## Lengths and Surface Area 

## Posted By: KristenPosey, Sanderling (Anaval architecturelesson) 

Lengths vs. Surface Area are two totally different things. Length in a unit, surface area is units^2, and happens to cover a volume of units^3. You add 8 feet, you add a whole lotta boat, and not just in length. 

The Outward Bound30s (OB30) are probably more robustly built by far for their intended mission. Not that Graham's a cheapskate, and not that I've seen his SS, but what I mean is the OB30s are for passengers, they're open, and they're supposed to take people away from land. The Southern Skimmer (SS) is more optimized for the EC challenge. The stresses and things asked of her are different. As much speed as possible. The OBs are focused on keeping crew safe and providing an ok experience with fairly minimal accommodations. For SS, an enclosed space provides some floatation as well as some closed structure, which the OBs don't really get to have--they have some enclosed spaces for floatation, gear, and such, but it's an open open boat. 

Now, could you take an OB30, re-write its mission, and come up with a boat that has sorta similar looks and sailplan, but more optimized for the EC? Oh heck yeah. She'd be lighter, the lines would change a bit--Southern Skimmer more approximates a sea sled than a long slender canoe, doesn't she? We could build our redesigned OB30 from carbon and Kevlar. Take out some of those seats, and simply raise the watertight sole and compartments some for increased floatation and storage for gear low in the boat. As for those masts, you might just get those carbon too, if you had the $. (And, uh, I prefer wishbones, but that's me.) 

Go back and look at the GA (General Arrangement, the sheet that shows the boat drawing from the side with the sails up) for the OB 30. In the Prinkers (me-speak Principle Characteristics) you'll find some intruiging data. 

First, LOA, LWL, BOA, BWL. Ignore the LOA and BOA for now, that just tells you what kind of dock size you need. LWL and BWL are Length and Beam at the Waterline. That's the meaty information. Length to Beam ratio for the boat is: approx 4.4. She's a long slender gall. Don't have the numbers on SS, but you can do the division if you do. Different missions for different boats. 

As for the weight... there are three weights given. Lightship is the boat, no gear, no fluids, no nuthin. Just the boat by her ownself. Running off the beach in a race, you'll have packed lightly, you'll not be on the boat, you'll be out pushing, so the push off the beach weight would be whatever your light gear is + the lightship, since your own butt comes later. They then show a 1/2 Load and a Full Load. This is people and their stuff, tankage, gear, toilet paper, toothbrushes all on the boat. 

That's quite the cargo. They mean to carry a lot of folks, and food for all those people for a week. 

Now, there are some other totally cool bits of information here. Prismatic and Block Coefficients. They're "form characteristics" that tell you things about the hull shape. The Prismatic Coefficient is, specifically: Cp = Volume of Boat when loaded to design Waterline /(Waterline Length * The cross sectional area of the boat at its fattest point). Basically, if you made half "cylinder" with the boat at its fattest point, the length of the waterline, what percentage would the actual volume take up of that. Then there's Block Coefficient: Cb = Volume of Boat/(Length * Beam * Draft) (That'd be draft without keel or rudder, or canoe body draft). That's drawing a box of the outer dimensions of your submerged body and determining the percentage of that volume taken up by the real body. These numbers are a little hard to make useful till you've been using them alot, but the best thing to do is make a list of boats, and start comparing the values. 

Wetted Surface Area. That would be, the square footage of an imaginary blanket covering exactly the submerged bits of the boat. You could, using a finite analysis, calculate out what that would be, approximately, if you had the lines plan to that old thing. Fortunately, engineers are lazy as hell and don't feel like that most of the time, so for different hull types we have different approximating equations that we use for the preliminary stages of design. Here is a wetted surface area equation for... a trawler, which won't work for you (um, my sailing books are not at work, mostly, and I don't have my thesis hard drive here at the moment, but I'll see if I have a better equation later) S = (Volume/Beam) * [(1.7/Cb) + (B/T) * (0.92 + (0.092/Cb))]. But that's for a hippo of a boat, not the sleek ladies we're talking about, so don't apply it for anything but hippos, and always remember that it's an AOF (Approximation from some Old Fart, that's another personal term), so take it with a grain of salt. (Equation from Schneekluth & Bertram) Generally, there are many grains of salt and the whole consideration of vessel design is black magic. 

Now... what else can we glean out of this... oh. Displacement Length Ratio. Ratio's a dumb word because it's not just a simple division, since they take it down to the same units first. It is: Displacement (in long tons) / ((.01 * LWL)^3). A long ton = 2240 lbs. It's a naval architecty thing. Now evil minded person that you are... since you have that "Ratio", what would the approximate new weight of the OB30 be, if it was the same length as SS? 

