Swept back hull appendages. A thing of the past?

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?

 

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.

 

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

 

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

 

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

 

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

 

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.

 

I’m curious; do you have pics of your boat with the new rudder ( out of the water, of course)? I’d like to see your set up. I’m doing something similar, and I’d like to see how you worked out some elements of the design; most notably protection from crab pots and the like.

 

The keel is swept forward, not the rudder. Sorry for the confusion. Crab pots are fairly easy to deal with, since lines, kelp, etc tend tbe swept up to the hull, where it’s easier to deal with them with a whisker pole. Once in a great while we get junk on the rudder, but again, it’s usually up by the hull. The rudder is vertical, and the tip is swept back a bit, WaterRat build. I’ll post pics if you’re interested.

 

Ahhh ok. Thank you. I appreciate that. The part about dislodging things that get caught on the foil is especially useful.

I’m doing an 8 degree forward swept rudder to help with the ventilation problem that can happen with transom hung rudders, and that was a major concern for me.

 

FWIW, consider the section design of the rudder- IIRR, our WaterRat rudder (in the pic above) is based on a 63 series shape, and has never spun out, or felt like it was ventilating (tiller feedback, and in big air & chop the rudder root sometimes sees air). There are some good threads on windsurfer skeg ventilation (spin out) that might be useful, as well as some dinghy and multihull rudder strategy threads- for example, tucking the leading edge under the transom. Personally, I like transom mounted rudders because I can see the rudder, and remove it easily with a cassette style case, which helps in clearing stuff, as well as using the cassette shape as an end plate, if that appeals. You might call WaterRat (If he’s still in business, I haven’t looked in a while) and see if he’ll make you a rudder?

 

I agree about the benefits of transom hung rudders. Thanks for the pointers. I’ll look into them. My plan is to have the rudder in line with the transom ( which would amount to an 8 degree of forward sweep ) and back cut the transom portion of the leading edge of the rudder; creating a semi balanced rudder, placing the leading edge of the rudder under the boat, and giving me the benefit of upwash.

I have a Pearson flyer rudder as the base to modify to make this rudder. I want to make sure I have as much info as I can find, before I start modifying it. I don’t want to cut it and then realize I made a mistake in my design. Thanks again for your input. It’s definitely helpful.

 

One thing I left out was the tendency of swept forward foils to twist, which means some attention to the engineering of structure and attachment points, as well as section shapes that aren’t particularly AOA sensitive. Gutelle points out that 3 degree forward (or backwards, for that matter ) sweep doesn’t mess with L/D efficiency much. IIRR, we have something like that ( you can get a guesstimate from the pic?), and most wise heads in the biz agree it’s giving us some upwash, at least, on the keel.

 

A transom hung rudder that projects a short distance forward of the pintle axis isn't too far from the mainstream.I would suggest that you search for a photograph of a similar boat sailing with a few degrees of heel before getting too firmly attached to any ideas.I have seen numerous images of boats that seem to heel more on the lee bilge than pivot around the centreline and this has the effect of lifting the upper portion of the rudder clear of the water.I have major doubts about the extent of any upwash that may be generated,beyond the general level of that in the flowpath around the hull and a shallow,low rocker hull may not have much at all.It could be educational to attach a GoPro to the transom and tape a few dark coloured tufts to the rudder to get an idea of what is actually happening.If the leading edge of the rudder hardly lifts at all when heeled,it might be a simpler solution to add a basic fence in a location that remains immersed.Otherwise a longer rudder might be another solution-subject to depth of water.

 

that does beg the question of where the upwash washes up to on the stern hung rudder- just on the leading edge? On the whole foil? If the upwash is mainly on the leading edge, can envision it climbing up on the leading edge, and collapsing into a trough mid section leading to
ventilation, to say nothing of a wave knocking the rudder into ventilation. Depends on the foil- one strategy among windsurfers is to have the surface piercing part of the skeg a different section that doesn’t develop as much lift, and that seems to tame ventilation.
Interesting read on Fins | Windsurfing Forums, page 1 - Seabreeze https://www.seabreeze.com.au/forums/Windsurfing/General/Interesting-read-on-Fins?page=1