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61 Foot Steel Brigantine

MERMAID

61' MERMAID - An Authentic 1700's Brigantine by Kasten Marine Design, Inc.
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Exterior Deck Plan | Interior Profile & Arrangement | Interior Section View
Starb'd Aft Perspective | Starb'd Forward Perspective
Hull Low Aft | Hull Low Fwd | Under Sail, Astern | Under Sail, Aft Quarter
 

Copyright 2010 - 2015 Michael Kasten
 

INTRODUCTION

The following is an outline of how the original design requests for the Brigantine "MERMAID" have evolved into its eventual form.

The original 'Design Brief' for the project outlined a 50 to 65 foot schooner for blue water sailing and living aboard, to be styled after 18th century working craft.  Additional requirements were to have a pilot house with a protected interior helm, and a flexible-use "trade space" below that could be configured as a studio / workshop or to occasionally carry cargo, ranging from Maine to the Caribbean and across to the Mediterranean. 

Early on, the decision was made to build the "MERMAID" in aluminum, and for all the good reasons we have articulated elsewhere on this web site.  It was also decided that the "cargo hold" should be able to be converted into cabins for potential future charter operations with capacity for six passengers under USCG inspection.  The original layout sketch seemed to fit into an overall length of around 60 feet, so that's where we started...

In keeping with the 'Privateers' of the 1700's, we settled on a brigantine rig and a true brigantine hull form, that of the Pacific Swift, as described below.  Having settled on a desired hull form, layout, and rig, we proceeded to develop the structure drawings and other documents so that we could solicit estimates from builders. 

One of the favored builders convinced the owners that steel would be a better hull material (mainly because that particular builder favored working in steel).  That builder turned out to be the chosen one, so the change to steel was made.  This naturally required major re-work of the design, the cost of which the owners hoped would be recovered by a more favorable build cost.

Subsequent to the conversion to steel structure after the design work was very nearly completed, in order to satisfy a newly evolved business model the decision was made that the MERMAID would have to qualify for carrying up to 48 passengers on day sails out of Baltimore...!  

Over time, the 'Mission Statement' had migrated from that of being an aluminum schooner for carrying six charter guests or a cargo of up to 5 tons in the hold below, to being a steel brigantine for carrying 48 passengers with their weight of 4.5 tons imposed one meter above the deck...!  

All of this required no small amount of design gymnastics and extra work around here... In order to comply with the strict USCG 48 pax stability requirements for sailing vessels it was necessary to widen the hull, and of course we had to do so at the last minute so that production could commence.  When all was said and done, we had effectively designed three different boats...! 

At this point, most yacht designers would have simply gone mad...   But we rolled with it as usual, and the result is one extraordinary sailing yacht. 

When she's launched, MERMAID will sail with a USCG certificate for carrying 48 pax as day guests on protected waters, or 24 pax on semi-protected waters - an excellent pedigree..!
  

RESEARCHING THE TYPE...

In order to approach this design request methodically, I first started with the owner's mission statement. This acted as a guide to how the vessel should take shape, including a description of its purpose, various inspirational illustrations and photos, a sketched interior layout, specific rig requests and, importantly, that it replicate a traditional 18th century (1700's) sailing vessel in terms of its appearance and its function - both hull and rig.

My first task was to research vessels of that period in order to provide authenticity in terms of styling and overall arrangement. In my view, t his has been quite successful. One styling exception is that vessels of that time period rarely if ever would have had a house on deck. Most examples of appropriate size during the 1700's were small naval vessels, blockade runners and privateers, with the privateers being the most versatile in terms of their original use.

The 75' American brig "SWIFT" of 1778 was chosen as a departure point for the Mermaid, mainly with regard to aesthetics and general form. This vessel is featured in Howard Chapelle's "The Search for Speed Under Sail" (Plate 18) as an excellent example of the high degree of refinement in hull form reached in America during the Revolution.

The 75' Swift carried one shore boat, forty men, and ten "three pounders" (cannons). It should be noted that there were a number of vessels named "Swift" during the 1700's, so the above reference is given for the sake of accurate identification.

Among the features of the boats of this period were a long, fine, relatively straight run, combined with displacement being carried rather farther forward than is common in more modern vessels. The 75' "Swift" of 1778 however had relatively more balanced lines than her peers with a fine forward entry, even though her center of buoyancy was still rather far forward.
 

ESTABLISHING THE HULL FORM...

First, an accurate 3D model of the "Swift" of 1778 was created in CAD, then that model was adapted to the smaller size required for the MERMAID, including a number of refinements to the hull form for the sake of improved stability, cargo carrying, balance, and all-around sailing ability.

In order to accommodate the interior spaces requested but to stay within the original size requests, the length on deck was limited to just over 60 feet, not counting the stem nor the rake of the transom above deck.

In modeling the hull form, the requests for extreme ruggedness, heavy weather seaworthiness, and seakindliness were primary considerations. Refinements include a more rounded bottom in section view for the sake of increased interior space and cargo carrying ability, and a center of buoyancy slightly farther aft as compared to the original "Swift."

The center of buoyancy on the final 61' MERMAID design is approximately 50.5% of the WL aft of the stem rabbet. This is still slightly farther forward than is common among today's sailing vessels, however this position was deemed desirable for a variety of reasons, including all-around sailing performance, the presence of a Brigantine sail rig, and the forward-of-center location of the cargo area.

The result is a nicely balanced hull form having a well formed entry without any hollow to the forward waterlines, a reduced forefoot as compared to vessels of the 1700's, a long fine run without hollow in the quarters, and a graceful tumblehome to the topsides aft.

The keel was made level on the bottom in the after half, and was given greater depth overall for the sake of improved windward ability, greater stiffness under sail, and improved directional stability. For the sake of enhanced steering control, the rudder is considerably larger than would have been the case on older vessels (which were often steered as much by sail trim as by the rudder).

There is a 'transom stern' arrangement typical of vessels of the 1700's, which includes an overhanging "upper transom." The space contained by this overhanging transom has been used to house the steering system.

The exterior will be painted in keeping with the traditions of the period and there will be a few bits of exterior wood trim to preserve the styling of the time, including carvings, wooden companionways, doors, and so forth. For the most part, the intent is that there be no indication that the vessel is built in metal when viewing the exterior.
 

PARTICULARS


SAIL RIG

Square rigged ships of the 1700's carried various rigs. The original "Swift" of 1778 carried the sail rig of a Brig, having a square tops'l and royal above a gaff mains'l aft; a main staysail with two additional staysails above; a square fore course, square top, and royal on the foremast; and three jibs forward. In way of spars, both main and fore had a lower mast, topmast, and royal. The bowsprit was fidded with a jib-boom fitted (outer bowsprit).

In order to simplify the rig on the MERMAID, to accommodate a slightly smaller overall size, and to optimize sailing ability for the sake of maximum independence from fossil fuel for propulsion, a Brigantine rig was selected, allowing generous sail area without the rig becoming too tall or difficult to manage. The Brigantine rig is characterized by having fore and aft sails on the main mast, square sails on the foremast and staysails in between the masts.

MERMAID has a gaff mainsail and gaff tops'l aft; a main stays'l amidships with large fisherman topsail above; a square foretops'l on the foremast with royal above; and a forestaysail with an inner and outer jib forward.  In the sail plan above, a raffee is shown in place of the fore royal, which allows her to still carry all sail when carrying passengers.  When running during longer downwind passages, in lieu of the forestaysail a larger lower square course can be set as shown on the sail plan.

This rig allows the lower sails and the main gaff tops'l to be self-tending with sufficient sail area for tacking to windward without requiring the square tops'ls or the square fore course - and, blessedly, without having to wear about as would be more likely with a Brig.

In order to allow sufficient sail area, as well as to accommodate a 60' bridge clearance along the ICW, the main and fore topmasts are arranged to be easily lowered, then raised again once the bridge is passed.

The masts, bowsprit, and all other spars are aluminum pipe. This allows the spars to be whatever length is needed without any restrictions imposed by available timber lengths. The masts and spars are therefore able to be much simpler than on the original "Swift." There are spreaders on each mast, also fabricated in aluminum, with a modest crow's nest platform of teak slats on each.

The rigging is rather traditional in its arrangement, except that the main topmast shrouds extend to the deck, as do a pair of fore topmast running backstays. The bowsprit is a single pole rather than being fidded. This is lighter, stronger, and vastly simpler.

Standing rigging materials will be traditional galvanized wire, parceled and served. Running rigging is also rather traditional, except for the use of modern Dacron three strand line and Dacron sails.

Most of the halyards are carried outboard to the rail, with halyards for the square sails, jibs and forestaysail carried to the base of the fore mast.

The Brigantine Mermaid is a true "tall ship" in every sense of the word.
 

SAILING PERFORMANCE

Sail areas are as follows:

With all sail (but without the Lower Fore Course) this provides an average of 2.8 for the Sail Area to Wetted Surface Ratio in the half load condition. This is well above the typical high of 2.6 for modern IMS boats. In other words, sail area is ample even among racing vessels - indicating excellent light weather performance.

The Sail Area to Displacement ratio for All Sail in the half load condition is 20.7, just below the IMS high of 21 for racing vessels - indicating ample sail area for all around sailing and fast passage-making.

Using weight study results for the half load condition with only the lowers, the Dellenbaugh angle calculation indicates a heel of 8.0 degrees in 15 knots of wind - in other words, very stiff by comparison with other vessels. With all sail (but not the Lower Fore Course)and 15 knots of wind, the Dellenbaugh angle calculation predicts a heel of 12.2 degrees, which is approximately typical for vessels of around 55' WL length - in other words not too stiff and not too tender, but excellent when considering that the Uppers are included.

To accommodate fast sailing; to best accommodate heavy weather; to improve stability and balance; and to more easily carry a variety of cargo weights, the prismatic coefficient was intentionally kept in the upper range. Prismatic varies from .574 at the reference WL, up to .583 fully loaded. The relatively high prismatic is appropriate for a cargo vessel, and favors fast sailing. That there is an excess of sail area vs. wetted surface indicates that light weather performance should also be excellent.
 

PROPULSION

A John Deere 6068-TFM diesel engine has been specified, coupled to a Dong-i marine gear having a 3.5:1 reduction. A Hundested VP-3 three blade CP propeller, shaft, and pitch control deliver power to the water, allowing full control over the pitch for varying load conditions, and for optimum motor-sailing.

In order to keep all onboard electrics agnostic with regard to the available shore power voltage and frequency, shore power will first pass through an isolation transformer, and be routed directly to a pair of battery chargers that can operate on either 50hz or 60hz.

Wherever possible, systems, lighting and equipment will be 24v DC powered. For typical AC usage onboard, the 24v DC battery bank will provide power via three over-sized heavy duty inverters, arranged for redundancy so that failure of one is inconsequential to the use of AC power. The battery bank will be sized to allow the vessel to operate in "silent mode" without the generator or main engine for up to 12 hours.

A 4kW generator will be provided for "high power" AC usage within the "trade-space" / cargo hold. The generator will also energize twin heavy duty battery chargers. The main propulsion engine will be arranged with heavy duty high capacity alternators. The charging system will be arranged so that any combination of battery chargers or alternators will charge the battery bank. Charging will be augmented by solar / wind / etc.
 

ON-DECK ARRANGEMENT

The aft deck is raised up to the height of the top of the bulwark, and a quarter rail is located around the aft deck. The fore deck is located at the height of the main rub-rail, and is continuous at that height all the way forward to the stem. A substantial bulwark surrounds the fore deck. Stanchions and life lines will be provided to a height of 34 inches above the deck edge throughout.

The on-deck arrangement includes a generous lounging area right aft, with wrap-around seating and a large skylight / table on center. Forward of that is a protected seating area arranged as an exterior helm station just aft of the pilot house. Ventilated storage is provided below the seating for propane and gasoline. A Bimini cover can easily be rigged for sun / rain protection as needed.

A pilot house is located just aft of amidships at the forward end of the aft deck, and is sunken into the deck in order to provide a low profile and less windage. Within the pilot house are a seat / pilot berth on each side, and a helm station right forward. The port and starb'd steps and seating provide headroom for the passage and engine space below. A door leads out and forward on each side of the helm. A companionway aft leads below into the galley / saloon.

On the fore deck is a skylight for the master / owner's cabin below, and a large cargo hatch / cabin trunk just forward of amidships. In the cargo hatch, a scuttle is located for convenient access to the hold / charter space below, and is shaped in order to provide maximum protection from wind and weather.

Forward of the cargo hatch / trunk cabin is another scuttle, which provides access to the crew cabins below. Right forward is a substantial windlass, which also acts as the bowsprit foundation. Preliminarily, a "Forfjord" type of anchor is shown forward, located in a hawse pipe, so that it will clear the bowsprit and chains.

Optionally, a substantial 'cat-head' can be located to port and to starb'd for handling a pair of kedge anchors, in keeping with the period of the design.
 

BELOW DECK ACCOMMODATIONS

The brigantine rig, in combination with the various interior restrictions, have dictated the mast locations, which have in turn imposed their own restrictions on the interior arrangement.

The cabin soles have been located fairly low down in order to provide approximately 6' 8" headroom below the main deck; around 5' plus in the engine room; and a work space headroom in the ER of around 6' 8" to port.  The soles at these heights naturally have a limited range to port and starb'd, so the interior has been optimized to take best advantage of the available flat sole areas. Step-up regions of the sole have been located outboard, also optimized to fit with the interior accommodations.

Starting right aft, there is a large dinette extending aft to the transom, and facing forward to the galley. The galley is largely to port, with a counter top and cabinet located to starb'd. Overhead cabinets are assumed, but are not illustrated.

The galley contains a top-loading cold-box, a large four burner range (which can be gimbaled if desired), a large single basin sink, and a top-loading bin storage aft. To starb'd of the galley is a day-head for general use and aft of that, more storage below a counter-top.

Forward in the galley there is a companionway to provide access to the pilot house, and to starb'd there is a passage leading to a passage. Forward of the galley, located o outboard and to starb'd, the passage contains a large built-in freezer aft, and a washer / dryer forward. Above those is a large flat work-table, which could become another pilot berth if desired. To port of the passage is the engine room.

In order to achieve sufficient room within the owner's cabin forward, the engine room was made one foot shorter than originally indicated. Access to the engine room is provided via an access panel from the passage, and also via a doorway to port, forward.

Forward of the engine room is the master / owner's cabin, having a queen / king berth on center aft (below the lowered headroom area below the pilot house), and a walk-around sole. There is a long bureau to port and to starb'd with a flat counter top above each. Aft and to starb'd is a tall wardrobe. Aft and to port is a head / shower. Forward to port is a door for access to the engine room.

Forward of the master / owner's cabin is the cargo / charter / studio / workshop / trade space, to be configured as needed – to be determined.

Forward of the trade space is a crew cabin with sleeping for four, storage for crew gear, and to port, a head / shower dedicated for crew use. In the crew head / shower outboard is a washer / dryer dedicated for use by the crew.

Forward of the crew cabin is the forepeak, containing a wrap-around work bench, and storage for the anchor rode.
 

HULL STRUCTURE

Originally designed as an all aluminum vessel, the hull and decks and deck house were subsequently converted to all steel construction, and in the end the hull was widened in order to achieve a wider margin of compliance with the USCG rules for carrying passengers. We have therefore effectively created THREE designs, one for aluminum and one for steel both with 17' beam, and another in steel with 18' beam....

In each case, scantlings were calculated to exceed the requirements of the applicable ABS Rules. The frames were placed in order to coincide with the interior arrangement so that bulkheads could be directly attached to frames. A few key bulkheads are all steel (or aluminum) for the sake of watertight integrity.

The keel is a box-section and contains the ballast, as well as fuel and water tanks.  Ballast is approximately 35,500 lb. with some ballast being movable in order to accommodate the various uses envisioned for the cargo hold, i.e. carrying cargo, or being a charter cabin, or being used as a workshop.
 

LAYOUT OPTIONS...

Our investigations into the stability requirements have shown us that MERMAID will qualify for a USCG certificate to carry up to 12 pax offshore, or 6 pax on oceanic voyages.  We have also assured compliance with MCA, IMO and ISO ocean service stability requirements.  Under the MCA rules, this would allow carrying up to 12 passengers on ocean voyages.  

Although adapting the cargo hold and crew area forward to carry that many divers would be possible, the vessel size is much better suited to carrying four to six guests on longer more intimate luxury cruises, say for week-long dive adventures in tropical waters.  To accomplish this, the hold could easily become a pair of nice en-suite guest cabins.
  

VARIATIONS ON A THEME

The 61' MERMAID design shown here has become the inspiration for a series of new designs intended for construction in tropical hardwood by the traditional shipwrights of South Sulawesi, Indonesia.  Their intended use is chartering, however the smallest vessel among them would make an outstanding private yacht.  These new designs range in size from 66' on deck to 145' on deck, with one in the middle...  Please see the following links for more information:
 

Four Schooners With Wooden Structure Suited to Being Built in Indonesia
20m Sulawesi Privateer  |  31m Komodo Privateer  |  42m Kalimantan Privateer  |  36m Tern Schooner
 

SUMMARY

As we have written previously in the Spring 1999 Editorial for the Metal Boat Quarterly, the materials of construction need not dictate the aesthetic presentation of a yacht. In the case of this vessel, the intent is to create a period-correct vessel in all but the materials of construction, and to take advantage of modern materials as well as up to date knowledge of naval architecture.

In so doing, it is not necessary to pedantically copy the details of exactly how such a vessel would have been built at the time, especially with regard to materials and structure. Since we now have much stronger materials, a better knowledge of naval architecture, and vastly more robust analysis tools, we have made generous use of these technologies. Among the many examples of new materials include the use of steel for the hull structure, galvanized steel wire for standing rigging, Dacron for running rigging and sails, nylon for anchor rode, and aluminum for all of the spars.

Still, in order to create a convincing period-correct "new" design, I feel it is important to be sensitive to the aesthetics of the type, the general hull and deck arrangement, and the elements of style right down to the carvings. And in so doing, to preserve as much as possible of the traditional logic having to do with the rig.

Inevitably, the question of authenticity comes up. In the Spring 1997 Editorial for the Metal Boat Quarterly I have outlined the rationale, summarized briefly as follows:

We have to keep in mind that the US colonial and post-revolutionary sailing vessels were made of what was most readily available on the East coast of the Americas at that time: wood. Today the situation is much different. We now have much easier access to metal than we have to wood.

Perhaps in order to approach the whole concept of building a highly traditional sailing vessel in the very most traditional of ways, we should ask our selves: "Would the colonial owner-skipper, keenly interested in turning a profit, have even remotely entertained the notion of building a vessel out of a more expensive, more difficult to find, more labor intensive, and weaker material?"

Without question, the answer would be a resounding "No!"

The metal hulled 1700's-styled vessel we have planned here will not only look like the real thing, it will in fact be the real thing...!

It just won't be built in wood.

______________________________

For more information, please inquire.
 

Please see the  AVAILABLE BOAT PLANS web page.

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