Turbo-ing
part 1

by Chris Cochran of Morrelli & Melvin

And who is Chris Cochran? He is a Naval Architect, Chris grew up near Detroit, Michigan, racing on his uncle's Beneteau 38. As a seasonal sailmaker, Chris attended the University of Michigan, raced on their sailing team, and graduated with a Bachelors/Masters degree in naval architecture and marine engineering. For a graduate project, he designed, built, and tank-tested a 1/3 scale Mini 6.50. His interests lie in designing and racing on high-performance monohulls and multihulls. Enjoy

My last article prompted a lot of questions & suggestions, mostly regarding ways to improve an older boats performance through keel & rig modifications. This so-called act of “turbo-ing” an existing design offers a convenient (and sometimes fun) alternative to buying a faster yacht. So in my next series of articles, Ill discuss ways to improve a boat’s performance through some hefty rig & appendage modifications, and what to expect from your designer, boat yard, sailmaker, etc…. The keel seems to be the first thing that people want to change, and since the last Yacht Design 101 article was about keels, I figure discussing those would be a nice segue to this series.

But before we delve into keel modifications, it might benefit everyone to have a refresher in stability. Righting moment, or stability, comes from the shape of the hull (form stability) and the location of the center of gravity (ballast stability). The ballast stability works like a lever – you can get the same righting moment by lowering the center of gravity while decreasing the ballast or raising the center of gravity while increasing the ballast. Form stability, on the other hand, has more to do with the overall shape and displacement of the yacht, regardless of the center of gravity. Only significant changes in displacement will affect form stability, so to simplify things, we’ll assume that the changes made to the ballast will not greatly affect the form stability.

There are a few characteristics common to “modern” keels. To begin with, the old trapezoidal/elliptical fin is usually replaced with a higher aspect stainless steel strut. The shape of the strut can be designed to have a better lift/drag ratio than the existing fin (see Yacht Design 101: The Deal on Keels for an explanation on why), while being stronger and lighter than the lead equivalent (stainless steel is 30X stronger than lead but 70% lighter). Most of the keel’s ballast is then concentrated in the streamlined lead bulb, which can come in all shapes and sizes. The lift/drag ratio of the strut alone is probably better than the original fin, but there is a significant increase in drag from the addition of a bulb. Also, high-performance keels are usually deeper in draft, which shifts the center of lateral resistance (CLR) downwards. Assuming the rig and sails are not changing, the heeling arm, or the distance from the sail plan’s center of effort to the keel’s CLR, will increase. This causes the heeling moment to increase, which counteracts the righting moment. As we will see, this can be good or bad.

So depending on the type of boat and the desired performance increase, there are a few options to consider when upgrading your keel.
  • Option A: Do you want more stability in the boat?
  • Option B: Do you want a lighter boat?
  • Option C: Do you want both?
  • Option A can be used if you already have a ULDB, and want to increase the righting moment because of a lack of crew riding the rail, a desire for more sail area, etc… In this case, the keel’s center of gravity will need to be lowered to get a better moment from the ballast. This can be done be increasing the draft and/or lightening the weight of the strut. But a careful balance needs to be obtained so that the increased heeling moment (from the increased draft) does not exceed the righting moment and make the boat too tender.
  • Option B could be used if you’re trying to race a heavy cruiser-racer (with the emphasis on cruiser) in a light-air venue. For instance, the boat already has enough righting moment, and most of the time the crew is on the leeward rail because it’s rarely windy enough to heel the damn thing. You can maintain the righting moment and lower the weight by increasing the draft of the bulb while decreasing the weight of the bulb. The heeling moment will be greater, but that might be OK up to a point. And a modern strut/bulb combo could potentially have less drag than an old school cruiser/racer keel, which is necessary if the righting moment is not increased
  • Option C is a good solution for a medium-displacement yacht that needs a little more power capacity or a bigger rig. It could stand to lose a few pounds for downwind work, but needs the power when going upwind. As you would expect, this is done by lowering the center of gravity while carefully slowly decreasing the weight of the ballast. The easiest way to do this is with a canting keel (which actually rotates the center of gravity to windward), but that’s another subject for another day. For a fixed keel, it’s a matter of controlling the weight of the strut, increasing the draft of the bulb and decreasing the weight as much as possible. A Velocity Prediction Program is a great tool for this because the designer can observe how all of the changes affect the performance of the boat as a whole, at all points of sail.

Sounds pretty cut and dry, right? Unfortunately, it’s not so simple to just cut one keel off and put a new one on. When grossly modifying a keel, there are some major structural considerations: the rig, the keel bolts/sump, and the boat’s floor structure in way of the keel. When sizing the rig, designers use the righting moment to calculate the mast compression and the loads on the stays. Increase your righting moment too much and the rig could get loaded beyond its design load and fail, and that would not be too cool. The keel bolts are another area to look at. A new stainless strut can be thinner (transversely) than its lead predecessor. So if the keel is thinner and has a narrower chord, chances are it will not fit the original keel bolt pattern. With any luck, you can use some of the original holes and just increase the bolt size. If not, you’ll have to rebuild the keel sump and start with a fresh bolt pattern. Lastly, the internal structure might need to be re-built if a new keel is installed. With more righting moment and a narrower “footprint”, a new keel might induce higher loads on the existing floor designed around your original keel. For racing boats, the keel modifications might be easier. The internal structure and keel sump can usually be rebuilt without tearing out any of the interior. The only exception for high-tech race boats is whether the rig is over-sized enough to absorb the higher loads. For cruising boats, the rig should be no problem. But the interior joinery is sometimes one with the internal structure, so there could be an extensive project to rebuild the keel sump or floors. Don’t worry, a designer will be able to tell you whether it’s possible, necessary, and worth it to modify the keel.

I only covered a few examples, but there are many more scenarios where a keel modification can be optimal. The important points to get out of this are that different performance requirements will require different changes, and to be aware that the keel might not be the only modification you need to make. Next month, we’ll look at ways to compliment the new keel with a new rig and/or new rudder.

06-May-2005