To add thrust calculation to the boat propeller calculator, we need to know the propeller's pitch and diameter. The formula for thrust is:

thrust (in pounds) = (0.5 x air density x (pi/4) x diameter^2 x pitch x RPM^2) / (792000 x gear ratio)

where:

- air density is the density of air, which is approximately 0.002378 pounds per cubic inch at sea level
- pi/4 is the conversion factor from circle area to blade area
- diameter is the propeller diameter in inches
- pitch is the pitch of the propeller in inches
- RPM is the rotational speed of the propeller in revolutions per minute
- gear ratio is the gear ratio of the boat's transmission

Here is an updated PHP code for the boat propeller calculator with the addition of thrust calculation:

```
<!DOCTYPE html>
<html>
<head>
<title>Boat Propeller Calculator</title>
</head>
<body>
<h2>Boat Propeller Calculator</h2>
<form method="post">
<label for="diameter">Propeller Diameter (inches):</label>
<input type="text" name="diameter" required>
<br>
<label for="pitch">Propeller Pitch (inches):</label>
<input type="text" name="pitch" required>
<br>
<label for="rpm">Motor RPM:</label>
<input type="text" name="rpm" required>
<br>
<label for="gear_ratio">Gear Ratio:</label>
<input type="text" name="gear_ratio" required>
<br>
<label for="throttle">Throttle Setting (%):</label>
<input type="text" name="throttle" required>
<br>
<label for="density">Air Density (lbs/in^3):</label>
<input type="text" name="density" value="0.002378" required>
<br>
<input type="submit" name="submit" value="Calculate">
</form>
<?php
if(isset($_POST['submit'])){
$diameter = $_POST['diameter'];
$pitch = $_POST['pitch'];
$rpm = $_POST['rpm'];
$gear_ratio = $_POST['gear_ratio'];
$throttle = $_POST['throttle'];
$density = $_POST['density'];
// Convert throttle percentage to a decimal value
$throttle_decimal = $throttle / 100;
// Calculate speed in miles per hour
$tire = 6; // Tire circumference in feet
$speed = ($rpm / $gear_ratio) * ($diameter * M_PI) * $pitch * 60 * 0.0000113636 / $tire;
// Calculate thrust in pounds
$thrust = (0.5 * $density * (M_PI/4) * pow($diameter, 2) * $pitch * pow($rpm, 2)) / (792000 * $gear_ratio);
echo "<p>At $throttle% throttle, the boat will travel at approximately <strong>" . round($speed, 2) . "</strong> miles per hour and produce <strong>" . round($thrust, 2) . "</strong> pounds of thrust.</p>";
}
?>
</body>
</html>
```