Deep-Vee Bottom

(Last of a three-part survey on powerboat hull design. Reprinted from the May 1979 Motor Boating and Sailing)

These days, when a designer lays out plans for a fast seagoing power vessel, he often goes to some variation of the deep-V Hull form.

When Ray Hunt drew the first Moppie in the earlv sixties, he must have realized he was on to something. The boat, in its prototype stage, was extremely able in all weather, possessing an incredibly “cushioned” ride in seas that would crack the jaws of passengers on all other fast boats.

The reason for the soft ride deep-V’s display is the consistently full deadrise (more than 17’) from forefoot to transom. As the boat slams into a sea, the wave is deflected upward and outward from the keel (or fairbody), and this deflection expends the energy that would otherwise shake your fillings out.
The major sacrifice made by the deep-V, however, is the ease with which it will attain planning speed.

DeepV.html_txt_8deepvee1The ideal planing hull is totally flat-sectioned. Examples of this can be seen in early Sea Sleds and hydroplanes. But the Deep-V is quite the opposite. Where the common warped-plane or V-bottomed boat will usually pop up on plane with modest horsepower expenditure, the deep-V needs more push to climb out of the hole.

But try to skip a Sea Sled over the surface in a heavy chop. You will have either a submarine or a flying, out-of-control vessel. Here’s where the deep-V shines.

Bugs and refinements

As the years since Moppie I have passed, engineers and designers have worked out many of the bugs in the deep-V concept. To deal with the upward deflection of water from the bottom – a problem leading to extremely wet running on early boats, with spray flying up and over the house in some extreme conditions – builders have added spray knockers or chine steps to flatten the angle of deflection.

To help the vessel get up on plane sooner, longitudinal strakes have been added to protrude in long, flat, buttock-like sweeps acting as planing surfaces, and the fairbody line (the profile of the hulls bottom) has been deepened aft to create a “wedge”, or increase in pressure under the stern sections.

But the problems of getting the deep-V on plane still persist, though much has been done to perfect the concept. For example, one typical 42-foot deep-V requires 302 hp to pull ten knots (the speed at which she starts to feel the hole deepening under her stern sections). A comparably weighted, flat-sectioned planing hull, on the other hand, needs only 133 hp to sustain the same speed at that pre-planing attitude.. When on plane, however, the situation is somewhat the reverse. The deep-V uses 552 hp to sustain a planing speed of 20 knots while the production modified-V in our example needs 567 hp.

Deep-V’s to tend roll easily when riding at anchor or while drift fishing or trolling. Short of rigging flopper-stoppers or adding bilge-keels there is very little that can be done about this situation. It’s just one of the sacrifices you make for the gain in performance and seaworthiness.

Ironically, a refinement lately seen in Deep-V’s is the shallowing of the deadrise angle. That’s right, cruising deep-V’s are less “deep” today than they were at their beginning. The major reason for this is the fuel situation. Designers are simply trying to compromise some of the heavy-weather performance qualities of the deep hull for the fuel efficiency of the flat-sectioned planning boat. Where the early Moppies pushed 20’ deadrise angles through the water, many of today’s hulls sport a modest 17’ deadrise (or less) angle. This is not a “cop-out”, just clear thinking on the part of today’s economy minded designers and engineers.

The deep-V is an excellent choice for all-around cruising. But it is at speed in heavy going that the true potential is realized. A cruise in one will show you what Ray Hunt was telling the world almost 20 years ago. -Doug Schryver