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I found an interesting site - pdq36.blogspot.com - where someone added hull extensions to a PDQ 36' cat. The idea seemed great from my point of view, in that a cat will probably be in my near future, and I don't want/need anything bigger than 36' for 2 people, but would like to have extra carrying capacity. The idea of hull extensions is very appealing, since the extra weight would be on the stern, with a generator and dinghy plus misc. hanging from the stern. What I don't know is how that would affect the design and performance of a cat. And if it would be so much better, why wouldn't the factory have done this? Is this a viable idea that could be used on any cat?
 

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It'll probably have some adverse affects, like reducing the effectiveness of the steering, since it effectively moves the rudders inboard a bit.

It may increase the chance of the boat pitchpoling, by effectively moving the center of buoyancy aft and effectively moving the center of effort forward.

It may cause problems with light air performance by increasing wetted surface area drag.

It may cause problems with lee/weather helm, since it is effectively shifting the center of lateral resistance a bit aft.

If the idea was really that good an idea, don't you think they would have done it already??? BTW, it will increase the cost of haulouts and marina slips, because you're effectively making the boat longer.
 

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Manta has done this with a fair amount of success. The Manta 42 actually started out as the Manta 38, then became the 40 and now the 42. You can tell by looking at the boat that the pontoons are extended farther aft than what you see in many brands. I don't think the only thing they did was to extend the hulls, but that was the major thing.

I don't have enough multihull experience to comment on SD's points, but we have very good friends who are cruising long term and fairly far afield in a Manta 42, and they report that the hull extensions actually reduce hobby horsing and do increase load carrying capacity because of the added buoyancy. In terms of close quarter maneuvering, I would think it wouldn't matter because you'll be doing the twin screw thing anyway, but again, I don't have personal experience with that.

The aesthetics are something that you either will like, you won't, or you're willing to deal with because of the practical benefits. The Mantas definitely have their own look, which is saying something in a catamaran world where the boats look quite similar across all lines.
 

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Seawinds 1000 became 1000xl, then 1160 by adding extensions on the back that also smoothed the water flow off the originally chopped off (straight drop) stern.
Performance was increased significantly. Too bad the rest of the boat design made it a non-starter in my view.
Seawind Catamarans
 
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