Thread: hull type
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Old 10-12-2008, 06:59 PM   #5
CODOG
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Join Date: Jan 2007
Location: Bournemouth, southern England
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If I understand your question correctly, then yes you can. But as already stated above, you will begin to lose the benefit of a planing hull design, and also not have the full benefits of displacement / semi-displacement hull design. Your requirements as to size and maximum speed would be a major consideration in this question too.
It is possible to design a planing hull that does not sink by the stern / rise at the bow as it gains speed, climbs the hump and comes up onto the plane as a normal planing hull would, either by adding considerable length or moving the LCG a long way further forward...but this design would therefore not be able to plane in the accepted sense. It would begin to plane due to its basic design, but the amount of hull surface still in the water would have high drag, its flat running angle and weight biased fwd would tend to force the bow to push the bow wave out of its way (rather than riding up and over the top of it) and the net result would be a higher power required to drive it at a relatively slower speed, and rather inefficient fuel consumption. This is by now a 'planing' design so compromised that the designer would have probably changed tack toward some form of semi-displacement design some time before.
There are artificial means that can be employed to 'flatten out' a planing hulls natural running angle (trim tabs / wedges / transom plates etc), but anything that will effectively lift the stern / push the bow down is normally used just for slower speeds, before or just on the hump (this is what I'm guessing you are getting at) Also, at higher speeds, too flat a running trim on a planing hull may result in bow steer, chine running and straight line tracking problems.
There are a million permutations ranging between full-on displacement right up to fast craft that run high speeds with only the tips of the props and the tip of the keel and chine in the water...but they all compromise one or more qualities for the reward of excelling at others. Planing hulls will rise up at the bow in their attempt to climb the bow wave in order to ride atop the water rather than through it...its what they do...if you deliberately design one that doesn't, it will suffer in other areas of performance / economy / handling too.
This brings us back to the size you have in mind and the speeds you want from the hull ? The design requirement you hinted at in your initial post suggests a decent speed but with minimal change between static and running trim angles ? So far its leaning toward a chine-bilged, semi-displacement hull with waterjets.
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