Seaslug, I am with those who disagree with you concerning the effect of adding transom extensions - the additional LWL will decrease
hobbyhorsing and thereby reduce slapping and slamming. Think of two teeter-totters with the fulcrum at the same height. Persons at the end of a 6 foot teeter totter will move up and down much further than ones on a 12 foot teeter totter, even though the forces exerted are higher due to the lever principle. Only if the fulcrum is raised proportionally will the ultimate heights reached remain the same.
Since hulls typically have some rocker, the extensions aft also operate so as to make use of the upward slope aft in the bottom of the hull
: extend this upward slope over a further distance and you raise the height of the transoms so that they are out of the water
. This improves hydrodynamic efficency.
This is to say nothing, of course, of the fact that theoretical hull speed
is directly proportional to LWL: increase one and you increase the other. In the end result, properly designed/constructed transom extensions can reduce hobbyhorsing, slamming/slapping and increase speed.
I also disagree with your formula for assessing bridgedeck clearance based solely upon LWL. Indeed, while such formulae are sometimes cited, the width of the tunnel is far more relevant to slamming than the LWL. Indeed, if once considers two boats where all else is equal (BWL, hull
shape, tunnel width, Cg, Cb, hull shape, loading, SA and Ce for the sailplan, etc.) then although both boats have the same bridgedeck clearance, the one with the longer waterline will tend to hobbyhorse less and therefore pound/slap less than the boat with the shorter LWL. Yes, on its own, increased LWL reduces
Tunnel width is the critical factor and a figure sometimes cited (including one by an earlier poster in this thread) of 1" per foot of BWL is a good starting point in catamarans that have BWL/LWL ratios of about 50%. Some suggest, hoever, that as beam increases much beyond that ratio ( further increasing relative tunnel width), the figure may actually need to be closer to 1.3" per foot. Why?
seas, a cat with an 8 foot tunnel width will permit
50% less water
into the tunnel than a boat with a 12' tunnel width. The impact will therefore be 50% higher. Furthermore, as there is a slick, or shadowing effect from the windward hull into the tunnel, that effect will also be greater where the tunnel is narrower. In terms of seas off the beam, a narrower cat operates much like a narrow jeep/offroad vehicle by being better able to straddle irregularites in a road.
Of course, a narrower tunnel can increase slapping to the extent that the bow waves can converge more readily (and closer to the front of the bridgedeck). To best optimize a relatively narrow tunnel, it is critical that the bridgedeck either start further aft, or that it have much higher bridgedeck clearance towards the front of the bridgedeck. It is also helpful to have small, upward sloping knuckles towards the bows in order to divert the spray downwards.
Factors such as bridgedeck shape are also critically important in determining the impact of seas in the tunnel, regardless of the width. A bridgedeck with a relatively blunt leading edge to the bridgedeck (as often occurs with transverse berths over the leading edge of the bridgedeck) is far more susceptible to slamming than one with a gradual curve aft (and preferably, also a gradual curve back up at the end of the bridgeck).
Once water comes into contact with the bridgedeck, it operates not much differently than the hull of a boat in the water. A flat bottomed scow with a blunt bow will pound much more than a hull with some rocker and a graceful entry. For this reason boats such as the Manta
, with relatively low bridgedeck clearance, nevertheless are known for being quite effective at avoiding slamming.
Anyway Alias, sorry but bridgedeck clearance is not just a matter of one simple equation that can be applied to all boats!