Forums: Whisstock Designs: Inshore vs. offshore New Topic Post Reply
Author Post
Posted: Aug 18th, 2014 at 03:37   |   Subject: Inshore vs. offshore
Could you please explain what the differences are between the two? I am interested in the 15'6 but would like to be able to sail the San Juan islands or to Catalina island off of California but want to know if this boat would be up to the tasks

Thanks, bill
Posted: Aug 18th, 2014 at 17:11   |   Subject: I found it
Design Category A ~ 'OCEAN' Designed for extended voyages where conditions may exceed winds of Beaufort F8 and significant wave heights of 4m and above, and for which vessels must be largely self-sufficient.

Design Category B ~ 'OFFSHORE' Designed for offshore voyages where conditions up to, and including winds of wind force 8 and significant wave heights up to, and including 4m may be experienced.

Design Category C ~ 'INSHORE' Designed for sailing in coastal waters, large bays, estuaries, lakes and rivers where conditions up to and wind force 6 and significant wave heights up to, and including 2m may be experienced.

Design Category D ~ 'SHELTERED WATERS' Designed for sailing on small lakes, rivers and canals where conditions up to and wind F4 and significant wave heights up to, and including 0.5m may be experienced. - See more at:
Posted: Aug 18th, 2014 at 22:40   |   Subject: Inshore vs. offshore
I see you found the official EU designations, which are pretty self-explanatory.

The thing about small boats is that for the EU Categories displacement alone puts them out of Category A or B, with minimum displacements of 3 tonnes and 1.5 tonnes respectively – Design 077 has a displacement of 0.6 tonnes – or 0.7 tonnes if you add the optional internal ballast.

So it doesn't matter how seaworthy a small boat is it cannot get classed as Category A or B.

The other major consideration is AVS (Angle of Vanishing Stability) which is theoretically the angle at which the boat capsizes. For category B this is 130º - 5 x displacement – so for the lightest boat possible at 1.5 tonnes, the AVS is 130º - (5 x 1.5) = 122.5º. As the boat gets heavier, the AVS can get less, but it can't fall below 95º.

To put that into context, 95º is a very low AVS – No. 077 has an AVS of between 106º and 127º, depending on whether you include the deck and superstructure in the calculation, or not. And with the 100kg of optional internal ballast the stability increases yet more.

I use the ISO Dynamic Stability calculations to arrive at the category. The ISO is a more nuanced calculation, but boat size still plays an important part.

While categories are a good guide to the design intent, they don't mean that a small boat can't sail long distances out in the ocean – in fact many small boats have made trans-ocean crossings. A lot depends on the design, the robustness of the construction and the ability of the crew.

No. 077 is a very robust small sailboat; she has a self-draining cockpit, a proper hatch and companionway, properly constructed cockpit lockers, and so on. So, within the constraints of her size, she is a very seaworthy boat.

Not an easy subject to be definitive on!