Replacement Boat - attempt 3

Replacement Boat attempt 3


Is now April 2011, and have made decision to bit the bullet and finally build a nice coastal boat. Here are some early sketches.

Not shown are the wheels that will fit under the crossbeams and pivot out when not in use. Because I want to get up a beach unassisted when neccessary the weight will be too much for me to drag. My feeling is that a pair of wheels should give me a lot of flexibility. Note that the crossbeams and outrigger can be dissassembled easily as connection is via lashings and large diameter dowels.

Based on my first boat, the outrigger will be lighter, yet be able of carrying water ballast (blue area) by way of 2L softdrink bottles filled with fresh water in a small raised open box. Crossbeams will be lighter than my first boat where I had over-engineered them. My best guess at present is box beams of 90mm x 90mm.

The plan for this week is to buy some timber and start building little things. I will start with simple rectangular hatch covers, then simple crossbeams. Once all the small items have been built only then will I start on the hull itself. By starting with the small items first, it gives me time to refine the lines of this boat before I start building. I am giving myself 6 months to build.


I have settled upon a rig of between 90 and 100 square feet sail area. One option is to go with my existing bermudan mast and sails (main and jib). The trouble with this is the difficulty of taking down the mast at sea in the event of a very heavy winds coming through. I feel that a dipping lugsail might be the best way to go.

One method for the mast of a dipping lug is a conventional unstayed vertical mast. The other option is a bipod. A bipod would allow me to pivot the rig forwards and aft a little to change the center of effort. In addition tacking with a dipping lugsail suspended from a bipod would be simple. I can leave this detail till later

News 14 April 2011

I have taken some advice and decided to narrow the boat a degree. Mostly this was done by raising the chine. I then put the narrower dimensions into a boat design software. Now waterline beam comes out at 57cm at 250kg laden. I think I would be hard pressed to make it much narrower, perhaps a few centimeters here. New lines are shown below. A narrower boat for the same given displacement means a deeper hull, more draught. These new lines took me a couple of hours to work out. I am very, very happy with them. If you have a monitor with 1280 pixels resolution, the images should come out crystal clear.

Why 18ft, and why this width?

The longer the boat the better it will handle in the sea. The more lenght though, usually means more weight. As I want to be able to get this boat up onto the beach with aid of some wheels, this seems to be about the best length I can get. Note that I have chosen this width as it gives me the chance to have a lay down in the boat under some shelter, if even for a short time. If I had a narrower boat, the sleeping place would have to be higher, resulting in more windage. 57cm is not super narrow, but is should be ok. My previous boat had a waterline beam of about 75cm, so this is much narrower. Additionally the moderate Vee amidships gives a good ride in rough conditions, has less surface area, and gives a degree on inbuilt lateral resistance, thus hopefully taking the strain off the board to some degree. I think this is a nice compromise between competing factors. Another reason for not going even narrower, is the need to carry a lot of equipment, (life jackets, anchors, tent, blanket, clothes, torches, emergency equipment, tools etc). A slightly wider boat has better carrying capacity as opposed to as faster narrower boat.

Why I chose not to go with a proa

I am not opposed to a proa. My trouble is that I lack experience in them. Another complication was that because proas are double ended, for equivalent size boat I would have to build something 20ft long, which becomes difficult to move about by road. Another complication is that because I want the width to lay down somehere out of the wind, this results in a boat a bit wider with a lower deadrise angle than otherwise. Now with a lower deadrise angle I really need to utilise a board for lateral resistance. In a small proa the board is likely to have to be in the middle which is where you want to sleep. A leeboard is an option but that has more drag? Another consideration was that going for long trips, the dual rudders of a proa adds for complexity. The other option would be to go for a steering oar for multiday trips, but that to me seems like it would be very tiring. Thus for a combination of these ideas I have chosen not to go with a proa. I do accept that proas are better sea craft and are more refined, I just do not have the nerve (gumption) to tackle one just yet.

Other possible changes

I am contemplating changing the rig to a traditional dipping lugsail. This way I can utilise the wide stay base of the multihull for three stays for the mast. Another change is to narrow the width of the ama and extend if aft a little more. The other change I am contemplating is to move the aft crossbeam forward 1ft. The reason for this is to allow placement of an outboard motor (I have a late model 3hp yamaha outboard) on the starboard side of the main hull. The support for the outboard would be a 4 x 2 timber threaded through the hull, and bonded to the bulkhead at the rear of the center compartment. By moving the aft crossbeam forward this will give me clearance to mount a motor. I do not like motors, but they can get you out of a lot of trouble if things go wrong, or if you need to make progress quickly into a strong breeze

Changes - 24 April 2011

Based on feedback I haved tried to make the hull narrower and deeper. Additionally I have been advised to go with a deeper board and deeper rudder. Changes can now be seen here. To get a narrower hull, I raised the chine even more to just above the waterline. I wanted the board to have a lot of chord, maybe up to 30cm. Now as I want 30cm or more shelter in the main hull, this means more than 2ft of depth amidships from gunnel to keel

The other change recommended was a higher aspect ratio board. I decided against following this advice as I thought that the lower aspect board would be better at lower hullspeeds, additionally the lower aspect board would be more robust. It must be remembered that this is a design optimised for cruising as opposed to pure speed

News 24 April 2011

I have bought a bandsaw and a drill press to build this boat, my wallet is $1000 lighter (ouch). Both items arive Friday 30 April

News 2 May 2011

Bandsaw and drill press are arriving today, delayed a fraction due to problems with the delivery. I have bought more wood. Mostly this is suitable for hatch covers, strong back and crossbeams. I do not at present have the light strong rot resistant wood that is suitable for stringers and chine logs for the main hull.

I was experiementing with the little handheld router on Saturday, should be useful in making curved over edges, it is a nice simple tool once you have the hang of it.

Based on feedback, I have both lenghened the main hull by a few inches. I have also reduced the flare angle above the chine. Angles below the chine remain unchanged. I think this is better, adding another 4 inches or so of length is a modest increase in weight, but assists greatly in obtaining better trim. I might add another 2 inches before I am finished, but no more.

Particulars of these hull lines based on 250kg displacement

  • midsection freeboard = 35.5cm, this is roughly OK..yes? (now increased to 38cm)
  • draught = 25.5cm (now increased to 27cm)
  • waterline beam = 57cm
  • wetted surface = 3.1 sqr meters
  • forward prismatic coeffiecitent = 0.617 (whatever this means)
  • aft prismatic coefficient = 0.592
  • maximum beam = 82cm (now reduced to 77cm)

What I have tried to do is go with a simple easy-to-build hullshape. I have tried to make the forefoot fine so that is cuts through waves. Yet I have also made the bow slightly full to give reserve buoyancy so that it does not dig in overly if pushed too far. There is a degree of rocker to allow easier tacking. The stern is a compromise between a wide planing stern, and a narrower stern that gives low drag at lower hull speeds. The flare angle is modest so that I should be able to use strong paddle strokes something not possible with too much flare. My general feeling is that it is a reasonably good shape and shoud do what I want it to do fairly well.

note that this table of offsets refers to the above image. This is the one with a maximum beam of 82cm. After doing this drawing, I changed the lines to something deeper and narrower. To work out what shape works best, it is a bit hard to tell. A deeper narrower shape will be faster, but something a bit wider might be a little more comfortable and have a bit more carrying capacity

Details of ama

  • Ama is a simple design, easy to make and low drag.
  • Max ama width = 38cm
  • Ama volume = 320L
  • ama was extended to 4.5m long (15ft), based on feedback
  • ama has a flat deck to assist ease of construction
  • It would be prudent to break this ama into six watertight compartments via use of thin bulkheads
  • Based on previous experience I want to make the ama as light as possible to assist in moving it around
  • Weight for the ama (ama to windward tack) will be essential, Idea is lots of softdrink bottles filled with fresh water
  • Support struts are simple softwood, utilising shape of the ama above the chine
  • Construction is 4mm plwood, epoxy resin. Fiberglass tape over chine, mulitiple fiberglass layers over keel
  • Inside of chines are filled with wooden chine logs