Original Concept and then what happened

The boat Solo Ocean Explorer (S.O.E.) was and is designed to be a safe, robust, affordable vessel for 1 person to cruise long voyages around the world. I think this size of 28ft is about the minimum that could be considered Prudent. It is true that a 24ft Piver Nugget has circumnavigated, as has a Tiki 21, however I think these are really a little too small. My idea was to have a larger hull, but have minimal accessories, with the idea to keep down on weight and complexity. Note that there is no standing headroom, only sitting headroom. There are no keels or boards, only a 2 inch mini keel. This means poor upwind performance, however the upside is good access to beaches in remote areas

The first idea was to take the Piver Nugget and then add a 4ft plug to the bow. My thinking was that this would increase speed, give better motion in a seaway and increase carrying capacity for stores etc. The idea being to have a larger hull but with minimal extras, thus putting less stress on the hull.

I tried some other layouts with external boards and internal boards. I discarded these in favour of the chined Vee hull. This has a good seakeeping hull, whilst the chine reduces wave resistance in a seaway compared to the pure Vee of Wharram Tiki designs due to the narrower hull width nearer the gunnel. Crossbeams are kept high so as to avoid drag and spray from wave crests. There is a long narrow foredeck which is good for seakeeping and gives more separation between the occupants and breaking waves.

Performance is meant to be modest. Boards and a U shaped hull would give higher performance, however I favour a simpler easier to build hull combined with a rugged strong hullshape that can tolerate beaching easily.

It was not meant to look like Arpex, but then it did

S.O.E. was meant to look like a stretched Piver Nugget, however it came out looking like Arpex. This was not intended, however there are some differences.

  • Arpex had a fin keel, S.O.E. does not. Thus Arpex will point higher.
  • S.O.E. can be beached more easily due to lack of a keel
  • Arpex has a keel stepped mast, S.O.E. has a deck stepped mast making raising and lowering the mast easier.
  • Arpex has a narrower waterline width and longer length, thus Arpex will be faster.
  • S.O.E. has more width at the chine giving greater weight carrying ability.
  • S.O.E. has greater distance between the two major crossbeams lowering the twisting forces on the hull-crossbeam connectives
  • Arpex has a higher cabin giving more comfort, S.O.E. has a more cramped cabin giving lower windage
  • Arpex has a fixed rudder, S.O.E. has a kickup rudder which assists in beaching
In general Arpex will be a faster boat that can point higher. S.O.E. should be a slower boat but should be better for carrying more loads and then beaching in out of the way places

Avoiding the Cost-Weight-Complexity Sprial

By keeping the size relatively small, weight is reduced. A small weight, small size, no moveable boards or cases, allows for a simple rugged craft. The modest weight combined with a waterline beam of only 75cm means that reasonable performance can be obtained on a small rig (250 sqr feet). The wide staying base for the rig means that mast spreaders are not required, also the wide stay base means lower mast compression loads. A running backstay is used, this allows the boom to move 90 degrees either side of center. Port and Starboard chainplates are not required as the stays simply attach to lashings around the ama. The lashings-stay connective is a ring. Turnbuckles are not used, instead the lashings can be tensionsed/eased to control the mast.


Construction is simple hard chine plywood stringer frame method. Main hull is 6mm good quality marine ply coated with 6Oz fiberglass cloth and epoxy resin. The ama is 4mm marine ply with fiberglass cloth and epoxy

Weight Estimate

The proper way of determining weight is to add up the weights of each individual componenet. This will work but will take a long time. A rough way is to look at similar boats and see if a rough estimate can be obtained.

  • Gaia 1 - 12m 1500kg unladen. Now using the cube rule, a similar vessel at 8.53m gives 540kg
  • Wharram Amatasi 28 450kg unladen
  • Wharram Tiki 26 700kg unladen
  • Wharram Tiki 21 360kg unladen
  • Piver Mariner 25 770kg unladen
  • Woods Shadow 24 Cat 700kg unladen
  • Madness 31 Proa 454kg unladen
  • Mark Hansen outrigger 600kg unladen (33ft LOA)

Looking at the above vessels, it may be that a reasonable estimate of unladen weight for S.O.E. is 550kg. Then add say 350kg for stores and crew, that gives a loaded displacement of 900kg. Now given that the advised minimum ama volume is 110% of vessel weight, this gives a minimum ama volume of 990L, which we can round off to 1000L

Note that there may be a degree of error in these numbers and perhaps it is better to assume for a loaded weight in the order of 950kg. If we assume that 50kg of that weight is supported by the ama that leaves 900kg weight to be supported by the main hull

Sail Area

Sail area is determined using the SA/D calculation. First lets look at bruce numbers for similar boats and see what we can find. Note SA/D refers to sail area in square feet divided by displacement is pounds to the power to 2/3

  • Norman Cross 24 Tri - SA/D = 24.2
  • Marples DC3 Tri - SA/D = 28.9
  • Constant Camber 26 Tri - SA/D = 24.8
  • Richard Woods Surfsong - SA/D = 20.6
  • Richard Woods Shadow - SA/D = 25.5
  • Piver Mariner 25 - SA/D = 28.1
  • Chris White Skyhook 39 - SA/D = 35.8
  • KIR-2 Outrigger - SA/D = 23.3
Looking at the above list it seems that an appropiate SA/D value for a smallish mulithull is about 25.
Now given that loaded displacement is approximately 2000lbs, this gives a sail are of 250 sqr feet when using a SA/D value of 25. A more prudent option might be to use a lower SA/D figure of around 23.2, this gives a sail area of 230 sqr feet. Note the KIR-2 outrigger has a SA/D value of 23.3. An aside the Woods Surfsong is a smallish bridgedeck catamaran, and hence the sail is mounted quite high above the waterline, giving higher tipping forces, hence the smaller rig to compensate for the high position of the sail plan.

The Mini Keel

The tiny keel/skeg is a 2inch by 2 inch hardwood timber. It provides a degree of protection for the hull when beaching. It improves upwind ability a small amount. Having no moving parts it is robust and simple. Being so small it still allows easy beaching in all sorts of places, thus avoiding the need to anchor far offshore and use a dingy to row ashore to explore. The small keel has a thin stainless steel cap of 2mm stainless steel sheet that avoids abrasion.

Rudder Size and Strength

When in doubt follow the experts. The Searunner 25 building manual has now been put online for all to read for free. In the manual there are notes on how to build the rudder for the Searunner 25. The idea would be to copy this design, possibly increase the rudder size a little and thus increase the strength of the rudder to cope with extra forces imparted on a larger rudder

Crossbeam Size and Strength

The front crossbeam is stronger than the aft crossbeam. The crossbeams are box beams. There are formulas regarding crossbeam stength online. Additionaly some physical testing of a crossbeam jig with same length, same height but only a tenth of the width would be prudent. Note that crossbeams and beam hull connectives need to be strong enough to handle shock loads. A high factor of safety needs to be applied to crossbeams and connectives. Additionally maximum loads imparted on the crossbeams and connectives in proportional to the volume of the ama. If the ama is smaller maximum loads are reduced because extra loads cause the ama to submerge

Note the crossbeams can be removed if required for example when moving the boat ashore for storage. The crossbeams sit in open boxes which support the loads imparted. The underside of the box has a strong timber in the hull that stregthens the crossbeam hull connectives. Note that extra reinforcing of the hull sides at the connectives is required to handle loads. This would mean more that a doubling of the hull thickness near the connections from 6mm to 12mm plus

Windage and Freeboard Comparison

Please find some information regarding appropiate Freeboard and Windage. The idea was to create a hull with a long foredeck to create a faster, drier boat with more carrying capacity than a smaller boat. The concept is based on the Piver Nugget but with a 4ft bow extensions

It can be seen that the freeboard and windage is low, comparable with a Piver Nugget. Although low, it is not excessively low. It should also be noted that the small cabin combined with good cabin shaping produces low hull windage, thus increasing upwind potential. It should be noted that pointing ability will not be high as there is no board nor any fixed keel

         Plan Elliptic..

Plan Elliptic is a refinement of S.O.E. it has a curved hull made from 2 layers of verical strip 3mm marine plywood. A curved hull is harder to build, however it gives less drag, has reduced interior framing allowing for a narrower hull. On the hard chine boat the frames intrude into the sleeping berths, thus the hull needs to be wider to compensate for these internal frames.

Hull Sides are defined by an elliptical curve

The hull sides are defined by the same elliptical curve throughout the entirety of the hull. The ellipse used is one that fits within a rectangle 3000mm high by 2500mm wide. This ellipse can be defined by the equation x^2/1275 + y^2/1500 = 1. Note that this equation has been solved for both x and y, and the results put into a spreadheet, giving exact offsets down to the millimeter for all 1500 increments in the y axis and all 1275 increments in the x axis. Note also that a cubic polynomial curve was trialled and found to have an inferior shape, the cubic polynomial shape may be suitable for strip planking, however for cold molding I much prefer the elliptical curves as it gives a more consistent radius. A too tight a radius and the plywood will have difficulty bending, a too large a radius and the hull side will be too flat and lacking the curve that gives it strength

Build Overseas to just to get it done

Technically I could build the boat where I live (Australia), however I know I dont have the time, inclination to spend years working slowly to finish a boat. Much better IMHO just to fly up somewhere cheaper, say Vietnam, and employ 4 or 5 boat builders on 10 or 15 dollar a day and get the boat finished in a few months. Go hard, go fast, get it done. Note that as a compound curve hull is more work, such a hull is best built overseas. A hard chine hull might be OK for Oz, because it is less man hours.

Advantages of cold molded hull

Compared to a hard chine hull the curved hull has several advantages.

  • Less drag
  • Narrower waterline beam due to less internal framing interfering with the berths
  • Lower weight
  • Stronger hull
  • Disadvantage - more work to build

More information to come..............