As promised, here are my observations after about 860 hours of cruising; Return trip from Lake Ontario to Florida Keys, and the Down East Circle Loop (from Lake Ontario, around Nova Scotia and back north at New York City). Although we try to avoid rough seas and storms in general, we had a good mix of weather conditions during the past year.
First, I have no regrets having done the extension.
The extra buoyancy in the stern improved load carrying capacity, returned vessel to level trim with dinghy (engine plus fuel), spare anchor (original set from PDQ), bicycle and kayak all at the back. It would be fair to say that Kama Kat is a cruising boat, perhaps heavier loaded than her sister vessels. Impact on boat handling has been positive in a minor way. Bearing in mind that water line length was only increased by less than 4.5%, no miracles were expected.
By shifting more weight astern within the boat also, and virtually running with empty bows (except for the generator) I could say that we rose easier/quicker to the waves and pitching dampened out a little more quickly when rocked by ugly wakes. This is a subjective assessment, but I believe it is not overstated.
As you have probably observed, our hulls start to squat down at the stern around 7kts and don't rise until about 9kts, making this speed zone "to be avoided" since most effort goes into wake/wave generation as opposed to forward speed. On both sides of this "no-go" zone, speed and engine RPM graphs follow a fairly straight line where we may cruise at will. I was hoping to minimise the "no-go" speed zone through the hull extension, by delaying the onset of hull squating and promoting an earlier rising. The "no-go" zone has been reduced by about 0.2-0.5kts at both ends (depending on sea and wind conditions) eg. hull rising at 8.5kts vs 9kts under favourable conditions.
Effectively, wake generation has been reduced (lower wave heights) and fuel consumption has been reduced.
When we compared our fuel consumption in 2008 with the previous year (on the same run), under similar conditions, we noted an improvement in the 4.5-10% range. We mostly cruise at about 7.5kts ....occasionally "fast cruise" at 11-13kts.
If I were to do this again, I would have the top of the extension about two inches higher to prevent swim ladders from being washed around 7kts.
