How to calculate my bilge-pump size?

[AndrewB]

I spent all Saturday fitting a small electric bilge-pump, which doesn't work. This was at the recommendation of the insurance surveyor - never mind that we have two perfectly good manual pumps. But I agreed a low capacity automatic pump would help to keep the bilge dry, even if it wouldn't cope with a major leak.

So I bought a Guardian pump rated at 500 galls per hour (½ litre per sec) through-flow. No other measure of capacity is indicated. I attached it to the end of a pipe that was pre-fitted in the yacht for this purpose. However, when it is turned on, although water rises up the pipe, it does not reach the outlet. There is no blockage, the pump simply seems unable to lift the water high enough.

The exit pipe is 19mm diameter and about 5 meters long. It rises 2 meters from the bilge to the top of a non-return loop at the outlet. This implies raising pi*1.9^2*500=5670cc=5.7kg of seawater by 2 meters.

Question 1: Should I demand my money back, or am I being hopelessly over-optimistic about the pump's capability?

Question 2: If I am being over-optimistic, then how do I calculate what size of pump I need?

To clarify, suppose I wanted it to deliver the ½litre per sec at the outlet, so it would need to empty the outlet pipe in 11.4 seconds. Am I right in supposing the following. The energy requirement is 5.7*2 = 11.4 kg-m or about 112 Joules of energy. This implies 112J/11.4s = 9.8 watts of power (or about 0.01 HP), not including frictional losses. But electric bilge-pumps do not seem to be rated in terms of output power, so how can I tell? On the input side, the pump I have uses 20 watts of electricity which sounds enough - but there are probably huge efficiency losses.

 

[Ships_Cat]

Rule, at least, provide curves for their pumps (they are, or at least were, on the internet). However, the submersibles typically have optimistic curves and flow collapses dramatically with head and with decreasing supply voltage. We recently had to double up the pumps in some compartments in some new builds due to poor performance, even though designed to their curves.

As an aside, I personally would not bother with them in a normally dry yacht (as a metal boat should be) that is either live aboard or is regularly visited. In most places small commercial vessels do not have automatic bilge pumps (to prevent oily water being pumped) and we don't find them sinking all around us - although most are attended daily, of course.

By the way, there is a steel yacht berthed directly across from us, I believe a Roberts and also called "Sentinel".

John

 

[Mirelle]

Change your surveyor

I agree with Ship's Cat - wholly unnecessary piece of kit (in a wooden boat, never mind a steel one) which serves only to promote stray current electrolysis.

Sounds like the chap does not know his stuff - none of the three surveyors who have looked over my boat in the last 20 years (she gets surveyed every 4 years, "whether she need it or no", because there are always things the surveyor sees and the owner misses) has ever suggested that there was any problem with just two manual pumps.

 

[AndrewB]

Re: Change your surveyor

I'm inclined to agree -- but once the insurance surveyor has made a 'recommendation', the insurers insist on it. We only felt relieved that the list of recommendations this time was mercifully short.

On a modern yacht with a mass of electrics like ours, a bilge pump is a minor item in the battle against electrolysis. The bilge sump is a box electrically isolated from the rest of the yacht - unless the yacht is sinking!

I'd like to have a totally dry boat, but its a council of perfection that due to flaws in the way it was designed and built, this yacht can never achieve. We are satisfied to have organised it so that everything flows to the sump.

 

[Mirelle]

Re: Change your surveyor

We have lights and a few instruments plus a couple of car plug sockets for chargers, fans, etc. Rewired a few years ago rather expensively with rubber covered large section cable, waterproof junction boxes, cable tray, etc. So far no sign of the dreaded electrolysis.