“Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did do. So throw off the bowlines. Sail away from the safe harbor. Catch the trade winds in your sails. Explore. Dream. Discover.” – Mark Twain

Tuning the Rig on a Catalina 310 with a Traditional Mainsail


Catalina does provide a pretty decent tuning guide in the owner’s manual.  But a lot of the specifics in that tuning guide are more qualitative then quantitative.  So it’s difficult to judge if you have tuned the rigging correctly.  I mean how do you really judge “a 50 pound push should deflect the upper shroud about 1″ at shoulder height”?

Loos & Co. Inc. does make tension gauges that give you some quantifiable numbers relative to your rig tension.  And we have the PT-3 Tension Gauge (a tool, of course I have it). But the answer isn’t to simply put every wire to 15-20% of its breaking strength.


To tune the rig correctly you need to account for rake (the distance aft of the mast base that the top of the mast will bend), prebend (the bow in the middle of the mast) and performance under load. Add to that the complication that the boat wasn’t constructed with all of the shrouds precisely located an equal distance from the mast base and that split backstays need to tensioned differently than a single back stay.  This can get more complicated then simply turning turnbuckles until you get to a number.

Side note on turnbuckles.

Turnbuckle with cover

These brass turnbuckles with a built in cover that Catalina used on our C310 are horrible.  There is not flat spot to grab these with a wrench, so you have to resort to putting a large screwdriver into the slot and using that to turn.  When you actually start to really torque down on them the soft metal bends and you can’t really tension them well.  I will eventually switch these out for standard, open body stainless steel turnbuckles.

Open body turnbuckle

Back to tuning.

Here are some resources that I used to put together a rig tuning plan: Selden Masts Hints & Advice (large pdf); Practical Sailor’s Article Boat Clinic: Tuning the Masthead Rig; the C34 IA Techwiki on Rig Tuning; and the C34 IA Rig Tuning Chart.

From what I have been able to gather from Catalina, riggers and other mast manufacturers when you setup a deck stepped, masthead rigged mast with perpendicular spreaders for cruising you want the mast centered between starboard and port, a 4-6 inch rake and a 0.5-1 inch prebend.

The forestay should be set at approximately 15-20% of the breaking strength of the wire.  This is impossible to measure directly because of the roller furler. You have to measure this indirectly by the tension on the backstays. This is where it really gets a little complicated.  The angle from the bow to the front of the mast is larger than the angle from back of the mast to the stern. This means the back stays will have more power in their pull than the forestay.  The forestay is 5/16″ wire and has a breaking strength of 12,500 pounds.  So 15-20% of this would be 1,875-2,500 pounds.  Based on the angle difference I estimated that the backstays are 20% more efficient than the forestay (this is a bit of a SWAG). So that would mean that I would be looking for a tension setting of 1,500-2,000 pounds on the backstays.  The backstays consist of a single 1/4″ wire from the masthead to approximately 12 feet from the stern.  At 12 feet from the stern the backstay is split into two 1/4″ wires at a stainless steel plate.  A 1/4″ wire has a breaking strength of 8,200 pounds; the tension setting of 1,500-2,000 pounds would be 18 to 24 % of the breaking strength of the wire.  Based on this, I set my goal tension at 1,500 pounds or 18% of the breaking strength of the wire. As I said, the backstay splits at 12 feet from the stern.  This point is too high for me to measure above the split.  So I have to take my readings below the split on each leg.  But that means accounting for the tension of both legs.  The angle of the split is approximately 25 degrees.  If the angle was at 45 degrees then each leg would just need to be set at 50% of the desired tension.  Using another SWAG, I estimated that each leg needs to be set at 60% of the desired tension.  That would be 900 pounds.

The upper shrouds are 5/16″ wire.  Based on my research I wanted these to be set at 15% of the breaking strength.  The intermediate shrouds are 1/4″ wire and should be set at the 10% of breaking strength. The lower shrouds set the prebend and are 1/4″ wire.  Since you want a 0.5-1 inch prebend you will have more tension on the forward lower shrouds then the aft lower shrouds. Neither should be more than 15% or less than 8% of the breaking strength.

Based on all of the above, here are my goal tension settings:

  • Backstays: 900 pounds, 10 on PT-3 Loos Gauge
  • Upper Shrouds: 1,875 pounds, 29 on PT-3 Loos Gauge
  • Intermediate Shrouds: 820 pounds, 9 on PT-3 Loos Gauge
  • Forward Lower Shrouds: 1,230 pounds, 13 on PT-3 Loos Gauge
  • Aft Lower Shrouds: 656 pounds, 6 on PT-3 Loos Gauge

In an ideal world these tensions would be equal on the port and starboard sides of the boat but that would not likely be the case since the boat isn’t laid out exactly symmetrical.

First thing I did was loosen all of the shrouds and backstays to hand tight.  I measured to two points on the toe rail that was the same distance from the mast base.  I then used the main halyard to measure if the mast was straight.  It wasn’t so I straightened the mast by adjust the upper and intermediate shrouds.  I sighted up the mast to make sure it looked correct.

Then I tensioned the backstays until I got to the desired rake.  But guess what? The turnbuckles bottomed out and I could only get about 3-4 inches of rake.  I measured the rake using the main halyard with some weight hanging from it (I used a water bottle).  The backstay is too long.  The easiest way to fix this is to cut about 6 inches off of the backstay above the split and use a Sta-Lok Eye Fitting to terminate the wire. But that will have to weight for another day.  I was able to get to 3-4 inches of rake and an 8.5 on the Loos gauge (around 800 pounds).  While not being perfect it would work.

Next I tensioned the forward lower shrouds.  At 12 on the Loos gauge I had about 1/2 inch of prebend.  I measure the prebend by attaching the main halyard to the goose neck for the boom and putting tension on it. I then made a visual estimate of the bow towards the bow at the point where the forward lower shrouds attached. I then took up the aft lower shrouds until they read 7 on the Loos gauge.

I then alternated port and starboard side, making a couple of turns on both the upper shrouds and the intermediate shrouds.  I kept checking that the mast was straight with the main halyard. Eventually I got to a point where I couldn’t tension the upper shrouds any more.  The brass turnbuckles were just warping under the pressure and I didn’t feel comfortable.  Both the port and the starboard were reading 25 on the Loos gauge.  But when I sighted up the mast and measure it with the main halyard it appeared that there was a slight bend to port. So I loosened the port a half turn at a time until it was straight.

The next step in tuning is to see how it performs under sail.  I simulated this at the dock.  When sailing in 10-15 knots of wind we are healing between 10-15 degrees.  So I attached the main halyard to a cleat at the dock and cranked it in until we were healing 10 degrees.  I then went and checked the “leeward” side.  Only the intermediate shrouds felt loose.  So I took up two full turns on it.  I repeated the process on the other side and got the same result.

I pinned the turnbuckles and I am ready to sail. The whole rig feels a lot tighter and more secure than it did prior to the tuning.  I had one of my sailing buddies that races a lot and has been crew on many delivers over and he took a look.  He was amazed at how straight I got the mast port to starboard.  He said he would feel very comfortable sailing on our boat offshore.

10 thoughts on “Tuning the Rig on a Catalina 310 with a Traditional Mainsail

  1. Wow… nice job learning and working out all that stuff!

  2. Great job Jesse – I can see that you have the potential for a future income stream to support long term sailing.

  3. Nice write up, I have my Loos gauge and plan to do this pretty soon.

    I bought all new turnbuckles just like the ones you have pictured from Defender and was very pleased with the price and quality, as far a rigging goes they were some of the least expensive parts.

    My concern is also forstay /backstay tension. Mine a pretty loose and I have only about a 1/2″ of take up left on the forestay. I hope I can get the proper tension on mine. On my lower backstay it is split and has a pully rig to adjust rake and tension dynamically, does yours have this? Something else I know little about…..

    • That’s a backstay adjuster/tensioner. My boat didn’t come with one from the factory. I thought about adding one but in researching them I choose not to put one on. Our boat is a masthead rig (I think yours is too) and backstay adjusters are not very effective on masthead rigs. If we had a fractional like the new Catalinas I would have added a backstay adjuster. Although I most likely would have gone with a piston style one not the pulley style.

      Also, in talking to some riggers and cruisers most long term cruisers don’t adjust the backstay tension that often. The constant loading and unloading of the headstay can lessen the life of those critical pieces. If your racing, especially in light winds, the backstay adjuster can be a very effective tool. But for cruising it seems that the best strategy is to tune your rig for heavy weather and just accept some reduced performance during light winds.

      It’s also part of the K.I.S.S. principal. Less things to break at the worst time.

      Good luck with tuning your rig and fair winds!

  4. Hi Jesse: 2 Old Pirates from Toronto ONT. I have a Catalina 310 2003 and I’m getting conflicting info about what guages to use for the shrouds. Will the Loos Gauge PT3 Pro deal with ALL my needs for tuning or will I need Loos PT1 & PT2.

  5. Thanks for a great post on rig tuning. I’m a week away from tuning my Catalina 387, and this post was very helpful. Now to amazon to by the tension gauge. Cheers!

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