dory catamaran hull

[frank smith]

In looking at catamaran hulls i see many different ways to go about their design.
I am interested in a simple dory hull form and wonder what the disadvantages of the hull form are.

I must have info right away for great project to be built in India .

Thanks Frank

[bill broome]

have a look at richard woods' site, he offers a 22' dory hull boat design.

they work just fine, and are relatively easy to build. if you must have a catamaran, his designs are good.

but in the 6-8m range, there is a lot to be said for a trimaran: the bigger main hull is more human-sized, and they are easier to put on and off a trailer.

[frank smith]

Thank bill , I'll check that out .

[guzzis3]

Waller has a range of "truncated V" hulls, essentially dories. The coral coast 25 is a very nice trailerable ocean capable design.

The dory is simple to build, not as fast as more complex shapes so not the best choice for a racer, but for cruisers they are a good choice.

Richard Woods has done a version of Gypsy without the cuddy and demountable for some clients. 28' with 6' headroom.

[rayaldridge]

Here's a dory cat that I like a lot:



I think the hullform has a lot of advantages, and fewer disadvantages than it might appear. It's easy to build, it makes for a dry boat, and it sits on a trailer or the mud flats with much less stress than a boat with rounded bottom.

Disadvantages might include more turbulence from the chine, but my opinion (and not just mine) is that the long slender hulls of multis develop less turbulence from chines than do the fatter hulls of monohulls.

The most interesting thing I've learned from sailing Slider is that she goes to windward fairly well even without her big daggerboard. I'd hesitate to ascribe this to her dory hulls, but as a data point, Bernd Kohler has had good luck with these hull forms (and what he calls "anti-vortex" horizontal fins.) Anyway, the whole thing intrigued me enough that I'm building a simple little cartop cat as a test platform-- to see how well a dory cat can sail to windward without conventional boards.

[frank smith]

Originally Posted by rayaldridge

Here's a dory cat that I like a lot:



I think the hullform has a lot of advantages, and fewer disadvantages than it might appear. It's easy to build, it makes for a dry boat, and it sits on a trailer or the mud flats with much less stress than a boat with rounded bottom.

Disadvantages might include more turbulence from the chine, but my opinion (and not just mine) is that the long slender hulls of multis develop less turbulence from chines than do the fatter hulls of monohulls.

The most interesting thing I've learned from sailing Slider is that she goes to windward fairly well even without her big daggerboard. I'd hesitate to ascribe this to her dory hulls, but as a data point, Bernd Kohler has had good luck with these hull forms (and what he calls "anti-vortex" horizontal fins.) Anyway, the whole thing intrigued me enough that I'm building a simple little cartop cat as a test platform-- to see how well a dory cat can sail to windward without conventional boards.

Ray, I have seen pictures of your cat on the web. Nice job .

The cat I have been thinking about would be a glorified pontoon boat that would carry small house about 20'X 10' , but I dont think that the displacement can be made large enough .

[rayaldridge]

Frank, what sort of displacement are you talking about? Is this boat to be powered by an outboard or by sails?

[frank smith]

Originally Posted by rayaldridge

Frank, what sort of displacement are you talking about? Is this boat to be powered by an outboard or by sails?

It would be a power boat , not fast ,12kn would be ok . Disp. looks to be about 8000lb

[rayaldridge]

Frank, just for fun, I blew up the hull I'm currently drawing to 34 feet and a cat built of those hulls can handle 8000 lbs. The hulls would still be 12 to 1, so a relatively small outboard ought to be able to push them along without a lot of fuss. Of course, these are sailboat hulls, not powerboat hulls, but the point is that dory hulls can carry a surprising amount of displacement. That's one reason I chose them for my 16' beach cruiser. Using another chine to approximate a semi-circular section would have lowered my displacement significantly, though the boat would probably have been a little faster.

[guzzis3]

Ray, damian here, we have corresponded on yahoo and other places.

Frank, the dory has a slightly larger wetterd surface area than a rounded hull. This matters little and the chine turbulence matters not at all on a power boat. It's a good choice.

[trev0006]

This looks gorgeous.

Originally Posted by rayaldridge

Here's a dory cat that I like a lot:



I think the hullform has a lot of advantages, and fewer disadvantages than it might appear. It's easy to build, it makes for a dry boat, and it sits on a trailer or the mud flats with much less stress than a boat with rounded bottom.

Disadvantages might include more turbulence from the chine, but my opinion (and not just mine) is that the long slender hulls of multis develop less turbulence from chines than do the fatter hulls of monohulls.

The most interesting thing I've learned from sailing Slider is that she goes to windward fairly well even without her big daggerboard. I'd hesitate to ascribe this to her dory hulls, but as a data point, Bernd Kohler has had good luck with these hull forms (and what he calls "anti-vortex" horizontal fins.) Anyway, the whole thing intrigued me enough cars that I'm building a simple little cartop cat as a test platform-- to tires see how well a dory cat can sail to windward without conventional boards.

[rayaldridge]

Originally Posted by trev0006

This looks gorgeous.

Thanks. It's a pretty place, at the east end of Perdido Key, just south of Pensacola. It's really only accessible by boat, since the eastern end of the Key is National Seashore. At night, facing the Gulf, you can't see the loom of Pensacola's lights, and the sounds are surf breaking on the Key and the clang of the bell buoys that mark Pensacola Pass.

I wrote about that trip here:

http://slidercat.com/blog/wordpress/?p=64
http://slidercat.com/blog/wordpress/?p=72
http://slidercat.com/blog/wordpress/?p=73

And a little video I shot when leaving that beach:

http://www.youtube.com/watch?v=VHjY5RKyTbQ&feature=user

[Chris Ostlind]

Originally Posted by guzzis3

...the dory has a slightly larger wetted surface area than a rounded hull. This matters little and the chine turbulence matters not at all on a power boat.

Just curious, Damian.... I know of the wetted surface issues, but how do you come to the following conclusions?

Would not all forms of drag have an effect on the performance/efficiency of an immersed hull. Wouldn't the added drag signature demand more fuel usage and/or a larger engine?

It would seem that if one were shooting for a high efficiency design, for instance, that all of these issues would jump straight to the top of the list.

[Guest625101138]

Originally Posted by Chris Ostlind

Just curious, Damian.... I know of the wetted surface issues, but how do you come to the following conclusions?

Would not all forms of drag have an effect on the performance/efficiency of an immersed hull. Wouldn't the added drag signature demand more fuel usage and/or a larger engine?

It would seem that if one were shooting for a high efficiency design, for instance, that all of these issues would jump straight to the top of the list.

Chris
There is negligible difference between a round chine and squarish hard chine from a drag perspective at displacement speed. As the speed goes up the better lift of the flat sections on the hard chine usually reduces drag compared with the round chine.

The hard chine will have smaller max Bwl for the same displacement and length. This lowers wave drag and more or less offsets the slight increase in wetted surface.

There will be unique aspects to each case but generally there is very little difference in drag between round or square chines.

Rick W

[Chris Ostlind]

Originally Posted by Rick Willoughby

... As the speed goes up the better lift of the flat sections on the hard chine usually reduces drag compared with the round chine.

Depending upon actual hull shape, boat weight, etc. and the potential angle of attack to gain significant lift.

Originally Posted by Rick Willoughby

There will be unique aspects to each case but generally there is very little difference in drag between round or square chines.

The unique aspects would seem to be far more than unique when it comes to a preference in hull shape for efficient, power cruising designs. Just casting about in the fleet of boats about the same size and purpose, you do see flat bottomed power cats, every once in awhile. They are not typically meant for displacement speed motoring. All of the really good displacement and semi-displacement power cats of this size and smaller, are more in the rounded chine environment, if not outright smooth hulled, much like those seen on a Tornado cat. This is especially true for the boats that are aimed at good, semi-displacement speeds and minimum engines.

Now, if the argument is ease of build, especially with commonly available materials then there are a whole bunch of new considerations and dory-esque designs start to make some sense. Others may jump-in here with their favorite build technique, so that discussion is pending.

Drawing upon the conversations I have had with Kurt Hughes, who has a considerable amount of experience with boats of this type; His suggestions are, if it's in the 30 feet LOA range, then the Tornado style hulls make the most sense. I'm seeing power cats with all kinds of different hull strategies above that size. A leisurely cruise through all the obvious designers will show a wide range of hull solutions that are supported by customer feedback and endorsements.