Some Correspondence Between Bill and I

[Kevin Keller]

A few months ago I was communicating with Bill regarding trying to tune my boat. I thought some of you might find these emails interesting and learn something from them. I hate to see corporate knowledge not shared. I have some other ideas on how to get maximum lift from my sails and will try them in the future. If they work I might share them.

Please feel free to comment or make suggestions.

Kevin





Hi Bill,

We were emailing a few years ago about refurbishing my SC-20 hulls. As you might have seen I put an ARC-22 sail configuration on my boat after fixing the hulls. The boat looks great but probably isn't as fast as I think it's potential.

That potential has mostly to do with my lack of knowledge on correct sail shape, downhaul position, and mast rotation for each point of sail and wind conditions. So I wanted to make a small shroud tension gauge that could give me immediate results on those factors. Tom suggested I just chart speeds with the different positions but I don't think the winds are steady enough out here in Hawaii to get accurate results.

You wouldn't happen to know the approximate tensile force in the shrouds, static and under sail?

Any thoughts on me doing this?

Kevin Keller


Hi Kevin,
The SC20 tall rig or ARC22 rig is designed for light to medium air, 15knts max for good speed.
When I think of Hawaii, I see winds of 15knts and up, 20 to 25knts often. The standard SC20 rig is much faster than the ARC 22 rig in winds over 15knts. The 38.5ft tall mast and sail plan were designed for boats sailed in light winds, inland lakes like Atlanta, Georgia, drifting contests.
When Eric and I race the 22 in the Steeplechase, we frequently sail with our mainsail reefed to standard 20 size and a small jib. We have won the Steeplechase with this sail plan.
What do the sailplans look like on the big cats and tris racing across the ocean at 30 to 40 knots boatspeed. The sails they have up are only about 50% of max sail plan size.
At max speeds sailboats are torque limited, turnover torque sideways and pitchpole limited torque forward. Torque in our case is sail force times height from CB to CE. Lower the sails shortens the lever arm and for the same max torque limit, the sail force can be larger and boat speed greater.
Good sailing,
Bill



Thanks Bill for the reply.

I guess I misled you a little. I am still using the 33' mast however I put the self-tacking jib, boom-less main, and auto-rotating mast that the RC22 has. I think it has a little bit of a experimental-ness in it.

I think I get too much mast rotation when I am out sailing, even with the mainsheet blocks attached all the way forward. I really don't know most efficient rotated mast position I should have for each point of sail, I can only guess. Instead of guessing I could do some runs noting each position but it is difficult in gusting inconsistent winds we have.

I want to make an instrument that will measure the tension in the side stay that can give me real time feedback on my mast rotation, down-haul tension positions, and possibly the diamond stay tension. I was wondering if you knew how much static side stay tension there is and maybe how much I can expect with a 15 knot wind so I can design something in that will read those ranges.

Kevin



Hi Kevin,,
Putting a strain gauge on a shroud can be done. The problem is that there will be alot of noise in the signal. Every time the boat sails through a wave or even a ripple will put a jerk in the shroud tension, in the output signal, and then you have to separate the noise from the force signal that you are interested in. Developing this device and getting it working so that it is useful to you will be a major project in itself.
The max shroud load you would ever see is like 3,000 pounds. A normal working load is like 1500 pounds. Sailing with the trapeze reduces the shroud tension.

A simple boat speed measuring device is much simpler. Install a pitot tube, 1/16th ID SS, at the bottom of the rudder, 1inch up, and extending forward 1/2 inch beyond the rudder leading edge. Route the tube up the rudder in a groove 2 to 3 ins forward of the trailing edge and bring the tube out of the rudder just below the notch that holds the rudder in the up position. From here use rubber hose to carry the pressure signal to a MPH gauge which can be purchased at any motor boating store. Get one that reads low numbers, 30mph max. I've used these speedos for years, The cut across the rudder and up well past max thickness will not compromise the rudder structurally. Tom H can build you rudders with the tube installed if you like.
Bill


Hey Bill,

Thanks for the reply. That is exactly what I was looking for. You gave me some things to ponder. I do have a couple of extra rudders maybe I can play with.

I might have mentioned earlier that I have a slight bend in my mast right above the hounds. I will try to straiten it sometime because it screws up the auto-rotation. I do have a tall rig mast also. It has the dual diamond stays. I am thinking about cutting the tall mast to 33' and using it since it is straight. Do you think it is best to change it back to the single diamond stay configuration if I do?

Kevin



Hi kevin,
I just saw your question. If you usually sail in winds less than 10 knots, fix the tall rig mast. If you usually sail in winds over 10 knots and sometimes up to 20 knots, the standard rig is faster. The windsped/ boatspeed crossover point is at 15 knots.
If you shorten your mast, two spreaders are a little overkill structurally but in your situation they are much less expensive. A new single spreader bracket, which is longer than what you have, costs around $500 to $600 dollars. You already have two spreaders and wires that can be shortened. This is the low cost way to go. You may need to undercut the leech of the jib a little to clear the spreader brackets in their new locations.
Bill


Bill,

Just so you are not confused, the current mast in use (the one that has a slight bend in it) is the 33' mast. The extra one I have is the TR, which is straight. I usually sail in winds 15Kt + so I will stick with the standard rig. So what I was figuring on doing was taking the tall rig and cutting it down and putting on the single spreader from the standard rig. I assume the standard rig spreader I have is the same one they are putting on the newer 33' masts (if they still make them that small. How does that sound?

Mast theory has always been somewhat of a mystery to me. I have been told that with my current short mast the diamond stays should be tight for light winds and looser for the higher winds. I believe Tom said they are not used to put a prebend to flatten the sail.

What is the ideal shape of the mast? Should it be straight (laterally)? so the sail is like a flat wing? or should there be some lateral bend in it that will cup the front edge of the sail. I realize that might be a hard question to answer because of the mast rotation but is there a side to side curve you should have in the mast?

Thanks for your help Bill.


Kevin,
Prebend in the mast is an effort to stop mast inversion with the spinnaker. Set prebend to factory specs. The mainsail that you use with a prebent mast must be designed for a prebent mast. The luff curves are different and they don't work well mixed up. Just do everything like the factory says and everything will be fine and fast and you won't break anything. Forget the local experts telling you to run tight diamonds here and lose diamonds there. Also remember this; Tom and I have tried many, many many combinations of rigs and diamond tensions and sail cuts etc. You probably can't think up a combination settings we haven't tried. So, when Tom suggests you do something "this way", know that it is the best of many, many trials.

[GeneHacker]

Intresting. Thanks for sharing.