Maximum beam ?

Max beam

All the things that you ask about are variables in the design of the boat. Using the published data and your experience you select the max beam and the position of the CLR and CE etc. There is no hard and fast rule about the location of these things. Some things in design are easy to select, such as the PC. It is selected to be optimum at the chosen speed. There should be no augment about that number. But the overall shape of the boat is subject to many discussions and augments about what is the “best” shape. The bottom line is there is no one best shape and it depends on many variables and opinions about the use of the boat and how far you are willing to go in terms of safety and comfort. Look at boats that have the style that you think is best and use those numbers as your starting point.

 

For Mr Coen

Vendee Globe boats are not unbalanced designs. The 60 feet monohulls are designed for singlehanded sailing in seas you can't even imagine and are driven harder than a common sailboat will never (happily) be.

If these designs were unbalanced, it would not remain any survivor. These very large planing hulls try to optimise downwind perfs in South Pacific with high winds and heavy seas as well as in Atlantic with lighter conditions and conserving good upwind perfs. A very difficult design exercise.

When heeled the underwater shape is balanced, and be not fooled by by the pic of the boat. The ballast can actuate very efficientely combined with the keel giving an enormous righting moment. If the boat was unbalanced the skipper could not work on the bow while the automatic pilot is making its job.

Idem for downwind sailing, the boat must be very stable as skipper has to work at the mast like sending alone a few hundred square meters spinaker in a 30 knots winds.

These boats are the result of a very careful and detailed design after 30 years of experience on planing sail boats. The data is not public, and jealously kept by the designers (try to ask to JM Finot or Pascal Conq any information and they will be as mute as turtles). Design of such boats is so complex that it is beyond of the most educated amateur designer and 80 % of the profesional naval architects. The remaining 20 % need to take a serious course and refresh their math skills.

Coming back to the object of this thread;
There are not any hard rule, except that the boat must be stable in its path. That depends on a lot of variables you have to integrate.
But it exists a bunch of half scientific/half empiric rules having an application on limited domain like the rules for a heavy cruiser, which very honorable goal is going gently at a decent speed from the point A to the point B with the max comfort of the passengers.

Gutelle has written a very good (and recent) book where you'll get very good "rules" for recent cruising boats.

Boats are moving three dimensional objects, it's very important to calculate all the centers at every situation of heeling upwind and downwind to verify if the boat is stable or can dampen any perturbation in the least number of cycles possible. A nice work of calculation of moments and frequencies.

A classic tramp was rithmic heeling and tendency to turn back upwind while going under spinaker (a defect of a lot of older sailing boats). Well designed modern boats do not suffer of that.

 

Robert Gainer,

Ilan Voyager really hit it on the head. (his post 8/31/04 @ 12:36PM)

"Boats are moving three dimensional objects, it's very important to calculate all the centers at every situation of heeling upwind and downwind to verify if the boat is stable or can dampen any perturbation in the least number of cycles possible. A nice work of calculation of moments and frequencies.

A classic tramp was rithmic heeling and tendency to turn back upwind while going under spinaker (a defect of a lot of older sailing boats). Well designed modern boats do not suffer of that."

A boat in a sense is a compromise..... My dad's 45ft fishing boat would not work well with a set of sails on it, but then a hunter38ft would not work well as a commercial fishing boat (these are extreams to make a point).

The beamMax is dependant on the application, is the boat a planning hull, a pure displacement, and displacement/planning hull. Is the sailboat going to be used for lakes, coastal, or bluewater? Is the boat a motorsailer, daysailer, racer...? The physics of the application will determine the beamMax and one should be a student what boat is used in what application.

A good example is the boat design I have been working on for the past year... The influence on the beamMax is state laws, it is a trailerable 28ft'er.

 

Thanks rjmac... you're perfectly right.

First thing in design is to ask yourself what's the program of the boat. What's is intented to do. Narrow the program, a boat can't do everything: for example comfortable boat with a lot of amenities means heavy, thus not very fast.

I shall add for not despair anybody that 98 % of times while designing small boats like sail and motor yachts the static analysis are simple and can be done fastly on a sheet like Excel. A lot of software (free and commercial) are able to give the centers in a lot of positions (heeling etc...) and the engineering basis is straightforward, mainly common sense, and can be found in many excellent books.

Calculations of dampening vertical, horizontal or rotating accelerations are a bit more complicated and useless in small boats unless in very extreme shapes, or speeds. If you stay within the limits of a known and proved shape, you have not to worry.

Also empirism is not a bad way. look, examine, analyse as many form plans you can find, and recollect infos about the behaviour of the boats you have the form plans. The best designer of professionnal fishing boats until 50 feet with whom I've worked was a naval carpenter who went to school until only 14 years old...but he had great intelligence, an elephant's memory, a sharp eye, a true instinct and most important a very old and very thick note-book quoting the behaviour of all his boats (and many by other) and hundreds of drawings over 40 years... When the old designer died, a friend of mine, specialist of work boats, bought the precious note-book at a pretty high price.

Dynamic analysis like those made on fast war ships or ferries need at least a big computer and lot of engineers. That's beyond the possibilities of most naval architects because the data are not public, the software has pratically to be written and the computer costs half million bucks for the simplest...