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Anti Siphon Valve Maintenance

Technical Articles

Tag: maintenance water

There are 3 anti siphon valves on our boats and they each have a critically important function and, as such, it is vitally important that the valves operate properly. I believe that the majority, if not all, of our C320’s use the vented loop with “duckbill” valve as opposed to the check ball type valve. The good news is that these valves are easy to access and easy to maintain.

The anti siphon valve in the port lazarette is used to prevent water from siphoning through the cooling water seacock, through the engine cooling system, into the mixing elbow, and into the Aqualift muffler when the engine is not running.

The other two vented loops are located in the “medicine” cabinet over the marine toilet. One is used to prevent water from being siphoned through the seacock and into the shower sump and the other prevents water from siphoning from the seacock into the toilet.

On the top of each of the vented loop assembly, there is a threaded plastic cap with a small hole in it and in which is a small buna rubber one way valve that resembles a duck’s bill. The valve is normally closed, but if a vacuum is drawn within the loop, the duckbill valve (which is pointing down, away from the cap) opens, allowing air to enter.

Maintenance is simple:

1/ Remove the cap and insure that the hole is clear. Never plug the hole and don’t replace with a solid cap. If water leaks from the cap, it means the “duckbill” is missing or defective.

2/ Remove the duckbill valve, being careful not to tear it. Clean with nothing stronger than white vinegar. Do not lubricate and especially avoid contact with solvents and solvent-based lubricants, such as WD-40. (Ref: Art Bandy, Forespar)



Shake, Rattle, and Roll

Technical Articles

Tag: diagnostics engine prop

Here’s an interesting story but not a good example of the usually good troubleshooting skills I pride myself on having...

The last couple of times I've been out, I noticed a sort of rattling under power but only at one specific rpm. The usually trusty, Yanmar 3GM was a little rougher at idle (in gear) also.

It sounded like I had caught a fishing line and the lure was whacking against the hull as the prop turned. The noise was louder in the cockpit, but barely noticeable in the aft cabin. Since it only occurred at 2000 (+/- 50) rpm, I was ignoring it.

Finally had a friend listen and he offered to dive the boat. No fishing line found. All zincs still tight to shaft. Shaft turned easily. No cutlass wear. No prop or shaft fouling (which was good since I pay a diver to clean every other month). No obvious prop damage.

So I decided to avail myself of the excellent alignment procedure and calculation spreadsheet that we have on our website (http://www.catalina320.com/article.php?story=2007053112223718&query=alignment). Opened up the engine to shaft coupling and it proceeded to open up to the tune of a .071" deflection at the top position!!! Side to side was fine.

So I said WOW!!! The front of the engine really needs to be raised (perhaps in combination with dropping the rear). Only then, was when I discovered that the jack nut on the portside front mount had come loose and was actually down to the base of the mount. So, that mount was doing nothing except constraining the engine foot from going any higher. What I couldn't believe was that the engine did not appear to be shaking wildly at the offending rpm, but apparently the shaft was whipping to some harmonic.

Anyway, I got the up/down alignment very close (.002) and gave up trying to horse the engine left/right once I achieved a .003 misalignment laterally. Runs smooth as silk now.

Funny thing, I was relating this story to a friend who had previously owned a C320, and he said that the exact same thing had happened to him.

So, checking the locknut and jack nut tightness, as well as the screws into the engine bed, are now on my annual maintenance checklist. Consider adding it to yours or to your pre-launch checklist. The alignment itself is non-trivial, but, checking mount bolts, checking cutlass play, checking thrust play, checking coupling bolts and wires, and changing the transmission fluid are simple things to do and should be done.

Winterization Tips

Technical Articles


Whether you winterize your Catalina 320 yourself or have your boatyard do it, the following can serve as a guideline for the work that should be accomplished.


Remember that the primary objective is to prepare your boat for the possibility (or inevitability) of freezing conditions. Fluids must be removed or protected and nothing should be aboard that might be damaged by low temperatures. As always, manufacturers’ instructions have priority. We’ll start with the engine, since that is best winterized prior to haul out, and then assume the other systems are winterized “on the hard”.

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Catalina 320 stemhead fitting sudden failure. Check your rig!

Technical Articles
Catalina 320 stemhead fitting sudden failure. Check your rig!
The design of the original stemhead fitting (the part that is bolted to the bow, to which the forestay attaches) lends itself to the initiation of crevice corrosion type cracks that progress through fatigue until the entire fitting gives way with a possible loss of your mast! This file shows where the crack originates, and some examples so you can better check your own rig.


This first picture shows side by side the original design (and a replacement without the side 'gusset plates'). The left end is the part that projects upwards towards the mast and to which the forestay, or bottom of your furler) attaches...

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Adding a Backstay Adjuster

Technical Articles

Tag: upgrades rigging

Submitted by Jon Vez


Following is the procedure I used to install a backstay adjuster on my 1999 320. This method requires no drilling and will work fine if you use a bimini.

The 320 has a substantial masthead type mast with no real ability to bend, so the purpose of the adjuster is not to induce mast bend, but to tighten the forestay when sailing upwind. Tightening the forestay allows you to point higher, reduce heel, and lessens the tendency to round up in strong breezes.

This configuration also allows you to back off the tension on your backstay for improved sail trim when off the wind. Many sailors see a backstay adjuster as an upgrade just for racers, but I believe that this upgrade is equally compelling for cruisers as well. Having a well trimmed boat with less healing and better upwind performance simply adds to the pleasure of sailing!

To create the attachment points for the adjuster, you will simply replace the clevis pins at each backstay tang with a D shackle. The pin that comes with the D shackle replaces the original clevis and the ‘D’ portion becomes your attachment points for the adjuster’s blocks and tackle.


I chose to create an adjustment on each side of the backstay. I originally took this approach to increase purchase, but there is the added benefit of creating less clutter in the cockpit. The starboard side is my ‘course’ adjustment and the leeward the ‘fine’ adjustment.

The actual installation time was about 3 hours for two people including tuning the rig.

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