Please read the following table in HTML format from a webpage and then answer the question in detail according to the table. Each table cell is defined by a <td> and a </td> tag. Each table row starts with a <tr> and ends with a </tr> tag. 'rowspan=m' and 'colspan=n' represents a cell spaning over 'm' rows and 'n' columns.

Webpage title:
German submarine U-775
Table title:
Summary of raiding history
Table:
<table id='hittask' contenteditable='False'>
<tr> <th colspan=1 rowspan=1 > Date </th><th colspan=1 rowspan=1 > Name </th><th colspan=1 rowspan=1 > Nationality </th><th colspan=1 rowspan=1 > Tonnage </th><th colspan=1 rowspan=1 > Fate </th></tr>
<tr> <td colspan=1 rowspan=1 > 6 December 1944 </td><td colspan=1 rowspan=1 > HMS Bullen </td><td colspan=1 rowspan=1 > Royal Navy </td><td class="highlighted"  colspan=1 rowspan=1 > 1,300 </td><td class="highlighted"  colspan=1 rowspan=1 > Sunk </td></tr>
<tr> <td colspan=1 rowspan=1 > 28 February 1945 </td><td colspan=1 rowspan=1 > Soreldoc </td><td colspan=1 rowspan=1 > United States </td><td class="highlighted"  colspan=1 rowspan=1 > 1,926 </td><td class="highlighted"  colspan=1 rowspan=1 > Sunk </td></tr>
<tr> <td colspan=1 rowspan=1 > 6 March 1945 </td><td colspan=1 rowspan=1 > Empire Geraint </td><td colspan=1 rowspan=1 > United Kingdom </td><td class="highlighted"  colspan=1 rowspan=1 > 6,991 </td><td class="highlighted"  colspan=1 rowspan=1 > Damaged </td></tr>
</table>
Question:
How effective were the U-775 submarines during the war? 
Answer:
German submarine U-775 sank one merchant ship, for a total of 1,926 gross register tons (GRT), one warship sunk (1,300 tons) and one merchant ship damaged (6,991 GRT).