Rob Shirley's Pro 48
When I was first introduced to one of Rob Shirley's initial model Pro 42 boats, I had mixed feelings. Its construction demonstrated an attention to detail, and it had a roomy, user-friendly interior, however, in terms of power (at the time it was only a single jet and engine arrangement) there was little to to get excited about. The single jet's handling characteristics, owing to its under powering, were similar to a truck running on bald tires over a slick road. Recently, I had an occasion to change that opinion. A meeting with Rob Shirley in Grand Cayman for a look at one of his newest models, a triple-jet powered 48 footer with a sharp, rakish profile, turned into an event where I would eat crow-by the bucket full.
The day was not one of Grand Cayman's best. Strong winds blew in from the northeast, producing five to six foot seas interspersed by the occasional eight footer, so we opted to stay near Seven Mile Beach. To get there, Rob left North Sound through Pappagallo Cut.
Even on a good day, this particular route out of North Sound is a difficult one, mainly because it is not much of a cut. It has a maximum depth of four feet and, when the wind blows hard, the waves really stack up at the mouth. The rough water, though, proved no problem for the Pro 48. Out in the chop, the ride and handling were stable, displaying no unsettling running characteristics as we traveled around the island's Northwest Point.
For a commercial dive operation, the dive boat is one of the most crucial pieces of equipment. On average, guests will spend more time on the boat than they will in the water. This insight led Rob to believe a boat must provide the best, most comfortable, enjoyable experience it can. From this experience customers will want to dive with that operation again and will pass along positive referrals.
Measuring 48 feet in length overall (from pulpit and swim platform) with a broad 16 foot beam, the Pro 48 is by no means a cheaply built boat. The hull bottom is solidly layered with fiber glass to approximately one inch thick in alternating layers to achieve maximum strength. In addition to using top grade vinylester resins and glass laminates (all hand layed), Rob prefers special fiberglass/foam/plastic composites in place of wood in all of the hull's stringers, bulkheads and deck areas. Since there is no wood, there is little chance of rot.
The result is fewer repairs and a greater degree of longevity for the hull. In its standard dive boat configuration (all models are configured to customer preferences), the 48's main cockpit, featuring a whopping 392 square feet of open deck space, is more than enough to comfortably accommodate 30 divers. Tank and equipment placements are provided by two extensive tank rack/bench seat arrangements (designed to hold 32 tanks per side) with enough seating for 44 passengers. Forward of the ladder to the upper deck and helm is a two tiered camera table with a holding rack for a five gallon cooler.
For access to and from the water, the transom's wide, cut-out opening features a heavily braced, 3 by 10 foot wide swim platform with twin drop down ladders. Like the rest of the aluminum work used on the 48's bow, grab rails and camera table, the ladders feature the same thick (inch and a half and higher), marine grade tubing, with excellent beadwork in the welds.
Some of the standard below deck equipment includes three 1,750 gph bilge pumps, stainless steel, 50 gallon freshwater holding tank and a 450 gallon fuel tank. The Pro 48 incorporates three 318 hp, Volvo TAMD 62 twin turbo diesels powering three UltraJet 300 series jet pumps. The question that some might ask is why jets, when propellers are still the most widely use frontline propulsion system for both recreational and commercial watercraft?
Exposed props have a rather significant achilles heel. In addition to the possible safety risk to divers, props are prone to severe damage from submerged objects, hard pan bottom, rocks, etc. For negotiating such obstacles in-water, jet-drive systems rule of the roost. In comparison to vessels propelled by props, jets seldom require more water depth than is needed for a bare hull. In fact, when on plane, jets operate well in far less water than the hull requires at rest. This is a particular advantage for a large vessel such as the Pro 48, which features a 24 inch draft should it need to run across shallow water.
Construction wise, the only real difference between a prop and an in-water jet drive system is the location of the propeller. The propeller, more precisely the impeller, is housed inside a tube creating what is called an axial flow pump. With an inboard arrangement, such as the 300 series pump manufactured by UltraJet, the pump is mounted well inside the 48's underside. Through this structure, water is drawn upward through the propulsion system's intake (flushmounted to the hull bottom), traveling slightly higher than bottom level of the hull, then expelled with great force through an aft-facing nozzle-an action/reaction principle of physics that works much the same as it does for a jet aircraft. Since most of the jet's vital components are enclosed, little is left exposed, thus greatly minimizing any damage that could occur from impact with sand bars, reefs, logs, etc.
From a performance standpoint, inboard jet drive systems, such as the ones produced by UltraJet, can be just as effective in terms of fuel efficiency, load carrying capability and speed as their traditional propeller driven counterparts. In fact, some of the jet systems incorporated in some of the big mega-yachts (60 feet and up) actually travel faster (some are able to obtain speeds of 60 mph plus) than their prop driven counterparts, while burning the same amount of fuel.
Can the Pro 48 do as well? In its 600 nautical mile, nonstop delivery hop from Key West to Grand Cayman, the triple jet 48 handled the journey more than well. >From dock to dock, the voyage took almost exactly 40 hours to complete, averaging 20 mph at 2,200 rpm, during which the vessel burned a total of 640 gallons of fuel, amounting to more than 1.1 nautical miles per gallon. For any boat this size, that is as good as it gets. Our top speed recorded in Grand Cayman with 43 of its 64 tank racks loaded with aluminum 80s, was a better than respectable 32.9 mph, while turning 3,200 rpm on all three engines.
From a handling standpoint, the 48's triple jet configuration can maneuver as well as any standard twin propeller inboard system, even at slow speeds. To back down (go in reverse), a cup shaped deflector called a bucket drops down in front of the propulsion end of the nozzle to change the directional stream. This also can effectively act as a breaking system, capable of stopping the vessel in half the time/distance of a comparable twin propeller craft-even at full throttle, with little to no harm to the hull, engines or jetdrives. Try that with a prop driven boat and you will likely tear the transmissions apart and/or loosen the engines from their mounts.
The reason jets can get away with it is because no transmission or reduction gears are involved with an axial flow pump. The engine instead is coupled directly to the impeller with a short shaft in between. When the bucket is dropped, the only thing altered is the directional flow of the exiting water.Between its exciting propulsion system and quality craftsmanship, this new model from Custom Dive Boats, Inc. is bound to become the top contender for dive operations worldwide. At present, since its introduction in 1996, dive operations in Roatan, Hawaii and as far as Micronesia have taken delivery of the Pro 48 with the triple jet powerplant.
For more information, contact Custom Dive Boats, Inc., 1416 Intrepid Drive, Deland, FL 32724; phone (904) 738-7300 or fax (904) 738-4455.