Not a new idea by any means; Tee section booms were used on the old J class yachts after all.  There is one fitted to an IOM sailing at our club, but I cannot recall seeing others. 

The idea is to form an end plate for the sail; as in aerodynamics this increases the effective length of the sail / wing.  My boom is  made from a cut down Sails etc section with an alloy plate glued on top. The existing clew fitting can be used as shown. The endplate is not 100%, being limited by the 20mm diameter rule and is therefore approx 10mm off the boom CL. The gauge is a piece of CF tube 20mm ID to show compliance with IOM Rule F.4.5. The yellow tape just allows it to be seen better. The pic shows the sail on my Alternative with the foot eased out for clarity.

As I say, nothing new here but was wondering if anyone else has quantified it's effectiveness.  I do not think it worth using for the jib boom however. There are several factors that may negate any advantages here but, once again, there may be other opinions.

Endplate effect on sails is common for speed sailing. Windsurfers, yachts, multihulls etc; often using the deck or the boom.

Richard

Good to see someone trying different ideas. I would expect the difference to be marginal with a loose foot

In the J's any crew hoisted to the masthead to sort any rigging issues often jumped into the main and slid down-  not sure if a T boom is worse to land on than conventional!

I thought the J's (Enterprise) used  a "Park Avenue" Boom, an equilateral triangle section with a flat side on the top?

1 hour ago, Guzzilazz said:

I thought the J's (Enterprise) used  a "Park Avenue" Boom, an equilateral triangle section with a flat side on the top?

According to my book on the America's Cup; Enterprise (the first J Class Defender) had a Park Avenue boom (so called because two people could walk side by side along it), whereas Shamrock 5 had a conventional boom. The primary reason for the PA boom wasn't as an endplate but rather it had adjustable slides to allow the foot of the sail to be adjusted to the desired curve. Endeavour 1 & 2 both copied this idea but both the respective defenders (Ranger & Rainbow) used a thin boom but with struts and wires so the entire boom could be bent to a curve.  (I'll get my anorak).

To put my other anorak on;  the effect of fences (and winglets) is to make the wing (sail) behave as though it had a larger aspect ratio i.e. it discourages air spilling over from the high pressure side to the low pressure side, which in turn causes an energy sapping vortex.  To be effective the boom would need to seal the gap at the bottom of the sail. Look at the current crop of AC boats and you will see that the mainsail touches the deck, which is made flat for just this purpose.

It's only my opinion but I think a loose footed sail that has a constant aerodynamic shape all the way to the foot is better than a sail that is constrained to be straight along the foot with an added endplate.  Sod's law of course applies and you want the endplate to be widest at around the point of maximum depth of the sail on the leeward side so a loose footed sail would need a very wide endplate to have much effect.  Given the 20mm rule it can't be very wide just where it needs to be.  Unless you have access to a wind tunnel or some fairly sophisticated CFD programs then I suspect the effect won't be large enough to notice.  Still you wouldn't be the first person to prove me completely wrong.