|A second hand Stiletto 25 - quick
but too much draft:|
A friend of mine by the name of Ray Ellard bought a second hand Stiletto 25.
Originally designed in the early 60s by New Zealander John Spencer it was one of the revolutionary boats of its era (right). It used the new-fangled material of Marine Plywood to produce a radically light and fast type of boat. Other examples of Spencer designs at the time were the 29ft Serendipity, the maxi Buccaneer and the maxi schooner New World - all notable boats.
Another designer working along the same lines at the same time was E.G Van de Stadt with his Black Soo ( red boat right), Zeevalk and later the first true maxi yacht, the mighty Stormvogel.
These boats were real flyers compared to the conventional planked boats they raced against. Faster upwind because of higher ballast ratios and low slung lead. Faster downwind in a breeze where they could plane at speeds unimaginable to contemporary yachting circles.
Additionally, these boats for the most part, could be amateur built and were quite seakindly - something had to make up for the lack of headroom!
Of course the yachting authorities at the time took a dim view of these fast, simple and relatively cheap boats so promptly altered rules to heavily penalise the speed producing factors or ruled them out on grounds of construction.
But I diverge
The Fall From Grace:
My friend Ray was sailing down the Port River in Adelaide when the boat shivered strangely and started rolling wildly - the keel had fallen off!
Ray is a man of action so he quickly dropped the sails and motored the mile or so back to the mooring.
I received a phonecall not long after this with a proposition from Ray - that I design a new keel that would be simpler to build than the original solid-lead tapered keel. He also wanted to lose some draft if possible. He certainly was not after "a race winning super streamlined job". In fact he wanted the simplest, crudest keel possible so he could sell the boat and build something with a shallower draft for cruising the Gulfs and just possibly the Murray River.
So I set about making his wish come true!
(The original keel is the deeper of the two outlines - one of the few bits of design I have done in imperial measurements - inches)
A simple approach - a single slab of lead with constant sweep and taper with a similar centre of gravity to the original (fig 1). The top had identical dimensions to the original so it could pick up the same bolt holes. I sent the drawings down to Ray.
I received a fax back. Ray was not happy with the idea of having to move such a monolithic lump of lead around by himself.
An "upside down keel" (fig 2) with the root chord (the part of the keel against the hull) the same as the original "fallen keel" (as it had been dubbed by this time). The upper section was to be a plywood box built hollow with three identical pieces of ballast bolted together and then to its base. There were to be some extra floors fitted inside the boat to take the additional load
Ray faxed back that he appreciated my efforts at accommodating his ideas but was now looking at the possibility of moving the boat down to the Lower Murray at Goolwa. Like many Adelaidians he finds it more interesting than the long sail down the Port River, followed by a couple of hours bouncing around in the Gulf - before the long sail home.
The criteria became to reduce the draft of the boat to 3'6" (1050mm) instead of 4'9" (1445mm). Preferably without sacrificing any stability.
This was made more interesting because the hull draft was 14 inches - which left only 2ft 4ins (700mm) for the keel itself. I muttered a few times about the risk that the boat may not go upwind any longer with such a shallow keel. Ray replied that he thought it would be OK if he sailed the boat "sympathetically".
Additional concerns are that as the boat is now going to be moved to fresh water that the boat will sink somewhat - probably by a bit more at the bow than the stern due to the depth of the bow. It was up to me to guess by how much and to move the ballast aft by the right amount to compensate.
In this version (fig 3) the plywood box part of the keel has a constant chord of 58 inches - 22 inches longer than the original. This would require Ray to fit two extra keelbolts and fit a couple of extra floors to the boat - I figured he would be happy with the extra keelbolts as they will reduce the risk of the keel falling off again.
The extra chord also allowed the area of the keel to come up to near where it should be -even though it was still very short on span (depth).
The ballast was stretched in a fore and aft direction. This allows the ballast to be longer and thinner, reducing its depth. The ballast is also slightly wider than the upper keel - I was hoping that the added width will reduce crossflow at the tip, reducing drag.
I sent it off to Ray with a scrawled repetition of my concerns about the damage to the boat's performance.
Ray faxed back saying it looks fine and would I be so good as to do the final calculations and construction drawings.
The Last Temptation:
I attempted to juggle the volumes and centres of the keel and ballast to get the boat to float on her original lines in fresh water without creating an unbalanced helm.
I did take the opportunity to experiment with the Gougeon Bros developments in hardware bonding - where bolts are glued in place rather than fitted with nuts on the ends. This method was used to attach the bolts for both the ballast and hull attachment to the box keel. The holes in the timber flanges are drilled oversize by a calculated amount then thickened epoxy is poured into the holes, gluing the bolts in place. There is a good description of the method in Chapter 29 of "The Gougeon Bros on Boat Construction".
Plywood construction for the upper hollow box is an excellent method as the surfaces are very fair (smooth without bump or undulations) allowing for a highly efficient foil section to be maintained. This makes a huge difference to the performance of any boat.
The final drawings (construction - above right) were posted down. They include full size computer plots of the frames for the keel box with ply thickness deducted, and full size plots of the pattern for the ballast allowing for shrinkage of the lead as it cools.
No News is Good News:
I didn't hear from Ray for some time.
Ray asks "You know those full size templates you sent down for the ballast"
"Well the poured ballast came out a little different"
"You said it would weight 405kg"
"It weighs 417"
Another call some weeks later
"You know that keel you drew for me" Ray says sternly
" Well I made it up and fitted it at the Port River before moving to Goolwa"
I manage to croak something
" We won our class in the Port River Regatta"
He went on to say that the boat was easier to handle, faster downwind (as expected) but seemed unchanged upwind. On final transition to fresh water the boat is back on its original lines, in original trim.
It all goes to show there are such things as Guardian Angels.
You cannot tell the condition of keel bolts by the condition of the nuts on the ends.
Dear Reader, please do not assume that the success of this project means that a keel of this style will give both shallow draft and race winning performance to any boat. My concerns about ruining the performance were quite real. However I struck a combination of the right boat and the right owner - Ray also happens to be a sailor of the first order. Guardian Angels again I guess.