Boom brakes on cruising catamarans
Steve, I have used a boom brake for 5 years I manufactured myself on a 30'' monohull. I used a section of a 3" diameter spinnaker pole I cut down to fit my J measurement. I used a pattern off of one I found at a Boat US store. It worked very well on that boat. Transferred it to my newer 32'' and find that with the larger mainsail it is not giving the same control!
Several things to consider:
1 - The diameter of the drum.
2 - The number of turns on the drum. I had used one before and now have increased to 1& 1/2.
3 - The line type. A hard finish line does not have the grip that a soft finish gives.
4 - Line size controls the amount of surface contact the line gives for each turn.
5 - The amount of tension you place on the line when in use. Before, I could just pull it snug by hand, now I have to really bear down on it.
6 - Distance from gooseneck where it is attached. The further away from the gooseneck, the greater the lever arm you have. On prior boat I used the chain plates as attachement points for end of line and block lead to cockpit. Placed fitting for brake directly over chain plate. On new boat I used inside sheeting track and this does not give the leverage I need.
I guess the answer to your question is that YES they can be very effective. I have done jibes in 25+ winds without any adjustments to the main sheet. However, getting it set up and tensioned to control takes some effort to find the correct variables. A commercial brake probably answers many of these variables I have addressed. Suggest reading installation directions carefully before purchase to be sure they give answers to above variables.
Good luck and let me know if this was helpful.