Fresh Water Pressure Switch

If you have played with boats much, you know that the least reliable item on board is likely to be the pressure switch on the fresh water pump. To make matters worse, these are usually difficult to change.

We have been averaging a year of use on our pressure switches, and a year ago decided to install an industrial pressure switch. But it wasn’t until a recent failure during Steve’s evening shower that action was initiated on this long overdue project.

Square D fresh water pressure switch

This is a “Square D” pressure switch, connected to a schedule 80 plastic PVC “T”, ready to insert into the pressure line just after the pump.


In case you are taken with this concept, the above photo shows the ordering data.

inside of fresh water pressure switch

An oversized look at the guts of the switch. There are two sets of contacts, each with many times the surface area of the usual marine pressure switches. You can use the two sets in parallel, or save one set for later use. The contacts can be cleaned with a flat file. The springs are used to set the cut-in and cut-out pressure.

industrial size fresh water pressure switch installed on cruising boat

Switch installed. It works great, and the pump is operating at a much higher flow rate, indicating the old switch probably had a voltage drop across it, reducing voltage and therefore motor speed.

Switches like this can be bought in industrial supply houses for about US$35.

Posted by Steve Dashew  (April 10, 2008)

2 Responses to “Fresh Water Pressure Switch”

  1. Stephen Says:

    Hi Steve,

    After reading and enjoying so much of your wisdom – I kind of feel like I need to give back.

    Yes, water pumps can be a problem and I have found the same problem you describe. I have owned a 75 ft boat, a home on Lake Austin, Tx and a home on Antigua that all used water pumps for house hold use.

    Then I found and love Grundfos MQ Series pumps. I think they offer a 12 volt model. What is most special about these pumps is the pressure switch and pressure tank is built into the pump. It is a variable speed model and will self prime. They very simply just work great and cost about $500 USD.

    I am a fan of your writing and have read almost every page on this website and read and reread every page of Practical Seamanship and Offshore Cruising and those are thick books. They are easily the best on the topics.

    I am also amused at how wasteful large yachts are in all reguards to your designs. Your boat is better at cruising with twin 150 HP and a 8KW Gen. I believe in “In form follows function” and if you look at ocean going fish none are short, wide and tall like so many of the wasteful yachts.

    I have wondered a few things that are no covered in your books.

    Have you ever considered using a Heritage Diesel Stove for cooking, hotwater and hotwater heating? Have you ever considered no genset and only using small DC Alts with a tiny Diesel? Why don’t you use Kiss type WindGens as your books are not so clear on this? Have ever considered a design that could do Blue Water and Freycinet gauge for Europe’s canals. Why don’t you carry a motorcycle, 4 wheeler or some simple land vehicle on deck? Have you ever looked at the old Gardner English Diesel Motors? I belong to KISS thinking and wonder how your designs would work without stabilization?

    Here is my big question – Please don’t laugh – What would take to build a boat that push its bow up on a sandy beach, then winch it’s way up on a sandy shore, then winch it’s way off like some old Navy designs?

    What would a buget FPB 64 cost and would be interested in trading for a boy as I see you only have girls?

    Sorry for so many questions at once – pent up demand for knowledge. If you need some land time, I would be honored to share with you our home in Antigua.


    Stephen Sidaras

  2. Steve Dashew Says:

    Thanks for the feedback on pumps. We are always looking for a better pressure pump. Re dc gensets, we have built several of these with dual alternators and watermakers with throttles to control RPM, dialing it down as load drops off if the watermaker is not running. Re wind generators, they make sense in some applications. Personally, we do not like the blades whirling around anywhere near our bodies, so have not seriously considered them. Might see you in Antigua one day. We are long time members of AYC and have not had a Cap Horne Pizza in a long time.