Savitsky Method

CGN, please see my reply in the other thread entitled "Savitsky Power Prediction". Ta.

 

I want to study about occurance and probability of porpoising in planning boat. I would compare between extant methods. Is there any one to help me by some refrences?

 

"For a given deadrise angle there is a specific relationship between trim angle and lift coefficient which defines the inception of porpoising. Because of such porpoise considerations it is necessary to design the boat to operate at an unfavourable trim angle from the point of view of resistance, as the minimum drag-lift ratio occurs at an angle greater than the angle needed to avoid porpoising.

The trim angle required to avoid porpoising increases with increasing deadrise angle. Increasing the deadrise enables a planning surface to operate at trim angles more closely approaching those required for minimum drag.

So, it may happen a 20º deadrise brings better drag results than a 15º one at high speeds, if it risens the trim angle to a more efficient one from the point of view of the drag-lift ratio."

I am sorry but I do not know the author, it was a cut and paste I put away for a rainy day, most likely someone on this forum, I am sure they will recognise the contents if that is the case, and I apologize for not giving credit.

 

porpoising

hi, I want to study about porpoising and its limits. is there any one help me by some refrences? also I would calculate hydrodinamic coefficients in heave and pitch fore a prismatic planning boat. please direct me by your knowledge and experience.

 

thanks for your good help. do you know when a bout start to planning and go on its motion how Trim angle treat? does it increase?

 

Title : The Prediction of Porpoising Inception for Modern Planing Craft
Descriptive Note : Tullio /Celano, III
Corporate Author : NAVAL ACADEMY ANNAPOLIS MD
Report Date : 1998

Abstract : The purpose of this project was to study porpoising,
one of the most common forms of dynamic instability found in planing boats.
In descriptive terms, it is a coupled oscillation in pitch and heave that
occurs in relatively calm water. These oscillations can be divergent in
amplitude, leading to loss of control, injury to occupants or damage to
the craft. The mechanics of porpoising have been studied sporadically
from theoretical and experimental perspectives for many years. Studies
have shown that the inception of porpoising is influenced by displacement,
center of gravity location, and various hull characteristics such as deadrise
and beam. Until now, Day & Haag's thesis provided the only systematic test
results concerning the porpoising stability limits of planing craft.
Although the Day and Haag model tests were brilliantly executed and thoroughly
reported, many users of these data are not aware of the size of the models
tested. The average beam of the three tiny prismatic hulls was only 3.8 inches.
As a starting point, these tests were recreated using a series of three
hard-chined prismatic planning hullforms approximately five times larger.
The tests included hulls with higher deadrise angles, more typical of craft
now employed for high-speed military purposes. Two models of actual full-scale
craft, complete with performance enhancing features including lifting strakes,
trim tabs, and variable drive angle were tested. These additions were found to
have a profound effect upon the conditions at the inception of porpoising.
Established planing hull analysis methods were augmented with techniques
developed during the course of the study to provide a basis from which to
design and outfit high-speed, heavily laden planing hulls with respect to
porpoising stability.

Title : Theoretical Determination of Porpoising Instability of High-Speed
Planing Boats,
Corporate Author : DAVID W TAYLOR NAVAL SHIP RESEARCH AND DEVELOPMENT CENTER BETHESDA MD
Personal Author(s) : Martin,Milton
Report Date : APR 1976

Abstract : A theoretical method is derived for predicting trim angle
and speed coefficient at the inception of porpoising of prismatic
planing hulls. Although equations are derived for the surge, pitch,
and heave degrees of freedom, it is seen that the effect of surge is
small at ordinary operating trim angles. Comparisons of theoretical
predictions with existing experimental data on coupled pitch and
heave porpoising show reasonably good agreement for a wide range of
speed coefficients, load coefficients, and deadrise angles. The
theory may also be used for estimating the natural frequencies and
damping characteristics of prismatic hulls in the stable, high-speed
planing range.

 

thanks I found it at DTIC website, But I dontknow how can I download it!!

 

hi Dingo
is it possible for me have your exell sheet?

 

hi
I real life how is the variety of center of pressure in planinig hull?especialy when porpoising occure? is it a oscilate motion forward and afterward of center of gravity?

 

Hi,
Would you help me to find these books.

 

This one? http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADB240994&Location=U2&doc=GetTRDoc.pdf

Joakim

 

can planning hull calculate use vortex lattice methode? I want to compare with savetsky methode.. Any have idea