This post intents to show that there exist an optimal hull length for a given displacement. This optimal point is defined in the way of overall drag ; wave-making and wetted surface (c.q. form drag and frictionous drag)

If such an optimimal point is found then the immediate result will be that some boats will be better off with shorter hulls and other with longer hulls when only looking at overall drag.

Do this mind experiment.

Stretch a flat plate to such a length that its volume is equal to the required displacement. Its crossection area is extremely small while its hull length is extremely long and so the wave-making drag (form-drag) will be negligiable. Hull speed laws (Froude's law) will predict very high hull speeds. But the wetted surface area will be inmensely large and so to the wetted surface drag. It will be even be much much larger then the overall drag of say a normall sized catamaran hull of the same displacement but with much less wetted area.

As a counter experiment place the same plate perpendicular to the movement. Now we have an extremely small hull length with a inmensely large crosssectional area. The wave-making drag (form-drag) will now go through the roof while wetted surface drag in the direction of the movement will be infinisemal small. Again the overall drag is much much larger then that of a normal catamaran hull. Try to pull this plate through the water and you'll experience the truth of the last statement.

Clearly both extremes are more draggy then a normal hull, these three points can never be linked up by a straight line, ergo the connecting line is a curve of some shape and will have at least one minimum (drag) value. This point corresponds to a single hull length where the overall hull drag of the boat will the smallest. This will then be the optimimal hull length from an overall drag perspective.

This is also the direct counterproof against the rule : "Longer hulls always have less hull drag (and are faster)"

The same proofs that "shorter hulls always have less drag" is wrong as well.

Simply put the use of "always" is wrong.

For us it will be interesting to find where this optimal hull length is located. Is it inside the 6.5 - 4.5 mtr hull length spectrum of beach cats or out side of this range ?

That is why I asked Gareth to produce the plots for several hull lengths.

It is my opinion that for some bouyancy levels the optimal hull length is to be found inside this normal hull length range. Prime candidates are the 105 kg bouyance (M20, eagle 20, F16) and the 75-100 kg singlehanded bouyancy boats as with A-cats and F16's. Boats being significantly heavier like
that will have optimal hull length laying beyond 6.5 mtr and are therefor always better longer hulled. That is when limiting our hulls to the range 6.5 - 4.5 mtr.


I hope this is clear enough for all

Best of enjoyment on your steps into boat designing science !

Wouter