Bilge Pump Selection

My modern Knockabout Sloop will be engineless so I will also be without electricity (other than hand held battery powered devices). Therefore my systems will be few and simple. I would still like to have a good system for pumping out the bilge.

The Original Shields Class One Design was designed to have positive flotation by incorporating a forward and aft watertight bulkhead and watertight compartments in the floor and in the seat supports. This early add shows a flotation test.

The modifications required to for weekending necessitated removing the forward watertight bulkhead but adding a small enclosed cuddy cabin.

My first thought was to find out what the maximum amount of water would need to be pumped if the Shields became awash in water. So I calculated the total volume of water displaced when the hull is almost completely awash. In this way I can find out what the additional weight in water must be to become awash. This is an esoteric exercise arrived at by talking the water plane areas at various waterlines and multiplying them by their effective height to get the total displaced volume of water.

The first cut is the Waterline at 5' 0" which is the design waterline.

Awp = 82 square feet

Waterline at 5' 6"

Awp = 102 square feet

Waterline at 6' 0"

Awp = 117 square feet

Waterline at 6' 9"

Awp = 132 square feet

So the volume displaced, Vdisp, is:

V-disp = (82)(3/12) + (102)(6/12) + (117)(7.5/12) + (132)(4.5/12)

V-disp = 194 cubic feet

The weight of sea water is 64 lb per cubic foot. So the total weight of water taken aboard to become awash, Wt-awash, is:

Wt-awash = (194)(64) = 12,416 lbs

Since sea water weighs 8.56 gallons per pound, the total amount of water that would need to be pumped is:
1450 gallons

The following table is a quick summary of commonly available manual bilge pumps.

The highest capacity and one of the most reliable marine pumps is the Edson 30 gallon per minute diaphram pump.

This work horse pumps 1 gallon per stroke. So the Edson pump will take 48 minutes pumping at 30 strokes per minute to empty the boat from an awash condition. I don't think this is very reasonable. Perhaps it could be done in an emergency, but...

So, where does that leave me? Well it leaves me in the same situation as most of the rest of enclosed cabin keel boats. Most modern keel boats will founder if an excessive amount of water is allowed to get into the cabin. So my solution, like others, is to keep the water out of the cabin. This will be accomplished by designing a high bulkhead at the entry to the cabin.

One last check. I want to find out what the volume of a full cockpit will be and decide what pump I want to have installed so I can quickly empty the cockpit should it fill.

The widest section of the cockpit from the floor to the waterline at 6 ft 9 in is shown below:

Cockpit Cross-Section Area = 16.2 feet

I assumed that the area was constant moving aft and that the seats and seat flotation aren't there. The length of the cockpit is approximately 6 ft 9 in so the cockpit volume is:

V-cockpit = (16.2)(6.75) = 109 cubic feet

So the amount of water in the cockpit is:

= (109)(64) = 6,976 lbs

= (6,976)/(8.57) = 814 gallons

I like this number a lot better than the awash number. Using the Edson pump at 30 strokes per minute the cockpit could be drained in 27 minutes. In actuality this number would be less using the actual volume of the cockpit.

Here is a summary of the pounds per inch immersion numbers as the Shields settles into the water.

0" - 3" = 438 lb / in
3" - 9" = 547 lb / in
9 " - 16.5" = 625 lb / in
16.5" - 21" = 707 lb / in

So the Shields would potentially settle about 12 3/4 inches with the cockpit full of water. This would leave 9 inches of freeboard with the cockpit filled. This is a situation I would rather not experience first hand but I feel more comfortable knowing that I have options if I should need them.

With all of this information I feel much more comfortable shelling out the bucks for the Edson pump. I will likely make the compromise of going with the Aluminum version and maintaining it regularly to prevent corrosion. The Bronze version is a gem, but is a bit heavy for my application and even more spendy.