Basic Calculation...

Could any of you help a mathematically impaired enthusiast with a little calculation formula.

I have seen various calculations posted on the web for displacement vessels…what I’m interested in is a formula for “planing hulls”. I understand the term “planing” is relative, but the boat in question will do approximately 80 mph and has a padded “flat keel” bottom and is fully planing.

Assuming we know total hull weight, CofG (if needed), wetted area of the pad, angle of attack in degrees, and speed…is there a basic mathematical formula that could calculate total drag or resistance???

[mmd]

Sorry, nope. Got a bunch of tech papers, books, monographs, etc., that offer up some pretty complex, interrelated formulae, but no simple, basic ones. If you understand that the simple term "planing" is indistinct and prone to interpretation, just imagine how complex it is to mathematically define a formula to decribe it.

Check out Lindsay Lord's or Peter du Cane's books on the subject, or Savitsky's papers. Whatever you find that you are comfortable with, bear in mind that most formulae are used to approximate a guestimate upon which a model is built for tank-testing to get close-to-real-world answers. Planing hulls are, unfortunately, one of those maddening phenomena that appear deceptively simple at first glance, but become wonderfully complex as you look ever closer.

Don't give up your quest in dispair; the journey is fascinating. Try Lord's book first - it is terribly out of date, but describes the different parameters well and is easy on the math part of the brain.

[gonzo]

Most calculations are based on a prototype. That is, they take a boat with known performance, figure out a formula that explains and calculates its performance and handling, and then use it to make predictions on modifications of the prototype design. This is the basis of all engineering. All data is experimental and related to a model.

[nevd]

If you use google to search on "fishmeal" and "kamen", you will be able to get a free Savitsky program for download.

This may help you if you are not wanting to get too complex. Bear in mind that aero drag will also be significant at your speeds and so will the benefit of running the chines dry at the transom.

Regards,

nevd

[dougfrolich]

Keep in mind that Savitsky (1964) is an empirical model and is intended for use with prismatic hull forms with constant deadrise. I think if you look into the work of Vorus, Marine Tecnology 1994? You may find a model that will take into account arbitrary hull forms.

Good hunting

appreciate the feedback.

I'll to a little more digging on this topic.

[Jimboat]

NEDV is right! The hydrodynamic characteristics will change as the aerodynamic loads (lift/drag) become more significant. Check out Secrets of Tunnel Boat Design book for full text of explanation, including formulae and examples.

[CDBarry]

The 1994 paper is a more rigorous derivation of the Savitsky method, but is also for prismatic hulls.

Zarnick entering wedge methods ("Powersea", for example) do warped, non-prismatic deadrise hulls, and some recent work by Taggart, Troesch, et al (recent Journal of Ship Research) does more arbitrary cross sections, though I'm not sure exactly how arbitrary.

[raceday]

Here is a simple calculator we use that gives us a close figure on speed and hull efficiency.

www.racedaymarine.com/speed.htm

Hope this helps you, if you have any questions feel free to write me.

John Cunningham
www.kanyun.com
john@webmediafx.com

[Jimboat]

Guest - I can give you a drag calculation for planing surfaces that is based on wetted area of the pad, angle of attack in degrees, and speed. Just send me an email.

hello gents.