Understanding Hull Forms

Posted on Apr 12, 2017

We boating writers are guilty of jargon about powerboat hull shapes.  To remedy that failing with respect to Eastern, Rosborough, and Seaway boats, here are some thoughts about their hulls.

Displacement Hulls

Boats float because their hulls displace a greater weight of water than they weigh themselves, even when we fill them with engines, fuel and water, equipment, and ourselves.  A displacement hull moves through the water, pushing it aside in waves that originate at the bow.  At slow speeds, all our hulls move this way, but there is a physical limit to how fast a bow wave can travel, determined by its length.  Thus a longer hull waterline allows a boat to move faster, all other characteristics being equal, but it still has to shove aside the volume of water that it displaces.  The general rule is that a hull in displacement mode can move efficiently at 1.0-1.3 X the square root of its waterline length, but adding more power to go faster simply drives the hull deeper into its own bow wave, increasing resistance exponentially with minimal gains in speed for the extra power.

Round Chine, Semi-Displacement Hulls

As internal combustion engines came onto the scene around the turn of the twentieth century, boatbuilders developed faster hulls.  For a time, they built narrower, slipperier hulls and applied more power, but gradually they figured out that hulls with flat or nearly flat aft running surfaces could lift partway onto their bow waves and beat the 1.3 rule.  One of the best known was Will Frost of Beals Island in Downeast Maine, who built legendarily fast workboats for lobster fishermen (and rumrunners) in the first half of that century.  They were the forerunners of the New England workboats that developed in the latter part of the twentieth century to take advantage of stronger, lighter gas and diesel engines.  Their sharp, “proud” bows cleaved through seas, and their rounded chines (corners between bottom and topsides) gave them a soft ride, while their flat running surfaces lifted them.  They appealed to thrifty Yankee fishermen because they were seaworthy, maneuverable, efficient, and fast for their time, cruising easily at 13-16 knots.  Their classic looks and no-nonsense capabilities appealed to recreational boaters too, so their builders soon were turning out “lobster yachts” as well.

And their evolution from Will Frost’s hulls was no accident.  The best-known designer of the 1970s and ‘80s was Royal Lowell, Frost’s grandson.  Want to see a Royal Lowell design?  Consider the lovely Eastern 18, this company’s original model, still in demand after 35 years.  Want more?  The Sisu 22 is considered by many the most able small powerboat ever built.  Lowell’s designs spread through the Eastern line to the 31 and 35 Casco Bay, also considered seaworthy classics.  Other designs in the Eastern and Rosborough lines derive from the Downeast/Canadian Maritime design tradition that Will Frost started.  One long-term index of their success is the number of Rosborough 246 hulls working in service to the Canadian Navy.

In the new century, the development of even more powerful, clean, super-efficient outboards has revolutionized the way we power 18-27’ boats.  Those 13-16-knot cruising speeds are still efficient and useful in well-balanced semi-displacement hulls, but Eastern’s hulls can cruise faster too if conditions permit, partly also because modern composite materials make them both light and strong.  They typically run well at speeds of 18-22 knots and top out around 30.  And they do so with moderate power, from 50-70-hp for the 18 to 300-hp for the 27’ models.  Those are winning combinations.

Hard Chine, Planing Hulls

Harry Farmer founded Seaway Boats with traditional round-chine, semi-displacement hulls in the 1980s, and some of those boats are still working in harbors all along the East Coast, but after twenty years, his thinking shifted to “hard chine” hulls with sharp corners at the edges of the bottoms.  Here he may have been influenced by Royal Lowell, who had begun experimenting with them in his later designs.  The Eastern 31 and 35 Casco Bay are well-proven Lowell examples.  These chines provide extra lift, and if combined properly with other design features, they allow the water flowing past to release more easily than round chines.  By the time Seaway became an Eastern Boats brand, all the models followed this form, and the Eastern 27 models ride on the same hull forms as the Seaway 27s.

Relatively narrow for their length, these hulls perform well at semi-displacement speeds, but with more throttle, they lift fully onto the water’s surface.   Here resistance to speed comes primarily from friction between the area (length X beam) of the hull and the water, and much less by the volume (length X beam X depth) of water it displaces.

The downside is that a hull riding on the surface is affected much more by wave conditions.  A completely flat bottom will rise onto plane easily, but it will “pound” against any waves it encounters.  The trick is to work out shapes that cleave seas apart while still allowing a hull to rise easily on top and stay there without requiring gobs of power…and fuel.  Both Lowell and Farmer worked around this issue effectively in their “semi-V” designs.

In general, they have a shallow V-shaped cross-section (deadrise) at the transom that becomes progressively deeper going forward, to present a sharp bow entry to oncoming seas.  That term deadrise refers to the angle at which the bottom rises on each side of the keel (literally, rise above dead-level).  Seaway and hard-chine Eastern hulls have transom deadrise of 10-16 degrees, tapering to around 30 degrees amidships and 40-45 at the bow.  The magic is all in that taper.  Their advantages are shallow draft, efficiency, and reasonable speed with modest power, but they will pound if driven hard into steep seas at the wrong running angle.

It’s important to trim all the Eastern, Rosborough, and Seaway boats fore-and-aft to fit sea conditions, lowering the bow to cleave head seas but raising it in following seas to prevent it from digging in and broaching as the wave pushes the rest of the hull sideways.  Adjustable trim tabs and engine power trim are valuable for getting the best out of these hulls.  Combine these tools with careful study of the hull’s shape, to gain a sense of what part of the hull is hitting the seas first in any conditions.

The final tool for an easy ride is the throttle.  Driving too fast in rough conditions will make any of these boats seriously uncomfortable.  Fortunately, these thoroughbred hulls reward any skipper who takes the time to learn how they perform best on any given day, using the trim tools and throttle.  That kind of partnership is what makes so many Eastern, Rosborough, and Seaway owners so loyal to their boats.