Offshore Optimising
August/September 1996
Sometimes nothing is better than going fast. When the handicap formulations get all too hard (IMS) or all too arbitrary (PHS) then the best fall back position is pure speed .The satisfaction is immediate, the gratification long lasting. Australian keel boat sailing seems to be entering its next "fast is fun" stage and the keys to success in this area are, as always, the big three : displacement , stability, and sail area. Historically designers have found it easy to keep the boat light and have lots of sail area but the displacement has always come at the price of stability as the lead mass had to be reduced to keep the boat light. Unfortunately this created the all to common scenario of what I call bicycle boats -fast downhill , struggle uphill. The net result is only partial satisfaction as the you catch up with the upwind oriented boats on the run but get passed on the work. Usually the bicycle boat is a long way back at the finish compared to comparable size all rounders.
The key to this problem is stability and the way to get stability is simple. Take all the lead in the keel and stick it in a bulb at as deep a draft as you can sensibly tolerate. This is nothing new but the idea is not as prolific as it should be due to fact that so many handicap formulations have favoured heavy boats in the first place and therefore the non-rating light designs of the period have keels of similar configuration to their heavier, internal ballasted brethren. Thus a Farr 11.6 has a keel as per the IOR designs of the period and is thus tender. However, being a lighter design they cannot tolerate heavier keels as they start to drag their sterns. The solution is to lower the center of gravity of the keel for the same or ideally less keel mass.
There are many solutions to a problem but the one that I suggest in this
situation is one that really takes the concept to its logical extreme. This
is the most severe but by far the fastest solution and one that will not be
soon superseded in terms of it's pure performance generating capabilities
on all points of sail. We have used it on a wide variety of boats from the
BOC 50 True Blue to restricted draft cruisers that want maximised performance.
Here are the steps :
STEP 1: Determine if your PHS or fast cruiser is overweight. Look at the
original drawings versus how the boat is floating now. Generally these boats
are burdened on their lines and any reduction on displacement will be welcomed.
STEP 2: Determine existing keel weight and stability. The former can be
lofted and calculated if not already known , the latter can be had from an
IMS or IOR certificate from your boat or a near sistership if you have not
been rated.
STEP 3: Draft ? Determine the optimal number for the area you sail in.
STEP 4: This is where the design engineers come in - optimise the stability
for the best upwind performance based on the assumption that all the lead
will be on the bottom of the keel fin. The result of this process will tell
you how much displacement you can loose with a reduction in overall keel mass.
STEP 5: Fabricate. What we advocate is quite simple. The keel should be
a vertical fin with parallel leading and trailing edges and a constant thickness
throughout to simplify manufacture. The load is taken through the steel skins
which are typically stainless or mild steel plate. These are brake pressed
into the desired shape and welded on the fore and aft seams. A bulb is cast
with a cut out for the foil and the two are bolted together. Alternatively
the bulb can be cast over the foil .
The bulb itself is generally radial in section (i.e. , any transverse cut
will reveal a circular cross section ). More exotic bulb shapes exist but
few are created on sound theory and many by the marketing department of Designer,
Inc. The squashed bulb has one reason to exist and that is for draft constrained
classes such as the IACC but when you look at the variety of solutions to
that basic problem you begin to appreciate just how hard it really is. Tread
lightly as the radial bulb from a sound laminar flow section works and works
well.
Attachment to the hull is a critical issue. Given that the foil is generally
quite small a larger flange plate is required to give adequate bolting area.
In a 40 footer this flange may be 20 mm thick and it needs to be recessed
into the hull into an area of solid GRP laminate. The surrounding floors and
longitudinals will need to be examined and brought up to adequate strength
as well. Generous washer plates in the hull which butt up against the floor
and longitudinal structure are also required. Suffice it to say that these
are issues well beyond the scope of this article but ones that need to be
adequately dealt with . I advise that the current ABS grounding standard be
employed in the keel attachment area.
What does it look like? The two drawings show two different interpretations
of the idea. One is for a restricted draft (2.1 m) 50ft fast masthead cruiser
and the other is for a 41ft fractional rigged PHS boat. Note the difference
in bulb placement. This is to do with the issue of helm balance versus the
location of the centre of buoyancy. In the case of the cruiser it was necessary
to have the keel further forward for balance so the CG of the lead had to
be moved back as far as possible. In the 41 footer this was not required.
This will always be a case specific area and it should not lead one to believe
that one configuration is better than another. Each has practical reasons
for being that preclude the other approach. Also note that the shallower draft
keel has a longer chord. This planform is not as efficient so the extra area
is needed to generate the desired lift.
The net result of this process is a minimisation of displacement and an
optimisation of stability . This means that two out of three speed producers
is the best it is going to get for your particular boat. If you want to go
faster that's a pretty good result.