Mike
Most hulls when pushed over hull speed will trim bow up. There are two factors that cause this.
The level of the water at the bow tends to be higher and this increases the lift in the bow. Likewise the level of water toward the stern tends to be lower so the stern sinks. Larger waterplane area in the bow and stern increase the amount of bow up trim. So KMl is a consideration here.
As the boat goes faster there is dynamic lift created that becomes significant. Having flat sections on the bottom of the hull gives better lift than round sections. A flat area in the bow with rocker has a higher angle of attack and will create more lift.
Michlet/Godzilla do not have the ability to model either of these effects. One of Leo's programs called Flotilla is able to produce the lift from the waves and adjust the trim. You can get it from here with some discussion:
There is an updated version here that I have not tried:
He also has a lifting surface program that I have tried but seems to under estimate lift compared with empirical programs I have and rough data from my V14 hull:
This is the V14 hull at 10kts:
Hard to see in the video but the bow is well clear of the water. From overall trim I estimate the dynamic lift to be about 20kgf. Leo's lsp does not give anywhere near this. Some old NACA flat surface planing formulas are closer.
Models are useful to see if a design works as a complete unit. Scaling to get meaningful performance prediction is very complicated. The most accurate model has a scale of 1:1. The more you depart from this the greater the opportunity for important elements to be lost and unimportant elements to seem important. For performance prediction it is best to use the testing to validate complex models like Flotilla for hydrodynamics and JavaFoil for sails, keels and rudders and then use these to scale from because they already have the key parameters nailed. But then these only give the smooth water case. The Delft Series testing has some good comparative data on performance in waves:
It is a big file but makes good reading.
With my slender hulls I have found windage to be more significant than waves from a resistance perspective. So I expect the lw hull on a harryproa could be made to have little added drag from waves.
Rick
On 12/03/2011, at 5:38 PM, Michael Gehl wrote:
I seem to recall that Rick Willoughby offered some hull and rocker alternatives a while ago for Dennis' design in which a bit of "planing" surface forward and/or suction effects aft would help counteract bow-down trim in the speed range of interest, modeled via Leo Lazuaskas' MIchlet/Godzilla suite.
Question: what are the scaling effects for hull and sail surfaces when testing physical models in real-world full-scale winds (which I suspect had outsize effects for Todd's models)? I suspect there are varied Reynolds number factors involved, possibly different for the water and air foils.
...and whatever happened to Dennis' project?
Mike
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