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Forums IJCA Forums
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IJCA Forums
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| Author |
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Steve Dallas
Posts:4
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| 03/20/2007 2:13 PM |
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So with the new mast on order, we now need to make sure that the new mast is to class minimum. We do not have a class measurer near by (closest is a 4 hour drive). Can anyone provide any clear instructions on the best way to do this? What do you use to actually cut mast etc. I really would like to measure 5 times and only cut once...the other way tends to get real expensive. TIA
Steve |
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Lorne Chapman
Posts:158
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| 03/21/2007 2:33 PM |
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A few things first:
- talk this through with your closest measurer by phone.
- if your mast comes with a pre-measured 7725 mm mark on the front - don't trust it. We have seen them off by as much as 8 - 10 mm when measuring.
- don't go by other boats - there is enough variation in the I-beam heights between boats to induce one of those "expensive" mistakes
- there are minor variations in measuring - it is not a perfectly repeatable process to 1 mm - so leave a few mm on the safe side for "insurance room"
- you need a 400 mm mast beam - if you can get a measurer to help - great - if not you need to borrow one
- keep the beer on ice until the job is done
To mark the mast (note - not cut yet)
1. Install the standing rigging on the new mast
2. Measure and mark the 7725 mm point on the front of the mast as per the Measurement Manual - use tape until you are absolutely sure where the mark is to be.
(available in Class Documents)
3. Have this checked by a measurer if at all possible - if not get some other sailors to read the instructions and repeat the process. Get them to look for systemic errors in how you are doing it. Even talk it through with a measurer by phone.
4. Step the mast - snug the standing rigging to minimize movement but there is no need to tighten.
5. Position the mast foot on the I beam approximately where you want it for tuning purposes and clamp to prevent movement
6. With some help - put the 400 mm mast beam on the boat in front of the mast. Be very sure that it is vertical and well seated on the gunwales.
7. If the 400 mm mast beam is BELOW the 7725 mark on the front of the mast you have that much room available to cut. If the 400 mm mast beam is ABOVE the 7725 mark you either have a short mast or a measuring problem - you need help!
8. If the beam is BELOW - mark the beam position on the mast front with a fine waterproof pen, move the beam, and put tape on the mark. Typically you might have 10 mm to 25 mm between the 7725 mm mark and the 400 mm mast beam mark. Measure this gap and write it down. This is what you have available to cut.
9. Do steps 6 - 8 again until you get repeatability that you are satisfied with. Take some digital pictures if you have to get remote help from a measurer - send them by email and then have a phone chat - before cutting.
10. When you are happy with all this - you can take the mast down, carefully preserving the two marks.
To cut
11. Get a steady platform to work on - a bench with a saw horse for the other end or something similar. Rolling around on a boat trailer frame isn't good.
12. Look at the dimension of the gap from 8. decide what you want to keep as "insurance room" and deduct it. Many use 5 mm, more aggressive folks go down to 1 or 2 mm. This is your cut distance.
13. Using a square - check the squareness of the cut on the bottom of the mast as delivered. If it is not square do your measuring for 14. from the longest point.
14. Measure up from the bottom of the mast (longest point if not square) the amount of the cut distance (12.) - make marks with the fine tip pen. Do this a few more times until you are satisfied with the repeatability and do not have any systemic errors in your measurement. Ask your crew or friends to check what you've done.
15. Using the square - draw line segments around the mast through the marks from 14. Going all around the mast - you should close the line at the starting point - if not work at it until fixed.
16. Measure the line to the bottom of the mast - is it the same as the cut distance? Are you really sure? Is the line around the mast square to the mast? If all this works - put tape with one edge lined up with the line on the mast to preserve the line.
17. The mast can be cut by hand with a good hack saw - by someone who is experienced using the tool. If you are not, find a friend who is. You get a much better cut with experience.
18. Finish the cut off with a file and some emery cloth.
Others will likely give you variations or add to this - watch this forum for more tips in the days ahead. You may get some more or better ideas.
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Steve Dallas
Posts:4
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| 03/21/2007 2:53 PM |
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Thanks so much for the helpfull advice. Quick question though, what is the 400 mast beam? Can one be purchased?
Steve |
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Tim Winger
Posts:49
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| 03/22/2007 3:35 PM |
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Steve,
Where are you located? Lorne is right about the measurement being difficult to do properly w/o experience. I also agree that the bands from Charleston Spar tend to be off significantly. It has been my experience on new masts from Charleston Spar that they are pretty close to min length already. You may want to consider the option of sailing it the way it came until you are somewhere a measurer can help.
There is a string jig that I use to get the height above the gunwale that is easy to reproduce and cheap to the point that I could send you one on loan...but it is not easy to use. It takes three people to use it, and is highly reliable once you know how. The key as Lorne says is to do the measurements over and over until you are abel to easily duplicate them.
Once you have your mark on the front of the mast and know how much you want to cut off, it is pretty easy to get a square line to follow around the mast. Simply take a piece of square copy paper or card stock and wrap it around the mast on your mark that is on the front of the mast, so that the ends meet at the back of the mast. This gives you the square mark. Buy a new hacksaw blade and cut it by hand. Start your cut on the waste side (below) of the mark. Cut very carefully and smoothly until you are through the wall of the mast. Then roll the mast and begin again adjacent to the first cut. Repeat this process all around the mast until you are through. It is almost impossible to cut straight through a mast section and keep it square. Clean up with a file.
Tim Winger |
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Steve Dallas
Posts:4
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| 03/22/2007 4:20 PM |
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Hi Tim,
Thanks for the reply. We are located in North Bay, Ontario Canada. We have a copy of the measurers handbook, and have several people that are willing to help (a few are skippers, so they can keep the beer cold). It seems fairly straight forward to measure the distance between the lower black band and the upper black band (unless i am missing something), the problem one is the 400mm below the bottom black band. If we didn't have to cut the mast that would certainly be the best, but I guess we have to go through the process to actually determine if it is necessary or not. Most certainly we would return the loaner string jig (we are Canadian eh!). If you have instructions on how to build one we could trade emails or even post it here as others might find it usefull.
For cutting the mast sqaure, what do you think about using an electric Mitre saw with an aluminium cutting blade. The mitre saws have a fence that if done correctly would ensure squareness? |
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Tim Winger
Posts:49
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| 03/23/2007 6:38 PM |
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Steve,
I think your idea to use the mitre saw to cut the mast will work fine. Just make sure it is square to the mast.
The mark on the mast to which Lorne and I have been refering is the lower band measurement point that is to be marked on the front of the mast at 7725mm below the center of the forestay fixing point - Rule 3.5.2 (e). Also see the Measurement Manual under rig measurement 5.1, Item #11 Rule 3.5.2e, and see figure 5.0. This is the point that must be 400mm above the hull/deck shearline intersect.
To get this measurement, you must use a very cumbersome hard jig with uprights that sit on the hull/deck shearline intersect on both sides of the boat and support a ridgid bar across the front of the mast. Or use the string jig, to which I referred above. The string jig sort of substitutes a tight string for the ridgid bar. Now you have introduced a lot more variables. The string must be very taught - no sag at all, so we use nylon with a little stretch in it. The uprights must be properly positioned at their base on the hull/deck shearline intersect, and stay vertical fore and aft as well as perpendicular to the string that runs between them. One person must position and hold each upright and maintain tension on the string while a third person directs the operation, making sure all is positioned correctly and then making the pencil mark on the mast where the string crosses the mast. The string and this mark may not be any higher on the mast than the bottom of the band we put on the mast in the previous paragraph. The distance between the bottom of the band and where the string crosses the mast below the band is the amount that may be cut off the mast. If you cut off exactly this amount, the string should cross the mast exactly on the bottom edge of the band when it is restepped after cutting. As Lorne suggests, you may want to consider allowing a 2 - 5mm cushion to allow for measurement mistakes, etc. If you find yourself a mm or two short after all is done, you can adjust this by putting a shim under the mast butt plate.
Tim |
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Don Merriman
Posts:14
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| 03/31/2007 3:19 PM |
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Steve,
Funny, I am working on the same project as this one along with some friends. Yes many skippers to keep the beer cold that is for sure. We have one person who is an IT person who should guide us with all of this. Well good luck with cutting your mast and welcome to North Bay! |
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Dave Barnes
Posts:27
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| 04/06/2007 10:11 AM |
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Joys of owning a J 24!
Welcome to my madness. I have just made a jig which I plan on using when the weather cooperates. I purchased an aluminum straight edge from Home Depot (in tool area next to tape measures). I will clamp the straight edge to two wooden legs. The clamps will allow the straight edge to be moved up and down to get the straight edge exactly 400 mm above the shear line. The bottom of the legs will be positioned inside the toe rail. Gravity and the toe rails will keep the bottom of the legs in place.
To find the shear line, I will tape two tooth picks on the boat corner. One facing out/down, the other facing up/out. The intersection should be the shear line.
I will then measure up from the tooth pick intersection point to the top of the straight edge using a ruler. I will adjust the top of the straight edge to 400 mm on each side.
Seems simple, but scary. If I screw up the wife will kill me! Can anybody see any flaws in my madness?
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steven clough
Posts:56
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| 04/07/2007 12:05 AM |
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Dave,
Take some pictures of this if you can. Sounds reasonable the way that you are performing the measurement. |
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Tim Winger
Posts:49
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| 04/07/2007 11:36 AM |
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Just a couple of comments on Dave Barnes' measuring device. Your idea is the preferred type of device with which to do this measurement. We typically use a 1" x 2" or 3" high aluminum box beam to construct this device in order to give us enough stiffness. A one inch beam will not work - I've tried it. The beam is about 103 inches long. If the limit of your straight edge is 8 feet, you can probably figure out how to make that work by adjusting the shape of your wooden verticals. it is important to keep that vertical measurement at 90 degrees to the beam.
When you construct the part of the wooden verticals that sit on the gunwale of the boat (the shear line), consider this surface is about 15 degrees from horizontal. So you want to notch in about two inches on a downward sloping angle of 75 degrees to the vertical. Then turn downward at an included angle of 95 degrees (which is 10 degrees inward from vertical). The clean sharp corner at the inside of this angle is your measurement point. When you put this in place on your boat, you will have to cut a relief notch to accomodate your toe rail. Measure 400mm straight up your vertical from the measurement point to mark where it should intersect the cross beam.
Mount one vertical with clamps or bolts, and sit it in place on the boat. Then position your other vertical so that both verticals are tight against the measurement point on the boat and perpendicular to the cross beam at 400mm. With everything clamped securely or bolted in place, remove the assembled device and check it by pulling a string very tightly between the two measurement points and measuring up to the bottom of the beam. You should use the bottom of the beam as your measurement surface so that the beam does not interfere with the mast boot or the deck.
Perhaps someday, we will at least have the vertical pieces available in the Class office to send out on loan to those who are making this scary cut on their masts. I suggest that you allow the mast to be about 2mm longer than minimum for safety. If you do go a couple of mm short, you can shim the mast with a couple of pieces of sheet aluminum under the mast base bracket. |
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Dave Barnes
Posts:27
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| 04/19/2007 3:21 PM |
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| I have pictures of the jig in use that I discussed above. dbarnes426@msn.com |
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