Arc bottom Vs "V" bottom. Pros and cons....

[skaraborgcraft]

Skimming through old design books,pre 1950, i note that there seems to be a lot of arc bottom plywood construction runabouts and cruisers,some up to around 24 ft. I dont recall seeing many new designs that are arc bottomed, and wonder why the V appears to be more widely used in preference?

Any clear advantages of one over the other in terms of performance or construction? I would have thought an arc bottom would be more liable to roll at rest than a V hull,but may have less wetted area for the same displacement when planing. I recall reading that an arc bottom can "skid" in hard turns,more so than a V, unless fitted with skegs and rails? Im generally interested in the use of arc bottom in outboard powered craft rather than under sail.

It appears the only downside of the arc bottom would be the rather flat entry,which could cause some pounding,and that in itself may be a reason why the V seems to be more in use, though i dont see why a transition from arc to a V forward could not be achieved?

Comments from all and especially those with experience of both welcome. Cheers.

[Peerie Maa]

Both Ray Hunt and Renato Levi demonstrated that deep V is the best for a planing hull. The transition from Veed forefoot into flatter stern quarters was termed warped bottom, the work of Hunt proved that this was not optilmal due to its wetted surface area drag and poorer seakeeping.

[Hunky Dory]

I think that as H.P. increased their was a shift from displacment to planing hulls. As speeds increased the v hull gave a more comfortable ride .

[Ian McColgin]

Arc versus V.

There's a limit as to how far from the stern you can run an arced bottom if the bottom is also to have any rocker, though the "tortured plywood" method pushes that a ways.

While some arc bottoms get to a fairly blunt bow, if you have a hard chine and move that to a pointy bow, you can achieve a nice sharp entry if you want.

The deep V bottom was pioneered by Ray Hunt for pleasure boats. A sharp V was used towards the bow in planing boats as diverse as the PT boat and the Chesapeake file bottom. Hunt's big breakthrough was to carry a deep V right to the transom, allowing the planing hull to bounce from wave to wave with less pounding than the previous shallower V, arced, or flat shapes at the transom previous boats had. The deep V is less efficinet for planing than flatter shapes and requires far more power to get up on a plane, even with lifting strakes. It's main value is that you can punch ahead at high speeds into seas that in another hull would either break ankles, puree kidneys, or leave the boat as splinters.

Flat surfaces have less surface area just as a straight line is shorter than an arc. If you have as fixed points your chines and your deepest spot at the keel, the shallow V will have less wetted area than the arc that hits the same three marks. However, the arc will provide a bit more displaced volume and might for the same depth provide a bit more capacity for more displacement, like a bigger engine.

Flat, V of any shape, or arc don't make any difference in rolling by themselves. Rolling at rest is more a function of deadrise and the resulting buoyancy in the vicinity of the waterline coupled with the boat's beam and resulting form stability.

Given how the prop is located, almost all power boats lean into a turn, especially at speed. Certain designs have prooven very dangerous if the outside chine does not get above the water and causes the boat to trip and roll over. Mostly pleasure boats avoid that issue easily. As a V hull gets deeper and the V is carried further aft, the boat's lateral surface will more resist skittering around the turn. Flat, arced, or shallow V sterns don't present much lateral resistance in a turn while planing and can be expected to cut a wider arc. This can be rather nice, allowing you to keep a higher speed through the turn, but if your net forward vector drops too far, you may come off the plane and the boat could roll or trip on it's chine amidships or on a deep forefoot.

Other things being equal, an arced shape in plywood is stronger and less inclined to panting and flexing under the stress of the water than a flat shape.

In short, which is better depends on many many considerations.