Impact of Modern, Triangular-Design on Boat Performance

Go round any boat show and look at the hulls from underneath—most now have a narrow, very pointed bow and a broad flat stern. Why? Where did this come from and what are the advantages and disadvantages of this shape compared to more traditional designs for cruising boats?

If you look at boats from the 60s and 70s, they tend to be narrower with less sharp bows and taper to a narrower stern. That was how boats had always been designed.

Some of the reasons behind this design shift have nothing to do with good boat design. First, berths are normally charged by length, but unless you have a catamaran, beam is ignored. So a shorter, wider boat will have the same internal space but will be cheaper to berth. Simple economics! Related to this, more beam allows the designer more options for layout. Twin aft cabins, bigger washrooms and more roomy galleys all become possibilities. Narrow boats, by contrast, can feel like living in a corridor.

WHAT EFFECT DOES THE TRIANGULAR DESIGN HAVE ON BOAT PERFORMANCE?

When a boat moves through the water, it has to push the water out of the way and then move into the space it has created. The more water it has to push, the more power it needs. In addition, the faster it has to push that water, the more power it takes. Speed is the most important factor here because the power required is proportional to the square of the speed—doubling the speed multiplies the power by four times.

A way to reduce the speed at which the water has to move out of the way is by making the bow very pointed, moving the widest part of the boat nearer to the stern and making the hull shallow.

The diagram shows how water flows around a traditional hull, and a modern, wide body, shallow draft hull. In the traditional hull, the stern is much deeper and often fuller so more water is pushed aside at the bow. The mid-section is nearly parallel so nothing much happens at that point. At the stern, all that water has to flow back into the “hole” the boat made. In the wide shallow hull, it skims along the surface.

As the hull moves, it displaces water sideways along most of the hull and at the bow. Although the water has further to go to get around the wide stern, since it is moving over the hull’s full length, the overall flow speed is slower. Some water is displaced downward but because the hull is shallow, again the water flow is slower.

While the hull shape does make a difference, weight is also very important. In order to skim over the surface, modern hulls need to be lighter. If you look at the displacement of a modern hull, it is about half of a similar-length boat made in the 1960s.

The next big difference is what happens to the bow wave at high speeds.

The fist two diagrams show a traditional hull shape, called a displacement hull, as speed increases. When a boat moves through the water it generates a bow wave. As the bow wave moves along the hull you get a trough midships and then a second peak at the stern. When the speed increases, the distance between the peaks increases. At its max speed (called “hull speed”), the second peak is at the stern. This speed is approximately 1.4 times the square root of the waterline length, so 6 kn for a 36-ft boat.

If you try to go faster than this, the bow tries to climb the front wave peak and the stern starts to fall into the trough. Effectively, the whole boat is now trying to climb uphill. By contrast, in a boat with a wide stern there’s a lot more buoyancy keeping the stern up. Equally important, the shallow and narrow bow does not have much buoyancy so it tends to cut into the front wave rather than ride up it. The result is that the hull has much less tendency to “squat” and can reach a higher speed. Taken to the extreme, this can allow the boat to plane like a motor boat and reach two or even three times its hull speed.

WEIGHT

A critical factor in this process is weight. In order to get the benefit of a higher top speed, a wide hull must be shallow so that it can skim over the water rather than ploughing through it. This has been achieved with the use of modern materials and construction methods. The weight of hulls of this design has dropped by almost half in the last 50 years. But you must keep the boat light for this to work.

That all sounds good but is there a downside—of course there is! A very narrow pointed bow does not have much buoyancy so it will tend to try and pierce through the waves rather than ride over them. In a steep head sea this can lead to the bow burying in the waves, bringing the boat to a shuddering halt. In a steep following sea, it can pick up that buoyant stern and the boat can take off surfing down the wave until the bow buries in the trough and she broaches. In extreme cases she can actually pitch pole—when the boat goes stern over bow. This does not happen on cruising boats but has happened to race boats.

Weight limits are another issue for cruising boats. In order to get these speed benefits, the boat has to be light. A positive is that means less materials in her construction, which saves on building costs. The problem is that when cruising we want lots of fresh water, plenty of diesel and lots of personal kit. We may also want to carry a decent weight in anchors and chains right at the bow. All of this will rapidly kill the performance benefits of this design and can make a boat unsafe.

A typical 15,000-lb., 1960’s 35-ft. cruiser would barely notice 2,000 lb. of gear and simply sink an extra inch or so. 2,000 lb. is a typical weight for an offshore cruising boat to carry. A modern, light 32-ft. boat will have about the same space below deck—maybe even more—but will weigh well under 10,000 lb. With these designs you need to pay attention to the max load limit set by the designer and especially avoid too much bow weight.

CENTER OF BUOYANCY

There is another significant issue with wide stern designs, which causes them to become unsafe if overloaded. When a wide stern boat heels, it starts to ride on the edge of the hull and lift the windward side out of the water. See the diagram below.

When looking at the boat heeled, notice how much of the windward side of the hull has lifted out of the water. That is part of the design. When she heels, the center of buoyancy moves rapidly outboard creating a strong righting lever and makes the boat very stable. The downside to this is that it also means the shape the boat makes in the water changes as well, and tends to make her less directionally stable.

If we look at the diagram above which shows the effect of overloading the same boat, you will see that when she rides low in the water, the lee deck can bury and the windward side of the hull no longer lifts out of the water. The result is the boat becomes a lot less stable. While it is unlikely that the boat will capsize, it will heel a lot more than it should and also roll badly, making her uncomfortable as well as killing performance.

These designs are not good load carriers. Remember that when looking at the boat’s load limit, which includes all the gear you put on board. It is not just the crew and their gear but also fuel, water, anchors and chains to name only some of the major items. The smaller the boat, the more this becomes an issue. A cruising couple is much less likely to hit the load limit on a 45-ft. boat than on a 35-ft. one. In the U.S., boats over 20 ft. are not required to display a capacity plate so this limit can be hard to find. One solution when shopping for a production boat is to look at the same boat sold in the European Union where capacity plates are required.

SLAMMING

A final point worth considering is slamming.

When boat goes over a wave it will naturally tend to rotate around its longitudinal center of buoyancy. The top picture in the diagram above shows a boat with a broad stern and fine bow. Most of the buoyancy in this boat is nearer the stern so it rotates about the line A-A.

By the time that point hits the wave crest, the bow is well past the crest with a lot of air under it. Therefore, the rotation is more sudden and the bow drops rapidly into the trough. This, combined with her repetitively flat bottom makes her tend to land with a crash. This is a rather uncomfortable motion referred to as slamming. The more traditional form shown beneath it, by contrast, has the center of buoyancy close to midships and a more V shape to the hull. She will follow the wave profile more closely and if she does land suddenly, the V shape will cut into the water rather than slam down onto it.

All boats can at times be prone to slamming in the right (or wrong!) sea state but the flatter the bottom and the further back the center of buoyancy is the more likely she is to suffer violent slamming.

If you own—or have sailed—a modern design production boat, let us know in the comments what your experience is with boat performance.