A question about hull shape on a sail boat

[Roland Stockham]

I have noticed in the last few years that many sailboats are now being built without what I would call a taper, from the widest part (max beam) to the stern. These newer designs, when looking from above, keep the beam of the boat nearly the same as it's widest point midships to the stern. Obviously, one advantage would be to increase the cubic ft inside the hull so that given a set length, say 40 FT there would be more room inside towards the stern. Can someone explain to me any detriments towards this design? I'm curious as to specific drawbacks to designing a hull in this shape. I will use this pic as an example as it seems that the transom is quite wide given the length of the hull

Thank you in advance.

Sean

This design pattern originated with race boats in the search for greater off wind speed. If you exceed the hull speed on a displacement hull the aft squats and you finish up trying to climb your own bow wave which effectively creates a speed limit of about 1.5 x sq root WLL. By crating a broad stern with a flat run and assuming you keep the boat very light that big stern allows the boat to skim over the surface and plane so greatly increasing max speed. Note the phrase "if you keep the boat very light", it does not work well for cruising boats which tend to be heavier anyway and then get loaded with all the stuff we need so the power needed to get up on the plane becomes unrealistic. The result is a badly behaved boat that does not like waves, buries its bow and is really prone to sudden broaching without much speed gain! But cruisers always want a 'fast boat' and more room below deck for the length so it has been copied to so called cruising boats (which aren't).

 

[Kayakmarathon]

Not really. As the reference Dolfiman gave shows, the actual waterplane is significantly finer than the hull, especially when sailing. This hull form is all about moving the CB well outboard of the CG on a set of sailing lines that are very fine. In a form without significant wall-sidedness initial static BM (i.e. Iwp/volume) is pretty meaningless.

I noticed that too. The longer effective lever arm with the wide stern hull allows the crew counteract the sail with a lighter keel.

 

[Paul Scott]

I wouldn’t be afraid of a big butt, if it’s well designed. Amati (below) will go high upwind at 20 degrees heel, fingertip pressure on the tiller, will plane under control at 20+ knots drinking tea. ( Personally, I’d suggest a narrow, deep boat. Better manners. Pinched sterns can get a bit wild downwind.) Amati is a custom Bob Perry design, 1999. Low wetted surface, doesn’t need a ton of sail area. Wood, 9600 lbs. Light air flyer. 10.5 foot beam, 8 foot beam at waterline. 40’ LOA. 8.5’ draft, fin keel. Coastal cruiser. 40’ is a bit small for offshore. Van de Stadt designed boats like this, heavier, which might work for your construction material, but still have nice manners. Amati’s in ‘Perry on Design’ for more info, along with a bunch of other Perry designs. Chined hulls with flat panel topsides will work easier with steel. Paul Bieker does hulls like that, although they are more oriented towards plywood or composite panels, and lighter construction. Take a look at the Wyliecats for inspiration- especially the 30, 44 & the 60. Not messing with jibs will help single handed.