# Calculating Draft

Discussion in 'Boat Design' started by matt24601, Apr 29, 2010.

1. Joined: Apr 2010
Posts: 1
Likes: 0, Points: 0, Legacy Rep: 10
Location: texas

### matt24601New Member

I am pretty sure I have a handle on displacement and calculating draft but I wanted to confirm with the community here. Hypothetically :

I have a craft which is flat bottomed and measures 42"W x 96"L (big shoebox)

Q1: How many pounds can I add before I sink down 1 inch? (freshwater)

Q2: My fully laden boat weighs 1200lbs. How many inches of it are underwater?

My answer for Q1 was 144ish lbs.

My answer for Q2 was 8 1/3 ish ".

Am I on target here?:?:

2. Joined: Dec 2001
Posts: 2,474
Likes: 117, Points: 63, Legacy Rep: 1728
Location: Oriental, NC

### tom28571Senior Member

Closer than the actual measurement will be because of varying density of the water due to temperature and other factors. In other words, you've got it. Now on to the next problem.

3. Joined: Jan 2006
Posts: 3,281
Likes: 426, Points: 83, Legacy Rep: 1279
Location: Lakeland Fl USA

From a purely theoretic point of view, Fresh water answers are..... 145.6 and 1237.6 pounds. As Tom says, your numbers are close enough. Keep up the good work.

This is one of the ways you can arrive at the answers.....42 x 96 = 4032 square inches....with one inch draft it is 4032 cubic inches....multiply by the number of pounds of water in one cubic inch....0.03611 and .03611 x 4032 = 145.6 pounds for each inch of draft. There are several other ways to get there, this one is a simple way to arrive at fairly small displacement numbers.

The process is a little more involved when the shape is other than a shoebox. If it is a conventional boat shape you need to guess what the prismatic coeeficient (Cp) is. Kayaks, canoes and fine ended displacement boats have Cp in the range of 0.5 to 0.53. Just multiply the shoe box dimension by the Cp and you are in the ball park. So a kayak that was drawing 3 inches and had a 30 inch beam and a 150 inch waterline would be about...........3 x 30 x 150 x 0.51 x .03611 = 248 pounds. This method is only ball park mind you. The math is OK but the Cp guess is argueable. Planing boats usually have much larger Cp...

4. Joined: Aug 2010
Posts: 1
Likes: 0, Points: 0, Legacy Rep: 10
Location: Seattle

### eelantraNew Member

Stability Calculation

How do I calculate the stability of rectangular box shaped container?
L=20 ft
B= 8ft
D= 9 ft