpm range.
"In other words, you may choose twin 150s for your boat and want to take it to the Bahamas from Florida," he says. "You'd like to cruise at 35 mph. Your twin 150s run at 4,800 for 35 mph, while a pair of 225s on the same boat only run at 3,600 rpm. Which do you suppose provides better fuel economy?"
Sandstrom supplies this equation so you can answer that question for your vessel:
HP / gross weight = X.
X = Y.
Y x 250 = pure mph.
Pure mph x 90 percent
(for prop slippage) = real mph.
This equation works amazingly well, but only for V-bottoms. By the way, gross weight is boat, fuel, tackle, passengers, food, bait, water, beer, cigars, etc.
Obviously, the larger the engine(s), the higher the initial capital outlay. So going smaller saves you money. According to Mercury's competitive intelligence manager (is that a cool title or what?), Jack Litjens, lots of boatbuilders put the minimum possible horsepower on the back of a new boat. "It lowers the initial cost, making the purchase more attractive," he says. "But most people who purchase that package end up being very unhappy. It may run great during the sea trial when it's just the buyer and salesman aboard. Then you put on three or four of your friends, a full fuel tank and the like, and suddenly the boat won't get up on plane anymore."
As we stated earlier, bigger engines don't work as hard at cruising speed as smaller ones do. Sure there are exceptions, and you'll need to determine if this holds true for your choice of boat.
Martin Peters at Yamaha offers his company's performance bulletins as a means to determine specifics on the best power for your boat. "Yamaha has a database of about 1,300 boat-and-motor combinations online. These are known as Yamaha Performance Bulletins, and they can be accessed from www.yamaha
outboards.com. Performance bulletins are the most visited area of our website," he says. "Customers can look for their make of boat and see the measured performance of a given boat model with a given outboard. Most bulletins provide information on time to plane, fuel consumption at rpm and speed at rpm."
Of course, not all boats run Yamahas. Martin suggests, "If the boat owner can't find the possible boat/outboard combination, they can find a boat of similar weight, beam and length, and come to some conclusions about the performance level of a given outboard on a similar boat."
Litjens reminds buyers of another crucial fact: "The engine industry has a rating cushion. Any engine can actually be 10 percent above or below the horsepower stated on the sticker. That means your 225 hp outboard may well be only 210 to 215 hp when you hook it up to a dynamometer."
Litjens also says that the type of boat makes a big difference. He claims that a deep-V boat takes more torque and power to get on plane than a hull with a flatter bottom like a bay or flats skiff. You can get away with less horsepower on the latter types.
You realize another advantage by using bigger horsepower: If an engine doesn't work as hard, then its longevity and durability should increase. That translates to less maintenance and repair costs.
Other advantages to higher horsepower come with top-end speed and midrange response. Should you ever really need to run fast to skirt a squall - or heaven forbid, get somewhere for an emergency - bigger power means faster speed. You don't have to use that power, but many people figure it's nice to have at your fingertips.
Perhaps more importantly, midrange response improves with higher horsepower. For instance, say you're running down-sea at 30 mph; then you start to climb the back of a wave. Higher horsepower may mean the difference between making it up and over that wave and bogging down like a three-cylinder Geo trying to reach highway speeds.
So the experts agree that going with the smallest power doesn't necessarily equate to better fuel economy, equivalent performance or increased customer satisfaction. In fact, the only thing they do agree upon as a reason to go with smaller power is that you'll enjoy a smaller initial capital outlay. That happiness probably won't last very long.