Skid Fins: The loads they carry

[Hydroplane Museum]

A historical look at skid fins and how they help hydroplanes turn left.

By Doug Ford
Reprinted from h1unlimited.com.

Everyone that has watched modern Unlimited Hydroplanes race has seen the so called "second roostertail" of spray that comes off the skid fins in a corner. That spray is very indicative of the tremendous loads these fins carry when the big boats enter a turn. It wasn't always like that.

Back in the '50s and '60s and even into the '70s most of the Unlimited were of the "round nose" type with the driver sitting behind the engine. These boats were generally more symmetric with both the left and right hand main sponson runners having some dihedral, that is, with the outside edges higher than the inner edges. The "skid fin" was nothing more than a small fin mounted on the inside airtrap at the back of the left sponson. These fins were only about four to six inches deep, and really didn't throw much water in a corner, nor did they carry all the side force needed for the boats to turn. Part of the side force was actually generated by the inside edge of the left sponson. These boats turned much like a dirt track sprint car, with quite a bit of yaw and sideslip.

An early 1950-1960s generation skid fin.

As the sport entered the late '70's and with the advent of the newer, pickle fork type boats, such as the '82 Atlas Van Lines, and the Juggernaut Miss Budweiser, larger skid fins became common, and the left side sponson runners were given anhedral instead of dihedral, that is, the outer edge of the left side runner was lower than the inside edge. This was done to help prevent "hooking" or catching on the inside of the runner when the boat was turning left. As a result, virtually all the side loads were then carried by the skid fin, which now was mounted more outboard and aft of the left sponson transom - pretty much as they are today. As the boat designs continued to improve, it was learned that bigger and bigger skid fins would allow faster cornering, and the fins moved further and further aft to avoid oversteer or hooking when a boat was turned sharply into a corner. It was also learned that aluminum fins were both high drag during a straightaway due to the thicknesses required to carry the larger loads in corners, and fatigue of the aluminum became a serious concern as well. This led to the large (and heavy) steel skid fins we see today.

An early 1980s generation skid fin.

What is impressive is the huge loads these fins carry today. To give an example, the first time Chip Hanauer drove a Miss Budweiser boat in testing the skid fin pulled off during one of the first corners he took at high speed. Following that, we decided to try to determine the forces involved, and actually measured the side accelerations (g-forces) that Chip put on the boat when he entered a corner at high speeds. Now, clearly, not everyone drives as hard into a corner as Chip did. Still, almost unbelievably, the side loads sometimes hit a high of 4 g's, that is, four times the acceleration of gravity. That meant that if the boat weighed 6,500 pounds, that the skid fin was subjected to 26,000 pounds of side load! And, since the skid fin was basically a "single sided" airfoil, with only the right side (inside surface) producing load, it dug a big hole in the water and shot up a huge amount of water in the air.

A modern skidfin.

Now with teams experimenting with larger and larger skid fins, the potential for the rods and brackets, and the hull itself being subjected to even larger side loads is possible. This may lead to a very unsafe condition, and something may break. Remember what happened to Dave Villwock at Valleyfield. So, technical design specifications and the required frequency of testing (magnafluxing) of skid fins needs to be looked at again. Further, the spray of water forced up in the air by these fins is becoming even more dangerous to other boats, so ABRA may have to look at the overlaps rule again, too.