Choosing Power Wisely

Posted on Mar 29, 2017

by Capt. John Page Williams

“Always choose the maximum power for a new boat or a re-power.”

That’s what a lot of boaters (and some dealers) say.  Here are their arguments:

  • “The largest engine(s) will let the hull perform up to its potential.”
  • “It’s important to have enough power to accelerate quickly onto plane with the maximum number of passengers aboard, especially if the boat is pulling skies, wakeboards, or tow toys.”
  • The engine(s) won’t have to work as hard as smaller ones, saving fuel and reducing internal wear.
  • When you trade in the boat, it won’t be worth as much or sell as quickly if the engine is smaller than the maximum.

Sound convincing?  Sure, until you begin considering the broader context of running your boat on big, coastal waters that can kick up rough, and the specific ways that you will use your boat.  Those four points actually do work well as arguments against under-powering your boat.  From there, though, it’s not so simple.  Consider:

  • What does “performing up to its full potential” really mean, especially on coastal bays?  Their often windy, shallow waters throw up steep, choppy waves that can turn almost any hull into a lurching buckboard if the skipper runs it too fast for the conditions.  Most days, top speed becomes irrelevant.  What counts in the real world is the boat’s range of efficient cruising speeds.  That range depends not only on hull form and power but how well the boat is balanced fore-and-aft and side-to-side.  The trick is to slow down until the boat rides as smoothly as possible over the seas.  It’s a feel that comes with getting to know how your boat behaves best in different conditions, how to “dial her in.”
  • When shopping for a new (or new-to-you) boat, ask for a sea trial, preferably on a windy day.  As the boat accelerates, watch at what speed it rises onto plane and the wake begins to flatten out astern.  Note the engine rpm and the speed.  By all means, throttle up to wide-open if conditions permit and note both rpm and speed.  Then throttle back and watch how the boat behaves.  The ideal performance profile is easy cruise at 60-80% of full throttle rpms with the most common load of people and gear you expect to be aboard, at which point the engine is running easily.  If you study the rpm/speed/fuel flow profiles on the Eastern and Seaway web sites, you’ll note that nearly all our models reach peak efficiency at speeds of 15-26 knots (17-30 mph) in that 60-80% rpm range.  The recent advent of strong but lightweight 200-hp, 4-cylinder four-strokes has provided a great new power choice for Rosborough owners whose boats are heavily loaded for long-range cruising.  
  • Here’s a purely subjective rule of thumb, though it’s based on a lot of time on the water: if the boat is 20’ or less, look for a minimum planing speed of no more than 13 knots.  For a 17-18’ boat, 10-11 knots is even better.  The Seaway 18’s performance with a 70-hp four-stroke is a great example.
  • Getting reasonable acceleration (also known as hole shot) is dependent not only on choosing enough power but mounting the best propeller for the ways you plan to use your boat.  As a basic rule, make sure that engine(s) can turn to the top end of the manufacturer’s specified operating range (generally 6,000-6,300 rpm on most modern four-strokes, 5,500-5,800 on direct-injection two-strokes).  Your dealer or service technician should be able to help you make a good propeller choice for your uses.  It may be useful to go to a four-blade propeller instead of three and install trim tabs for extra lift and trim control that give you more tools for dialing in, especially in rough waters.  Incidentally, it is also possible for max rated HP to supply too much acceleration.  Some standard twin-engine combinations can actually cause passengers to lose their grips on grab handles and T-top rails.
  • If your boat/engine combination passes these tests, your engine is going to be happy, especially if you use the manufacturer’s oil in its crankcase or fuel.  That oil can be more expensive, but it’s a tiny element in a boat’s budget, and a cheap investment in its longevity.
  • If the boat performs well, the trade-in question becomes irrelevant.  If it satisfies your needs, why worry about selling it now?  And if it does its jobs well, that performance should be its strongest selling point, no matter the size of the engine.
  • By the way, Eastern, Rosborough, and Seaway boats perform extremely well with smaller engines than most other mainstream hulls in their size classes.  A good example is the 24’ Eastern Center Console, which matches naturally with a 150-hp four-stroke, while other hulls of that size mount 300-hp or more.  That efficient performance with modest power reflects their origins as Downeast workboat designs.  The thrifty Yankee fishermen behind those designs were more interested in efficiency than flash.  Take a ride and you’ll see what we mean.

In the end, choosing power carefully can save serious $$$$, in the initial purchase, fuel efficiency, and even maintenance.

Adapted from an On Boats column published in Chesapeake Bay Magazine, June, 2016