Marine engineering students wanted for computer modelling for solar electric boats

[Leo Lazauskas]

Originally Posted by rxcomposite

When a catamaran moves through water, the waves created by one hull interferes with the wave created by the other hull. Hull spacing is associated with interference effect between the component hull. The wider the hull spacing the less interference effect. But how wide is wide without necessarily creating an ungainly wide boat that will turn out to be heavier than needed?

Tasaki has published a chart based on mathematical hull form showing the interference effect between the two hulls in relation to the hull spacing plotted against Froude number. The graph shows that there are two bands (sweet band) where the waves cancels each other and is considered “beneficial”.

I have prepared a program that searches this sweet band with a given hull spacing, length, and speed. Purist call this a spreadsheet as it lacks the macro or structured program to run it but I will leave it that way so that others may be free to modify or improve on it.

The hulls are analyzed separately using parametric design lanes published by Saunders. The hull design is guided by a narrow band of workable parameters plotted against Froude/Taylors numbers. These parameters coincide with form coefficients by Lindblad and Todd.

The program does the finger walking on the tables and is designed to trap errors when the inputs are out of range.

The master program defines the general hull coefficients while the hull population sheet evaluates a set of hulls that will fit the constraint. This is the “what if scenario” wherein slight variations of breadth, draft, and maximum cross section area will give an output of deck surface area or total resistance. It is also possible to exceed the constraint to check if other variation maybe missed out.

The work was inspired by Mike D, a former member of this forum when he posted several widely used formulas. Other formulas are standard NA’s formulas.

The formulas have been validated against sea trial report of boats that have been built and works out close. Be cautioned though that the ehp is a bare hull resistance and the shaft hp has only 40% loss. In actual practice, appendage resistance may go up to 20% on bare hull resistance and propeller may attain only up to 67% efficiency in open water.

The simple resistance formula as suggested is accurate only up to S/L ratio of 1.1. The Metric value of f in resistance formula was fitted to the English f as it is more accurate.

The program will only guide the user into tight design parameters. As Leo has said, if you want to be accurate, go mathematical. Michlet will give a dead on result.

Attached is the program and instructions.

If Michlet is too slow, you can also try some empirical formulae I came up with. These formulae give the "optimum" width-to-length ratio for catamarans where it is assumed that the optimum spacing is one that eliminates the largest peak in the free wave spectrum. It is not exactly the optimal width-to-length ratio, but it is a reasonable first cut. The graphs associated with these equations also show that there is a band at which no finite width-to-length ratio is beneficial.

Click on the first publication at:
http://www.cyberiad.net/hybrid.htm

and look around page 32.

Leo.

[icetreader]

Originally Posted by rxcomposite

...Attached is the program and instructions.

Good stuff!

Yoav

[pirooz.moradnia]

I have already taken my Bachelor's in Marine Engineering and Naval Architecture, and am taking my Master's in Turbulence, Mechanical Engineering at Chalmers University of Technology, Gothenburg-Sweden. I wish to be eligible for this position.

[JonathanCole]

Originally Posted by rxcomposite

Mine is a personal approach to the subject. maybe the others have a different way of tackling it.

I have just started digging into your spreadsheet, Rene. It is really an excellent piece of work. I hope others will be inspired by your example. This is a very logical approach to the problem of determining the sweet spot for solar catamarans. Of wider scope is the obvious usefulness of such spreadsheet programs for comparative boat design.

Any comments Leo, Brien?

[rxcomposite]

Thanks Leo. You always have been helpful and encouraging.

I have used the hull spacing as the fixed parameter in the design. I have noticed that Michlet and Hull form uses the centerline of the hull to define hull spacing. Should i change the spreadsheet to conform? What do you think.

Thanks in advance.

[rxcomposite]

Icetreader,

Thanks. Glad you appreciated it. I have also done a similar work for monohull. Because there arent too many constraint unlike the catamaran, this one outputs not only the coefficients but the hull form as well. The waterline shape, section shape, entry angle, ect.

The correct hull form for every S/L ratio.

Will post it soon.

[Leo Lazauskas]

Originally Posted by rxcomposite

Thanks Leo. You always have been helpful and encouraging.

I have used the hull spacing as the fixed parameter in the design. I have noticed that Michlet and Hull form uses the centerline of the hull to define hull spacing. Should i change the spreadsheet to conform? What do you think.

Thanks in advance.

I haven't had time to look at the spreadsheet yet, but I don't think it should matter much whether you use centreline spacing or the tunnel width, as long as you are consistent and the user is clear on what is being used.

Cheers,
Leo.

[rxcomposite]

Have updated Cat design so that output can be validated against a standard such as michlet.

Constraint is on hull breadth instead of hull spacing and advance filtering made easy.

[icetreader]

Thanks Rx,

You deserve credit for creating this fine tool, and you should publish it under your name

Yoav