Picking Power

Published on Jun 21, 2007

by Tim Banse of Boats.com

Shopping for a new gas-engine boat means making choices. One of the biggest quandaries boat buyers must resolve is how much horsepower is enough - or, conversely, how much is too little. But it doesn\’t end there. The questions - and the choices - continue: Should you opt for single or twin engines? Should you go with electronic fuel injection or choose an engine with a carburetor? Ultimately, you\’ll wind up with a power package that best serves your needs, but only if you thoroughly understand the inevitable trade-offs involved in choosing one configuration or one technology over another. Most people have a vague idea of the pros and cons of the issues they\’ll confront when buying a new powerboat, but there\’s always more to learn.


If you want an inboard gas-powered boat that will go as fast as possible, you\’re talking big-block power. Where speed is concerned, there\’s no substitute for cubic inches. The good news is that the current generation of big-block motors runs smarter than previous models. The bad news is that the bigger the engine, the more it costs. There - we\’ve already encountered the biggest trade-off: cost. Electronically fuel-injected engines cost more than carbureted engines. But don\’t think that carbureted power plants have stood still since the inception of EFI. They now boast sophisticated engine-management systems that maximize output at the same time that they extend engine life by protecting against overheating, over-revving, low oil pressure and detonation. As for EFI, it adds a few hundred dollars to the price, but pays big dividends in improved fuel economy and flatter torque curve. For my money (so to speak), the extra cost is worth it. Nonetheless, if a boat I was in love with were only available with carbureted power, I wouldn\’t let the absence of EFI deter me from buying it. Two- or four-barrel carburetors are entirely adequate, and are likely to be with us for a very long time.

As opposed to sportboat buyers, cruiser and sportfishing boat customers are usually more pragmatic - not that they don\’t appreciate crisp acceleration and a robust top end, but they tend to be willing to forego maximizing speed in the interest of minimizing purchase price. In other words, they try to achieve a balance between cost and performance. They weigh the variables in arriving at the "proper" horsepower for their boats.
Besides cost, just what are these variables? First, there\’s acceleration, and to judge that, there\’s no substitute for a test drive. Don\’t trust numbers and spec sheets. Rely on hands-on testing instead. To decide between a V-6 and a small-block V-8, for example, first drive one and then the other, even if you have to travel some distance to do so. Pay particular attention to how the boat feels getting up on plane. Keep in mind that a demo boat is usually lighter than your boat will be after you\’ve loaded on the coolers, the gear, the canned goods, and the kids. After running at cruising speed for awhile, slowly back off on the throttle until the hull falls off plane. The slower the speed at which you can keep on plane, the better.

Attempt to achieve a similar hands-on comparison test if you\’re trying to decide between EFI and a carburetor. EFI motors run more powerfully than their carbureted equivalent, with shorter times-to-plane and measurably crisper acceleration. They also offer better fuel economy. Where EFI really shines is at the launching romp. Power is there instantly, whether the engine is cold or hot, so you won\’t have to deal with a sputtering engine that might die at any second while you\’re trying to get out of the way of other boaters impatiently waiting to launch.

Stern Drives

Because stern drives utilize inboard power plants, all of the above is applicable to them as well. The big question is I/O or not I/O. On the plus side is shallower draft and better maneuverability (with a single). The drawback to a stern drive has traditionally been that more components have to be in the water - especially in salt water. Here there is very good news, however. Outdrive corrosion and failure is nowhere near the problem it was just a few years ago. Tremendous progress has been made in the protection of lower units from the detrimental effects of the sea.

If you\’ve settled on a stern drive, how about counter-rotation? This term actually refers to two distinctly different situations. The first is a twin-engine installation, and here counter-rotation means that the port propeller spins clockwise and the starboard propeller spins counter-clockwise, which balances engine torque and makes steering easier. It also keeps prop wash from pushing the bow off course. In this sense of the term, there\’s no choice to be made - counter-rotation is a standard feature on all current twin-installation stern drives.

In the second sense of the term, however, there is a choice, because counter-rotation also describes two props on a single drive leg, turning in opposite directions. Whether your stern drive is powered by a brutish V-8 or a perky little V-6, you should be aware of the advantages of this technology, which was pioneered by Volvo Penta. Consider foregoing a standard configuration for a dual-propeller stern drive such as Volvo\’s Duoprop or MerCruiser\’s Bravo Three.
Both Duoprop and Bravo Three spin two propellers on a single axis. One propeller spins clockwise, the other counterclockwise. The configuration effectively doubles blade area, which translates into increased thrust. This extra thrust optimizes performance by converting every last drop of horsepower into crisper acceleration and higher top speed. Perhaps even more appealing than this performance boost is a significant improvement in handling, not only at top speed where it can be critical, but also at displacement speeds - docking and maneuvering in tight spaces is much easier.

In addition to performance and handling gains, fuel economy is also substantially improved, which extends range as well as cuts costs. The bottom line is that this type of "counter-rotation" can transform a V-6 into a V-8 - one with exceptional fuel economy. Like all good things, Bravo Three and Duoprop drives cost more than conventional drives. Also, there is a limitation on which boats are available with which brands. At the very least, find out what\’s available. Better yet, run the numbers ... and run the boat.

Outboard Motors

Deciding on outboard horsepower is easy because outboard boats are rated for maximum allowable horsepower. The appropriate horsepower package will be anywhere from about 70 percent of this rating up to the max.

If you want to go as fast as possible, specify the biggest legal power package. If you want respectable, but less expensive performance, back off to around 75 percent of the maximum rating. But don\’t drop below that number or time-to-plane and top speed will be disappointing.

Before signing up for a pair of 400-horsepower V-12 outboards, keep in mind that the more cubic inches the carburetor must feed, the faster fuel will drain from the tank and the shorter the boat\’s range will be. Roughly, figure that at wide-open throttle (wide open throttle) 10 percent of the horsepower rating will equal the number of gallons burned per hour. Twin 150s burn about 30 gph at full bore, while twin 100s only burn about 20 gph.

This brings us to the question of whether a single 200-horsepower outboard is better than twin 100s. The answer is that each offers distinct advantages and disadvantages. For example, a single installation costs less and weighs less, is somewhat easier to steer, boasts slightly better fuel economy and does not require throttle synchronization. The difference in price will be about $4,000. With twins, lay-up, tune-up and repair costs are doubled. Also, a pair of 100-horsepower outboards will not push a boat quite as fast as a single 200 because the extra lower unit doubles underwater drag.

To my way of thinking, the only reason to resort to the added expense of a twin installation is for safety\’s sake. If you regularly fish or cruise far offshore, a second engine is good insurance. It\’s a question of how much you are willing to pay for peace of mind. This is definitely a personal call.

An option worth considering is to rig a single, large outboard and a "kicker" outboard. The four-stroke 9.9 high-thrust motors are perfect for this (depending on boat size) because of their wide-diameter propellers and lower-ratio gears. With the new oil-injection systems, both engines can run directly off the same fuel supply.

Another consideration is a transom bracket. A transom bracket is simply a motor-mount that bolts onto the stern. It holds following seas at bay (allowing a full transom), and increases acceleration and top speed because the propeller(s) is farther aft and in relatively undisturbed water. The ride is also quieter.
Owners who operate in salt water should definitely take advantage of the new breed of saltwater-specific outboards with their enhanced corrosion resistance and torque curves that retain plenty of mid-range horsepower - which is exactly where you need it when negotiating rough seas.

Tags: Bow/Stern Thrusters , Power Take Off/Clutches , Propellers