Posted on 30 January 2009
Written by Eric Sorensen
Taking a lesson from a behemoth
Even if your boat isn’t an oceangoing passagemaker, a look at the Moloka’i Strait 65 is instructive.
Chances are most of you aren’t going to do your cruising aboard a steel-hulled oceangoing passagemaker, but there are many lessons we can take away from such a vessel and apply to a 35- or 40-foot displacement yacht.
Since naval architect Eric W. Sponberg is an old friend who specializes in out-of-the-ordinary projects — heavy-weather motoryachts, houseboats that serve as floating apartments, sailboats designed to fit in a shipping container, retro runabouts — we’re going to have a look at what may be his crowning achievement: a big boat notable for its integrity and intelligence of design.
Why in the world Mr. Sponberg, you ask? Well, I’ve worked with Eric through the years on different projects (since he is an actual engineer who can do math), we’re both from New England, he has a beard and looks the part, plus we have the same initials. Anyway, I thought a look at one of his displacement motoryacht designs — the Moloka’i Strait 65 — would be instructive, since we think a lot alike when it comes to rugged offshore vessels.
Let’s look first at Sponberg’s design brief — the laundry list of owner’s requirements, or at least nice-to-haves — that includes trans-Atlantic range (at least 3,500 miles to comfortably cross without refueling). In this case that worked out to 4,000 nautical miles at 8 knots. She’ll go 10 knots at full power, a speed determined by the hull’s waterline length (56 feet, 7 inches), but the range will plummet as drag increases exponentially. If you want a faster displacement hull, just make it longer; you can hit a dizzying 13.4 knots in a boat that’s 100 feet on the waterline.
When I asked Sponberg the seaworthiness requirements, he basically said the waves get really big out on the ocean, and the Moloka’i is designed to come through unscathed. This is a “one-compartment” vessel, which means any one compartment (of the five total) can flood completely, and she’ll retain enough stability and buoyancy to remain afloat and upright in moderate sea conditions. The boat is also designed to completely roll over and come back up, provided the windows hold or there isn’t too much downflooding while she rights herself.
This points to tremendous righting arm energy in normal operating conditions, and it’s also indicative of a hull with ample (but not too much) metacentric height that does not roll too deeply. Freeing ports on deck are very large, so the free surface effect up high on deck caused by boarding seas will be short-lived.
The yacht was to accommodate up to eight people in four cabins, three of them below deck and one up on the bridge. This lets an owner-operated boat be run without the owner leaving the vicinity of the pilothouse, while also accommodating a hired crew, perhaps a related couple or at least one on good terms.
Sponberg designed the bridge with an eye toward good horizon visibility, plenty of room to store paper charts (rolled so they lay flat without creases), and a dumbwaiter to the galley below. The bridge was to have wing doors for topside access and a Portuguese bridge, basically a low bulkhead forward of the superstructure that protects people topside forward of the pilothouse and protects the pilothouse from boarding seas. The aft deck was to have room for a few chairs, as well as a hatch through which the engine could be replaced (along with another hatch in the saloon below).
The deckhouse has big windows, an indulgence you wouldn’t see on the original working trawlers, of course. But on the Moloka’i, they’re thick and laminated to encourage them to hold together even if shattered, which is pretty good insurance against breakage at sea. The boat was built to American Bureau of Shipping standards, though it was not certified as such, because of the considerable costs involved.
Now let’s have a look at the result of Sponberg’s design brief. The Moloka’i Strait 65 is as bulletproof as they get — an oceangoing vessel with a 19-foot-beam, steel hull and aluminum superstructure, displacing 181,000 pounds at full load. That’s a lot of refuge in a 65-foot package. (A note about the name: If you have an atlas, you’ll see that Moloka’i is an island in Hawaii, and it is known for rough water. The “strait” is a figment of the builder’s imagination). A 4,800-gallon fuel supply provides a 4,000-nautical-mile range (0.83 nmpg) at 8 knots with a single derated 440-hp Caterpillar 3406. There’s also a 500-gallon water tank, kept topped off with a 600-gallon-per-day watermaker. Compare this with a planing vessel like the 40-knot, 120,000-pound, 4,000-hp (approximately 0.25 nmpg) 68-foot Viking convertible.
The Moloka’i Strait 65 is loosely patterned after the Romsdal (Norway) fishing trawler. These vessels have long, open foredecks where the fishing takes place, with the deckhouse in the aft half of the hull. This is definitely the way to go if you want the most comfortable ride while at the wheel, as the vertical motions are much less emphatic aft. The Moloka’i is remarkably true to its roots in that sense (see www.romsdal.classictrawlers.net/index.php); the superstructure is a little larger, the hull a little wider, and the propulsion engine a lot more powerful than the original, but this is a true oceangoing vessel that has not been bastardized (made unseaworthy) by a marketing-driven purveyor of “trawler” yachts. This is the real enchilada.
The hull is built of 1/4-inch A36 mild steel plating and the main deck 3/16-inch steel plate. The hull is of steel because of the material’s tremendous strength, impact resistance and low cost (in relation to aluminum). Also, having lots of weight down low improves stability.
The inside of the hull below the waterline is painted with POR 15, a rubbery anti-rust coating. POR stands for “paint over rust.” (My kind of paint — wish they had it when I was in the Coast Guard.) Above the waterline, the inside of the hull and superstructure are sprayed with a two-part polyurethane foam that seals the surface, deadens sound, and prevents condensation.
The superstructure is aluminum, also quarter-inch, because it’s much lighter than steel (171 pounds per cubic foot vs. 490 pounds), yet still strong and corrosion resistant. A steel superstructure on a boat like this would have to be very small to preserve adequate stability. The two metals are joined by an explosive-couple strip (used on Navy ships for decades), where great pressure created by an explosion bonds the two metals — and in a way that doesn’t lead to galvanic corrosion. The steel side of the strip is welded to the hull, the aluminum side to the superstructure.
The hull has a round bilge, achieved by using relatively narrow steel planks that can more readily follow the curvature of the hull. Filler and paint follow to make the hull look like it came from a fiberglass mold. The round bilge decreases resistance to a minimum and maximizes the interior volume of the hull, which really helps with the capacity of the integral fuel tanks. A single or double (even triple) chine hull would be a lot easier, faster and cheaper to build, but it would also add wave-making resistance and decrease interior hull volume. The round bottom also reduces rolling accelerations, which is a big part of what makes people seasick, making for comfortable motions at sea.
The Moloka’i also has a bulbous bow, which serves two purposes. First, it increases the waterline length of the hull and interferes with the formation of the bow wave, so the boat can go a little faster, or the same speed with a little less power. (Eric figures on a 6 percent increase in efficiency for this boat.) Second — and every bit as important in a boat as beamy as this one — it dampens pitching, which is achieved by making the bulb flatter on the top and more of a cleaver on the bottom.
The flattened bulb top slows the bow as it rises back up on the next wave, which cuts down on the pitching motion for a smoother, dryer and more efficient ride. (If it pitches up with less energy, it will immerse less on the way down, lessening the demands on propulsive power.) Sponberg also increased the hull flare forward to pick up buoyancy quickly as the bow dips, which also helps diminish pitching. A canoe stern minimizes resistance at displacement speeds.
To make sure they had the right drive train (engine, reduction ratio and propeller size) and because the shape of the bulb influences performance so much, as does the choice of bilge keels designed to reduce rolling, a good bit of tank testing was done before settling on the final design.
The upper deck includes the bridge and adjacent captain’s cabin with head. Forward, where these boats were originally designed to be fished from, is room for a 14-foot tender. This vessel has very high freeboard, especially forward, so reserve buoyancy (the volume of the watertight hull above the waterline) is tremendous.
Forward in the superstructure is a dining area, breakfast counter and galley, as well as stairs leading up to the bridge and down to the accommodations. Aft is the main saloon. Down below we find a day head and the master suite with its own access to the laundry room. Forward are port and starboard guest staterooms that share a head. Forward of the head is the anchor locker and, below that, the bulbous bow compartment.
Another door leads to the standup engine room, with its workbench and outboard fuel storage tanks. The 3406 Cat has a “B” commercial rating of 440 hp at 2,100 rpm, an engine that will last a lot longer than those of us writing or reading this article. It connects via a 3.3-to-1 gear to a 3-inch shaft and then to a four-blade 47-inch Hundestadt controllable-pitch prop. The Cat also drives hydraulics for the bilge stabilizers and power steering. (You can count on the original Norwegian trawlers not having any of that nonsense on board).
To make sure there’s plenty of ice for cocktails, 32- and 12-kW gensets are included, and the big unit hooks up to the shaft to provide 4-knot get-home power. The 4,800-gallon fuel supply is carried in wing tanks in the engine room and in the double bottom forward. Stainless steel tanks hold 500 gallons of potable water, with the desalinator keeping those tanks topped off all the way from Chatham, Mass., to Sardinia. There are dampers on the combustion air intakes that close off in the event of a fire, allowing the Halon equivalent to do its job putting out a blaze.
This is one serious oceangoing yacht. I’ve been aboard it alongside the dock, and it feels more like a 90- or 100-footer from the sheer interior space. We’ve only had room to touch on some of the design highlights, but everything we’ve talked about here applies to your next yacht, especially if it has a displacement hull and you intend to take it well offshore. Have a look at Sponberg’s Web site for more illustrations of this fine vessel (www.sponbergyachtdesign.com).
Eric Sorensen was founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached at
This story originally appeared in the February 2009 issue.