Calibrating Your 505

by Ali Meller
Updated March 26, 2006

Why Calibrate

The 505 is a very adjustable boat that gives those racing it the opportunity to setup the rig and foils to best suit the conditions, their weight, and the course the boat is on. Correct settings will make the boat faster and incorrect settings will make it slower. While some choose to set the controls by feel, use of a calibration scheme will make it much easier to set up the boat to that it is the same as the last time you sailed fast in these conditions. It will also make it much easier for you to exchange tuning/setup information with other 505 sailors. This should make you fast more consistently. Use of a well understood and easy to use calibration scheme, in conjunction with a tuning matrix for the correct sails, will help newcomers to the class determine basic settings and get on the pace, more quickly, and will make existing 505 sailors more consistently fast.

About This Calibration Scheme

A group of 505 sailors including Ethan Bixby and I have been discussing a calibration scheme. We believe that some of the measurements used earlier, such as measuring rake from the mast tip to where the hull and transom meet, are not very reproducible across different 505s. We decided to alter the way some of the measurements were taken so they would be more consistent across different 505s.

We have tried to develop a calibration scheme that supports the following:

This calibration scheme is not yet complete. So far we have ways of calibrating mast rake, mast bend, centerboard angle, and jib lead fore and aft position (same as up and down), though this last requires a North jib with the double trim lines, at least to determine where the "0" mark goes.

North Sails has a Tuning Matrix that tells you what numbers to set each control to, for a given wind range.


Mast Step
Most -- but not all -- tuning sheets suggest the mast be stepped as far aft as allowed by class rules, and we agree. Since altering mast butt position will change your mast bend calibration, check the butt first, and move it if necessary. The back of the mast (back of the extrusion) measured at the height of the mast step/centreboard cap should be just over 10 feet zero inches from the aft end of the boat. Measure from back of mast (extend the sail track down if it is cut away) along top of CB cap, aft, over transom, to aft most point of hull (may be slightly aft of transom). Ignore rudder fittings. The mast should be stepped as close to 10' 0" as you can get without being too close to transom. 10' 0.375" is close enough.

Note 1. The aft end of the hull is where the class rule measurements are taken, not the actual face of the transom. This value is more reproducible across different 505 hulls.
Note 2. Many 505s have a mast gate positioned such that you cannot step the mast at 10' 0" and still ram it straight at the standard rake. If this is the case, move the mast butt forward in the step until you can rake to 3'4" with a straight mast. You may wish to consider rebuilding the back of the mast gate area so that you can rake to 3'4" with a straight mast.

Mast Rake
We measure rake forward to the bow, Here is why
1. Ensure your spinnaker pole topping lift enters the mast just below your jib halyard turning block. Remove the topping lift from the pole, and tie a small loop in the spinnaker pole end of the spinnaker pole topping lift. Insert the end of a tape measure in the loop. If you happen to have a shackle on the topping lift, use that to fasten the tape measure to the topping lift. If you have a stuff luff boat, you can use the jib halyard instead of the topping lift. Cleat the topping lift or jib halyard such that the tape measure inserted in to the loop or shackled to the line starts a t the top of the "gooseneck band" on the mast, with reasonable tension applied. Don't have a gooseneck band?

It helps if you have a low stretch spinnaker pole topping lift, as the measurement will vary less with the tension you put on the tape measure. Try to pull a similar tension

2. Take the tape measure forward and measure from the end of the halyard/topping lift (at the same tension) to the top of the rail at the bow.

3. Adjust forestay/jib halyard and shroud tensions until a measurement of 3'4" is obtained.

The shrouds should be just tight; I use 180 pounds, which is number 18 on the Loos gauge. At this low tension the mast should be straight without having to play with the mast ram, but check anyway and use some ram down if necessary to get it straight.

4. Mark that setting on the forestay tackle in the boat, where you can see it. This is the standard rake number; mark it in the boat as "4". By the way, this is the same as the old 25'8" aft rake measurement.

If you have a stuff luff jib system, you can also mark the luff of the jib at the height of the cotter pin connecting the forestay to the forestay extension.

5. Ease forestay/jib halyard and tighten shrouds until the measurement is 3'5" (tighten the shrouds to keep them at 18 on the Loos gauge). Mark that setting as "5". Mark 3'6", 3'7", 3'8", 3'9" (if you can rake that far) as 6,7,8 and 9, respectively. If your boat can rake further forward than 3'4", mark that as well. We are currently using rake positions as far forward as 3'2".

Mast Bend
We measure mast bend at deck level. Here is why.

Using the marks that you put on in the Mast Rake step above, set the mast back to the 3'4" rake, with the Loos gauge at 18 (180 pounds of tension).

Check that the mast is straight fore and aft (it should also be straight sideways!), and adjust the ram if necessary to get it straight fore and aft. You can check the mast by sighting up the mast track after taking the boom off the gooseneck. A more thorough way of checking is to run the main halyard down the back of the mast with enough tension for it to be straight. Then check for any gap between it and the back of the mast part way up the mast.

With the mast straight, put the deck scale on the deck, with "0" lined up with the back edge of the chafe plate, with 3/16" increments, positive going forward and negative going aft. You will find it hard or impossible to read these numbers while sailing the boat, so you now have to create a "repeater" set of numbers that you can easily read.
A good way to do this is to mark lines on each side of the mast, such that you can line up the line and the top of the ram car, while sitting in the normal skipper's position on the seat tank.
Some of the European 505 sailors make a pointer to reduce any parallax error.
You are going to end up with a nice scale at deck level that you cannot read, and a bunch of magic marker marks on the side of the mast, but the point is that when comparing two 505s, the marks on the side of the mast are in different places, with different increments unless the boats happen to have very similar ram geometry (fittings in same location, same length tube, etc.). So we cannot make a pre-fab scale for the side of the mast that works on all 505s.

We have ready made scales for deck level mast bend and jib lead you can print off, cut out, and tape to the boat or mast with clear packing tape. We also have scales for centreboard angle.

Shroud Tension
Shroud Tension: I don't calibrate shroud tension, simply feeling the tension by pulling on the shroud while sailing upwind. However, if you want to calibrate it, try this. With the rake at 3 and the mast ram at -6, use a Loos gauge to set shroud tension to 200 lbs. Mark the shroud tackle here as 3,2.

While a new or a well built older 505 can handle high rig tension, many older boats do not have the structure or fittings to handle very high rig loads.

With the rake at 3 and the mast ram at 4, set shroud tension to 300 lbs. Mark the shroud tackle here as 3,3.
With the rake at 4 and the mast ram at 3, set shroud tension to 500 lbs. Mark the shroud tackle here as 4,6.
With the rake at 5 and the mast ram at 3, set shroud tension to 500 lbs. Mark the shroud tackle here as 5,5.
With the rake at 6 and the mast ram at 3, set shroud tension to 600 lbs. Mark the shroud tackle here as 6,6.
With the rake at 7 and the mast ram at 3, set shroud tension to 700 lbs. Mark the shroud tackle here as 7,7.
With the rake at 8 and the mast ram at 3, set shroud tension to 800 lbs. Mark the shroud tackle here as 8,8
If you've gotten this far without breaking the boat, congratulations!

Jib Lead Fore and Aft (Up/down)
In light or no wind (or in the lee of something), with the mast rake set to "4" as above, and the ram set to "0" as above, sheet the jib in tight. Snug up the jib cunningham to a normal sailing tension and so there are no wrinkles in the luff. A loose jib cunningham leaves the jib slightly higher or lower (depending on how the cunningham is rigged) and could affect your jib sheet calibration. Sight down the trim lines drawn on the jib clew (The North jib has two trim lines, use the upper one). Move the jib cars fore or aft (or move the lead block up and down if that is how your boat is rigged) until the jib sheet is lined up with the more vertical trim line. I sight the trim line by standing on the leeward side of the boat in the dinghy park and pushing my head against the jib until I can line up the trim line with the jib sheet. Mark that hole on the jib track as "0". Number holes with positive numbers forward from there, and negative numbers aft of that hole. The angled aft (slightly closer to horizontal line) is for heavy air depowering when raked. In practice, you may not need to actually move the jib car for heavy air, as rake changes the lead angle for you. You may need to move the jib lead aft up to 3 holes for light air, flat water conditions, and forward 1 or 2 holes for light or medium air and lumpy conditions. I normally sail forward one hole, so at the +1 setting.

Note the Ronstan track used on many 505s has holes spaced at 3/4" intervals, so our scale does too.

A capability to move the lead inboard may be an advantage, particularly on a forward tack boat.

Jib Sheet Tension/Jib Twist
Put a mark on your jib sheets where you can see it against the seat tank, when sheeted in close hauled. Put a scale with 1" increments on the tank such that if you snap the jib sheet tight with one hand on shore with the jib not filling (light air or in the lee of something), the mark is at zero.
Roll the boat over on its side. Pull the centreboard out and put it in the full down position. Set the leading edge of CB perpendicular to bottom of boat at centerboard trunk. Mark head of board (perhaps aft end of head) level with the top of the CB cap. Mark this as "0". Mark increasing numbers (board going back into the trunk) in 1" intervals up the back end of the head. Mark "1,2,3,4,5,6,7,8". Also mark a position with the board as far down (raked forward) as possible.

Note - we're open to suggestions on this. This measurement will result in different effects with different CB planforms. I think what we are really trying to measure is where the center of area or pressure is fore and aft and below the hull

Set spreaders to 17" length (measured from side of mast to shroud), and for 5.5" poke (Note, these are for the current North mainsails, the poke for the older CE2 mainsail series is 4.5"). For lighter teams, and in very light or very heavy air, increase poke to 6". Lighter teams can also shorten spreaders to 16".

Note that this measurement is from the shroud to the closest part of the mast, along the spreader. This measurement assumes that your spreaders are 124 inches above deck level (both UK and US specs call for this) AND assumes your shroud chainplates and mast step locations are the same as other boats. If any of spreader bracket height, mast step position and shroud chainplate location are different on your boat, these measurements will not result in the same effects we get.

Squaring The Rig
It is worth checking that your rig is square in the boat. By coincidence there is an article in the March Sailing World about doing this on keelboats. Our jib is actually easier. Hoist a tape measure up the main halyard and cleat the halyard. Measure to the shroud chainplate, or rail at the chainplate on one side. Use a bit of tension on the tape measure. Walk around behind the boat to the other side and measure to the equivalent spot. Are the measurements the same? If not, the tip of your mast is not centered over the boat (We assume the rails are level). Adjust shroud lengths until you get the same measurement side to side. Now ram the mast straight fore and aft and check for sideways bend. You may need to shim one side of the mast gate or the other, or for more drastic adjustments sand one side of the mast gate, or even move the mast step slightly to one side or the other, to get the mast to setup straight sideways. Before doing anything drastic, take the mast out of the boat, stand it vertical and site up it to see if there are any permanent bends. Try to straighten those out, and then go back to checking the mast in the boat. Take lots of measurements of mast gate and mast step before moving either. No boat is perfectly symmetrical, but some are off by more than others. I have moved a mast step sideways about 1mm on one boat, and removed 1.5mm of material on the side of the mast gate on the other to get the mast to setup straight sideways. We had a coach take photographs of the rig from astern confirming it was bending more on one tack than the other before we sanded one side of the mast gate.

Squaring the Spreaders
With the boat sitting on its trolley, stand to one side about 20-30 feet away and line up the two shrouds near their base. It may help to have some one pull the trap wires out of the way while you do this. One you are positioned in line with shrouds, slowly look up to the shrouds just below the spreaders. Do they still line up? If they do, great. If one is now forward of the other, that spreader on that shroud is set further forward. You want the spreaders to have the same settings so the rig bends the same on both tacks.


Mast butt position:
Almost all 505 sailors like to set up the boats with the mast butt as far aft as possible. I believe this is because it reduces overlap between the main and jib, and makes the slot larger. Some sailors consider this to be so important that they specify low mast steps in their new boats. Then when they rake the mast back to the normal sailing position it is actually slightly further aft in the boat. It is important to check the mast butt position -- and move it if necessary -- as the mast bend calibration is changed whenever the mast butt is moved. You need deck, gooseneck and mast top bands to measure in at major events. Check the class measurement rules online for where the deck, gooseneck and top of mainsail bands should be, and mark them on your mast. For now put a mark on your mast level with the top of the foredeck (the mast should be vertical in the boat when you do this), and then measure up the mast 381 mm and mark that. The top of the gooseneck band should be at or very slightly above this second mark.

Note that the forward rake measurement is much less sensitive to mast bend, shroud tension, hull rocker, and height of transom than the old way of measuring aft using the main halyard. Most 505s have straight rails, whereas transom height and rocker are different. This measurement is sensitive to the height of the deck and gooseneck band above the rail, but I believe most 505s have maximum round in the deck, to raise the deck and gooseneck bands as high as possible.

For comparison purposes the following table compares mast rake measured aft and measured forward.

Measured AftMeasured Forward
25' 0"3' 8"
25' 2"3' 7"
25' 3"3' 6"
25' 5.5"3' 5"
25' 8"3' 4"
25' 9.5"3' 3"
25' 10.5"3' 2"

Mast Bend:
So why not simply put 1/4" marks on the side of the mast? The reason is that 1/4" marks are not reproducible across 505s with different mast ram heights. If I have a low ram track, a 1/4" change in mast ram height will be a greater change in mast bend than on a boat with a high mast ram track. By basing the numbers on deck level measurements, we get settings that are reproducible across different 505s.

You will note that these marks on the side of the mast have different spacing between them than those on the deck, which were all 3/16". On the side of the mast the increments will be smaller at the bottom of the range than at the top, and they will be larger than 3/16".

For reference to Ullman numbers.. The Ullman/West Coast system had "7" as the straight mast setting, while the North system has "0". The Ullman system uses fixed 1/4" increments on the side of the mast. With typical Waterat mast ram height on mast and tube length, these 1/4" increments are very roughly 3/16" inches at deck level, though as you go lower on the mast 1/4" translates to less and less measured at deck level. You might try the approximation of the Ullman bend number as 7-North number, as they use "7" as the straight mast number and increase the numbers going down the mast. The Ullman mast bend number that corresponds to our mast bend of "3" is "4" (7-3=4).