Drag forces

[C. Bailey]

Hi,

Are there equations for calculating drag forces on a boat hull at any given
speed (i.e. both when the hull is in displacement mode and when it is
planing). I am primarily interested in wave drag as compared to skin drag.
Are there generalized relationships or is this something only solved by
numerical analysis? The hull is a simple design - flat bottom (similar to a
air boat).

Any help is appreciated. After several days of roaming around the web, I am
empty handed.

Thank you,
Chris

[Gérard Delerm]

Hi Chris,

As you do, I am looking for some equations for calculating drag forces for a
planing surface.

It's easy to find softwares to calculate drag for an hydrodynamic profile
which works in water, but for the interface air/water I dont find anything.

If you find something, please send it me.

Thank you,

Gerard

gerard...@free.fr

[Adrian Field]

Hi,

There is some information on planing surfaces,
cavitation etc. for surface piercing struts and
other shapes in Hoerner, Fluid Dynamic Drag. It is
mainly for air but there is a chapter on
hydrodynamics.

Adrian

[Morgan Flexer]

For this type of hull try the NASA online papers, I don't have the address
to hand, but think it is fairly easy to find (will post it when I find it).
Many of the paper are quite old but they have a lot of work on flying boat
floats.

Regards

Morgan
Adrian Field <aif...@unixg.ubc.ca> wrote in message
news:38C8B622...@unixg.ubc.ca...

[Chris Pollard]

C. Bailey <Ple...@Reply.to.newsgroup> wrote:
: numerical analysis? The hull is a simple design - flat bottom (similar to a

: air boat).
: Any help is appreciated. After several days of roaming around the web, I am
: empty handed.

Most of the stuff is in the old books on naval architecture:-

I have -

Principles of Naval Architecture ed Comstock - larger ships but pretty
complete

Modern Ship Design - Gillmer - OK overall but not as much theory as above.

Principles of Yacht Design - Larsson - Modern info but mostly to do with
high performance sailing yachts rather than planing.

It's hard there isn't a lot out there and what there is is spread rather
thin. Many smaller boats are (or were) designed by trial and error.

[Hoyte Raven]

Chris Bailey wrote:

Are there equations for calculating drag forces on a boat hull at any
given
speed (i.e. both when the hull is in displacement mode and when it is
planing). I am primarily interested in wave drag as compared to skin
drag.
Are there generalized relationships or is this something only solved by

numerical analysis? The hull is a simple design - flat bottom (similar
to a
air boat).

========================
The 'generalised relationships' are not always so generalised. Usually
these are simple equations derived by statistical analysis. For the
displacement mode, there is the method of Holtrop & Mennen of MARIN
(www.marin.nl) which works well for a variety of ships. For planing,
there are simple relations such as those of Savitsky, which will work
for sufficiently high speeds.

A prediction that takes into account the details of the hull form (which
at least for lower Froude number have a large influence on wave drag)
and therefore is more precise, requires numerical solution methods for
the nonlinear free-surface potential flow problem or even more refined
flow models. Even then, accurate resistance prediction is hard. For an
example of such a code, you might look at
www.marin.nl/services/softwaredevelopment/cph_rapid.html
but I do not think that this is the sort of tool that you are after.

Good luck,
Hoyte Raven

[David Wilkinson]

Is there not a method due to Savitsky for planing craft and one due to
Gerritsma for displacement yachts?

In article <38C8B622...@unixg.ubc.ca>, Adrian Field
<aif...@unixg.ubc.ca> writes

>Hi,
>
>There is some information on planing surfaces,
>cavitation etc. for surface piercing struts and
>other shapes in Hoerner, Fluid Dynamic Drag. It is
>mainly for air but there is a chapter on
>hydrodynamics.
>
>Adrian

--
David Wilkinson

[C. Bailey]

Thanks everyone. I will dig a bit more into the given references. It's
surprising there isn't more equations of this sort given that most anyone
designing a boat would like to know how to equate horsepower to speed!

Chris

"Chris Pollard" <cpol...@teal.sni.net> wrote in message
news:%8dy4.196$vy2....@wdc-read-01.qwest.net...
> C. Bailey <Ple...@Reply.to.newsgroup> wrote:
> : numerical analysis? The hull is a simple design - flat bottom (similar
to a
> : air boat).

[Chris Pollard]

C. Bailey <Ple...@Reply.to.newsgroup> wrote:
: Thanks everyone. I will dig a bit more into the given references. It's

: surprising there isn't more equations of this sort given that most anyone
: designing a boat would like to know how to equate horsepower to speed!

Most boats use just a few formulas for their design. Drag is related to
length and surface area. So you make boats as long and as thin as you can.

If you want a fast boat - you design an aircraft.

[MorgIG]

If you are looking for a quick way to estimate the required hp for a planing
boat and you have another similar boat to work from, the Barnaby/Levi method
goes a long way.

V = (L^(0.25))*k*sqrt(HP/displ)

V = Knots
displ = tons (long not US)
HP = Horse power
L = feet

If you don't have a similar boat to work from you can use

K = 1.4 ( 4 conventional shafts)
1.5 ( 2 shafts)
1.6 ( stern drives)
1.8 (surface drives)

It should be noted that this method should work best with deep vee boat, see
Levi's work

regards

Morgan

[Richard J. Roake]

>If you want a fast boat - you design an aircraft<

its not quite that simple....but its close ;-)

see www.hydroski.com
--

Cheers

Richard Roake