HMY Rhiannon
| Name | HMY Rhiannon |
| Type | Navy Yacht |
| GRT | 137 tons |
| Country |  British |
| Built | 1914 |
| Builder | Hawthorns & Co., Ltd., Leith |
| Operator | Royal Navy |
| History | Peace-time owner: Baron Howard de Walden, vessel registered in Leith. |
U-boat attacks on Navy Yacht Rhiannon
| Date | U-boat | Loss type | Position | Location | Route | Cargo | Casualties | |
| 1 | 20 Jul 1915 | UC 3 (Erwin Weisbach) | Sunk | Mined off the Longsand LV. | 5 |
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