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HULL

Although the hard chine form was specified in the requirements, it is not by any means universally accepted that this represents the optimum for this class of vessel. However, certain considerations which affected the selection may be of interest.

The physical dimensions and weight involved in the mount¬ing of the C.F.S.2 gun and turret would have absolutely precluded the possibility of making use of a round form.

For example, the maximum beam on the W.L. for the case of the “Bold Pathfinder,” a round form craft 117 ft. W.L. overall is 18ft. 6 in. For the case of the “Brave” form as eventually selected, the maximum beam at W.L. is 20 ft. 6 in. for a W.L. length of 90 ft.

Apart from this the speed called for was 50 knots, or more, if it could be obtained and which, assuming W.L. length about 90 ft., is well beyond the speed at which the round form craft can compare at all favourably with the hard chine.

Beyond speed-length ratio [V/√L] = 2.5— 3.5 the hard chine form will be substantially less resistful. (See Fig. 3.)

As regards seakeeping properties it is frequently claimed that the round form is greatly superior. This claim is, in the writer’s opinion, open to debate, or at least there is no entirely clear cut answer if speed is a requirement. Obviously con¬cessions must be made in the planing form in the interests of seakeeping so that a considerable element of “vee” or deadrise is incorporated in the forward sections. Equally the round form in its high speed variant makes concessions to the
necessity for speed by virtue of the relatively fiat run of the buttock lines aft and in many cases there is even a “hard chine” worked in the aft sections.

A fair number of comparative tests in waves have been conducted in recent years to help to establish the relative merits of the hard chine versus the round form. The hard chine is sometimes referred to as the “planing” form, but of course at speeds such as we are discussing both types are in fact planning to a very considerable extent.

By this it is inferred that a lift is generated to sustain the hull mainly by dynamic considerations arising from the passage over the water at high velocity. In the more normal types of craft almost the whole of the lift required to support the weight of the hull is provided by the buoyancy or hydro¬static forces.

The series of curves shown are extracted from the report of a series of model tests in a realistic irregular wave formation carried out recently at the Davidson Laboratory of the Stevens Institute of Technology (Ref. 1).

These curves represent the data arrived at from analysis of the recordings and presented in statistical form as (a) plot of maxima, (b) plot of average values, (c) plot of “significant” values of bow and c.g. accelerations. (See Figs. 4 (a) and (b).)

A study of these curves will reveal remarkably little difference certainly at high speeds, while it is of interest that in this case the round form model (58) was tested at a length nearly 20% greater in order that all forms should be tested at the same displacement and military load carrying capacity. (See Figs. 5 (a)-(c).)

 

In general it may perhaps be conceded that a somewhat rougher ride will be experienced in the hard chine boat, which is in large measure due to the element of concavity near the chine. On the other hand most seagoing comparisons are not made at the same speed for the obvious reason that most round form craft cannot reach within 10 knots of the speed of the hard chine. Also, while the element of concavity near the chine may induce the occasional slam it certainly will keep the boat drier in a head and beam sea and much pleasanter and safer in a following or quartering sea. Furthermore, it is very doubtful whether any slight advantage possessed by the round form will in fact make all the difference between, for instance, a gun being operable and not operable. Running at really high speed in either type is not helpful to good gunnery except in
a reasonable sea.

Some very interesting and exhaustive experiments were carried out by the Admiralty at the Admiralty Experiment Works, Haslar, reproducing this boat at model scale, self propelled, and steered at a range of speeds in waves of varying size and character varying between λ = ½ L to λ = 2 L from ahead and astern. An attempt was made to reproduce spray by means of fans blowing from the side of the carriage on to the water surface. Accelerations were measured at various points.

The particular hull form selected for the “Brave” class was one originally developed by Vosper in 1944 as a result of war¬time experience with these boats and tested in the Haslar tank. This form was subsequently used in M.T.B. 1601 as well as for a number of other craft, slides of which are shown. (See Fig. 6.)

In order to facilitate the fitting of transom flaps to this form the bottom of the transom was made horizontal, thereby eliminating the element of deadrise previously incorporated. It was not considered that this would influence the perfor¬mance in turning to any substantial extent, while it has been shown on many occasions that the control of trim by means of a transom flap can do a great deal towards reducing vertical accelerations in a seaway, especially from ahead.

Fig. 7 will demonstrate this point (Ref. 2).

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