Trans RINA, Vol 156, Part B2, Intl J Small Craft Tech, Jul-Dec 2014
angles and low lift coefficients (which occur with lightly loaded
boats at to provide pitch damping. high speeds). Porpoising typically
becomes worse with increasing speed, because (although the trim reduces with increasing speed) the lift coefficient is reduced by the square of speed. Thus, a very useful parameter in planing hull designs is the inception speed of porpoising, which can severely limit the operability of an otherwise good design.
recreational stepped hulls naturally operate near the porpoising inception limit; however because presence of an afterbody, the oscillations are dampened.
of
Many the
Figure 12 shows a typical
time history of a run in which porpoising was observed. The time begins just prior to the towing carriage reaching the maximum speed. After the towing speed is reached, 2-degree pitch amplitude porpoising began to occur.
This porpoising observations from this test provides an opportunity to compare monohull porpoising criteria to stepped hulls.
The stable and unstable porpoising
conditions for this stepped hull were compared with the prismatic planing hull porpoising stability limit equation given by Lewandowski [7], based on the Day and Hagg stability limits published by Savitsky [6]. Figure 13 shows that for this configuration of stepped planing hull, stable behaviour was observed during conditions that monohulls would be expected to be stable.
This
indicates that the step in the hull did not cause any additional porpoising instabilities.
step configurations, but at speeds below The transition from
stable to porpoising occurred near the monohull porpoising limit line. The few stable runs that are above the porpoising stability limit line also correspond to the two largest
which porpoising was observed.
Figure 11: Experimental Results of Wetted Area versus Speed Coefficient as a function of step height
Figure 13: Comparison of observed
stepped hull
porpoising stability with monohull porpoising limits (solid line) of Lewandowski [7].
3.5 EFFECT OF STEP DEPTH
Figure 12: Time history showing the onset of porpoising (H = 5.71% b, Cv = 4.016, Steady Speed Achieved at 6.25 seconds)
The four runs in which porpoising was observed occurred during high speed tests of the two largest step configurations. These conditions had the highest running trim, as well as the smallest amount of afterbody wetting
©2014: The Royal Institution of Naval Architects
These tests have shown that while a large step allows for total un-wetting of the afterbody, it results in an unfavourable large trim angle, causing
a resistance
penalty at hump speed and porpoising at high speeds. Smaller step depths result in some afterbody wetting and hence reduce the running trim to a more optimum value. The optimum step depth will vary for each hull and loading combination. For the present tests, the smallest step depth tested (1.42% beam) had minimal effect on
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