Two amateur questions....

[dkubiak]

1. I have been reading Gerr's "Nature of Boats", and he has a semi-witty story about a boat with too little planning surface to actually plane. However, he has neglected to include any kind of formula for figuring this out on my own. Could someone please tell me how to figure this out?

2. I do not really understand the advantages/disadvantages of varying degrees of deadrise in a v-bottom hull. I would appreciate it if someone could explain to me how a shallow v behaves in comarison to a deep v.

Thanks!

Dan

[tom28571]

Look at it this way. The dynamic planing lift is, according to Newton, exactly equal to the momentum imparted to the water by the passing of the boat. A flat bottom hull accelerates the water straight downward laterally and slightly forward due to the trim angle. A V hull directs the water forward at the trim angle and laterally at the deadrise angle. Since only the vertical component of momentum supports the boat, the deeper the V, the less efficient at generating lift the hull will be.

Since flat and very shallow deadrise hulls will pound and ride rougher in waves than a deep V, the designer makes a choice on the intended use and comfort required to arrive at an acceptable compromise.

There is no single simple formula for computing the answer to Gerr's example. That is how he makes his living.

[dkubiak]

Okay. My primary concern is a smooth ride; so, I want a deep-v. However, I would also like the boat to have the potential to plane with the appropriate horsepower attached.

At what ange is a hull consider to be a shallow-v? When is it considered deep? And when do I go off the end and make a ridiculously deep-v?

Also, while looking through Gerr's book again I found a section where he discussed speed to length ratio and which ratios will allow planning. Can I figure out this ratio at various speeds for my design, or is this another way you pros earn your keep?

Thanks for all of you help!

Dan

[Note: right now my design has two pontoons 17.5 ft long at the waterline, 2 ft wide at the waterline, constant deadrise, and parallel buttocks in the aft two thirds. I'm still working on the v, obviously, and how much I think the boat will weigh.]

[Tad]

dku;

As hull design cannot be explained in a couple of sentences or even a couple of paragraphs, Tom was not trying to be difficult. Your two questions turned into four, or maybe six new questions. The answers involve perhaps a dozen major variables, and perhaps a million minor variables. But I often exagerate.

You appear to be talking about a catamaran.

Try this for starters,

hull forms

All the best, Tad

[dereksireci]

There no substitute for experience. Go to a boat show or a dealer and look at the hull forms out of the water. Get behind the wheel and run the boats in similar conditions.

All the formulas in the all the books in print are not a substitute for time actually running different vessels, in my opinion.

[dkubiak]

Originally Posted by Tad

Tom was not trying to be difficult.

I spent my years of formal education studying philosophy, so I am very familiar with an infinite web of questions. We always knew a fellow student had begun to understand when he no longer thought himself able to write adequately an answer to the questions asked by our professors.

Ignorance expects short answers. While I do not understand engineering, I also am not ignorant of its complexities.

So, I did not take Tom's answer as "difficult" but rather as an indication that he knows what he is talking about.

I sincerely appreciate all of you taking the time to help us occasional amateurs design and build boats that are, in the least, not death traps.

Dan

P.S. Thanks for the link. I'll read it this evening.

[tom28571]

I'll add ony one more comment to your particular situation. The very low beam/length ratio of approximately 8:1 for your catamaran means that the boat will act differently from "normal" planing boats where this ratio is usually about 3:1. The fact that there are two parallel hulls also means that individual hulls do not need to be symetrical. Go to a boat dealer that sells planing power cats and look at the shape of the hulls. Frankly, I have only looked at these boats and have never seriously studied their characteristics. It is clear to me that they have a different set of problems from monohulls.

Study the material that Tad referenced and anything else you can find.