Log in
Article search:

Q & A

What depth should the rudder be?

We bought a 10 year old 58' semi trad narrowboat for cruising this year which had previously been used as a liveaboard in a marina, complete with washing machine. Now that the boat is lighter, it has been commented that the rudder is not sitting as deep in the water as it should. What depth should we be looking for and how can we achieve it? Also, what power will we need to run the washing machine whilst cruising, or do we need to remove it (which will make the stern even lighter). Thanks

Asked by: chrissy  | 10.47am, Monday 23 September

WW says:

Although how much the rudder is sitting in the water is important, perhaps more significant is if the counter is immersed. If there is a gap between the counter and the waterline, then ventilation can occur, where air is drawn into the propellor, which makes for noisy cruising and poor handling.
Many boats often have their rudders too deep (often as increased weight carried by owners as they get older) byt a general rule of thumb is that the blade should be immersed, with just the raised "tail" of the rudder above the water. If any part of the blade is exposed, the boat probably needs trimming a little more stern first.
As for washing machines, if it is running through an inverter, you will generally need a 3000W unit, that produces a pure sine wave. Over a normal wash, most modern washing machines consume between 1 and 1.5 units of electricity (kWh; kilowatt-hours). At a nominal 12V, this means that around 80 to 130Ah of current are needed; given inefficiencies of alternators and inverters, a starting point for budgeting for electrical consumption would be around 150Ah per wash- the hotter the wash, the more energy (spinning and rinsing take less current, but might require a higher start-up current; hence the need for a sizeable inverter).
A good battery bank, and decent alternator, will be able to buffer the energy used, and with, say, a 70A alternator, you might get 40A current at cruising speed (or possibly quite a bit more), so the take from the battery over the, say, 90min wash cycle would be fairly limited.
Hope that makes sense- please let us know if you want further detail.

Mark Langley  | 1.52PM, Monday 23 September

It is quite likely that a live aboard boat will end up deeper in the water than a holiday boat with all the extra possessions stored on board. Now that they have been removed the boat will be higher in the water; but this will only affect the handling of the boat if the counter is out of the water.
The counter is the flat bottom of the stern above the propeller which should be on or just under the surface of the water. The rudder directs the wash from the propeller, and if the counter is out of the water this will be inefficient, and stopping will be impaired. Generally the rudder should be entirely in the water with the exception of the taller part at the end, if the rudder blade is partly above water it would indicate that the boat is too light at the stern. Ballasting the boat will solve the problem.
A domestic washing machine will need a 3kW supply, but often lower rated appliances are fitted to boats. The rating will be on the back of the machine or in the instructions.

Rupert Smedley  | 1.57PM, Monday 23 September

Readers say:

Thought I was sinking, rudder almost under water, then filled up my nearly empty water tank and rudder emerged.

Anthony Burnell  | 9.54AM, Thursday 3 October

You must log in to post an answer.