Generally speaking, is there a "rule of thumb" as to where beam should be located?

[choppy]

Generally speaking, is there a "rule of thumb" as to where the beam/widest part of the boat should be located? I think I read somewhere that the widest part of a boat should be somewhere aft of the halfway point of a boat's length. Is there some formula--generally speaking as I know a lot of different factors can come into play--that designers use to determine the location of beam when a boat is being designed?

[Hamish_A]

Im new to yacht design but I normally end up with it 5-10% of LWL aft of midships. This is normally where the LCB and LCF are as well.

[lewisboats]

Depends on what kind of boat you are designing. Over the years the trend has been to move the beam further aft but it has been everywhere from ahead of midships to well aft. Boats that are designed to be fast...ie with the possibility of semi planing or planing will have a large beam aft to support it. Those that are designed for slower speeds and more comfort will have less beam aft and generally more displacement...and the max beam will be further forward. This is a general statement and there are many designs that will not fall into this 51% plus category.

[Tad]

200 years ago max beam was way up forward, station #2-3, the so called "Cod's Head & Mackerel Tail" shape. We were still seeing that 30 years ago but it's pretty well gone today.

Now max beam is at the transom. So as a general rule it's somewhere between station 2 and the transom.......Station 6 is a good start but it will be a lot further aft on a very modern type.

This is one of those mixes of art and science (remember it's the sheer line in plan) that will never be addressed by any formula.

[alan white]

Widest beam can be anywhere within reason, and the position should compliment other factors such as hull shape, sail plan, specific usage and so forth.
Almost every measurment factor on a boat has both advantages and disadvantages. Even if, for example, the beam has been placed forward contrary to the current trend, there are some definite instances when that forward beam design could perform better than one with an aft max beam.
As said, wide transoms are the latest design trend, but in truth, having a big wide aft end isn't necessarily an example of forward scientific reasoning. In fact, if you plan to circumnavigate, you might consider a boat with relatively balanced ends so that the bow is more buoyant and the stern is less likely to present a broad face to following seas. If you plan to race, you'll be more likely to plane if your transom is wider (with beam well aft). The safe bet is to put the widest beam where boats with attributes you like have placed it. It's also important to remember that the waterline beam can be in a different place than the deck level beam.
If you don't have a lot of design experience, I say copy the best, or at least don't stray too far from good examples that have come before. And don't just look at the modern designs, but look at the last 100 years or more. There are some old designs that still are considered among the best for certain attributes, and usually these are designs that are usually the most seaworthy, if slower under certain conditions such as sailing tro windward, or most economical to run (at slower speeds) in the case of power boats.

[u4ea32]

Parametric studies of displacement vessels do indicate a clear advantage to having the beam just about midships.

Fashion based design indicates a current fad of having the beam very far aft, but this fad is about to fade, so if you designed such a silly shape, it will look obsolete by the time its built (follow what's happening in the mini transat -- the new shape of speed for box ruled sailing craft will be, predictably, a box, commonly called a scow).

Full planing hulls that are going for maximum speed, not handling nor seakeeping, work pretty well having max beam at the transom. This encourages the powerboat fashion of bax beam carried to the transom, as does the packing of boats into rectangular slips: if you want a boat that fits a slip like a glove and sells well, put the max beam at the transom.

Planing vessels that actually handle well in a sea way have the beam much further forward, even a bit forward of midships. Look at all surfboards. Widest at the tail was of course tried by many, and it fails every time.

A planing surface is a supercavitating wing, where all the lift is provided by the high pressure pushing up, nothing (virtually) on the top pulling up, unlike a typical airfoil at low angles of attack. As we all should know, the higher the aspect ratio, the better the lift to drag. Hence sailplane and albatross wings. So an efficient planing hull has the max beam right where the pressure is highest, which is just about at the leading edge, or where the hull enters the water.

Contrary to current popular perception, a wide transom far aft of the point of maximum dynamic pressure is very, very slow. A narrow transom planes much, much better. Again, the surfboard evidence. The pressure on the bottom decreases and goes negative (Bernoulli) aft of the high pressure at the leading edge. So that wide transom is not lifting, its sucking the boat downward. This can nevertheless work somewhat, because it increases the angle of attack at the bow, so total lift may increase, but drag too (its a lousy airfoil shape, low lift to drag).

If the transom is as wide as max beam on a planing craft, handling is simply awful when compared to a similar craft with a narrower transom. The cheap experiment is again surfboards or windsurfer boards, but older powerboats (ones designed to move instead of to maximally fill marina slips) also very clearly demonstrated this characteristic.

So: If you want it to sell, put the max beam well aft. Make it shaped as closely as possible to a marina slip. Follow the current design fads, regardless of how poor such designs actually are.

If you want the boat to actually work well underway:
a) make it a box (scow) for a box rule,
b) make it balanced beam near midship with as narrow a transom as your other requirements allow for a sailing yacht or displacement speed powerboat or a planing sailboat,
c) make it with max beam a bit forward of midships for a cruising planing hull,
d) make it max beam aft for a pure performance powerboat.

Of course, none of the above are likely to sell well. But you already know that the best way to make a small fortune building boats is to start with a big fortune.

[lobsterman]

I'd say look to nature for the answer, imagine the underwater profile of a duck, then design in the propulsion,keel, rudder and above waterline hull shape to your desired usage. Does that make any sense at all ???.

[Manie B]

Very good topic
with some excellent information

but

a current fad of having the beam very far aft, but this fad is about to fade

the VAST amounts of money that the "Volvo racers" have poured into research shows the aft beam way for future, this has changed yacht design forever

but we must also keep in mind that a lot of this technology goes hand in hand with modern epoxy and carbon fibre construction
all this is not easy for the home builder or cruiser
so think more about what you want to do
and most likely the beam will be "centered"

[daiquiri]

Well, honestly I think that before giving answers there's a fundamental question which has to be asked first: what type of boat are we talking about?

[Submarine Tom]

Oh, oh, oh, I know, I know: At the widest point.

-Tom

[river runner]

I'm just a baker in a grocery store, but I do know one thing, almost all seakayaks and flat water canoes have the maximum beam just aft of the middle. After listening to all the science and logic, my own conclusion (for displacement boats) is that for speeds well below the boats hull speed, having the maximum beam in the middle, or even forward of middle is probably more energy efficient. For speeds approaching hull speed, having the beam aft of the middle is probably more efficient.

[viking north]

Being a fan of double enders(sail) I agree with midship to slightly aft of midship as the max beam location and it never hurts to keep in mind the good characteristics of fore and aft symetry in such a hull at points of heel. On sailing hulls,I don't like max beam far aft as from experience it results in too much aft boyancy which encourages the broaching or tripping factor in big following seas. Again I qualify here I am not a designer just a builder with sea time.--Geo.

A yacht is not defined by the vessel but by the care and love of her owner

[choppy]

So an efficient planing hull has the max beam right where the pressure is highest, which is just about at the leading edge, or where the hull enters the water.

Just so I understand--the highest pressure is at the bow?

Contrary to current popular perception, a wide transom far aft of the point of maximum dynamic pressure is very, very slow. A narrow transom planes much, much better. Again, the surfboard evidence. The pressure on the bottom decreases and goes negative (Bernoulli) aft of the high pressure at the leading edge.

So that wide transom is not lifting, its sucking the boat downward. This can nevertheless work somewhat, because it increases the angle of attack at the bow, so total lift may increase, but drag too (its a lousy airfoil shape, low lift to drag).

This is going to be a 15' plywood sailboat. It will have a daggerboard and I would like for it to plane very easily. I was planning to have the max beam 3/5 back from the bow and for max beam to continue to stern. I am now rethinking this as I am wondering what would be a good compromise so that I create maximum lift but the least amount of drag?

My design is "loosely" based on the following link.
http://www.finot.com/bateaux/batprod...penanglais.htm

Also, what comes first--the placement of the mast or the placement of the keel? I know if I place the keel at "x" then I have to place the mast at "y" to create slight weather helm. So one will determine the placement of the other. But which one is the most important to place first?

I care if the design fails but it will not be the end of the world. I enjoy experimenting with boat design and then taking the boat out on the water and see what happens. If she capsizes, breaks apart or sails badly I will be in a protected cove on an inland lake so I should be okay.

Thanks for all the help,
choppy

[choppy]

Originally Posted by viking north

Best advise for the small amount of cost --buy a set of plans, why gamble with wasting money and time --learn from the plans/build and it will give you top schooling to attempt your own design and build down the road--geo.

We already know how those boats sail. Please don't take this the wrong way and I do appreicate your help and reply but why would I want to build by plans just to get the same result?

Thanks for taking the time to reply,
choppy

[daiquiri]

Originally Posted by choppy

Originally Posted by viking north
Best advise for the small amount of cost --buy a set of plans, why gamble with wasting money and time --learn from the plans/build and it will give you top schooling to attempt your own design and build down the road--geo.

Oh... Can't believe that Viking North has given that advice...