Replacement boat for my 18ft tacking outrigger - early sketches

Design Requirement
A craft that is seaworthy enough to cruise the SE coast of Australia, can sleep one person aboard in shelter if desired, and be able to land through moderate surf onto a beach and be taken up above the high tide mark by one person. This boat is my solution to that design challenge

My solution to the design requirement is an improved version on my present 18ft tacking outrigger. These sketches shown here are preliminary, however it appears at face value to be greatly improved over my present boat.

When compared to my existing boat some key points are

  • A steeper stem to cut through waves better
  • A vee bottom to reduce pounding
  • Larger storage compartments
  • Foam buoyancy in all of foremost compartment and in a section of the aft storage compartment
  • Rig at this stage is sprit and jib, combined sail area of 80 square feet as opposed to 45 sqr feet at moment

  • A more vertical stern that allows for conventional rudder fittings
  • The ability to quickly disassemble the boat into 2 parts.
  • Water ballast compartment in outrigger
  • A narrower hull with less flare
  • Provision to store items between the 2 hulls on meshing - useful if sleeping in boat

  • A double bottom that adds to structural integrity and is just above the waterline
  • Provision of gratings in cockpit sole to raise sleeping mat and sleeping bag above any water in the cockpit.
  • Provision to cover the cockpit with a rain and spray proof tarpaulin
  • Crossbeams are matched to bulkhead position for structural reasons and to provide an unobstructed cockpit
  • Outrigger has a flat top to assist in people stepping on it, volume approx 260L, length=16ft

The above sketches represent a better boat. It is the same length as my present boat but ought be much improved. Time frame to building is still a fair way out, when I save up enough money I guess. The hull shape is heavily influenced by the Norman Cross 18ft trimaran, which in my opinion has superb lines. Note that the transom is of modest width that creates less drag at lower speeds.

Disassembly method
The boat is designed to be broken into 2 parts, so that each part can be dragged up a beach single handed. The crossbeams and outrigger are one part, the main hull is the other part. The crossbeams have strong bolts securely glued into them. These then slot into pre-existing holes in the main hull. Then 2 nuts per bolt are added. This is a superior method to my boat where the 2 holes have to be aligned and then the bolt threaded through. The outrigger is permanently attached to the crossbeams.

The outrigger is a deep V in cross-section type. This is the simplest type there is. It is just an extended version of my present outrigger. Approximate volume is 260L maximum width=40cm and maximum depth=40cm. Unlike my present outrigger this projected boat has a flat top to simplify the building process and to provide a flat surface when stepped on. The outrigger has sufficient volume to withstand people stepping on it without it sinking. The outrigger has a water ballast compartment that can be filled with a bucket and drained via gravity. Alternatively a small hand operated pump and tubing could allow the compartment to be filled from the main hull. The compartment can drain via gravity by releasing a pinch operated valve. The outrigger is divided into many compartments for greater strength and to provide watertight integrity

Is 18ft the ideal size
If the craft is too small it will not go trough waves too well. If it is too long weight and cost to build becomes too high. The length seems an appropriate figure and is derived by having 6.5ft for a cockpit, then adding another 40cm for the daggerboard section. Additionally sufficient storage is needed for camping equipment, food, tools, sail accessories, emergency equipment.

Appropriate freeboard and windage
Freeboard is 38cm at midsection at 250kg displacement. The stem is 51cm above the waterline. The freeboard is comparable to my present boat. The bow is a bit lower at 50cm as opposed to 60cm, but this is to reduce windage. Having a slightly higher freeboard than other craft means that more volume is available for storage

Sail Rig
Not shown but rig is planned to be a spritsail mainsail and a jib with a combined area of 80 square feet. A sprit sail (balanced lug or standing lug would work too), without a jib would also work, My thoughts are that the long length of the hull, combined with improved upwind performance, indicate that a main and jib would be better suited than main alone. Much of this is I have to admit an educated guess. I found that my 45 square foot lugsail had insufficient sail area in light winds.

Boat weight
These weights are estimates. I think that the main hull would weigh more than my present boat, and the outrigger would weigh less than my present outrigger. My estimate for main hull weight is 85kg. My estimate for outrigger is 25kg, when crossbeams and rig is added my estimate for total weight comes to 140kg unladen. The idea is that because the crossbeams can be easily separated from the main hull, the 85kg hull is light enough to dragged up a beach by one person. As background, I can drag my 70kg dory from the water and up onto the wharf single handed with ease, but with the combined outrigger craft (sailing mode) I can only manage a few feet

Hull Construction
Boat is built on a strongback with frames and stringers. Then 4mm marine plywood sides and 6mm marine plywood bottom is added. All components are covered with epoxy. Reinforcement with screws where appropriate (screws embedded with epoxy to prevent rust). Hull construction is heavily based on the Norman Cross 18ft trimaran

I have shown a conventional fixed rudder. The idea is that this is at a depth where it is protected from damage by the depth of the main hull. Prior to going onto a beach the rudder can be removed and the boat steered by steering oar for the last 70 yards or so. A kick-up rudder would be a reasonable alternative. Rudder is controlled via a yoke and a long tiller.

Timeframe to building
To be realistic I will not be building this year. Actual building time would be modest as I have my experience building my present craft to assist me and speed up the process. It is a question of saving money and then finding a good place to build and store the craft

I am fairly happy with the crossbeams on my present boat.  I made mine by splitting a 4x2 timber lengthwise to make two 4x1 (in reality about 110x19mm). I then made some 70mm long spacer blocks on the table-saw so that were all exactly the same size. I then made a box beam with width of about 110mm and height of 110mm. I covered the 2 opened sides with plywood which was both epoxied and tacked on. 

At right can be seen a crossbeam under construction from my present boat.  These proved to be overly strong, as we tested a 3m crossbeam with 300kg load at midway and ends supported on bricks (the beam passed easily). 

The spacer blocks are secured into the timbers using heavy gauge coach screws. Where through bolts are in the beam wood reinforcement is required to make a solid beam at this point to avoid it crushing. The beam for any future boat would ideally be scaled down a fraction to minimise weight and windage,  Future box beams should be build in a jig to ensure the timbers are straight and do not warp. A jig can be as simple as a ladder

The board shown has a bit more chord that my present boat, and also has a bit more depth. Significantly it incorporates the innovative idea as used by Joe Dobler in his Lissa skiff. 

The upper part of the board is cut diagonally, allowing the board to swivel back by a small amount in the event of a heavy grounding. The gap is filled with a soft collapsible material (crush blocks) which absorb the energy of a heavy grounding

In regards to the two crush blocks. Logic dictates that the forward one be bonded to the board, and the aft one be bonded to the case. This allows for conventional vertical raising and lowering

Freeboard and Windage
If it was not for waves freeboard and windage could be less. My projected boat has more freeboard than the dory hull I have now. So perhaps I can take some ideas from the dory, and some more sheer, and reduce windage and freeboard a fraction amidships. The trouble is that a degree of height is required to provide protection from the elements when the crew sleeps aboard. Additionally some height it prudent to give some puchase for the unstayed mast.

Looking at the below comparison it can be seen that the projected boat has a much longer waterline length. This will improve speed that can be obtained. Additionally it should slice through waves as opposed to just going over them.

Main hull crossbeam linkage
The method proposed is to use embedded bolts that are tight fitting and glued into the crossbeams. These bolts slot into corresponding holes in the main hull. Obviously there a large stresses at this linkage position and attachment points and timbers need to be reasonably substantial

Because the bolts are fixed into the crossbeams it is easier to align the two structures. This is a better method than I use at the moment of having to align two holes, and then thread a bolt through. It may be that the first bolt than goes on is a simple butterfly nut that secures the beam temporarily then conventional hexagon nuts are added. Also note that where the bolts pass through the crossbeam, the beam is under great compression. Thus wood blocks are used here. Grain of this timber should lie horizontal, not vertical, so as to avoid splitting. Ideally these blocks would be of layers with grain at 90 degrees to each other

Sail Rig
Here is a preliminary attempt at working out a sail rig. Mast is shown at 11.5ft high, which is 2.5ft higher than my present boat. Sail area is a combined 75 square feet. This is a modest sail area, but the boat is meant for voyaging not racing.

The Sprit rig has the benefit of having a low center of effort, it has a good reputation, perhaps not the fastest, but certainly capable and robust. Another option might be a lugsail, either balanced or standing. I have a balanced lugsail on my present boat and fine it very easy to use. The sprit sail would be laced to the mast. At least one line of reefing points would be used.

My thinking at the moment is to add another foot of mast height to allow for more sail area, I am not sure. Mast is a simple un-stayed affair of solid wood, round section as per my present mast (I used the cheapest pine that I could find, planed then sanded it round,  then coated with epoxy). Very simple and very cheap. I find my present mast is quite light enough to be lifted into and out of the boat single handed with great ease. One feature of a short un-stayed mast, is that it allows the crew to remove the mast as sea and store it lengthwise on the crossbeams. I have done this on my present boat and find it very convenient. 

Another viable option is to do away with the jib and have a single sprit or lugsail. My guess is that upwind performance would be a bit less, but set-up and takedown time would be less. The trouble is that I do not know how well a lugsail or spritsail will go to windward without a jib. The small cat I used to sail did poorly to windward compared to the larger hobies with a small jib.

16ft Outrigger
Below can be found the lines for the 16ft outrigger. The length is derived from having the rear crossbeam being located at the back of the cockpit, and the front having to be located near the main hull bow, so as to provide resistance to pitch poling. Does the bow of the outrigger have to be at the same position as the bow of the main hull? Cant say for sure, but based on what other tacking outriggers do, and based on photos of traditional craft I have seen, the far forward position seems prudent

This hull has a volume of approx 270L. Beam is 41cm and maximum depth is 40cm. Based on some feedback the position of maximum width has been moved forward. The V section may not be the ideal shape for an outrigger, but it is very simple to make.

Outrigger weight. When working at a float the outrigger is ideally as light as possible, when acting as a counterweight, the outrigger is best if it has some weight. A solution to this is to use water ballast. Unladen weight is estimated at 25kg, added to that is another 30L of water ballast, combining to 55kg of ballast. Now more weight could be added, but I feel 55kg is a useful weight without going to the extreme.

Water ballast can be added via a small hand pump and tubing. Draining can be accomplished by having a small hose at the bottom of the tank, normally the outlet is high up and kept there via elastic. To empty the tank a line is pulls the outlet hose down, stretching the elastic. The water will then drain via gravity out a small hole in the outrigger. A one inch diameter hose should drain a 30L tank very quickly. When sailing in moderate conditions, it may be simplest that the tank is half filled and left unchanged with the extra 15kg of weight penalty being relatively modest

Another Method of draining the tank is to use a short length of pipe on a swivel. Pipe is kept high via elastic, and pulled down via a line to drain the tank through a small hole

Mast Partner
My thinking was that if the  mast partner was raised a few inches above the deck, this would increase bury, reducing stresses on the mast slightly, and in theory permitting a slightly smaller diameter mast.

A pyramidal style mast partner seems logical as it has low windage and is simple to make. Additionally it would provide some protection from water and spray coming into the cockpit from the bow

Newest Changes - May 2010
Based on some new information, more reflection and from experience of my sailing trip earlier this year I have made the following changes

  • There is now a low aspect centerboard as opposed to a daggerboard (no more breaking daggerboards in shallows!)
  • The outrigger has been reduced in size so as to lighten it, to allow for it to be more easily righted single handed in event of a capsize
  • The overall lenght of the hull has been shortend 6 inches to reduce weight
  • The hullshape has been made more full forward, and straighter aft.
  • The supports for the meshing on the side have been changes to allow for paddle strokes
  • The main hull - crossbeam connection has been changed from nuts and bolts to pegs and holes secured with lashings, a superior method

I think this latest version is better than the previous version. The outrigger is smaller and thus lighter. This means that it can be righted more easily in the event of capsize. The water ballast method has been changed from an internal ballast tank, to a simpler version of ten 4L water bottles secured with lines, is a shallow compartment.

Why is the board the shape that it is?
The board is of moderate aspect ratio so as to give good performance at lower boatspeeds. The shape has been doen this way so to give a longer unobstructed sleeping area in the main cabin, whilst retaining a robust strong shape. I think any board is better than no board. The centerboard will not break on grounding, as happened with my daggerboard on my most recent sailing trip.

Similar craft - first seen May 2010
I recently saw this craft from Solway Dory, it is a double outrigger sailing canoe called the Osprey. I think it fits the design requirement well, a highly seaworthy craft, that can accommodate gear and people, can access a beach through moderate surf, and still be dragged above the high tide mark by one person. I emailed them asking for an approx weight, and they replied with outriggers it weighs approx 180 pounds, or 80kg. Having moved my 70kg dory around a lot, I can say that this is a comfortable weight for a single fit person to manhandle and drag around. Solway Dory have kindly updated their website and included unladen weight. Their website can be found here.

Noting that I drew the replacement boat without seeing the Osprey 'trimaran', I can see that they are quite similar. The external leeboard opens up the hull to allow for more space. Probably lowering performance a little but with the positive of more hull space. The craft features large buoyancy/storage compartments fore and aft, an excellent safety feature. The beam looks wider, my guess is about 3ft. Again Solway Dory have decided to trade off a little boatspeed so to increase carrying capacity. I like the rig setup with 2 simple unstayed masts. This frees up the center of the craft for the occupants. The bow seems a bit fuller, probably to avoid excessive burying of the bow in a seaway. If I lived in the UK, buying a center hull from Solway Dory, doubling up the crossbeams, and having a single larger outrigger would appeal to me. Of course I would add twelve 2.5L softdrink bottles with fresh water in the outrigger to provide easily removable water ballast. Of course it would be prudent to keep a little fresh water in the main hull in case the lines securing these bottles came loose, and they all floated away.

If you search through the Solway Dory website, you can find a YouTube video of one of these craft sailing. From memory it can be found on a blog.

Another similar craft - seen October 2010
I stumbled across this modified Wharram Melanesia at the forum page on the Wharram website recently. I have included it here, because to me it looks just right. The outrigger has been increased in volume, and a kick-up rudder installed, a more conventional rig is used and the main hull has been half decked. As to which way is the best to go is a matter of opinion. It seems some Wharram Melanesia users prefer the excitement of the original low volume outrigger and of using a steering oar. To me, this one shown above, just looks very good (but that is just my humble opinion). A link to the relevant page can be found here.