top of the old one. We call it reshaping. For this we design a new keel profile and mill out some horizontal ribs from a fibre- glass plate. The ribs are then glued onto the keel fin, and the area in between is filled with foam, then finally covered with a few layers of fibreglass; or if the changes are small we can sometimes get away with using just filler without extra laminate. ‘For a boat of around 45ft this keel
reshaping can be completed for around ⇔6,000-⇔8,000. ‘There is an even simpler way of opti-
mising keel profile, though, and that is to add a pre-fabricated strip of extra area onto the trailing edge, then using filler and sandpaper to create a nice hollow shape.’
Hollow, welded fins ‘If you want to take it a step further we can produce a new keel. The new fin or shaft is then made from welded steel plate, with a hollow core. This offers potential for a sig- nificant weight saving if required. Typically some extra lead will then be added to the bulb to compensate for the stability lost, but for a 45-footer the general displace- ment can be close to a ton lighter. That’s the “high fidelity” solution, with a price tag four to six times the reshaping solution. ‘Rudders are a bit simpler. Most often
we make a new rudder by milling a foam core to the profile we want, then mount it on the existing rudder stock before adding the necessary layers of fibreglass/epoxy or sometimes carbon. ‘In the design phase most of our working hours are spent adapting the profiles
52 SEAHORSE
individually to the boat. We have different programmes for this but mainly use XFoil and our CFD. A so-called generic optimisa- tion algorithm is also used to finesse the profile until it precisely matches the boat. ‘I know from experience that quite a few
other designers employ one profile that they know works, and they stay with that for all kinds of boats; in my world this is the wrong way to approach the optimisation. ‘We also have a base profile that we
know works, but we tweak and adjust it individually for every project by testing sev- eral thousand profiles using the algorithm. ‘Then sometimes a yacht will perform
best with something entirely new. To get it right you also need to know what speed range the boat mostly operates in; but that information is now easily accessible using the ORC VPP and factoring in the relevant sailing venue and anticipated conditions. ‘Obviously keel depth is a very big
factor. We have seen a number of boats where the owner wanted to reduce draft – not only for ORC optimisation but also because a 3m draft is often not desirable, especially here in Denmark. But when the draught is changed you need the same surface area to arrive at roughly the same hydrodynamic lift, so a deep keel obviously ends up narrower with a higher aspect ratio compared with a more shallow keel. ‘But a short-span keel with a long chord
operates very differently from a deep keel with a short chord. It makes a big differ- ence in terms of the flow it is operating under, whether it is laminar or turbulent. ‘To estimate this we go back to the
Reynolds Number, which is particularly helpful in estimating when and where the transitions occur between laminar and turbulent flow. All of this knowledge goes into identifying an optimum profile which is also suitable for the crew’s sailing style… and ability! ‘Finally, there is the fact that generally
keels tend to be narrower at the bottom than at the top! Quite a few of the other designers will scale the same profile and use it along the whole span. But the correct pro- cedure is to have several profiles that fit the local chord length moving down the keel. ‘If you want a truly optimised profile you
need to look at all these things, especially the conditions the foil is supposed to operate under. You can’t just use the same profile for a long and a short foil, for a fast and a slow boat and so on.’
Ballast ‘Looking inside the boat, there’s always the question of ballast. You can choose to make the boat heavier if you believe the gain on the rating certificate is bigger than the actual loss under sail, or you can move the centre of gravity which obviously also changes stability. ‘A large number of the boats at the top
of the ORC fleet still sail with extra internal ballast that makes them slower and less stable. Simply because it pays to be a bit slower and cash in on the rating advantage instead. This is not something we should be encouraging and for very obvious reasons – especially yachts that race offshore. ‘We can also play with crew weight. On
TROND TEIDEN
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