Pitching motion

In my comments on the New Hampshire races, I had mentioned that the pitching motion in waves may be amplified by the effect of the pendulum formed by the deep keel. The pendulum frequency should have a period around 1 second. I have since measured the pendulum period on 2 finished boats, my 3-Liter Torpedo, and my Modified Bob-About 2. In both cases, the period was about 0.6 seconds, which is significantly shorter than expected from the pendulum effect. This made me remember that the effect of the flotation in the ends of the hull adds a great deal of stiffness to the pendulum, increasing the frequency and reducing the period. The proper calculation of the frequency is based on the angular momentum of the hull (including the bulb) and the overall pitch stiffness (including pendulum and flotation). I didn’t attempt that calculation, as the actual measurement will suffice, and is free of calculation errors.

I have also attempted to minimize this effect by adding a horizontal vane to the bottom of the rudder. The vane was fairly large (about 4 inches x 1.25 inch). It has been tested in the bathtub, but appeared to have no effect in damping the pitch oscillations. It may have a secondary value in preventing nose-diving, as Andy T has claimed, but has not yet been tested for that. It will probably have a cost in windward performance. It also creates problems in transporting the boat, as it is very vulnerable to damage unless the rudder is removed.

Don’t forget that the rig probably makes a significant contribution to the moment of inertia in pitch as well.

A reasonable way to measure the moment of inertia in pitch of the entire boat is to find the balance point when it’s sideways, and suspend from a wire of known size and stiffness. Then rotate it and see what the frequency is. I’ve done this for model aircraft. A variant is to suspend on two strings. An additional complication much harder to measure is the “added mass” effect of the water next to the hull. However, I think there are some empirical formulas for this.

I think this may be another place where the lighter displacements would be superior. I would guess the damping and increased resistance from the hull in the water is related to the area of the waterline, i.e. if you took a slice of the hull even with the surface of the water. I am not sure, but the term waterplane comes to mind. I have no idea if that’s correct.

Walt and all - I tried three different horizontal foils on the end of the rudder (t-rudders) to try to resist nose diving and downward attitude while before the wind. I started out with one the size of Andy’s and the last one had roughly the same area as the rudder itself (and anyone who has seen my boats know that I like big rudders).

The smaller two didn’t effect either the nose down attitude or the pitching motion to any noticeable degree. The largest one sort of worked but kicked up a massive wave following the boat. The boat didn’t seem to pitch as much but it also seemed slower. When testing this big foil in stronger winds (10 mph as opposed to the more moderate days of 5 and under) the boat would start to dive and the big foil would act like a brake which in turn put the boat up on its nose and spin it around to windward.

I know that T-rudders work on r/c F-48 tris and on full size hydroplanes, but I feel that the length of the Footy doesn’t have the leverage to resist the forces generated by the sails. As to pitching motion, that is more pronounced when there is little or no way on. In this case a T-rudder will just move with the boat and won’t have any dampening effect at all.

Maybe Andy has had more luck than I but for me T-rudders on Footies are a waste of time. Better to focus on hull form and sail design to minimize these inherent characteristics of really short boats.

Hi everyone
Yes you are right I did try foiling rudders on two boats
The first was on my very 1st Bottle Footy that kept thinking it was a submarine and thoroughly enjoyed spending most of its time with its nose buried up to the mast step.

Due to the bulbous bow it just kept going and was easily as fast as Trevor’s Minstralette across the wind 18 months ago.

At Cheddar Reservoir the sailing club have a proliferation of foiling Moth racing boats that come right out of the water on two foils

Seeing this gave me the idea that I could possibly get the back end to kick down.

Having a carbon batten for a rudder it was quite easy for me to add a foil to the base ~ I have to agree that it easily gets damaged

However when last sailed and viewed from directly astern the effect of the foil could be seen working on the stern creating a bobbing action.

I tested wings on rudder and on fin a few years ago. I found the same results as Niel. While some reduced pitch, they slowed the boat on the beat. I decided, as Niel, that it was better to design a boat that was fast on the beat, and reasonable (doesn’t broach when the bow dives) to control on the run.


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