606 W. J. Marwood and A. Silverleaf 
Up to a certain speed, usually about F, = 0.7, both pitch and heave in ahead seas are 
greater for the round-bilge boat, and less than the hard chine boat beyond this speed. This 
applies in waves up to 1.5 or 2.0L. In longer waves the round-bilge form continues to pitch 
more but there is little difference in heave between the two forms. 
In the following seas the pitch of the round-bilge form is slightly greater in waves up 
to 2.0L and slightly less in longer waves. The reverse is true of heave. 
In shallow seas the accelerations at the fore end of the round-bilge form are slightly 
higher than for the hard chine design, and this corresponds with the slightly greater pitch 
and heave. Accelerations at the after end are much the same for both designs. In steeper 
seas the accelerations of the hard chine form are greater both at the fore end and at the 
after end. 
Slamming is generally more pronounced in the hard chine form for the same still-water 
loading condition as regards displacement and running trim. 
With regard to wetness, as would be expected the water is thrown low and clear by the 
hard chine forms and higher and nearer the hull in the round bilge form, although both are 
generally dry in seas dead ahead. The effect of wind on the bow is to increase the degree 
of wetness, and the round-bilge form is wetter than the hard chine in this condition. 
Finally there is one feature of high speed craft, of which the paper makes no mention, 
which I should like to remark on briefly, viz., the use of a transom flap in the form of a 
plate hinged at the lower edge of the transom so as to be adjustable and generally about 
1,/20th the length of the craft in width. 
Model experiments show that in calm water running, stern trim and rise are reduced by 
the use of a transom flap at all speeds, from which it might be expected that the resistance 
would also be reduced at all speeds. In fact the measured model resistance is reduced by 
the use of a flap only up to a moderate speed, beyond which it increases, compared with no 
flap. 
Nevertheless sea trials have shown that the flap improves the speed up to full power 
due to the change in interaction between hull and propellers having a favourable effect upon 
the hull efficiency elements. For a model of a 70-foot-long hull the augment of resistance 
was reduced and the wake increased to give an increase in hull efficiency of 8% at top 
speed, for a flap angle of 5 degrees. 
Both model experiments and sea trials have demonstrated, however, that as the maxi- 
mum speed increases, or as displacement decreases, the flap becomes less effective and 
smaller angles of incidence of the flap are required to obtain maximum effect. Ultimately a 
condition will be reached when the flap is of no propulsive advantage. One explanation, or 
part explanation, for this may be that in designs with high top speed, not fitted with a flap, 
the running trim ceases to increase above a certain speed and then begins to reduce, although 
rise may still continue to increase. Full consideration must be given to this fact when con- 
templating the incorporation of a transom flap in a new design. 
As regards behaviour in a seaway in waves less than ship length and height, in the 
region of L/30 the flap has an appreciable effect upon pitch or heave at any speed. In 
waves of critical length about 2.5L when motion is usually severe the flap reduces the 
pitch and heave by about 10 percent. 
