Get the value for a form select

Hello folks, im trying to create a simple select box for one of my forms.

So far i have created the select box option array by doing this:

$companies_result = db_query("SELECT nid, company_name FROM {node_companies}");

while ($record = db_fetch_array($companies_result)) {
     $companies_arr[$record['nid']] = $record['company_name'];
}

//and the form below like this

$form['vessel_owner_id'] = array(
    '#type' => 'select',
    '#title' => t('Vessel Owner'),
    '#options' => $companies_arr,
    '#description' => t('Please choose an option.'),
);

Now this form works fine and stores the vessel_owner_id into my table but if i want to edit this node, i would like the select box to be already selected on the option that was chosen when the node was created.

The HTML would be something like this:

<option value="4">Company 1</option>

Any ideas?

Cheers