It is with a heavy heart that I consider to take the rig off my boat which will become a motor boat again after I have rigged it in its first version about 14 years ago. The reason is that my boat has got a poor stability with respect to roll when the sail is not hoisted to give a roll damping effect. I believe that it is the combination of hull and rig which is not the best in my case, and that the rig in itself can be made functional on a different boat. There may be a slim chance that there is someone who has thoughts about converting to Junk rig and who might see a way to use the mast and other parts. For me it would be nice if the rig can be utilized in some form, as I have put quite some thought into details and therefore I feel that the scrap metal dealer shouldn’t be the first option for the rig. So, if you have some plans or know someone who might have, or if you are just curious, you will find some detailed information about my rig, and my reasoning for removing it, in the following. Price can be discussed, but I intend to be reasonable. Freight will be from Trondheim, Norway.
If someone has suggestions to improve the seaworthiness so I can keep the rig, I would also be happy for that, and any comments about boat stability will be welcomed.
In the first version I had designed a flat H&M style sail with 25 m2 sail area. As the motor boat hull has a keel that is deepest far aft, the mast was placed rather far aft to give a forward lead within the recommended range in Practical Junk Rig. With this rig style the boat had a good rudder balance, and the boat behaved harmoniously and well even in rather rough seas. I had designed a “partners support frame” to transfer the forces from the partners down to the gunwale to prevent large loads from the partners acting on the cabin top and window frames. However, although structurally sound, this frame would not have helped my boat in any beauty contest!
In 2008-9 I moved the mast forward 1.6 metres forward to get the partners on the foredeck. There were two reasons, a) I could then remove the partners support frame which I didn’t like the looks of, and b) I wanted more sail area, and could start experimenting with a wingsail. The present version of the sail has a good drive and gives a good rudder balance when hoisted fully up. The mast is extended with a 0.9 m long section in spruce to allow for the higher sail, and in its first version I had a sail area of about 41 m2 which was quite a lot on this boat version which with its lightweight keel could be bought with a Bermudan rig at only 17.5 m2 sail area. I later shortened the wishbone length by 0.5 m, so the area of the sail is now about 35 m2. Total length of each set of wishbone plus batten is now about 6.1 metres except for the top batten which is shorter (no yard here).
However, this wingsail sail with slimmer top section develops a serious lee helm when reefed, which has made it difficult to control the boat in some cases. I here did a big mistake in designing the sail, and now I finally appreciate the Junk sail’s top section with triangular panels: The triangular top sections behind the inclined yard give a reduced forward lead when reefed, while the wing sail with a shorter top batten gives an increased forward lead when reefed. This is naturally not a problem for boats with two sails where the forward lead can easily be adjusted by reefing the two sails differently.
Also, moving the mast that much forward worsened the behaviour of this motor boat hull in another way. When I had to furl the sail fully down and was motoring, the boat was rolling unpleasantly. My suspicion is that having the heavy mast and also furled sail with their centre of gravity placed much forward and high up changes the main axes of inertia in an unfavourable way: When rolling, the hull also gets a slight yawing motion which when motoring forward, amplifies the rolling, in other words a vicious circle. An observation I made while this boat was still a motor boat points in the same direction: When we were out and three or four teenagers were sitting on the fore deck, the boat lost its normal directional stability – when I looked down a few seconds, the boat had turned 90 degrees and the guys on the fore deck shouted that we were about to hit land!
The conclusion I have come to is that I have pushed the limit for roll stability too far. A colleague in the local boat club has told me that the skippers on costal freighters had a rule of thumb for how much deck load they could carry. They used the deck crane to start a light roll of the ship at the quay, and were clocking the roll period. The roll period is seconds had to be less than the width of the ship in meters to have acceptable stability. I have done similar exercise for my boat. With my sail furled down, the roll period was in excess of 3 seconds, while the breadth at the waterline is about 2.5 metres. In other words, my experience seems to fit with the old freighter rule of thumb; I have exceeded the limit for acceptable stability. It would be interesting to hear how others in the JRA look upon boat stability and if other similar rules of thumbs exist. For sure, yacht designers must have criteria, it is just me that don’t know them.
One may ask, why not always have the sail partly up to give a roll damping effect? That is possible if the sail has a low forward lead so it could act almost like a mizzen example when fishing, but that would at least mean a new redesign of the sail, and the outcome is uncertain. By going back to having my old Viksund 27 Columbi as a motor boat, which as such is a really seaworthy boat, it becomes more attractive for my family members for fishing and other pleasure tours on the fjord.
Some details of the mast and sail:
· Hinged mast in aluminium, built to lay it down for passing under bridges. Four POM (plastic) bushings keep tight albeit low-friction fit between sleeve and mast sections.
· Mast scantlings [Diameter (Ø) x wall thickness] and length:
Lower section, up to hinge: Ø125 x 5 mm, 3 m long
Sleeve: Ø150 mm x 5 mm, 2.0 m long
Section above hinge: Ø100 mm x 10 mm in lower part and Ø80 mm x 4 mm in upper part, 6.8 m long
Top section in spruce: Ø80 mm, adding 0.9 m to height
· Partners with wood wedge retaining rings (preventing the wedges become loose)
· Wishbones and curved battens to make a so-called wingsail with an efficient camber
· “Sheetlet lifter” on the shorter top batten to lift the uppermost sheetlet clear off the battens below, thus preventing fouling of the upper sheetlet on the battens below while tacking.
· The sail is only a cheap tarpaulin of a type used to shield construction scaffoldings. It is fixed to the battens by numerous small plates screwed to each other in pairs.
I have pictures, drawing sketches etc. for those who are interested.
Nils Myklebust