32' cruising cat-ketch from scratch.

[Seafarer24]

I want to design, build, and cruise a Cat-Ketch sailboat. I have attached two pictures of the design thus far. Yes, I know it needs a lot of work.

Design parameters that I am working with...
Loa: 32'
Beam: 10'
Draft: 4'6"
Displacement: 10,000 lbs. Hoping for 50% ballast ratio.
Headroom: 6' minimum - 6'6" maximum.
Sail Area: 600sq. ft. equally split between two equal-height freestanding spars. Likely a 400sq. ft. reacher set off the mizzen for long downwind runs.

Additional features... ~18" wide side decks with 6" tall bulwark. Bow well at the front dips down another 6" for a 12" tall bulwark.

The first area that I see that needs work is the bow, I should have drawn it with the bow ~1' below the waterline.
The second area that I see that needs work is the rocker, it has too much displacement forward and will float bow-high.

However, I'm not really sure how to draw around an intended displacement, or how to draw to make the boat sit level. This is one of the reason there is no bow/stern view.

I put the max beam at 50% of the length, put the center-of-effort of the sails in line with the max beam, and am trying to get the center-of-displacement there as well. I am hoping this would produce an easily balanced boat, and one that would handle well when reefed. I also wanted the increased interior room. How much will this effect the speed of the boat? I know that the further aft the maximum beam, the longer the water flow will remain attached and the faster the boat should be....

There is no provision for a typical engine. I want to use a small diesel-powered generator which will power a pair of electric motors located inside the keel, each spinning a 2-bladed folding prop (one above the other).

In the deck plan, the black lines are for travelers (if I use traditional booms instead of wish-booms). The sole purpose of the "radar arch" is to move the traveler out of the cockpit (I hate shin-knockers). However, it looks like it might be a useful support for a bimini too. I'd at least try to make it look like a boom crutch or something classy, I promise.

I plan to design an 8' dingy to fit the forward cabin-top area, between the main-mast and the main-traveler. That is in my "Design-a-dingy" post.

[lewisboats]

What kind of boat design library have you got...(hint hint)

Steve

[Seafarer24]

"How to design a boat" by John Teale
"Elements of Yacht Design" by Norman Skene
"Cruising Sailboat Kinetics" by Danny Greene

It seems to me that they design a boat and then figure out what it's draft, displacement, center of bouyancy, and center of lateral resistance is. I want to know if there is a way to plot out where you want these to be, and then design around that.

[lewisboats]

Normally folks know what they want in accommodations and this is the starting point...along of course with the general style, length, beam and draft they are looking for. Adding the other stuff into the mix one at a time generally causes you to rework the prior stuff a bit to get things to mesh. It is called the design spiral. You normally wouldn't pick Displacement as your starting point because that is a pretty flexible criteria and basically for a yacht...why? Displacement as a starting point would be good for something designed to carry a specific weight of cargo. Same with LCB and CLR...those are a function of hull shape (which would need to be defined first) and positioning of appendages which are also relatively flexible and can be worked arount things that are more important. Accomodations, Length, Beam and Draft are the basic starting points to which the other elements are derived from and to an extent can be modified by these elements to come to a balance of compromises. It is all a matter of what you want the boat to be defined by...and most yachts are defined by the interior space and the comfort of the passengers first, then you wrap a shell around that space and start forming it into a boat shape. With that basic down, you start working with the #'s and adding your other criteria and start on the balancing act of getting all the compromises to an acceptable point.

Steve

PS: which edition of Skene's...The most cited would be the one revised and edited (basically rewritten) by Francis S. Kinney...this is the one to have.

[sharpii2]

If you want to have your Center of Boyancy (CB) at 50% of the waterline, you have to make the area moments on either side of the half way point equal. (You do know how to do areas and moments, don't you.)

First, you draw the hull as you imagine it will look. Draw the profile, then draw the main section, paying especial attention to the underwater part.

Next, fair waterlines from that center section into the bow and stern. This will give a frame work from which the other underwater sections are extracted.

Extract them, find their areas and moments from the half length mark. The bow ones should equal the stern ones. Just remember that the water has to get around whatever shapes you draw easily. With dilligent work, you can come very close to be right on the money.

If they don't, either the bow or the stern can be trimmed or fattened to decrease or increase the areas of the sections there, especially at the ends. Its best to do this work with as few drawing tools as possible. You will be erasing a lot! I use a scale ruler and a cheap 0.7mm lead holder

HINT: Having the top veiw of the stern look pretty much like the top veiw of the bow really helps a lot. (it also helps in maintaining the same CB as the boat heels.) But, as with any other design feature, This one too has its drawbacks. A boat drawn this way will not have as much sail carrying ability as a boat drawn with a fine bow but really wide stern, so don't expect to be the first one around the cans with a boat like that. Increasing the top veiw fullness of both the bow and stern will improve sail carrying ability too, but may make the boat harder to push through head seas.

Hope this helps.

Bob

[Seafarer24]

You can see from the second image that I have an interior layout drawn up. This did provide the basis for some of the hull shape.

The stern is 6' wide, at least at the top, and it will be pretty wide near the waterline as well.

I don't even know what you mean by areas and moments. Unless by areas you mean underwater surface area, and by moments you mean weight x distance from the center of bouyancy? I'd imagine that I would have to get the boat 3D before doing these? Right now I just have an overhead and profile drawing, and the profile drawing needs revision.