Draft

Just to keep the creative discussion going!

The Footy rule allows very deep draft for the size of the boat. Is this worth exploiting?

Given that the weight of an extra inch or so of fin is negligible, the obvious reaction is to use as much draft as you can get - bigger righting moment = more sail-carrying power = more speed.

On further reflection, this may not be the case. Putting the bulb lower down increases the fore-and-aft moment of inertia - which is tantamount to saying that the boat will hobby-horse more. Increased power to carry sail is offset (at least partially) by decreased efficiency of the rig.

Furthermore, lower ing the bulb lowers the centre of forwad resistance. If we incease sail area to take advantage of the increased righting moment, we allso raise the centre of effort - so the tripping couple (the force causing the boat to submarine) is increased very substantially. Downwind performance suffers - and not just because of the additional drag of the longer fin.

Anyone have any thoughts?

On the other hand, just a wild thought that comes to me as I type… The longer fin should give more lift for less drag going to windward. What happens if we put the ballast in the fin rather than in a bulb? Or mount the bulb at some intermediate point on the fin? These are at first sight both thoroughly bad ideas - but are they?

A.

Angus wrote >
>The Footy rule allows very deep draft for the size of the boat. Is this worth exploiting?

Yes

>Given that the weight of an extra inch or so of fin is negligible, the obvious reaction is to use as much draft as you can get - bigger righting moment = more sail-carrying power = more speed.

Agreed

>On further reflection, this may not be the case. Putting the bulb lower down increases the fore-and-aft moment of inertia - which is tantamount to saying that the boat will hobby-horse more.

I disagree. Just the opposite. Putting the lead on a longer fin gives a slower rate of inertia. Hobby-horseing is caused by wave action. A slower rate of inertia should resist the wave action. Hobby-horseing should be reduced.

>Furthermore, lower ing the bulb lowers the centre of forwad resistance.
If we incease sail area to take advantage of the increased righting moment, we allso raise the centre of effort - so the tripping couple (the force causing the boat to submarine) is increased very substantially. Downwind performance suffers - and not just because of the additional drag of the longer fin.

The higher rig and therefore higher centre of effort will cause the boat to submarine sooner. But if this boat is for racing, I would take that risk for upwind performance gain.

>On the other hand, just a wild thought that comes to me as I type… The longer fin should give more lift for less drag going to windward. What happens if we put the ballast in the fin rather than in a bulb? Or mount the bulb at some intermediate point on the fin? These are at first sight both thoroughly bad ideas - but are they?

Yes! Put the weight as low as possible. Any additional area below the bulb is needless skin friction. Also drag would be increased due to the second fin/bulb interface.

IT WILL PRODUCE SOME LIFT WITHOUT INCREASING THE RADIUS OF GYRATION. HOWEVER, I DID SAY I THOUGHT IT WAS A VERY BAD IDEA!