We had looked into these units when Swan Song was earlier in the stage of re-construction. At the time, according to the factory man in Japan, we'd need two of the mid-sized units to keep Swan Song from rolling less than 25 degree each side. If a sea state was at the level that we were rolling this much the power required to keep them both spinning was just over 8 KW. This all according to them. In a really nasty sea state they would need to be turned off as if they hit the stops often and with too much force they might self destruct. Suffering and surviving a knockdown wasn't in their vocabulary! All in all we opted not to go in that direction as the predictability of sea state isn't within our ability in making a passage. As most of you know we went the route that few have taken and for the life of me I don't know why. An Anti-roll tank, ART, is our only roll reducing system on Swan Song. A totally passive device with no maintenance to date, 5 years, and only one moving part, water. I wish I could say exactly what the numbers are for roll reduction but I can't. What I can say is that in 15,000 odd nm we have never rolled more than 30 degree and perhaps on 20-25. This includes a tough patch coming into Hawaii last year in waves that were occasionally in the 30' range. We can sit with no way on in 8-10 ft. seas with very little movement. So little that you can work on things and not be chasing tools all over the place. This is a very comforting fact especially when you have to shut down the engine for daily checks at sea. For some reason Naval Architects hate them. They are afraid that they will somehow encourage a capsize or something similar. We have found all of this is bunk. Swan Song is stiff and has a high righting moment but it is round bilged and likes to roll and once started would go on for 12-15 rolls as a minimum. We tested all of this with full inclination tests, etc. Our boat parameters were fed into the computers that Dr Bass up in Newfoundland runs with his software and came up with the design tuned to our boat. $10,000 later it was constructed and put in place on top of Swan Song's Pilothouse. 1500 lbs. of water over our heads ;-) Everyone who has ever been aboard either on the hook or underway is astounded by how Swan Song behaves in the water. At first they are puzzled and then as they watch other boats nearby whether they are sailboats with the masts waving in the sky or power boats showing lots of bottom paint they realized that Swan Song doesn't hear the music that the other boats are dancing to. As Seahorse John (now departed) once said in Bequia after an evening on board with the ferryboats passing 50' away without spilling the wine, "I thought you must be aground as you aren't rolling like the others". Then as he watched us round the point heading north out of the harbor the next day waiting for the first African tradewind swell to pin us down Swan Song just went on her merry way. "Damn I got to get that system". Bob Phillips, Another Asylum, has the same system done by Dr Bass. He is the one that sold me on it. Check with Bob as I think he'll compliment just about everything I've said. I can't compare if paravanes would be as good but I don't think so especially at slow speeds, stopped or in shallow water plus they do require effort. Active fins are nothing but trouble long term and because of our size, propensity to roll, and slow speed the size of a system for Swan Song is large and in the order of $75K with all the attendant maintenance over the years. We would already be due for an overhaul. So for us the option was the paravanes or ART. Glad we made the choice we did and have never looked back. Also as to significant wave height measurement, sig wave height is the average of the highest 1/3 of the wave. The actual wave height can be as much as 3 times that on occasion and from my experience usually is. Our height of eye is 12' off the water sitting in the pilothouse and beam waves up to this height aren't comfortable but offer no problem on the beam. Even an occasional crest coming aboard just tosses you sideways. Bigger waves that this and we are either taking them further forward or further aft. Dead down we are fine even in that 30' stuff near Hawaii. 20-30 degrees off our stern is our weak point. We get both the pitch and a roll so the corkscrew motion gets uncomfortable pretty quickly As always YMMV..... Dave & Nancy Swan Song Roughwater 58 Limin in Waikiki Beach