Freeboard & Sheer Line
Freeboard in HullForm is defined in three places, the bow, midship and stern and it is worth considering each of these in turn. A high bow freeboard will give extra buoyancy at the front, part waves and help resist diving. The Viking longships knew this long ago. The earlier Iambus had a 40mm bow freeboard and this was not enough as it would dive whenever hard pressed and run out of forward buoyancy quickly. However, the limit is set by the whole boat having to go in a measuring box a foot high. Every extra cm of bow freeboard means a cm less fin length and moment arm for the lead in heel. This gives a permanent reduction in sail carrying power for a feature that is not in the water most of the time and only comes into play in extremes of gusts. The best value can only be guessed but as 40mm was clearly too little the Iambus2 went for 70mm.
Stern freeboard need not be very large. With a flat run towards the stern and a wide transom the stern wave tends to be suppressed and occur behind the boat. When heeled the boat tends to pitch down and the stern lift slightly so again there is very little tendency for the stern to be under water. A Value of about 30mm seems to be enough.
At the midship station it might be thought that the near maximum beam at heel at hull speed would go under water and require a large freeboard. However this neglects the fact that the standard ship wave pattern with bow and stern waves also includes a trough at midships so that the water level is actually low there and only a moderate freeboard is actually needed. The result of a high bow freeboard and relatively low midship and stern freeboards is the traditional Stollery banana shaped sheer line with a high upswept bow. Together with some rake of the stem this gives the most resistance you can get to diving without using a flared bow.
Stern freeboard need not be very large. With a flat run towards the stern and a wide transom the stern wave tends to be suppressed and occur behind the boat. When heeled the boat tends to pitch down and the stern lift slightly so again there is very little tendency for the stern to be under water. A Value of about 30mm seems to be enough.
At the midship station it might be thought that the near maximum beam at heel at hull speed would go under water and require a large freeboard. However this neglects the fact that the standard ship wave pattern with bow and stern waves also includes a trough at midships so that the water level is actually low there and only a moderate freeboard is actually needed. The result of a high bow freeboard and relatively low midship and stern freeboards is the traditional Stollery banana shaped sheer line with a high upswept bow. Together with some rake of the stem this gives the most resistance you can get to diving without using a flared bow.
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