Race tuning

[Piffle4179]

Have recently purchased a DSI and am attempting to tune the rig for racing. Have started with the North Tuning Guide, but can not reach the mast heal placement measurement or the Rake measurement from tip of mast to transom. (Would require moving mast step forward and thus creating a very large rear rake angle, just does not look or feel correct.) Any thoughts or ideas?

[K.C. Walker]

These old boats are hand made and do vary in their measurements. The North tuning guide is a good place to start. My boat did not line up with their measurements either. What you want to do is get it set up kind of close, or as close as you can get to those measurements, and go sailing. With the breeze at 8-10 kn, hiked out hard in the straps so that the boat is dead flat, crew all the way forward crammed against the cuddy, and close hauled, with the board all the way down, check your helm. You should have a minor amount of whether helm. If the helm is off, adjust your mast rake. When you get it down, measure it so that you have a base point to return to.

If you are using North sails you are trying to get the proper prebend in your mast to match their sails. I'm guessing that when you say the mast heal you mean the mast base. So, part of those measurements take into account that you are putting bend in the mast with your setup. You could ask North what is there recommended prebend, if it's not listed, and at least set that up. I have set up mine with 1.5" to 2.0". Some people use a movable mast base to achieve different mast bend according to conditions.

[Piffle4179]

Thanks for your insight. North recommends approximately 1 inch positive prebend as a start. As to the mast base, my base is locked in a track both fore and aft. I do not see any adjustment to that while the rig is fully tensioned. I do see how prebend would move the tip of the mast aft, thus shortening the measurement to the transom. Does anyone use a plumb line to measure the distance that the top of the mast is behind the mast at the deck level? Might anyone have a starting measurement for the rake?

[K.C. Walker]

Right, these boats aren't set up to adjust the mast base location while under tension. Some of the early DS 1 boats had a mast Jack to be able to lower tension enough to move the mast base without having to pull the rig. Using a lever on the fore stay may be enough to lower tension enough to move your mast base. Regardless, the rules do not allow changing it while racing.

There are a lot of top sailors that don't adjust the mast base for conditions.

[Mariah]

I am trying to tune the mast to conform to the north sails tuning guide for my newly purchased main and jib. The transom to the aft mast measurement calls for an 11’ 1/2” measurement and mine measures 10’10” or 2.5” shorter than specified. The mast butt is mounted on a base plate that has the mast mounted as forward as the adjustable base plate allows. Also, the masthead to transom measurement is about 4” more the the north tuning guide. My question is : should I remove the mast base plate forward to achieve the 11’ 1/2” transom to mast spec. or should I try to get the 1” prebend some other way?

[GreenLake]

I am trying to tune the mast to conform to the north sails tuning guide for my newly purchased main and jib. The transom to the aft mast measurement calls for an 11’ 1/2” measurement and mine measures 10’10” or 2.5” shorter than specified. The mast butt is mounted on a base plate that has the mast mounted as forward as the adjustable base plate allows. Also, the masthead to transom measurement is about 4” more the the north tuning guide. My question is : should I remove the mast base plate forward to achieve the 11’ 1/2” transom to mast spec. or should I try to get the 1” prebend some other way?

There are actually two things that you want to control: mast rake and prebend.

The mast rake is controlled by the mast foot in relation to the mast partners (deck opening). If the position of the mast partners is known (and it should be uniform for all DaySailers), then the distance from transom to mast foot gives you the desired amount of mast rake.

Mast rake affects the fore/aft position of the center of effort. More rake moves it back. The further back it is, the longer the distance between it and the center of lateral plan, or the notional point where all the forces on the hull combine. A longer lever arm means the boat has a stronger tendency to round up, so you get more weather helm. With your measurements, you should see less weather helm than the tuning guide calls for.

It may be something that you can feel on the water.

Prebend is caused by the tension from the shrouds, which pull slightly aft. If the mast foot is in the correct position, then the mast top, will be in a known position when the rigging is slack. As you tighten it, the mast top should come back a bit.

Both mast rake and prebend can also be measured on the water. You can tie a weight to the main halyard at the dock and see the angle it makes with the mast (=rake) and you can hold it to the mast at the partners to make a straight line and observe the bend. Those measurements may not be as precise, but they give you an idea that the "magic" numbers actually do what they are designed to do.