High Speed Displacement-Type Hulls 605 
A narrow transom gives increased running trim but the effect on resistance is compli- 
cated. There is an optimum transom width depending on speed, trim, loading, and other form 
parameters. For high speed a wide transom is desirable, but if the width is excessive stern 
trim is impeded. If the width exceeds 0.8 of the maximum beam the sides of the hull may 
not be “clean” at top speed. 
Running attitude at a given speed is mainly governed by loading and static trim, 
although chine dihedral, curvature, and width of transom can have appreciable effects as 
noted previously. Excessive trim causes high hump resistance which can lead to “locking 
up” of the engines. The effect of propellers is to increase the running trim by as much as 
1 degree. 
Next as regards resistance and motion in waves, as the authors have shown, a form 
which has a low calm water resistance by virtue of high running trim can be a poor boat in a 
seaway, particularly as regards pitch and slamming. On the other hand too small a trim 
leads to wetness and pounding even though slamming and motion may be reduced. 
In L/30 waves maximum motion in ahead seas usually occurs at A/L between 2.5 and 
3.0 when the pitch is about 1.] to 1.2 x maximum wave slope, out to out, and the heave 
about 1.1 x wave height. 
In L/20 waves the maximum motion occurs at A/L about 2.0 or less, pitch being about 
1.35 x maximum wave slope and heave 0.9 x wave height. 
The motion is characterised, at planing speeds, by notably small movement at the stern. 
Mention is made in the paper of accelerations as high as 9 g and this has been referred 
to by previous speakers. Such accelerations can be produced of course but conditions on 
board would hardly be tenable and there would be grave risk of structural damage. Local 
accelerations of this order occur of course when the craft slams. 
For a given maximum acceptable acceleration, which may be decided for strength or 
other reason, there is a limiting speed and wave slope; e.g., in a 70-foot boat if the maxi- 
mum acceptable acceleration is 2 g the boat should not be driven into seas steeper than 
L/20 at speeds greater than F, = 0.7. Steeper waves can only be encountered, to keep 
below 2 g, if they are shorter than ship length. 
Now as regards the advantages of round bilge and hard chine forms there is much that 
can be added to the information given in the paper and among some of the more important 
facts are these. 
In calm water the round bilge form is less resistful than the hard chine form up to at 
least V//L = 3.2 (F, = 0.96). Thereafter the hard chine form shows to advantage. This 
agrees with Fig. 25 of the paper being discussed. 
In waves the resistance of the round bilge form is still less than the hard chine form up 
to V/VL = 3.7 (Fn = 1.1), but only in waves up to 3.0L when the difference is of small 
magnitude only. 
As regards motion in waves it is necessary, I think, to qualify the statement in the sub- 
section of the paper titled “Design of a Round-Bilge Form for Good Seakeeping” in terms of 
Fe and E/Ls: 
646551 O—62——40 
