bilge pump discharge thru hull - Page 2 - SailNet Community

   Search Sailnet:

 forums  store  

Quick Menu
Boat Reviews  
Search SailNet 
Boat Search (new)

Shop the
SailNet Store
Anchor Locker
Boatbuilding & Repair
Hatches and Portlights
Interior And Galley
Marine Electronics
Other Items
Plumbing and Pumps
Sailing Hardware
Trailer & Watersports
Clearance Items

Advertise Here

Go Back   SailNet Community > On Board > Gear & Maintenance
 Not a Member? 
Old 12-02-2010
Omatako's Avatar
Senior Member
Join Date: Sep 2003
Location: Auckland New Zealand
Posts: 2,505
Thanks: 1
Thanked 31 Times in 28 Posts
Rep Power: 12
Omatako will become famous soon enough
On my boat the bilge pump pumps into the cockpit drain about a foot above the waterline. I've never had a problem with it. No chance of a syphon because the drain is open to atmosphere in the cockpit.

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.


"Two possibilities exist: Either we are alone in the Universe or we are not. Both are equally terrifying."

Arthur C. Clarke
Reply With Quote Quick reply to this message Share with Facebook
Old 12-02-2010
jrd22's Avatar
Courtney the Dancer
Join Date: Nov 2000
Location: San Juan Islands., WA, USA
Posts: 3,877
Thanks: 4
Thanked 22 Times in 21 Posts
Rep Power: 15
jrd22 will become famous soon enough
RichH- I understand what you are saying but isn't that all based on when the boat is level, not heeled over one way or the other? You got me thinking about why Brewer designed our discharges to be a foot or so below waterline. If the discharge is at the water line when level it will be quite high when heeled to the other side, depending on the angle, which would increase the total head by the number of feet above the heeled waterline (assuming the pump to be in the center of the boat). If the discharge is about a foot below waterline when level it will be app. that same foot lower when heeled over, increasing performance. Conversely, when heeled toward the discharge it will be lower in the water which doesn't seem like it would decrease performance much if at all. I agree with you that there must be some increased resistance discharging into the water as opposed to into air, but it seems like it would be minimal at the flow rates that most bilge pumps put out and the relatively large diameter of the hose (I could be wrong about that, I'll have to ask my brother the pump expert). The biggest disadvantage I can see to being below water line is the potential for growth, like you said, to block or partially obstruct the opening which could really decrease performance. The only other thing that I see as a possible negative to having thru hulls at or just above water line is the potential for damage from hitting objects (logs here in the PNW) in the water which generally are floating at the surface.
Interesting thread. I kind of like the idea of the bigger second pump discharging into the cockpit.
SV Laurie Anne

1988 Brewer 40 Pilothouse

Reply With Quote Quick reply to this message Share with Facebook
Old 12-03-2010
RichH's Avatar
Senior Member
Join Date: Jul 2000
Location: Pennsylvania
Posts: 3,262
Thanks: 20
Thanked 125 Times in 115 Posts
Rep Power: 15
RichH will become famous soon enough
jrd - all this is probably based on 'averages' because there will probably be that for an immersed discharge, there will be an equal average time when such discharge is above the water. Also if there is a 'siphon loop', and because the static head is from the *bottom of the bilge to the waterlilne ---- INSIDE the discharge hose* the relative 'relative static head' will remain quite 'constant'. When the discharge is above the waterline, it will take MORE energy to pump (because of the reduction of the 'siphon effect).

If the system is 'sized' correctly the velocity of the flow through the pipes/hose should be 'non-turbulent' and therefore the friction/resistance due to the flow in (excess) hose/pipe would be at a minimum (approaching whats known as essentially frictionless 'laminar flow'. That usually means (as a wild-assed guesstimate), using the next larger pipe/hose diameter and plumbing components than the discharge nozzle diameter of the pump, omitting any sharp bends or 90 degree 'elbows' whenever possible. When the 'velocity' of the discharge becomes 'high' and the flow in the hose is 'turbulent' there arises a lot of needless 'pressure drop' in the piping due to this 'friction', ... the (common centrifugal) bilge pump begins to 'slip' and automatically reduces its volumetric flow vs. increased discharge pressure. Most 'bilge' pumps are centrifugal ... and by this argument would favor a more 'constant displacement' type pump such as the old fashioned diaphragm or vane bilge pumps (ignoring their vulnerability/ poor performance when loaded with 'trash'/debris).
I use 6 ft. /second velocity as the max. limit to avoid 'turbulent flow' as a 'guesstimate or 'go-by' instead of doing laborious calcs. of each component, etc. 9 ft./sec. usually results in full turbulent flow --- high resistance and corresponding loss of centrifugal pump capacity.

"....... I kind of like the idea of the bigger second pump discharging into the cockpit".
Me too, I run my 'last chance' high capacity pump to the cockpit ... I want to SEE that the hi cap. is ON ... when you SEE it, your 'sphincter' tingles a bit 'faster' and encourages you into 'action' a little faster than waiting to be surprised to see the hatches in your sole floating by.

Last edited by RichH; 12-03-2010 at 11:23 AM.
Reply With Quote Quick reply to this message Share with Facebook
Old 01-02-2011
montenido's Avatar
Senior Member
Join Date: May 2008
Location: Los Angeles area
Posts: 323
Thanks: 0
Thanked 1 Time in 1 Post
Rep Power: 7
montenido is on a distinguished road
Hi all, interesting thread. I am getting ready to replace my primary pump and float switch, and will also add a larger "de-watering" pump. I did this on my last boat, adding a discharge thru-hull high up on the port side. I also tied in a 12v horn that would sound when the big pump went on. I like the idea of having the larger pump discharge into the cockpit, but have one reservation. If you get pooped will the additional water in the cockpit flow down the discharge hose and down into the bilge? I guess you still need to loop it above where it discharges into the cockpit.

Does this sound reasonable to you guys?

Thanks, Bill
Odyssey, '79 CSY 44 Cutter
Channel Islands, CA

"There is no unhappiness like the misery of sighting land (and work) again after a cheerful, careless voyage."
Mark Twain
Reply With Quote Quick reply to this message Share with Facebook

Quick Reply

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now

In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.
User Name:
Please enter a password for your user account. Note that passwords are case-sensitive.
Confirm Password:
Email Address
Please enter a valid email address for yourself.
Email Address:


Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
Thread Tools

Posting Rules
You may post new threads
You may post replies
You may post attachments
You may edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Similar Threads
Thread Thread Starter Forum Replies Last Post
Thru-hulls and bilge discharge (Coronado 25) BubbleheadMd Gear & Maintenance 8 04-19-2012 12:46 PM
Installing automatic bilge pump in series with diaphram bilge pump... sweetdreamyamaha Gear & Maintenance 6 11-07-2009 11:36 AM
Bilge Pump Discharge Location, Other Qs montenido Gear & Maintenance 2 02-04-2009 06:26 PM
Bilge Pump Discharge - New Thru Hull ??? Terry Clarke General Discussion (sailing related) 4 05-10-2003 05:07 PM
Bilge Pump Discharge - New Thru Hull ??? Terry Clarke Gear & Maintenance 3 05-01-2003 06:25 AM

All times are GMT -4. The time now is 01:04 AM.

Add to My Yahoo!         
Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2015, vBulletin Solutions, Inc.
SEO by vBSEO 3.6.1
(c) LLC 2000-2012

The store is owned and operated by a company independent of the forum. You are now leaving the SailNet forum. Click OK to continue or Cancel to return to the SailNet forum.