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DESIGN & CONSTRUCTION
(Specifications & Technical data)
GENERAL DESIGN INFORMATION
The most difficult task to be faced by Spirit of the Bahamas is the 3000 mile open ocean passage from the Galapagos islands to the Marquesas and the decisions made in regard to size, configuration, layout, and power have been based largely (but, by no means, solely) on this requirement.
In regard to size (based on the rather radical proportions we have chosen to utilize in our designs) a length of 82 ft (25 m.) was chosen as the minimum size vessel capable of catrrying the required fuel load, at the targeted speed of 12 knots - using one of two 100 hp. (75 kw.) engines, during the longest passage.
Fortuitously, 82 ft (25 m.) also happens to be (in our opinion) about the minimum length vessel capable of travelling relatively comfortably to windward in moderate sea conditions at a targeted speed of 18 knots - using both engines during the shorter passages.
In regard to the small steadying sail, it's main purpose is simply that - to steady the motion of the vessel. Our goal is to promote the reduction of environmetal impact on the planet by all reasonable means. We have previously pointed out that it seems unlikely that significant numbers of owners and operators could be persuaded to revert to sail power as their primary motive source.
However, all vessels will, from time to time, be assisted in their forward progress by the wind and it would be irrational not to utilize any cost effective means to take advantage of these situations when they present themselves. We, therefore, see no inherent conflict with our goal of promoting econmical motor vessels, in the fact, that this steadying sail will, on occasion, assist in driving the vessel forward.
Really, really, long and skinny hulls, with very sharp forebody, deep V midsections and flat, slightly rockered underwater sections aft, will provide: a comfortable ride with little pitching, relatively high speeds with minimal horsepower, and the ability to take full advantage of any surfing conditions. Multi compartmented hulls, with an abundance of foam flotation, and strategically placed ballast tanks will ensure control and ultimate safety in extreme sea conditions.
SPECIFICATIONS &TECHNICAL DATA
Length overall 82 ft. (25 m.)
Length waterline 82 ft. (25 m.)
Hull beam (width) 4 ft. (0.9 m.)
Length - hull beam ratio 20 -1
Overall beam (width) 20ft. (6 m.)
Length - overall beam ratio 4 - 1
Draft (light) 18 ins. (20 cm.)
Draft (1/2 load) 22 ins. (25 cm.)
Draft (full load) 26 ins (30 cm.)
Weight (light) 16,000 lbs (7,000 kg.)
Weight (half load) 20,000 lbs (9,000 kg.)
Weight ( full load) 24,000 lbs (11,000 kg.)
Weight - length ratio (half load) 244-1
Engine (one) 100 h.p. (75 kw.)
Weight - power ratio (half load) 200 - 1 (120 - 1)
Ultra eco-Cruise speed ( half load) 12 knots
Engines (both) 200 hp. (150 kw.)
Weight - power ratio 100-1 (74 - 1)
Eco-Cruise speed (half load) 18 knots
FUEL CONSUMPTION & RANGE PROJECTIONS (NAUTICAL MILES)
Fuel capacity (5,000 lbs. 2,250 kgs.) 800 gals (3,000 litres)
Fuel consumption (one engine, cruise speed ) 3 g.p.h. (11 l.p.h.)
Gas mileage (one engine, half load,12 knots) 4 m.p.g (1.68 k.p.l.)
Range (one engine, 12 knots, nautical miles) 3,200 miles
Fuel cunsumption (both engines) 6 g.p.h. (22 l.p.h.)
Gas mileage ( both engines, half load, 18 knots) 3 m.p.g. (1.26 k.p.l.)
Range (both engines, 18 knots) 2,400 miles
(25% 1 engine 75% 2 engines) (nautical) (statute) (metric)
Average speed 16 Knots 18.4 m.p.h. 29.5 k.p.h.
Average gas mileage 3.5 m.p.g. 4 m.p.g. 1.7 k.p.l.
CAUTION DEEP END
All information, directly related to this venture, has been included on the 2 HOME
pages. Those seeking further, in depth, information on a particular aspect of
this project are invited to click on any of the following links:
CONSTRUCTION
In choosing the type of construction for Spirit of the Bahamas and hopefully for a whole range of vessels to follow, the overall goal was to facilitate the establishment of numerous, low capital cost, low enviromental impact, production / repair facilities throughout the world (including the smallest and least developed population centers) . In order to achieve this goal, the following requirements head the list of priorities.
(a) Low production set up and infrastructure costs.
(b) Utilization of a high percentage of renewable resources.
(c) A high level of flexibility in regard to product size and type without
accompanying high capital and production costs
(d) Minimum labor reguirements in terms of both man hours and skill levels
(e) Low cost, widely available materials.
(f) Low cost, low tech, repairs.
(g) Durability and low maintenance.
We have chosen composite construction utilizing a marine plywood core epoxy /glass fibre skins with judicious use of small quantities of high strength materials such as carbon fibre and kevlar. For fastenings, stainless seel screws and epoxy adhesives will be employed.
In regard to the use of marine plywood as a core material, the following observations will hopefully help in dispelling several, commanly held, myths regarding boat building materioals.
1. Solid "fiberglass" (or, more correctly, fiberglass reinforced plastic "F.R.P.") while still used in many small production boats, is too heavy (in relation to it's stiffness) to be successfully employed in larger vessels - other than slow full displacement types.
2. The alternatives are traditional wood construction, aluminum, and composites.
3. Traditional wood construction, while marginally lighter than "fiberglass" suffers from all the well known disadvantages associated with this type of construction (which are often unfairly applied to all other types of construction uilizing wood.)
4. Aluminum is favored in many commercial applications because of it's ability to absorb rough use (or abuse) and still retain it's structural integrity - if not it's aesthetic appeal. Impact with floatig debris, docks, other vessels or even rocks which might hole a fiberglass or composite vessel will often (in the case of an aluminum vessel) result only in an unsightly dent.
5. On the other hand, like F.R.P., aluminum is not a particularly stiff material in relation to it's weight and requires a complex, and labor intensive array of structural members to provide adequate panel stifness. What with the need for high tech equipment and highly skilled welders ( particularly in regard to the thinner plates required in high performnce vessels) and the high cost of creating and maintaining a decent finish over a distorted welded surface, aluminum is an expensive choice. Other drawbacks to aluminum include difficulty of repairs, susceptibility to electrolytical corrosion and metal fatigue, and virtually total lack of any insulating properties.
6. Wood/epoxy construction (often refered to as West System) while not usually lumped with "composites" can clearly be categorised as such. Combining thin layers of wood (one of the stiffest strongest materials available) with epoxy resins (the strongest and most waterproof adhesive) allows for the construction of very strong lightweight vessels which are far superior in every respect to traditional wooden boats.
7. Drawbacks include the susceptibility of wood, like all organic materials, to rot. Consequently, great care must be taken to ensure that the relatively thin, soft waterproof epoxy coating is not damaged. Clearly this many layered construction system is capable of producing even the extreme compound curves demanded by many modern designs. For this very reason however, it is also very labor intensive and requires highly skilled craftsman.
8. Composite construction usually implies the employment of "sandwich" construction to create the neccesary panel stiffnesss - i.e. compining two outer skins of relatively thin, heavy, high tensile strength materials with a relatively thick, lightweight, low tensile strength inner "filling" or "core"
9. Skins include aluminum, laminates of fiberglass with polyester resin and various combinations of kevlar and carbon fibre with epoxy resin. Cores include aluminum honeycomb, a myriad of synthetic foamed products, end grain balsa and plain old plywood.
10. Plywood, while being the the heaviest of the core materials commanly employed, is also, far and away, the least expensive and the strongest. In fact all the other core materials are virtually without strength in their own right. i.e. Aside from providing sufficient compression and sheer strength to keep the sandwich intact they contribute virtually nothing directly to either "panel stiffness"or "global" strength.
11. "Global strength" is the ability of the vessel as a whole to withstand the forces
imposed by the seas - as opposed to "panel stiffness" i.e the ability of the local plates or panels to resist deflection. Global strength becomes more and more critical as the size of a vessel increases. (Supertankers virtually never suffer damage from the impact of the seas on their steel plating, but it is not uncommon for them to be broken in half by the global forces placed on the structure as a whole.
12. Because of their configuration, catamarans are very susceptible to damage from global forces and every effort needs to be made to ensure that the two hulls do not sail off in different directions. In a plywood cored vessel, the plywood plays the major role in regards to global strength and also provides a significant direct contribuion to panel stiffness.
13. While plywood is utilezed to some extent as a core material in most composite vessels, it's use is usually restricted to local highly stressed areas - such as where deck fittings are to be attached, in cockpit floors and transoms. Clearly, plywood, unlike other core materials, is not at all amenable to conforming to the multitude of compound curves in most current designs.
Spirit of The Bahamas, however, with it's extremely simple design, nearly devoid of curves (simple or compound) presents no restriction to the use of plywood as a core material and it's use is seized upon. Nor are the benefits derived from this choice limited to it's low cost, high strength and the fact that it is a renewable resource - as sigificant as these characteristics are. These benefits include:
(a) The ability to be formed into a rigid fair structure without the useof costly
space consuming molds and jigs.
(b) The ability to accept fastenings anywhere throughout the structure.
(c) Wide spread availability
(d) Ease of repair.
(e) Good insulating properties.
(f) Very high resistance to mechanical fatigue . (sress imposed by repeated flexing)
(f) a "feeling or ambiance" that only natural wood can provide.
The drawbacks to the use of plywood as a core material are as follows:
(a) Heavier weight - which can be readily compensated for by design which takes advantage of it's greater strength.
(b) The risk of rot (shared by end grain balsa) which can be virtually eliminated by due diligence - first and foremost in the design stage and, secondly, in regard to constructon and maintenance. It should be noted, that, regardless of core type, all forms of composite construction are subject to structural deterioration if moisture is allowed to enter the laminate.
MAINTENANCE AND REPAIRS
In regard to maintenaance, contrary to popular opinion, the cost of maintaining a well desighned and constructed plywood cored vessel in A1 condition is actually less than for a production "fiberglass" vessel. The reason for this is that the epoxy resin, in combination with the linear polyurethane painted finish, used in the plywood cored vessel is virtually impervious to moisture and U.V rays.
In the production "fiberglass" vessel, on the other hand, the polyester resins and gel
coat, used in it's construction, in addition to being far more "brittle", are, relatively speaking, quite porous - in regard to both moisture and U.V. rays. This lesson is made clear on a daily basis to many "no maintenance fiberglass" boat owners - faced with huge boatyard bills for correcting blistering problems below the waterline, finish problems above the waterline, and crazing or cracking problems throughout the structure.
In no way is the above inended to refute the claim that a poorly designed, poorly
constructed, and poorly maintained plywood (or balsa) cored vessel will deteriorate far more rapidly than a "fiberglass" vessel. We freely acknowledge this to be the case.
On the other hand , a well designed, well constructed, and well maintained, plywood
cored vessel, besides being a one piece structure with no seperate parts to wear and eventually come apart, is virtually impervious to U.V rays, moisture, corrrosion and material fatigue (failure due to repeated flexing). As such , it is difficult to place any intrinsic limit on it's life span.
Naturally, with this type of construction, any repairs that might be required would be relatively simple. If our project is successfull in encouraging the establishment of wide spread construction and repair facilities, such repairs would also be readily available at a low cost in terms of both time and money.
Like most vessels, Spirit of the Bahamas, will utilize examples from virtually all of the categories of boat construction systems. Indeed, ultra lightweight foam will be used throughout the structure to provide labor savings, insulation, flotation, and panel stiffness. The predominant construction system, however can best be described as "plywood cored composite".
PREFACE
This web page, while including many design and construction specifics, is by no means intended as a short concise description regarding the particular design and constructin details of Spirit of The Bahamas - which are really of little import. It is, rather, a quite lengthy treatise (including all the "whys and wherefores") on the major choices and decisions underlying the design and construction of both Spirit of The Bahamas and, hopefully, a whole range of vessels yet to come.
Those looking for a short "boat report" as one might find in a popular boating magazine will not, I am afraid, find it on this web page - or on this web site.
2007
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