Almost ready to build test boat - Methodology for design

I am close to being ready to start building a very small catamaran for testing on lakes.

What is the methodology for figuring out the most efficient hull configuration for best efficiency at a given design speed and displacement?

How do you scale from a small prototype to a larger vessel? What needs to be taken into consideration when scaling?

 

This is a very intelligent question, because scale can be very interesting and complicated for sailing.

Winds, waves, viscosity of both water and air don't scale with the model. A light, five knot breeze to a full size boat can be like twenty knots over the water to a scaled model. This effects ballast and sail area.

I only have a minimum experience with experimentation of these concepts, so I would suggest keep asking these questions and do a lot of your own thinking about what is different for a model than for the vessel that is four to a hundred times bigger.

Common sense and experimentation will likely give you the best answers. Those need not necessarily be your own.

 

What size is "very small"? Most "model" testing requires very accurate (not necessarily precise except as precision relates to the value) measurements; which for most people rules out quantitative analysis for qualitative ones.

 

If you just want to design a sailing catamaran, assuming that is the type of catamaran you require.
Google for "HOW TO DIMENSION A SAILING CATAMARAN?".
It will get you started.

Regards AlanX

 

Hi,

This matter is really well covered by the ITTC (International Towing Tank Conference).

Test procedure, results mitigations, material, measure,examples.

Cheers,

 

The first version would be about 12'x6', likely try two hull forms, and get some real data for powering. The second version for island hopping the bahamas about 24'x12'. If I really enjoy the cruising life, I want to build a 40'x20' version to cross oceans. The goal is to test hull forms for seakeeping and efficiency.

 

Great, thank you.

 

Most excellent, thank you.

 

You could perform a survey of boats that are designed for your purpose, and compare your design to the sample population.

 

A 24' boat will perform completely different than an enlarged version of 40'. Boat behavior does not scale up directly. It takes sophisticated engineering to predict the behavior of a vessel from a model. If you are really set on designing your own boat, paying a test tank facility would be your safest bet. However, building a boat is a job that normally takes several years for an amateur. Designing a boat is much more complex. The initial question you pose means that you need to get a complete course on Naval Architecture and Engineering. Even professional shipwrights and reputable yards buy their designs, or sometimes have a specialized design team on staff. Also, a tank test facility will charge about $20,000 for a preliminary study.

 

I guess I'll just give up because no mere mortal could possibly take on this task.

I'm not looking for precise predictions, if I wind up within 15-20% in terms of performance I will be happy. Which leads me to the question; what is the typical discrepancy between scale models and full size in terms of performance? And is performance typically better or worse when scaling up?

I may not be a naval architect, but if the conditions are scaled with the size of the boat I would have to disagree with this statement. The physics you are dealing with don't change much at those scales. If it was a 4' model compared to a 40' I may buy that.

 

Keep in mind, there are approximately 6076 feet in a nautical mile, which means, when a ten knot breeze blows a 24' boat across the water, that breeze take about 1.4 seconds to blow one boat length. For a 40 foot boat, it takes a little less than twice as long. That seems like a difference worth thinking about. Don't forget the relative wave length and timing, air movement at the top of a thirty foot mast and the air movement at the top of a fifty foot mast may also be significantly different.

That isn't to say that modeling isn't worth it. Just understand there are a number of factors that are hard to scale, even when the model size is close to the full scale design.

 

Thank you, Will, for the practical input. I'm taking these things into consideration. From what I understand the wind gradient isn't especially large. I imagine this is more of an issue in large seas?

I'm going to build a small boat anyway for cruising around on lakes, so might as well try things and gather data.