Sportboat Buying Check Points No. 6: Beam

Few if any sportboats under 28' (8.5 m) have a beam greater than 8'6" (2.59 m) which is the maximum beam that can be trailered without a permit in all 50 U.S. states. But some popular sportboats have a beam as narrow as 7'0" (2.13 m). So the questions arise: how much beam is enough, how does more or less beam effect performance, comfort and safety, and how much more does a beamier boat cost?

Test Captain Rob Smith is 6' (1.84 m) and 230 lbs. (104.5 kgs.) yet he is not too big for this Glastron GSL 235 BR with an 8'6" (2.59 m) beam. Note that there is plenty of room for him to pass through the opening to the bow. (To see the test video of the GSL 235... )

Most everyone wants as big a boat as they can afford, but most people only look at length and rarely consider the effect of beam on the size of a boat. In fact a longer boat with less beam could actually have less interior volume than a shorter boat of greater beam.

A prime example, and an exaggerated one, is comparing an 18' deck boat with an 18' bow rider. Everyone realizes that the deck boat has greater deck space, but what they may not realize is that some deck boats actually have 20% or more area than a sport boat the same size. The same phenomenon also occurs among sportboats where beam generally runs from about 7' to a maximum of 8'6".

The Boating Golden Ratio

In architecture 1 to 1.61803 is considered the golden ratio; is there such a thing in boat design? If there is, it is probably a beam/length ratio of 1 to 3. For certain types and sizes of boat a 1-to-3 ratio might be pleasing to the eye and practical as well. For example, consider a 30-footer with a 10' beam. While there are not many 60 boats with a 20' beam, most come close to it with a 18' or 19' beam.

In smaller lengths where the sportboats reside, the 1-to-3 proportion would make an 18' boat 6' wide, a 21' boat 7' wide, a 24' boat 8' wide, and a 25.5' boat 8'6" wide. In fact, these are not far from the proportions and beam widths that most boat designers employ in sportboats.

Narrow is Fast

All things being equal a boat carrying less beam will be faster than one with greater beam. A wider boat on the waterline presents more frontal area and resistance to forward motion, and the greater surface area on the bottom generates more drag. Of course, all things are rarely equal, and usually a beamer boat will also be a heavier boat, meaning it also has more mass to move.

These are the basic reasons in laymen's terms as to why builders who emphasize speed in their unique selling proposition, also often have the narrowest and lightest boats in class. Less weight also means less cost, and light narrow boats, even though it is just a few inches, can reduce the price of a boat in several ways. In addition to the material in the boat, there is also the size of the engine. If it can be smaller and still generate adequate speed still more weight and cost can be saved.

Wide is Comfortable

As illustrated in the picture with Capt. Rob Smith in the Glastron GSL 235, the 8'6" beam allows an interior space that is comfortable for even a large man to move around easily. Seats and passageways can be wider. There is more space for coolers, people, and gear. The GSL 235's beam is slightly greater than the "golden" 33% at 35.8%, but that seems to be fine with all concerned.

Greater beam also means more stability. While we rarely hear of sportboats capsizing or swamping, that is likely to be because they both have relatively low centers of gravity and their owners do not take them out in problematical conditions. Stability in sportboats comes into play most importantly when the boat is anchored, drifting or just idling along, with its occupants enjoying the day. Beamier sport boats will simply roll less in these conditions and thereby provide a more comfortable ride for the passengers.

Just as narrow boats are generally less costly, conversely beamier boats can be more expensive because there is more material in the hull, and more of everything inside the boat, as well. Not surprisingly, the engine might be a higher horsepower. Taken together, the boater pays handsomely for his extra room and his comfortable ride, because most sport boat owners do not want to go any slower in a beamy boat than a narrower one.

This is one of the reasons that deck boats were introduced two decades ago -- to increase interior volume and deck square footage without increasing length. Some deck boats actually have a deck area that is 20% or more greater than sportboats of the same length.

Does that mean that everyone should run out and buy a deck boat?

In the early days of boating powerboats tended to be long and narrow because with the engines and the hull shapes of the day they would go faster than shorter, wider boats.

Beam:

In the United States all 50 states will allow a boat with an 8’6” beam to be trailered. In some states boats up to 10’ can be trailered without a special permit. Other countries have other requirements, so check with your local marine authorities.

Generally, the greater a boat’s beam, the more stable it will be and the larger its interior volume will be. A boat with greater beam will also be heavier and be harder to push. Not surprisingly since it has more material in its hull than a boat of equal length with less beam, it is generally more expensive.