This 48-foot 66 MPH luxury power cruiser was beautiful inside and outside, but during trials and during the test ride, a squeaking "humming noise" was noticed in the proximity of the icemaker cabinet door. The frame of the cabinet door was attached to the hull and the salesman convinced the potential buyer that the noise would disappear after the boat was operated a few hundred miles. According to the salesman, there were built-in residual stresses that will be released by operating the boat with motions and slamming for several hours. The boat was sold despite of the squeaking.
After some short familiarization runs, the new owner left for an extended trip in Lake Michigan, but on the first day, a personal watercraft that was stored on a platform aft of the transom "popped off" (broke free) when the speed of the 48 foot was between 35 to 50 miles per hour.
On the second day, upon running into some adverse weather conditions, the following happened:
In addition to the above, several other damages developed, the majority was of items attached to the hull structure, indicating as the main cause of the possibility of large hull elastic deformations and probably damage to underlying structure.
The vessel was a 2003 model Fountain 48 Sport Cruiser. Hull construction: molded fiberglass. Deck construction: cored molded fiberglass. Core material: balsa with plywood reinforcement.
Vessel was powered by triple Yanmar turbocharged model 6LT2A-STP turbocharged marine diesel engines of 257 kw at 3100 rpm and the maximum speed was 66 MPH. Note: The port engine had 42.2 hours, center 42.6, starboard 42.7.
Vessel was powered by one marine genset contained in a sound shield.
Sleeping accommodations were for 4-6 passengers as follows: forward cabin with double berth and port master head compartment; starboard galley; head compartment; port settee. Aft cabin with double berth.
Luxury power cruiser are not usually designed for such high speeds as the 66 mph capability of the 48 Express Cruiser, therefore, the equipment generally installed aboard cruisers is not designed or intended by its manufacturers to withstand the impact loadings experienced at such high speeds. Other cruisers of this size are powered for top speeds in the range of 30 to 35 mph with cruising speeds in the range of 23 to 30 mph. Purchased components for such boats, such as heads (marine toilets), refrigerators, stoves, etc. are designed for the stresses encountered in the operation of the normal range of boats operating at speeds of 20 to 35 mph.
The Fountain 48 Express Cruiser was designed for a top speed of 66 mph and can cruise at 45 to 55 mph. Impact accelerations in high speed planing boats increase nearly as the square of the speed. The significance of this relationship is that impacts from waves on a boat traveling at 60 mph will be roughly four times those in a similar boat at 30 mph. The relation between wave heights and impact accelerations further exacerbates impacts related to the speed of the boat. Impact accelerations from waves increased as the square of the wave heights.
The Fountain 48 Express Cruiser was a design aimed at high-speed performance compared to other luxury cruisers. In order to achieve the top speed of 66 mph, there are a number of design features that are developed to achieve that high top end speed. One is a stepped hull bottom. The hull's planing surface is broken into three distinct planing surfaces by two transverse steps in the bottom. Such steps have long been designed into racing hulls in order to control the trim or angle of attach of the planing surface. Such a stepped planing surface also reduces drag at high speeds. Another feature is that the longitudinal center of gravity is well aft of the middle of the boat's length. This is accomplished by locating all of the major weights in the stern of the boat. The three propulsion diesels, the diesel generator and fuel tanks are all in the stern of the boat.
This concentration of weight in the stern combined with the stepped bottom enhances top speed, but performance at lower speeds becomes a problem. There is a range of speeds between idle speed and the minimum planing speed of the boat that is impracticable to navigate. The minimum planing speed was something around 25 knots (29 mph). Twenty-five knots may be an uncomfortably high speed to maintain in certain sea conditions as the boat may pound very hard into waves at such speeds. For more typical cruiser designs without a stepped bottom and with a more normal weight distribution, planing speeds in the mid-teens are feasible giving a more comfortable ride. The alternative for the operator of the 48 Express Cruiser, however, is to drop to idle speed of ten knots or less, where the concentrated weight in the stern of the boat results in the deep immersion of the stern makes it possible for waves to wash into the cockpit.
In order to create space sufficient for the luxury grade accommodation, the depth of the structural support members consisting of longitudinal girders and floors has to be reduced to a minimum. The result is that the forward bottom of the hull is somewhat less rigid than in a race boat or a sport boat where the grid of longitudinal girders and transverse floors can be proportionately deeper and stiffer. Such a lessening on the stiffness or rigidity of the bottom will allow the forward bottom to flex under the pressure generated by impacts with waves. The flexure of the forward bottom may contribute to the observed breakage of the relatively stiff furniture items that are attached directly to this relatively flexible structure as well as the observed cracking in the gel coat.
The continuous squeaking noise of the icemaker cabinet door should have been a hint to the manufacturer and the buyer to the fact that the hull deformations were excessive, and, although nobody was injured, the excessive deformations could have created delamination or structural grid failures at the intersection of individual stringers and bulkheads or floors. Additionally, small deformations of the hull can create a "hook" that can result in dynamic roll instability.
It is a fact that substantial elastic deformation of the hull occurred. These deformations: namely hogging, sagging, torsional and shear can crated compression in the bottom, which combined with the peak hydrostatic pressure when the bottom hits a wave crest, would result in buckling of bottom panels.
The above actions will create inward deformations in panels of the bottom, which if are not symmetrical could result in an overturning torque.
Therefore, excessive deformations create not only the risk of structural failure, but also the risk of capsizing.
Hector Pazos, is a Naval Architect, Marine Engineer and a Registered Mechanical Engineer and has been engaged in Accident Investigation/Reconstruction for more than 40 years. He has been retained as an Expert Witness in over 1,200 Maritime cases, related to both commercial vessels and pleasure crafts, for both defense and plaintiff.
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