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October 25, 2001

A Question of Power

Should you choose an inboard or outboard engine?

When selecting a fishing boat, we compromise. At least, most of us do. We choose between speed and economy, between open fishing cockpits and live-aboard accommodations, between the convenience of keeping a boat in the water at a marina and the convenience of keeping a boat on a trailer at home. Some decisions are easy to make, such as between big boat and small boat - which often comes down to economics.

In the past, power choices were also simple. Trailerable boats up to about 25 feet usually had outboards - although boaters once considered stern-drives more dependable for offshore use. Most inboard boats in the 25- to 35-foot range were gasoline-powered and serious sport-fishing boats over 35 feet usually ran diesels. While this rule still holds true to some ex-tent, our power choices have greatly expanded in recent years.

In days gone by, gasoline inboards and stern-drives offered more speed than diesels, more dependability than outboards. Today, lighter diesels with higher horsepower ratings give performance equal to or even better than gasoline engines in some hulls.

Gasoline inboards for fishing vessels are generally marinized automobile engines with cast-iron blocks, not designed for prolonged operation at the high rpm necessary to attain peak horsepower. They have a price advantage for owners who don't use their boats often, seldom range far offshore or have experience maintaining and repairing this type of engine.

Marine diesels were adapted from industrial engines designed to power trucks, road machinery, agricultural equipment and electrical generators. Diesel engines generally produce peak horsepower at lower rpm and are engineered to run for long periods of time under heavy load. They also produce more torque than comparable gasoline engines for more usable boat-moving power, especially in the lower rpm ranges. Diesels usually produce better fuel economy, which translates into more range. However, even lighter diesels outweigh comparable gas engines.

New, lighter diesels coupled with stern-drives offer trailerable-boat owners diesel economy and dependability along with respectable performance. The new breed of higher-rpm diesels, adapted mostly from light truck engines, fit in the same space as a big-block gasoline engine, often use the same transmission and running gear, and provide more horsepower per pound than lower-rpm diesels.
New diesels may be more expensive than gas engines, but rebuilds of popular models like the venerable 671 Detroit are surprisingly affordable. From my experience, diesel engine longevity is the best of any marine type and gives a boat higher resale value as well as lower insurance rates.

The Outboard Alternative
The high-tech outboards available today are as reliable as any powerplant can be. Fuel economy on big outboards is much improved, and government-mandated pollution control regulations helped yield fuel consumption figures that rival those of diesels. Although diesels will always have a safety advantage, at least outboards keep the engine and most other sites where fumes and sparks could ignite outside the boat, in the open air.

Outboard engines are the only powerplants designed specifically for marine use. Their all-aluminum blocks provide more corrosion resistance when properly maintained, they don't need a separate cooling system, and they contain engine, transmission and complete drive train in one easily serviced and replaced unit. Two-cycle outboards require no engine oil changes, and the only fluid level that normally needs checking is the outboard oil in the remote tank.

Designed to run at high rpm for long periods of time, outboards are available in horsepower ranges suitable for most small to medium offshore boats. Of course, all the high-tech engineering that goes into a modern outboard - oil injection, computerized engine controls, multiple carburetors or fuel injection - does cause the price to soar. The price of a top-of-the-line outboard may seem even higher than a diesel unit until you take into account that the diesel needs a separate transmission, prop shaft, rudder, etc. Most outboards also come with instrumentation and engine controls included in the package.

Four-cycle outboards are now available up to 90 hp and you can expect more powerful models in coming years. Though heavier than two-cycle engines of the same power rating, four-cycles yield better economy and run much more smoothly.

Making the Choice
Boats in the 25- to 35-foot range can be suitably powered by either outboard or inboard engines. But, considering the advantages of speed, maintenance and trailerability, I personally prefer outboards.

Speed: Even though sea conditions restrict offshore speeds at times, regardless of power, anglers welcome the ability to run hard and fast on calm days. The run from my slip on Oyster Creek to the Freeport jetties is around 15 miles. That quick trip in my 40-plus-mph outboard boat takes an extra hour coming and going in the single-diesel-powered, 34-footer charter boat I sometimes run. Not all diesel boats are slow, but they are slower on the average than a boat with outboards in the same power range. Also, most big center-consoles or small express-type boats running twin outboards would be powered by only a single diesel engine.

Maintenance: I lose less time down for engine maintenance and fewer fishing days each year to mechanical problems than do the majority of inboard boats berthed around me. Most inboard boats leak around the packing glands (though I haven't tried the new mechanical seals). Through-hull fittings for cooling-water intakes can leak or be plugged by bottom growth and debris, causing the engine to overheat.

With outboards, I don't have to worry about the boat sinking in its slip if the bilge pumps fail to come on or the batteries die. Instead of spending time below deck checking engine oil, transmission fluid and water level in the heat exchanger expansion tank before each offshore trip, I just check the remote oil reservoir and I'm ready to go.

Outboard owners don't have to worry about cutlass bearings, rudder assemblies and shaft alignment. They can check all their mechanical components visually at any time and clearing debris from a prop can usually be done from the boat.
The main disadvantage to outboard power for larger boats is that advances in these engines change as fast as computer technology, and changing outboards to keep up with the latest technical doo-dads would be very expensive.

Trailerability: The majority of inboard boats, especially diesels, stay in the water. This means bottom jobs - an every-other-season chore in my home waters. It doesn't take long for barnacles to grow on the metal running gear - props, struts, shafts and rudders - and it doesn't take many to affect performance.
Fishermen often keep their outboard boats on a trailer or in a sling, but even docked in the water, the entire engine assembly - including props - can be tilted clear of the surface. Some installations require a hydraulic jack plate to get the tilt and trim cylinders out of the water. With outboards in the water, it's important to keep an eye on the underwater zincs or the trim and tilt cylinders will corrode.

The subject of outboards versus diesels is very much on my mind these days as I'm restoring an older 34-foot boat. Although originally powered by diesels, its hull lacks sufficient buoyancy for heavy engines and such niceties as generator, air conditioning and large amounts of fuel. By choosing outboards - a pair of V-6's weighing less than a single diesel of most types - the weight savings will allow me to add a generator, oversized fuel tanks, large live well, huge fish boxes and all the gear six anglers can carry - and at a greatly increased cruising speed. The belowdecks space saved will hold most of the items mentioned above without giving up cockpit room.