Choosing the right water filter for your boat comes down to one question: what do you need it to do? A filter that improves the taste of your drinking water is a completely different product from one that protects you from water-borne illness like giardia — or one that protects a watermaker membrane. Using the wrong filter doesn’t just mean wasted money; it can mean real damage to expensive equipment or water that isn’t as safe as you think.
Having filtered water for drinking, watermaker prefilters, and dock fill connections across two boats and many years of cruising, here’s what I’ve learned.
This article covers the filter cartridges themselves — the types, what each one does, and which to use for which purpose. If you’re still deciding which filtering system to install, start with my companion article on boat water filters.
The Basics: How Water Filter Cartridges Differ
All 10″ filter cartridges look roughly similar from the outside, but they work very differently inside. Before choosing one, you need to understand three things:
Size. Most boat filter housings use a standard 10″ by 2.5″ cartridge. That’s the size I discuss throughout this article and the most widely available worldwide. Always confirm your housing size before ordering.
Type. There are six common filter types, and they fall into two broad categories: sediment filters and carbon filters. They do fundamentally different things, and knowing which category you need narrows your choice immediately.
Micron rating. Every filter has a micron rating — the smaller the number, the finer the filtration. Ratings commonly range from 0.5 to 40 microns. Finer filtration slows water flow slightly, which is rarely a problem on a boat — except on a watermaker prefilter, where inadequate flow can damage the expensive high-pressure pump. This is one reason watermaker manufacturers specify their required micron rating carefully, and why you should always follow it.
A Critical Note About Watermakers
If your boat has a watermaker, its membrane is extremely sensitive to chlorine. Even trace amounts will permanently damage the membrane, which is expensive to replace. The only filter suitable for a watermaker freshwater flush system is a 0.5 micron carbon block — it’s the only type that eliminates chlorine completely rather than just reducing it.
The Six Types of Water Filter Cartridges
The six types are evenly divided between sediment and carbon filters. Sediment filters remove particles, debris, and rust — but do nothing for taste, chlorine, or microorganisms. Carbon filters improve taste, remove chlorine, and depending on the type, can remove bacteria and other microorganisms. Knowing which category you need is the first step to choosing the right filter.
Sediment Filters
On boats, sediment filters serve as prefilters — for watermakers, dock fill connections, and filtering caught rainwater between the catchment point and the tank. They are not used in drinking water systems.
On our boats, we used all three sediment filter types for our watermaker prefilter depending on what was available where we were cruising. The most important thing is getting the correct micron rating for your system — don’t stress about which of these three types you get.
Pleated filters
Pleated filters come in two varieties: pleated paper and pleated polyester. Paper is cheaper but deteriorates quickly; polyester is far more resistant to bacteria and chemicals and is the better choice for marine use. Neither does anything for taste or microorganisms.
Pleated filters are the most commonly recommended watermaker prefilter, and many cruisers try to rinse and reuse them with varying success.
String wound filters
String wound filters look exactly as they sound — string wound tightly around a core. Like pleated filters, these are for sediment only. Many now come in dual-layer versions that catch larger particles first, then finer ones, extending the filter’s life before it clogs. They cannot be rinsed and reused.
Polyspun filters
Polyspun filters are the third style of sediment filter. Reviews suggest they last slightly longer than string wound and may cost a little less. Like the others, they cannot be rinsed and reused, and they do nothing for taste, chlorine, or bacteria.
Carbon Filters
Carbon filters improve taste, remove chlorine, and — depending on the type and micron rating — can remove bacteria and microorganisms. These are the filters used in drinking water systems and in watermaker freshwater flush systems.
Carbon wrapped or impregnated filters
These are the most affordable and easiest to find of the carbon filters. They reduce chlorine and improve taste — but they do not fully eliminate chlorine. As noted above, even trace chlorine will damage a watermaker membrane. Do not use these on a watermaker freshwater flush system.
Carbon wrapped filters are typically polyspun or pleated filters with a carbon coating, usually available at 2 to 20 microns. They will not remove bacteria or microorganisms. For a boater who just wants drinking water that doesn’t taste like the tank, these will do the job.
Granulated carbon filters
Granulated carbon filters have a solid plastic outer casing and are easy to identify. They are good at removing chlorine, volatile organic compounds, pesticides, and many man-made substances, but they do not filter sediment effectively and do not come in a fine enough micron rating to remove giardia or cryptosporidium.
Like carbon wrapped filters, granulated carbon does not fully remove chlorine — do not use these on a watermaker freshwater flush system. Best use is simply improving the taste of drinking water.
Carbon block filters
Carbon block filters are the most capable of the six types — but not all carbon block filters are equal. Only a 0.5 micron carbon block filter will remove giardia and cryptosporidium, both of which can cause severe illness. A 0.5 micron carbon block is also virtually 100% effective at removing chlorine, making it the right choice for a watermaker freshwater flush system when you use bleach in your tanks or take on city water that may contain chlorine.
If you’re installing a standard plumbing filter housing for drinking water, a 0.5 micron carbon block is what I’d use. It’s what most watermaker manufacturers specify for their freshwater flush systems as well.
A few things worth knowing about carbon block filters:
- Effectiveness at removing sediment depends on the micron rating — very fine carbon block filters will clog quickly if your water has visible dirt or sediment. Consider a sediment prefilter if that’s an issue.
- Carbon block filters can remove small quantities of heavy metals but shouldn’t be relied on for high concentrations.
- Coconut shell carbon is the best material; it costs a little more but is worth it.
- Change according to manufacturer directions — carbon filters lose their effectiveness long before you’d notice any change in taste.
On our boats, we could never be certain of finding 0.5 micron carbon block filters in Mexico and Central America. We’d buy them on trips back to the US and carry them in our luggage. If you’re heading offshore, stock up before you go.
Images in this section are courtesy of DiscountFilters.com, which has a good selection of 10″ filter cartridges at reasonable prices.
Which Filter for Which Purpose
Use this table as a quick reference. If you’re using a watermaker, always confirm the manufacturer’s recommendation — micron requirements vary significantly by system.
| Purpose | Filter type | Micron rating |
|---|---|---|
| Drinking water — taste and chlorine improvement | Carbon wrapped or granulated carbon | 2–20 micron |
| Drinking water — full protection including bacteria and cysts | Carbon block | 0.5 micron |
| Watermaker prefilter | Pleated polyester or string wound* | Per manufacturer spec* |
| Watermaker freshwater flush (if using bleach or city water) | Carbon block | 0.5 micron |
| Rainwater catchment prefilter (between catchment and tank) | Polyspun, pleated, or string wound | 5–10 micron |
| Dock fill or tank prefilter | Polyspun, pleated, or string wound | 5–10 micron |
*Watermaker prefilter micron rating varies by system. Some manufacturers specify 5 micron; others require 10, 20, or even 30 micron to maintain adequate water flow to the membrane. Using too fine a filter can starve the pump and damage the membrane. Always follow your watermaker manufacturer’s recommendation.
Don’t Learn It All the Hard Way
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Carolyn Shearlock has lived aboard full-time for 17 years, splitting her time between a Tayana 37 monohull and a Gemini 105 catamaran. She’s cruised over 14,000 miles, from Pacific Mexico and Central America to Florida and the Bahamas, gaining firsthand experience with the joys and challenges of life on the water.
Through The Boat Galley, Carolyn has helped thousands of people explore, prepare for, and enjoy life afloat. She shares her expertise as an instructor at Cruisers University, in leading boating publications, and through her bestselling book, The Boat Galley Cookbook. She is passionate about helping others embark on their liveaboard journey—making life on the water simpler, safer, and more enjoyable.
Carolyn Shearlock says
Hi June! That’s one topic that I haven’t written about, partially because there is such variety between systems. Most systems have some way to divert a bit of product water into a glass and then you can use a tester like this one (this is the one we had and many other boats did too, costs under $20) — there’s a sticker on the back of it to tell you what acceptable tds (total dissolved solids) levels are.
Membranes will fail in one of two ways — either plug up so that the output just gets lower and lower, or be “destroyed” so that it’s not doing as good a job and the tds level will get higher and higher until it’s unacceptable.
The Boat Galley says
It really depends on the size (microns), the water quality, and how much water you put through it. Putting a large volume of somewhat dirty water through a very fine filter will cause it to plug up much more quickly than half as much clean water going through a coarse filter. Typically, they’d last about 3 months for us . . . but whenever we were heading to out of the way places, we’d assume they’d only last a month and stock up. The non-charcoal filters will slowly allow less and less water through, and you can simply change them when it gets unacceptably low. The charcoal filters will usually be labeled with how often to change them.
Stu Meisner says
Can a hand pump such as a small fynspray be used with a system with fine filters one could buy at a plumbing supply. In your initial article you said that these plumbing supply systems could be used with a foot pump, but does that include both a hand pump and fine filters ?
Stu
Carolyn Shearlock says
I haven’t personally tried it. A lot will depend on how much “suck” the pump can develop — a foot pump is pushing the water through the filter, which is much more efficient than a hand pump, which by its very nature, is right at the outlet and hence trying to suck the water through the filter. I’d check with the manufacturer of the hand pump if it’s critical.
Carolyn Shearlock says
I like the extra protection that the 0.5 micron filters give. Assuming you have a standard 10″ housing, this is the one I’d go with (be sure to check the specs to make sure it’ll work for you, I’m making assumptions about your system):
http://www.discountfilters.com/sediment-carbon-water-filters/everpure-dev910853/p111751/
Ivan says
Ok thanks I ordered that filter, now I’m looking at standard housings… Many to pick from… Do you know of any solid rated models?
Carolyn Shearlock says
I don’t really know that any are particularly better or worse than others. A handy feature to have is a pressure release button — makes it easier to unscrew the bottom when you need to change the filter. Also, get one that uses standard plumbing parts to connect to your water hoses, not any special proprietary quick connects (if one fails, it can be hard to find a replacement).
Read more in my article about Water Filtration Systems:
https://theboatgalley.com/water-filters/
Matthew Dehn says
There is another type of filter cartridge out there called a DI filter– you can run water from a Reverse Osmosis watermaker through one of these DI filters and you have water that is suitable for your batteries! especially nice not to have to find/carry distilled water back from the grocery store.
Carolyn Shearlock says
Some people do re-use them after back flushing and spraying. If you’re just using them to remove sediment, it should be okay to reuse them if the flow is okay.
Carolyn Shearlock says
Storing water for any length of time — particularly in clear containers — will let algae grow. The best way to stop it is bleach . . . but that’s one of the things we’re trying to filter out. Instead of filtering into containers, we just filter (with the dispenser) as we need it.
Gene Magee says
Hey Randy! Many years later, but what UV system do you use?
Carolyn Shearlock says
Glad it helped!
Carolyn Shearlock says
I’d use a 0.5 micron solid charcoal block filter OR get a Seagull filter, which is the best filter I’ve ever found: https://amzn.to/3JGtPlP
Carolyn Shearlock says
1. UV should be the last thing before the faucet.
2. I don’t know that any filter removes lime — it seems you need a water softener and/or reverse osmosis (that’s different from a reverse osmosis watermaker)
3. Remove filter and use food-grade antifreeze (aka RV antifreeze) in the hoses: https://amzn.to/3CW3Tii
Marla says
Thank you for all of the wonderful information. We went with the Seagull filters for our pressurized system. Still struggling with figuring out how to filter our manual foot pump water in the galley and head. It is a dual stroke pump so it gets stuck using a Seagull filter. Any suggestions?
Carolyn Shearlock says
If you have a problem with the Seagull, I think you’ll have a problem with any sort of a filter. I know several people who have Seagulls with foot pumps, but I think it’s the fact that you have a different type pump. In order to put a filter on that line, you might have to change the pump.