Shoal Draft V. Deep Keel, Part II

[sailingdog]

How fat is your keel if you could shave 800 kg off of it???

Originally Posted by Giulietta

In my case the keel was originally intended to be like it is now, and both hull and keel were "matched" from start.

But on the first trials we realized that we had added too much lead, and the boat was "too stable" for what we wanted. Also weight was a factor.

We found out in early tests that the postion of the bulb in relation to the boats's CofG was not right. Since we couldn't make holes to move the mast (which affected rating), we could only work from the water line down.

Being a prototype, we had only two options:

a) Spend a fortune in water tanks modulating the keel (which we did only on the first keel)

b) Install a "1st step" keel, and based on sea trials, test the keel and use comon sense and boat feel to "trim and "shave the keel.

As a consequence, we built an easy to remove keel, that inserts in an upside down box, so we could remove ti for alterations.

We modified it 3 times, and ended up lowering it almost 1 foot, and removed 400 Kg of lead from the bulb. I now have 9,5 feet draft.

Pointing is absolutely unbelievable and the last mod, made a 50% improvement in boat handling and responsivness.

One note: We had to adapt the rudder to the new keel.

My keel is designed for what it is intended, and where I sail draft is not a problem. A good thing.

We sail without reefing in up to 30 knots, and I bet, that no matter how hard we could try, a shoal keel would be un-sailable (SP)?

Each design has its own requirments

[Diva27]

With respect to lift and draft, it might be worth noting (as Giulietta has) that the rudder often is overlooked in such discussions. As a foil in its own right, designers of contemporary "fin keel" yachts assign about ten percent of the lifting surface under water to the rudder. (splitting the total induced drag between rudder and keel also lessens the overall induced drag.) It's sometimes been the case that when a deeper draft design has been sawn off keelwise for a shoal draft version, the deep draft rudder gets a docking as well, and not always properly. The rudder's area has to increase when its draft has been shortened, and that by default gives you a lower aspect ratio with less efficiency. Overall, you have to remember your rudder's design and finish etc when you start worrying about windward performance.