POWERBOAT DESIGNS

MOLOKA’I STRAIT 65

for voyaging

Built by:

MOLOKA'I STRAIT MARINE
Sales: American Global Yacht Group

Annapolis, MD

Tel: (410) 315-8156

Hull #1 of the MOLOKA’I STRAIT 65, Atlantic Ranger, runs down river soon after launch.

Painting by Steve Davis

PARTICULARS

Loa                                                             65’ 0”
                        Lwl                                                              56’ 7”
                        B                                                                 19’ 0” hull
                                                                                       19’ 8” rubrails
                        D                                                                  6’ 0”
                        Displ.                                                    181,636 lbs. at Dwl,

                                                              with appendages 165,000 lbs. mid-cruise                         Block Coefficient                                        0.421
                        Prismatic Coefficient                                  0.602
                        Lbs./inch immersion                                 4,226 lbs/in
                        Displ./Length ratio                                    406.6 mid-cruise displ.
                        A/B area ratio                                           2.040
                        Installed power       Caterpillar 3406C, B rating, 440 HP @ 2100 RPM
                        Speed                                                            9.0 knots, cruise
                                                                                               8.0 knots long voyage cruise
                                                                                            10.2 knots, max.
                        Speed/length ratio                                        1.2 cruise
                        Range                                                      3,500+ miles
                        Fuel capacity                                            4,800 gal
                        Water capacity                                           500 gal

specifications subject to change

Introduction: Designed and fabricated for serious voyaging and live-aboard, the MOLOKA’I STRAIT 65 is constructed of the highest grade marine steel and aluminum alloy. Moloka'i Strait Marine is seeking serious buyers for the first few vessels. Owner participation throughout the entire design and construction process is welcomed and encouraged to truly customize each hand-crafted yacht. Inquire about either the Three-Deck or Two-Deck accommodation layouts.

Genesis of the design: I had done some design work on the Moloka'i Strait Marine (MSM) 50' trawler over the last few years, and Geoffrey White and Lee Cherubini (the company principles) decided they wanted to get into bigger trawler yachts. They felt that the market was pretty open for a high-quality metal yacht of the Romsdal style (as in Romsdal, Norway, fishing trawlers). This type of design is characterized by a relatively clear foredeck, a superstructure set amidships to aft, and a rounded (what we call "cruiser") stern. Most of the metal yachts in the mid-60' range were coming from Canada, and MSM felt they could do a really good job of construction in the US, much better than coming out of Canada.

Metals: I've designed the hull for steel construction, transversely framed, with an aluminum superstructure. The two metals are connected by a Detacouple, which is an aluminum/steel strip that is explosively clad together, much like how our dimes and quarters are made. Detacouple was invented by DuPont at least 30 years ago, and is now made by Dynamic Materials Corporation. The steel side of the strip is welded to the main deck, and the superstructure is welded to the aluminum side of the strip.

Hullform: The hull is a heavy displacement type with a bulbous bow and cruiser stern. Cruising speed is 9 knots, with maximum speed of 11 knots. Therefore, the hullform is designed for generous interior volume with standing headroom in the engine room.

A fair bit of thought went into the shape of the bulb and bow. Many trawlers of this size suffer from uncomfortable pitching and heeling. The pitching (almost like hobby-horsing) seems to come from the fairly straight, wall-sided shapes of the bow plating. When the boats get into significant waves, there is little bow shape to dampen the motion. The wall-sided bows sink readily as the bow goes down, without absorbing much energy. On the MOLOKA’I STRAIT 65, I put generous flair into the bow plating so that as the bow pitches down, the submerging volume increases more quickly than on a wall-sided bow, thereby absorbing more of the pitching energy.

Also, the bulb is not a simple round cylinder as is so common on trawlers of this type. Round cylinders are easy to build, but they slap on the waves and also have little damping effect by virtue of their shape. On the MOLOKA’I STRAIT 65, I have a Nabla-style bulb which is characterized by being V-shaped on its bottom surface and flatly rounded on its top. The V-shaped bottom cleaves the on-coming waves without slamming, and then as the bow pulls up on the return motion, the higher drag of the rounded top of the bulb slows the bow's lift. The resulting difference in speed between the bow sinking and returning is the dampening effect of the bulb. Pitching motion bleeds out quickly, making for a very comfortable ride.

Bulb proportions are also critically important. A bulbous bow is designed primarily to reduce the wave-making resistance of the hullform which means you can reach the design speed with less horsepower. Resistance is reduced because the bulb produces a wave that is opposite to the normal bow wave. The two waves cancel each other out. This occurs only if the bulb proportions (length, beam, depth, volume, and cross-sectional areas in profile and transverse) are correct. If they are not correct, then it is likely that the bulb could have no effect on resistance at all, or it could even add resistance to the boat. This is why model testing is so important when you have a bulbous bow, so that you can determine precisely what the resistance effects are.

The bilge keels and stabilizers will add resistance to the hull, and we are hoping that what is added will be less than what is reduced by the bulb. The bilge keels are large enough to let the hull stand upright when sitting on the hard. This is important for people who like to voyage to far-away places where tides are significant, but shore facilities are not. The stabilizer blades, nested in line with the bilge keel blades, are therefore protected against damage from grounding. The generous projected areas of the of the bilge keels also create extra dampening effect against rolling. The round-bilge hullform makes for gentle rolling motion (as opposed to quicker, snappier motion of a chined hullform), and the bilge keels dampen the roll quickly by virtue of their drag against roll. Therefore, we expect the motion of this boat to be exceedingly comfortable, compared to other designs on the market.

Model testing: MSM contracted with Oceanic Consulting Corporation at the Institute for Marine Dynamics, St. John’s, Newfoundland, to carry out model tests on the MOLOKA’I STRAIT 65 to determine the horsepower requirements of the final hull design. This was accomplished with a multipart hull model at 1/5^th scale (13’ long), milled out of foam by a numerically controlled milling machine using the hull shape data from the original computer design.

The hull model was comprised of one stern section and two bow sections, one of which had no bulb, the other with. First, the stern was assembled with the no-bulb bow and towed in the tank from 6.5 knots to 11.5 knots at half-knot increments to determine the baseline resistance of the basic hull form. Then, the first bow was replaced by the bow with the bulb, and the tests repeated, to determine the net resistance reduction effect of the bulb. In the next step, tufts of yarn were glued to part of the submerged area of the bulbed model so that, when towed at cruise speed, the flow could be studied for best placement of the bilge keels and stabilizers. Finally, the model was towed at the original speed range to give readings of resistance for the final hull configuration. Knowing this information, we were able to precisely determine the best engine selection, horsepower tuning, and propeller design. See the second and third technical briefs for a more complete description of the speed power predictions and the results of the model testing.

Accommodations: We have two arrangements for the interior, a three-deck version and a two-deck version. The three-deck version has the pilot house up one full level from the main deck accommodation. This gives the most spacious interior and is the primary general arrangement of the MOLOKA’I STRAIT 65. The two-deck version has the pilot house 22" lower than in the three deck version, so that the motions in a seaway will be slightly less. The two-deck version also trades off interior accommodation space for more enclosed deck stowage, and this area is located under the pilot house.

From the top down in the three-deck version, the pilot house has a console and helm station to the front, with a captain’s cabin at the back. A small head with shower is in the starboard quarter which may be used by the watch and the captain as his personal head.

On the main deck, the foredeck is clear for deck stowage of the dinghy which can be a hard-bottomed inflatable up to 14’ long. Inside the deck house, the dining area is forward, separated by a small breakfast bar from the galley. The stairway to the pilot house is to starboard, alongside of which is the walkway to the main saloon aft. This area includes a small study nook and entertainment center to port, with the main settee in the starboard quarter.

Nested below the stairs to the pilot house are the stairs to the below deck spaces. At the end of the stairs is a common head for daily use. Go aft down the hallway to the engine room and the laundry room to starboard. Opposite the stairs is the master stateroom to port with a queen bed and ensuite head. Forward of the stairs are two guest cabins, port and starboard with a common head at the end of the hallway. Forward of the head is the anchor locker above, and the bulb space below.

The interior accommodation below decks for the two-deck version is similar to the three-deck version. Some variations of the spaces can be customized to individual owner’s desires. One option is to turn one of the guest cabins into an office with a Pullman berth. Another is to eliminate the hallway aft in favor of a bigger master stateroom and larger laundry room. In this arrangement, the access to the engine room is through the laundry room, and the main engine room access is from a hatchway in the cockpit aft.

Power: The boat is powered by a Caterpillar 3406C in-line 6 diesel engine with a "B" rating at 440 HP at 2100 RPM. Standard shafting and propeller is a Hundested controllable pitch propeller with a 2.43/1 reduction pitch controller. The engine room has full stand-up height with a workbench forward and to port. The main fuel storage tanks are port and starboard. A 32 KW Northern Lights genset supplies daytime electricity, and a 12 KW Northern Lights genset runs for nighttime power. The larger generator is fitted with a power take-off to drive a hydraulic pump that, in turn, drives a belt on the cardan shaft between the engine and pitch controller. This "get-home" system will drive the boat at 4 knots. Fuel capacity is about 4,800 gallons, expecting to give us a cruising range of 3,500+ miles at 8 knots, longer, of course, at lower speeds. This makes the boat truly capable of transatlantic crossing on its own bottom.

Tanks: Besides the main fuel storage tanks, two additional fuel tanks are located in the double bottom below the master stateroom. One of these is used as the day tank. Below the guest cabins are independent stainless steel tanks for various water services. The two fresh water storage tanks have a total capacity of approximately 500 gallons. Two gray water tanks have a total capacity of about 81 gallons, and three black water tanks have a total capacity of about 110 gallons.

Construction of the first hull began in September, 2001.

All of the framing for the hull and deck house was cut by numerical control (NC) cutting. NC cutting involves a completely independent design effort by a third party design firm that specializes in that type of service. Basically, it involves completely recreating our 2-D AutoCad structural design drawings in another computer program in 3-D. The design engineer then disassembles all the structural parts right down to the last bracket and nests them onto plates, complete with etched part numbers and cutting sequences.

The plating on the first hull was laid out by hand. This is because the hull’s compound curvature makes plating layout particularly difficult to do solely by computer, and having an actual hull right in front of your eyes makes plate layout a lot easier. Once the plate pattern was established on the first hull, it was completely recreated in the NC modeling program for cutting of all subsequent hulls. Boats 2 and 3 are under construction now, and they have NC-cut plating.

In September 2002, we launched the first MOLOKA’I STRAIT 65, empty of its interior, but equipped with all its primary machinery. We conducted a stability test and found the boat to weigh almost exactly what the weight estimate predicted for that stage of construction. The center of gravity was lower than expected, which is good because it means the stability at that stage is better than predicted.

Just before delivery to the owner, MSM puts the boat through a complete sea trial to prove that everything works the way it is supposed to, and we do a final stability test to determine the boat’s final weight and location of its center of gravity. The first MOLOKA’I STRAIT 65 will be delivered to its owner the summer of 2003, and it can be seen at its worldwide debut at the Annapolis Power Boat Show in mid-October.

Here are some publicity photos of the interior:

The pilot house.

The engine room.

The main saloon.

The galley and dining area.

The master stateroom.

The next version of the MS 65 is being redesigned with a multichine hullform for easier construction, and it will include a buoyant swim platform which extends the hull by three feet. The name is changing to the MOLOKA’I STRAIT 68.