Phoenix III Hull Structure

Glued-lapstrake is a wonderful method of construction for small-craft - the appearance is elegant (as long as the lining-off of the planking is tastefully done), the interior of the boat is relatively clear of structure, making maintenance of the paintwork easy, the method makes efficient use of sheet plywood, and the amount of epoxy work is relatively low. In addition, the plank overlaps on the outside of the hull perform very effectively as a series of spray rails.

Periwinkle showing the 'spray-rail' effect of the plank laps....

...and Phoenix III doing the same

In addition to all of the benefits I've mentioned above, what I think is the the most important element of the glued-lapstrake method of construction is that each overlap in the planking produces what is, in effect, an integral stringer! This stringer effect is caused by two characteristics - firstly the thickness of the hull planking is almost doubled where the planks overlap, and secondly, on a round hull, the adjacent planks are at a different angles relative to each other. The angular difference gives additional stiffness in exactly the way that corrugations in roofing iron add stiffness.

The structural benefits of the overlapping planks mean that a lapstrake hull can (within limits) be built with reduced internal framing, and in the case of 'Phoenix III' I specified no transverse framing from the semi-bulkhead at the forward end of the centreboard case through to the bulkhead at the forward end of the stern seat (i.e. 'sternsheets') - a distance of 2280mm or 7-1/2 feet. However, there is other structure present in the form of a substantial transverse thwart structure, the keelson, and the centreboard case.

I had some minor misgivings about whether I was taking the matter of a structurally clean interior to an extreme, but until very recently I have had no reports of problems. In fact, the very first Phoenix III built is now more than nine years old, and has been used heavily, going on the water weekly for that entire time, and having travelled long distances on a trailer (trailers damage boats more than anything else).

Despite the lack of structural problems reported, in 2007 I placed this entry into the instruction manual which accompanies the plans:-

Planking

One of the design aims with Phoenix III was to have a clean and uncluttered interior. This has the benefit of making sanding and painting easier, makes cleaning easier, and produces a smooth interior hull surface for sleeping aboard.

Another design aim was to make the boat as light as possible. The lighter a trailer boat is, the more she will be used. The clean and frameless interior is in line with the quest for light weight.

There are no transverse floor timbers (frames) specified in the plans between the half-bulkhead at the forward end of the centerboard case, and the half-depth bulkhead at the forward end of the stern sheets (seat). Between these two points, the hull relies upon the strength of the planking, the glued plank laps, the keelson (or hog), the centerboard case, and the main thwart.

The resulting structure is strong, clean, and slightly springy. But, it is very important that the builder pays close attention to the standard of gluing – particularly along the plank laps. Use of epoxy fillets along the internal lap lines will prevent water sitting in the laps, and will add considerable strength to the joint. This is only really important in the lower planks.

 For those who prefer additional strength, or who need a base upon which to place raised floorboards, transverse floor timbers can easily be added during construction. Consult one of the suggested test books, or contact me for details. My preference would be for bent floor timbers (steam-bent if necessary) glued across the top of the keelson, extending out to cover the first two laps. Size is arbitrary, but I would be thinking of 9mm x  22mm/ 3/8” x 7/8” as recommended by John Brooks for his design, ‘Ellen”. 

When Woodenboat Magazine asked me to write a 'How to Build' article about 'Phoenix III', I re-drafted the plans to make them better suited for magazine publication, and while I was at it, I drew a pair of half frames to go under the main thwart, on either side of the centreboard case. That would deal with any lingering concerns about the expanse of unsupported planking once and for all, but at the expense of the clear interior.

Now, having told this overly long story, I have to tell you that my customer and friend (email friend, that is - we live on opposite sides of the globe) Jonathan McNally, has reported a persistent cracking problem in the joint between the garboard strake (i.e. the plank closest to the keel) and the keelson on his 'Phoenix III'. Jonathan's boat was built without any extra framing structure, and is a good example of the 'pure' original design. The cracking is minor, but it does indicate a potential problem - as I had feared. 

Very fine crack in the paint work on Jonathan's boat just above the keelson in this photo

Jonathan believes that the cracking was initiated by heavy foot-falls on the planking, and he intends to put in some steam-bent ribs and light floorboards to distribute human foot pressure. This is very similar to my comments listed above in the except from my building instructions.

I'm hoping that this whole business is a case of me worrying too much, as nobody else has told me of the problem. I asked Jonathan about the eopxy products used, and they were all perfect for the job. However, the plywood came from a very highly regarded European manufacturer, but there is a twist. A very good friend of mine who is a Naval Architect conducted engineering tests on a range of plywoods on the Australian market, one of which was this fancy European brand. The results of the testing were surprising. This particular 'super high quality' ply came out as:-

• lowest bending strength;
• lowest peak load at breakage;
• lowest modulus of elasticity;
• lowest strength-to-weight ratio;
• 2nd lowest stiffness-to-weight ratio;
• lowest structural efficiency (adjusted)

So, many lessons to be learnt. If building a 'Phoenix III', I am now changing the status of the two half frames under the main thwart from, 'Optional' to 'Recommended', even if they may not really be required.