You could reduce the total reserve buoyancy considerably less than 300% and concentrate reserve in the ends. This certainly helps when going into waves and is not detrimental going with the waves.
The ship with the Axe bow that I linked to is preferred shape to the Wharram bow that rob linked to. The Wharram has a flat deck so that will dive if pressed into a wave. However the high stem and flare gives greater margin before burying. It also has a tendency to hobby horse; what goes up has to come down. This video compares ships with buoyant bows but one is wave piercing and the other is heavily flared with flat deck:
If you put more buoyancy in the ends it should still be wave piercing.
Slender hulls tend to slap rather than slam. The dynamic lift will only support a small proportion of the displacement in the usual speeds of interest. Buoyancy in the ends keeps the nose up. Also the KMl remains high for higher pitch angles relative to the surface; noting the surface of a wave is anything but flat.
With reserve buoyancy less than 100%, heave becomes negative in waves. Essentially the boat can fall faster than it can rise so it inevitably sits lower in the water. That means that clearances need to be higher. However it does not take much reserve buoyancy in the ends to improve the surface following ability to avoid diving and gain benefit of dynamic lift. Of course this a function of the moment of inertia as well. If the inertia is low then less moment is needed to change pitch angle. Obviously the accelerations in the ends can be much higher than 1g.
My use of heave is to mean vertical motion of the boat.
I have attached a table that gives wave height relative to wind speed in open water. The model has parameters that generate heights similar to the table for the nominated windspeed. It gives an idea why you will not be particularly interested in carrying full sail in winds above 20kts.
The 18m proa starts dragging the well noticeably in 15 to 17kts of wind on open water. The cross beam ploughs through crests in the leeward side in winds above 20kts. The clearances are 600 and 900. The well actually slams because it is a large flat area.
Considering the situation with low reserve buoyancy there is the prospect of the negative heave adding to roll as the ww hull is lightly loaded and will ride high. In fact the windward hull is more inclined to plane (and tends toward slamming) because it is lightly loaded.
Rick
On 29/04/2016, at 6:19 AM, "lucsimard@ymail.com [harryproa]" <harryproa@yahoogroups.com.au> wrote:
Rick, that is so interesting !
"One way to increase reserve buoyancy without increasing deck height, length or waterline beam is to flare the sides. This also reduces the wetter surface compared with vertical sides. You could also just put more volume in the ends using raised foredecks and leave the middle lower. The height in the bows has much more influence on avoiding a dive than the volume in the middle:"So the peak vertical acceleration would not change by moving the volume to the end (since volume does not change) but the pitch up moment going through each waves will increase ... would'n that make the ride bumpier ? (same as dynamic lift)"The hull with lower reserve buoyancy has less added drag from pitching up to the point where the heave is net negative."I want to understand but with my mother tongue being french and the load of boats terms I am a bit lost ... so need to ask more questions.Definition of heave I found is a lift force ... then a heave of 0 mean no net force upward (boat then ride at a certain equilibrium height ?)I could not find (look for a good 1 hour) either a graph, table or equation for "normal" wave that link wavelenght, height and windspeed together so I have an idea for your numbers in context. Would you have something on hand ?I am getting at profiling the hulls soon ... I am still messing around with the folding geometry which does not cooperate :-)Many thanks!
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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