Michell v CFD and EFD: Round 2007

many thanks for the reply very interesting

cheers

 

I've heard that too, although I don't necessarily agree with it. 25 years, perhaps.

Aerodynamicists have the distinct advantage of working in a single-phase medium. To add to their luck, they also have virtually nothing other than the body of interest to consider, except during the takeoff/landing process. They also have a considerable fortune to spend on their simulations- when you're building a plane that sells for three hundred million bucks a pop, a killer supercomputer and an army of programmers is not a big deal.

I have yet to see a simple, elegant means of simulating an arbitrary two-phase flow (if anyone's aware of one, let us know....), and yet this ability is essential to developing useful CFD models of ships. In many respects, simulating a ship is substantially more complicated than simulating an aircraft, or even a space shuttle, simply because there are so many unpredictabilities (waves, currents, wind, the aforementioned multi-phase flow). And there just isn't the same kind of money going into this work, because, well, ships do tend to work more or less OK as they are (after all, they have thousands of years more refinement behind them than an Airbus 380 does), and the payback from an enormous investment in CFD methods isn't as obvious.

 

I've been playing with Michlet for a few weeks now (after helpful prodding from Leo and Rick). I'm hoping to design a low drag canoe for the USCA 3x27 pro boat catagory. '5.64m length and 0.7m minimum beam at the 0.075m waterline. It seems the optimum shape lies outside of the version 8.07 Godzilla design space. How should I proceed to widen the design space for my specific project?

For example, this hull (also attached in.mlt) beats any I can generate with Godzilla:
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# Displacement Volume (cubic metres)
0.175
# Length (metres)
5.6
# Draft (metres)
0.1082

 

John
I am not sure what you mean by Godzilla design space.

It does not surprise me that you have developed a shape to beat the rule that Godzilla does not arrive at. Godzilla can be set to produce a hard chine but it will only produce a smooth curve from the fore body to the aft body.

It is not immediately apparent to me how to set constraints in Godzilla to achieve the beam constraint you have.

Your design will be marginal for stability but I expect it will be close to the best given the rule.

How does this design compare with what is currently in use?

Rick W

 

John
You can get a lot of debate on this site about abrupt changes in water plane. There are some who believe that water finds it hard to negotiate angles. For me I am confident that the results from Michlet will be accurate.

Looks like a simple shape to build. Just 8 flat panels. You could make it up out of 1mm aluminium sheets. Need two 3m by 1.2m sheets. Would weigh around 35kg. Could be built in a weekend very easily. One join in the middle. I have attached the development of the bow or stern.

Rick W.

 

Re: ...Godzilla design space...

I mean the set of possible hulls using the variables of hull type 7.

Re: Hazelboat vs current

The hull I posted with it's flat facets was inspired by the large viscous proportion of the total calculated drag. After the first try, (minimmum surface area) which was a hull having diamond shaped waterlines above the beam, I noticed that the Cp was very low. Many sources have reported minimum drag occurs when Cp is in the 0.66-0.70 range for Fn around 0.4-0.45. After a couple tries it became clear that increasing Cp by adding extra volume in the extreem ends gave the smallest increase in wetted surface and also reduced the wave drag.

This was good news as we are currently running in boats that require the bowman to place one foot on top of the other because the ends are so fine. It looks like something close to what I posted will reduce both wave and viscous drag plus allow more power from the bowman by better ergonomics.

In general the two fastest existing designs are close to having a diamond shaped waterline but have a hollow entrance rather than my convex entrance.

 

It would be worth your time to draw up the existing designs and compare with what you have using Michlet. It will be a good indication of the merit of what you have arrived at.

Rick W

 

John
One more thing.

I have found that any boat designed to exceed the celerity of the transverse waves having wavelength equal to LWL (so-called "hull speed") will have convex ends if beam is not constrained. The higher the design speed relative to LWL the fuller the ends and the narrower the beam.

I have not looked into the case where beam is constrained to a value but I expect having fuller ends would be traded for reducing draft to get an optimum.

Rick W

 

Yes and I am also concerned that the boat I posted will go fast only if it is on totally flat water and kept near zero yaw and perfectly flat...

Typical racing boats are made from 4.5oz/yrd^2 CF facings on a 3/8" nomex honecomb core for bottom and ribs. Between ribs is a single skin of 4.5oz CF with a lighter weight kevlar. The boats are 5.6m long and weigh less than 15kg. I've done some tests of the construction and the samples I have measured out to less than 35oz/yrd^2 for the completed sandwich. Tough to beat...

I'm considering a prototype of flat panels made with fiberglass skins on 2.5cm thick foam joined to 10cm balsawood bow and stern. It should be possible to get an inexpensive boat below 20Kg that is strong and stiff enough but will be vulnerable to puncture compared to the CF/kevlar/CF boats.

 

Yes I should get out there and measure my current boat to make that comparison. It's a "Corbin". There's an article attached that compares the boats currently in use including several pictures.

 

John
I suggested the aluminium for prototyping. You could get away with 0.8mm if you are careful in how you use it. It is so much faster than other methods because there is no need for surface finish. It is about as good as you can get.

There is no doubt that CF on Nomex would be the best way to go to get minimum weight for strength. As I said the boat would be suitable for flat panel as well.

I find that if the boat goes well in calm water it also performs best in a bit rougher. Might be different if conditions suit surfing but I have not done enough of this to compare.

I did not check the real simple developable design with Michlet. I just drew it similar to yours.

I have done a few "prototypes" in aluminium and find it the best for this. There is very little lost in having developable shapes with hard angles in the waterplane. The attached hull is 0.6mm aluminium. It is made from three developable sections. (I holed it when I hit a submerged branch sticking up from a log doing about 12kph)

Rick W.

 

I see your point. I'll check the sources here for materials.

 

John
Those boats beg the question - what constitutes a beam measurement.

One of the things that has been discussed on the forum are faux trimarans.

Have you thought about making a trimaran underbody with monohull topside.

You could make an optimised single hull with two outboard long slender hulls for the beam measurement. If you predominantly operate in smooth water you will only need a couple of inches clearance for the connecting wing.

On top the boat would look similar to the existing boats but in the water it would be a trimaran. The central hull would be wide enough to make foot wells. The outboard hulls would be set 75mm above the keel line to meet the beam criteria. This would be a real rule beater with way better performance potential than the awful shapes the rule currently forces.

Rick W.

 

There is a no concaves below the 3" waterline rule and max 1/4" concave above that. This applies to cross section and longitudinal section lines.

http://www.uscanoe.com/Download.html

 

John
Given the relative blunt entry you might gain a few watts by placing a bulb in the bow. Something the diameter of the draft at the bow and say twice the length to diameter half ellipsoid. This just reduces the lift of water down the sides at the bow wave. It is effective in eliminating the bow wave. Not a lot of energy but everything counts. It models in Michlet so you can see if it is better.

This is another thing that Godzilla will not evolve as the hull functions do not have enough parameters.

Rick W