How should I size and install a holding tank in an LM 27, and where should it go?

For an LM 27 used by two people, aim for a 40 to 60 litre holding tank close to the toilet, with short hose runs and accessible connections. Check behind or beside the heads for space to fit an elevated tank; if there is too little room for useful capacity, assess the nearest suitable space under a berth for a pumped installation.

Measure before choosing capacity

LM 27 interiors and previous owners' alterations vary, so no single tank shape or mounting position will fit every boat. Expose the toilet discharge hose, locate its seacock and measure the surrounding lockers, including their access openings.

Make a cardboard mock-up of the tank, including its fittings and hose bends. A tank that fits inside a locker may not pass through the door. Sanitation hose also needs considerably more turning space than a bare hose tail suggests.

• 40 litres: a reasonable compromise for two people with frequent access to shore facilities and pump-out.
• 50 to 60 litres: the more useful target for weekends with two aboard.
• 70 to 80 litres: consider this for longer stays if the boat has enough space and adequate support.

For planning, allow roughly 8 to 12 litres per person per day with a reasonably economical manual toilet, then check actual use. Flushing habits and electric toilet models can change that figure substantially. Leave about 20% capacity in reserve: a nominal 50 litre tank then provides approximately two days for two people using 10 litres each daily.

Position and discharge options

An elevated tank beside or behind the toilet works well if there is room. The toilet pumps into the tank, and a bottom outlet can drain through the existing discharge seacock where discharge is permitted. For gravity emptying, the tank outlet must remain above the outside waterline, allowing for sailing heel and loaded trim. The discharge route must fall continuously.

Do not sacrifice most of the capacity to achieve gravity discharge. A narrow tank squeezed behind the toilet may be less useful than a larger tank mounted nearby.

A tank beneath the nearest suitable berth can preserve space in the heads compartment and keep weight lower. Deck pump-out works with this arrangement, but overboard emptying normally needs a separate sewage discharge pump. Keep the tank close to the toilet and avoid long hose runs through the accommodation.

Check the effect on trim, access to existing equipment and the strength of the supporting structure. Sixty litres of contents weighs approximately 60 kg before adding the tank and fittings.