Uncategorized

To Mod or Not to Mod?

design it

To Mod or Not to Mod?

Over the past few years, Dibley Marine has been involved in a number of Keel , Bulb and Rudder modifications for existing yachts.
These yachts have ranged from 25 feet up to 150 feet and from various design houses around the world. The two main reasons our
clients have approached us for new Appendages is for either a performance gain, or draft restrictions.
Other reasons, and some are related to the above, are: Reducing Leeway, Minimizing Drag, Increasing VMG [Velocity Made Good), To
correct a Trim Issue, To correct a Weight Issue and to help with Helm Balance. Yachts are usually designed for a specific service to
their original owner, or to a Marketing Teams vision of what the masses want in a yacht. But when a yacht has been on-sold, sometimes
the total package doesn’t quite fit within the new owners requirements, and changing the appendages can be a good way of getting a
great yacht that performs to their expectations.
We recently did a new keel for a client who had increased his sail area by 20%
and found that the existing keels
profile area couldn’t resist the new sail
plans side force. So they were pointing
higher and going faster over the water,
but they were slipping sideways a lot
more and thus their VMG took a big
dive as compared to their previous
performance numbers.

A good rule of
thumb is that most performance keel
profile areas should not be less than
2.5% of the sail area [Main and
Foretriangle]. Cruising yachts may have
more, and some high performance
racing yachts have less. With the
latter, unless you keep the yacht moving at all times, and allowing the keel to work for you,
the leeway loss from hitting a bad wave, or having a bad mark rounding can ruin any gains you
had built up through having less wetted surface and drag.
Another project involved a cruising yacht that was stern heavy due to large fuel tanks aft.
We re-designed a keel so that half that fuel could be stored in the fin. The result was better trim and a bonus of higher performance
through a lower VCG [Vertical Center of Gravity]. A lot of the time, if new keels are designed, we look at a new rudder as well. This
depends on what the performance increase is going to be though. With increased speeds,
better sectional and profile shapes can be used to better effect. If more draft is designed
in, sometimes a more efficient aspect ratio can be used in the rudder, which previously may
not have been ideal as it might have been as it had to work around the old keels draft.
For those who are unsure whether it can be done, best to drop us a line and we’ll review the
options and benefits for you. – Kevin Dibley