Swept back hull appendages. A thing of the past?

[xarax]

There was an era where swept back fin keels and rudders were really popular. Nowadays it seems that the tide is reversed, at least at high-end racing classes as well as cruisers-racers. America s cup keels, for example, are suspended at the end of long fins with perfectly parallel vertical edges, and this is the norm in many other less exotic classes. Why is this so?

[Milan]

Basic role of the fins are to prevent hull moving sideways, produce lift so boat can sail to windward and to provide volume for ballast as low as possible. Modern configuration is more efficient in getting the ballast low, providing maximum lift (high aspect ratio), combined with low wetted area. The only advantage of the swept back shapes is lower risk of fouling with a flotsam as ropes, nets, sea grass and similar.

In the fifties and sixties when designers just started to work with fins and separated rudders to minimise wetted area, theory was long known but there was little practical experience available, so for a time being all sorts of shapes were tried is search for optimal combinations. Many designers draw fins with curved sweeping lines inspired by fins of marine animals, but as was already known in theory, experience shown that plan form is not very important, and fins shaped like this don't get ballast very low.

[Paul Scott]

My 40' cruising sled has a forward swept (5ish degrees) foil. Rudder normal sweep. Really great in light airs, and hangs on like you can't believe in the lumpy stuff. I like to think I can feel vortexes being swept away, and the hull lifting because of the upwash. And we designed it that way from the clean sheet. Dinghy ideas!

Paul

[sharpii2]

Originally Posted by xarax

There was an era where swept back fin keels and rudders were really popular. Nowadays it seems that the tide is reversed, at least at high-end racing classes as well as cruisers-racers. America s cup keels, for example, are suspended at the end of long fins with perfectly parallel vertical edges, and this is the norm in many other less exotic classes. Why is this so?

Speed sells.

And, other than the appearace of the boat, that is its most noticeable feature. The drawbacks one must face in order to get this one feature are far less obvious.

A short, deep keel fin with a ballast bulb on the bottom is probably the most effiient keel fin design one can hope for. The greater depth as well as the higher aspect ratio gives much more sideways thrust (lift) for its area than any other design. Because it has higher lift, it can hve less area and still be effective.

And because almost all the ballast is in the bulb at the bottom, it also has much more righting leverage than the older IOR keel.

This is all good, but it comes at a price.
1.) the draft will be generally deeper,
2.) The virtical leading edge as well as the bulb forward of it are very efficient at catching kelp, lobster pot bouys, and even anchor rodes, and
3.) having almost all the weight at the very bottom puts enormous loads on the keel/hull joint. Such joints must be carefully engineered to take very extreme loads. The best way to do this is to carry the fin itself way into the hull like a dagger board on a dinghy and distribute the load through not only the bottom, but the sides and bulkheads as well. This approach is not popular. This reason is that doing it this way would raise the cabin sole unacceptably high. so, instead, a plate is usually attached to the top of the fin and this plate is, in turn, fastened to the hull. The joint between the fin and this plate is of vital importance. Should it fail, the keel will drop off like a fishing sinker.

So, for higher performance, you have to put up with these shortcommings.

I, myself, would never do it.

This sort of reminds me of the WWII B17 and B24 bombers.

The B17 had a tapered wing that was very wide at its base. It had a LOT of wing area.

The B24, on the other hand, had a more modern wing that was narrower and more rectanguler in shape.

The B24 could fly considerably faster than the B17 and it could carry almost twice the bomb load. But the highly loaded wing was much easier for enemy fighters to destroy, making the plane easier to shoot down.

the B17, with its much lower wing loading, could withstand an unbeliveable amount of battle damage (you should see the old pictures).

The B17 is still much revered, even to this day. But the B24 was probably much more responsible for winning the war. But you hardly hear of it.

If I were designing a cruising sailboat, I wouldn't even consider putting a short, deep bulb keel on it.

If I were designing a racer, I wouldn't even consider doing without one.

Bob

[xarax]

Originally Posted by Paul Scott

I like to think I can feel vortexes being swept away,...
Paul

I believe this was the intention of the designers too, but if it really happens, then why swept backward or forward fins is abandoned?

[Paul Scott]

I asked the guys who designed it, and if I understood correctly, it boils down to the difference between upwash and downwash. There's more, but I can't find my notes. Gutelle has some stuff on this in his book.

Paul

[Milan]

For those who are really interested in this, I would recommend Marchaj's book Aero-Hydrodynamics of Sailing, (which I have at home), he explains every thing in great length.