Wednesday, December 6, 2017

Design Essay - Periwinkle







How Periwinkle came to be designed

This design came about as a direct consequence of the pleasant handling and satisfying speed of the sail-and-oar beach-cruiser, Phoenix III. I guess this will come as no surprise to those who know Phoenix III - a slim hull (L/B ratio of about 3.4:1 in the case of both Phoenix III and Periwinkle) with a five-strake-per-side glued-lapstrake (clinker) half-decked hull arranged for oars as the primary auxiliary, and a self-draining outboard motor splash well mounted off-centre on the transom in case motorised propusion is considered necessary.

Phoenix III was my very first lapstrake design, and marked a huge step forward in my self-education in small boat design. I had started drawing many years earlier - around 1990 - but it wasn't until 2003 that Phoenix III started to take shape on my very basic 2D CAD program (AutoSKETCH 8 by AutoDESK). Until then, my design work had been for personal learning so that I would be able to better understand the work of established designers.

Drawn at the unexpected request of Paul Hernes, Phoenix III came along at a time when domestic circumstances prevented the use of my large drawing board, so I took the leap into 2D CAD, treating it as a combination electronic drawing board and very accurate scale rule. Paul Hernes did a miraculous job of turning my sketchy drawings into a functioning boat named Willie Wagtail.

Paul Hernes sailing his Phoenix III in the early days
Paul's sailing attracted a number of like-minded people on the beautiful "Sunshine Coast" of Queensland, Australia, and before long there was a regular group of semi-retirees sailing on Tuesdays.
...a Tuesday morning
One of the regulars was John Shrapnel, and although he was well-equipped with his own boats, John took a shine to Phoenix III and Paul found it difficult to get a sail in his own Willie Wagtail!
John Shrapnel sailing Paul's Phoenix III in an enthusiastic manner. Note the excellent longitudinal weight distribution, and the nicely setting sails. The rig is free-standing, but the compression in the sprit puts the head of the mainsail in tension, which is in turn transferred to tension in the luff of the flying jib. This is one of the few un-stayed rigs which will allow a flying jib to stand effectively without back-stays or angled shrouds.

John soon decided that he needed his own boat of similar design, but he specified that she be slightly larger than Paul's, and that a condition of the design specification was that Paul Hernes must be beaten in every sailing encounter. Another preference was for a split rig using balance lugsails. I was unnerved by the design constraints, because balance lugsails are not known for being either close-winded, nor particularly fast when hard on the wind. John was philosophical, and liked the romance of the cat-ketch lug rig - so Periwinkle was born!
Design Elements

1. Easily-driven Hull  Just like Phoenix III, I wanted the new design to be an effective sail-and-oar boat, so correct rowing geometry and a slim hull were essential. At 17ft LOA and 5ft beam, Periwinkle is relatively lean and long. Although being a bigger boat than Phoenix III by about 132% volume and carrying capacity, she is pleasant to row as long as proper oars and oarlocks are used. A critical part of using oars in a boat such as this is to get the rig down completely, and keep the centreboard and most of the rudder up;

2. Choice of Rigs  As with many of my designs, I laid out the mast locations and position of the centreboard so that a number of quite different rigs may be used without having to perform carpentry on the boat. It takes a bit of effort to work through the compromises and proportions, but it is well worth the time.

Sail arrangements possible with the standard balance lug Cat-Ketch. The boat has a third mast stepping location inbetween the normal main and mizzen mast positions.

Mizzen left at home, and the main-mast moved to the third position. In this configuration the boat sails superbly well, with good hull-balance and an uncluttered rig.


Mizzen set on the main-mast in the third location, although the mizzen mast could have been used. With just 51sq.ft. of sail set, the boat recorded up to 8 knots by GPS this particular day. But speed is not the aim - safety and comfortable sailing in a hard chance is the desired outcome.


Just the mizzen set in the middle position. John is sitting comfortably inside the boat in a relaxed manner. Believe it or not, he was actually overtaking the boat in the background. This is a good lesson in physics. Once the wind gets to a certain speed, a boat will often be capable of higher speeds if the sail area is reduced! Read "High Performance Sailing" by Frank Bethwaite for the science.

Some of the possible rig options, all of which use existing mast locations, and all but two of which use the same main mast

3. Ability to cut through a chop and remain dry  Periwinkle (and Phoenix III) have very fine sections forward. I drew them this way because I grew-up on Moreton Bay, which is known for its short, steep chop. Having spent my youth getting soaked by water smashed into the air by conventional bow shapes, I was determined to keep everything sharp and fine up front. The clinker laps also help by performing the function of a series of spray-rails - it is amazing just how much water they knock down.

Topcoat on the hull and primer on the deck. This shot shows how fine she is in her forward sections.


Plank laps working well as spray-rails. Note the flat and undisturbed wake behind the stern.


I consider that for comfortable cruising, this combination of fine forward sections and clinker laps is of great importance. In a racing dinghy one doesn't worry about getting wet, but for cruising, it can mean the difference between a happy or a miserable day!

4. Emergency Buoyancy  Being able to right a capsized boat without external assistance is absolutely critical to safe and responsible dinghy cruising. Racing boats are designed to be righted by an athletic crew and sailed away instantly without bailing being required. The situation is very different in a cruising dinghy or recreational day-sailing boat. Carriage of supplies and equipment dictate dry-stowage volumes which are easily accessible, while at the same time being effective emergency buoyancy.

Many people make what I consider to be the mistake of using side buoyancy tanks and/or enclosed side seating. This sort of emergency buoyancy is fine for a racing boat because it keeps her floating high in the water after a knock-down and means there is little water aboard after righting. For cruising and day-sailing there are several problems with such an arrangement - the mast is held high above the water at deck-level, and the tip touches the water at quite an angle. This means that the boat is floating at well over 90 degrees to the surface - say 100 or 110 degrees - and there is a strong tendency for the boat to continue to roll given that only the tip of the mast is floating on the surface. Not only that, but when the boat is righted, having taken little water, she floats at her normal water line. A strong nineteen-year-old may have no difficulty getting aboard, but for someone like me (in my mid-sixties) it can be a real, life threatening problem.

In Periwinkle, the emergency buoyancy is contained within two bulkheaded compartments - one forward and the other aft. In the event of a capsize, the boat settles much lower in the water than would be the case with side tanks, and the bow and stern buoyancy compartments mean that she is longitudinally-stable. The lower floating condition means that the masts are floating for most of their length, making them - in combination with gaffs or yards- very effective outriggers. Vulnerable members of the crew can simply swim or roll into the flooded hull before righting and then the skipper simply puts weight on the centreboard to right the boat.

Here is a video from Gerry Lavoie, who built a boat from my First Mate design. Although First Mate is much smaller than Periwinkle and Gerry was playing in smooth water, the principles I've described are well demonstrated.


In the case of Periwinkle, the aft tank is unusually large, providing exceptional buoyancy and storage space in a part of the boat which is easily accessed. The main hatch is on centreline so that even if the hatch cover is left open, the opening is above the capsized waterline.

Large aft buoyancy/storage compartment. Note how the hatch is on centreline, which means it is unlikely to take water in a capsize, even with hatch-cover removed.
During capsize trials, my procedure was:-

  • release the main-sheet;
  • swim the bow head-to-wind;
  • right the boat;
  • tighten the mizzen sheet to hold the sail tightly on centreline. This ensured that the boat would reliably ride head-to-wind;
  • push down on one gunwale to bring the deck close to the water-line and simply roll into the boat;
  • sit comfortably on the aft seat (stern sheets) and bail at my leisure.
Secure and comfortable aft seating. During capsize testing, I found it convenient to sit on the seat and bail between my legs at leisure. Note the handy open storage under the seat. When lying on the floorboards, this space is very easy to access, and it can hold a lot of loose gear
Good ergonomics for camping aboard. The nice floorboards and the gentle curve of the planking make for a secure sleeping position with easy access to the stowage under the aft seating.The centreboard case makes for a sense of privacy from someone on the other side! I'm just an inch short of six feet tall, and you can see there is still length available at my feet. 
 5. Simple Mast Stepping  The easier it is to set and strike the rig, the more often you will actually get on the water. Almost all of my rigs (not just for Periwinkle) are fee-standing - i.e. no stays nor shrouds - and the result is a rig which can be erected and lowered in minutes. The heaviest component is the main mast, and when placed in the normal location, it is simply a matter of popping the heel into the mast step and pushing the mast forwards into a horse shoe-style mast partner.

The main-mast steps on the keelson and is simply pushed forward into the mast partner at deck level. In the forground you can see that if the third mast location is used, the mast has to be lifted to thwart level and dropped through the partner in the forward thwart.

Third mast location in the foreground
The setting-up of transport supports for a rig can be a real pain, but in the case of Periwinkle, the short length of the masts, and the lack of tangled stays and shrouds made it fairly easy.



6. Other rigs in use  I mentioned earlier that most of my designs make provision for alternative rigs, using existing masts and mast locations where ever possible. Here are a few photos and videos in what is an on-going journey:-

Original rig a few days after launching

Launching day

Original rig

Original rig

Gaff-Headed Cat Rig

Gaff-Headed Cat Rig

Mizzen removed - original main-mast and mainsail moved aft to third mast location as a balance lug

Mizzen removed - original main-mast and mainsail moved aft to third mast location as a balance lug

Mizzen set in the third location

Bruce Drever's beautifully-built example of Periwinkle sailing with the gaff-headed cat rig with small jib set flying. This is one of the rigs featured in the plans package

Bruce Drever's beautifully-built example of Periwinkle sailing with the gaff-headed cat rig with small jib set flying. This is one of the rigs featured in the plans package

John Shrapnel trying out a windsurfer rig just for fun

John Shrapnel sailing Periwinkle with the mizzen removed, the mainsail left in the forward (standard) location, and with the tack of the sail pulled aft to the mainmast. This makes it a Standing Lug, and John says that if he is going out for a solo sail, this is his rig of choice. John has found that when sailing alone, a 25kg (55lb) ingot of lead secured on the keelson is of great benefit.



John Shrapnel's Periwinkle sailing in very light conditions, but showing just how easily-driven is this hull.


John sailing his Periwinkle with only the mainsail set, arranged as a standing lug. The boat behind is an Oughtred Fulmar with one reef tied into the main. Note that Periwinkle is holding boat-speed, while getting up to windward of the Fulmar. The mainsail of Perwinkle is only 104 sq.ft., which is a very modest sail area.

I will be writing some more about hull forms, so you will be hearing more about Periwinkle.

Monday, June 5, 2017

Three Brothers - a 3D Virtual Assembly



As part of my attempt to learn how to use a 3D CAD program, I have been using my Three Brothers powerboat design as an exercise. Although the design is still under development, I thought that some people may be interested in seeing a stage-by-stage stitch-and-glue assembly sequence.

This assembly sequence is only one of several approaches to stitch-and-glue construction, but most have a common theme of not requiring a strongback or mould set-up. The shape of the boat is determined by the accurate design, marking-out, and cutting-out of the primary parts - if everything is done correctly, the boat assumes the correct three-dimensional shape without a strongback and set of station moulds, greatly increasing the speed of construction.

Three of the six main panels cut from plywood and laid on the floor. The accuracy of the design and cutting of such panels is the key to a successful build.

Two bottom panels stitched together along the stem (bow) and centreline of the bottom. Stitching is carried out with the two panels laying on each other, and when complete, the panels are opened up 'book fashion'. The assembly will sit on the floor as shown, but it is sensible to have some cradles for a project of this size (details will be provided in the plans).
Pre-fabricated bulkheads, frames, and transom are positioned on station marks and loosely sewn into location using cable ties.
Topside panels stitched into position. By this stage the glass-taping of joints will be taking place.

Cabin sides, including coamings, stitched into place.
Longitudinal webs glued and taped into position. Ventilation holes are suggestive only, and may be changed depending on style of emergency flotation employed
Outboard motor splash-well structure added.
Floorboards and other horizontal panels introduced.
Longitudinal deck-beams and roof structure in place.
Fore-deck and aft-deck panels attached.
Plywood cabin roof and front panel of cabin attached.
Gunwales, outer stem, and structural trim around cabin and coaming finish the basic job.
This is a very quick illustration of the basic assembly method. For more detail, I suggest reading Sam Devlin's wonderful book on the subject, "Devlin's Boatbuilding.

Sunday, June 4, 2017

Getting 16 feet Planks from 8 foot Panels...

In 2002/2003 I was becoming increasingly aware of my lack of bench space in the Wynnum workshop.

At one stage we had six boats under construction at one time, and when commencing the planking of a glued-lapstrake (clinker) sailing dinghy, I realised that I simply did not have the bench space to scarph together 8ft x 4ft sheets of plywood into 16ft x 4ft sheets, which had been my standard procedure.

The space being taken up by 16 foot-long panels had been on my mind, but on this occasion the chickens had come home to roost! Necessity being the mother of invention, I experimented with an alternative approach, and the result was highly successful. A recent call for help from a builder in America has brought the subject back to mind, and I thought the method may interest others.

Lapstrake plank patterns laid out on a 16ft x 4ft panel in my first rented workshop - that is a lot of space taken up!
In order to produce accurate planks from separate 8 foot-long sheets of plywood, I decided to capitalise on one of the by-products of my favourite method of spiling (i.e. method of determining plank shapes when laid out on the flat), and that is the production of lattice style spiling battens. You can find this method of spiling in a number of text books, but it is also shown in illustrated form in the instructions which accompany my plans. Here is a brief explanation of the method I use to produce accurate 16 foot-long planks from 8 foot-long sheets of plywood. Not only does this method save bench space in the workshop, but it also allows for more efficient utilisation of the plywood - i.e. less wastage. (click on images to see full detail)

Here is a plank pattern laid on a single 8ft x 4ft sheet of plywood in a manner which produces minimum wastage. The pattern shown uses diagonal truss-type bracing, but I often use rectangles of thin MDF hot-melt glued to achieve the same result more quickly.  See earlier photo above. Trace accurately around the pattern with a sharp pencil or a fine ball-point pen.
Where the pattern crosses the edge of the 8ft x 4ft plywood sheet, carefully and accurately mark the pattern as shown above in red.

Next, mark a line eight times the thickness of the plywood from the edge of the next 8ft x 4ft sheet. This line is drawn to allow for the amount of plywood which will be taken up by the chamferred  edge of the ply to allow for a scarph joint.  In this case, the plywood is 6mm thick, so I have shown the line 48mm from the edge (if using butt-straps or a "Payson Glass Joint", just make sure the marks are clear of the edge of the sheet).  Lay the remaining portion of the plank template on this sheet with the red marks which were added in the previous step inside the line. 
Adjacent to the marks on the pattern, and inside the line marked eight times the distance of the thickness of the ply from the edge (or from the edge itself if using a butt joint), mark the plywood accurately as shown by the black arrow-heads in this drawing. Then trace accurately around the rest of the pattern.
Remove the pattern to leave the tracing on the plywood and mark a line eight times the thickness of the ply beyond the arrow heads. In this case, with 6mm ply, the distance is 48mm (see drawing above). If the joint is a butt, disregard the last mark.
Cut the half planks from each of the 8ft x 4ft panels, and plane an 8:1 scarph on each half of the plank (disregard for butt joints).
Glue the plank halves together using the scarph or a butt-joint and before the epoxy has cured, lay the plank template over the assembly to ensure that the two halves line-up accurately with the full template (ensure that you place waxed paper or plastic sheet between the glued panels and the template to prevent gluing them together).
This procedure is more difficult to illustrate and explain that it is to carry out. I can assure you that with the application of common sense, it is a simple and practical process which will save you time, space, and plywood.