I have seen boats with chainplates that differ as much as 2”!

I believe that it is best to center the mast with regards to the hull and keel. The wind and water won’t know if your chainplates are off unless the mast and headstay react differently on opposite tacks. The goal is to keep the mast in column side to side, and then to control the degree of mast bend. I wouldn’t worry too much about the chainplates.

Make sure the mast is straight on both tacks. You also might want to measure the distance from each spreader tip to the top center of your transome. Check for any rotation in the mast relative to the hull.

Good luck!

Brian,
Not all boats are straight, additionally, some were in collisions and have had their sheer and toerails repaired ( often appearing cosmetically fine ) but resulting in a hard to center ( in the partners ) rig. Center the mast between the chainplates. Not to cause further confusion, ( assuming that your bulkhead is in good shape ) while you are measuring, try the belowdeck measurement from the stem fitting bolts to both the Port and Starboard chainplates ( this may at least provide some peace of mind about the positioning of your chainplates! )
Good luck and best regards!
________(\____(\__(\_(\(\___/)/)/)___/)__/)________

This whole discussion has made me get out the tape measure and now I am really confused. I have always centered the mast based on two marks on the teak toe rail that were perpendicular to the mast and an equal distance back from the tack shackle on the bow. Then I tried measuring using Harald's method using the chainplates. I came up with a different position, off by a 1/2 inch. After careful checking, it looks the chainplates are not equidisant from the side of the hull. So now I ask you, which do you think is more important, centering between the chainplates, or centering based on the hull?? My thought is to go with the chainplates, but I am not sure.

Any thoughts.

Brian

I have used Harold's method, only to discover that my mast is in exactly the right position.

Instead of using rope and fish scales I used a wire coat hanger which I straightened. By making a hook at each end I could attach the wire to the chainplate and put a pen in the other end.

The wire does not stretch and gives a very accurate reading.

Thanks

Ron,
Like I mentioned in an earlier posting directed at John: "there is a lot more to this that I'm certain you'll take into account, items such as: even spreader length/sweep angle, properly positioned mast tangs (not wallowed out), a plumb/straight mast bearing beam". The assumption was made that among those items to take into account are straight mast, tang/chainplates, correct bearing beam height ....etc.....etc. Essentially it helps to have measured to ensure you have a straight boat.
Good luck, and use a low stretch tape!

_____(\_(\_(\__(\_(\_(\(\_____(\_________________(\(\_

Its all very well, but how do you know your chainplates are accurately positioned?

Thanks Harold

Ben,
This clarification should help you along: Think of the line mentioned in my previous posting as a compass (the type you would use to draw a circle). Make sure you make some loops to accommodate the pencil and the scale. Put the scale through the loop and attach the pencil to the scale. Hook the other end of the contraption to the Starboard chainplate. Scribe the foredeck and seahood with the pencil while maintaining constant pressure (what is described as the "scribe" is actually a portion of the circumference of a circle that could be drawn with this "compass". The chainplate is in essence the center of the circle. Repeat the same process for the Port chainplate. The points where the "circles" (i.e.: the "scribes") bisect can now be connected using a yardstick to determine the center of the rig between the chainplates side to side. Now all you'll have to do is determine max J dimension. Good luck!

Harold,

Would you mind explaining, in a little more detail, where to mark the points on the deck. I know my mast is not centred, because better sailors owned it before me, and told me so.

Your method may help me sort it out.

Thanks

John,
The process resembles something you may have done in physics, or geometry back in high school. When the rig is down use a very low stretch piece of wire or line and hook it to a fishing scale (the kind you hold to weigh the fish). Hook the contraption through the starboard chainplate where the upper shroud attaches. Now put a pencil through the other end, and keeping a constant even pressure (check the scale gauge!) scribe on the foredeck, and on the seahood. Do the same for the port side. The intersection of these four lines provides you with two points, which, when connected should provide you with the correct placement of the center of your rig through the partners (it is now centered between the chainplates). The longer the line or wire the more accurate. If you're certain you can apply constant pressure you may not need to use the scale. There is a lot more to this that I'm certain you'll take into account, items such as: even spreader length/sweep angle, properly positioned mast tangs (not wallowed out), a plumb/straight mast bearing beam. Done right you'll be seeing the same results on both tacks. Best regards and good luck!

I'm curious about how you found out that your mast partners (molded in the deck) were off to 1 side by 7mm? Did you run a plumb line from stem to somewhere at the stern? Did you first align your keel and your rudder, and then plumb the stem to the rudder fittings? The boat I race on has UHMW partners, but I'm not so sure that the mast partners are down the middle of the boat...

Doug,
Very creative, but no it's not Ultra Hard Melted Wax. UHMW-PE Stands for Ultra High Molecular Weight - Polyethylene. HMW-PE or simply UHMW is regarded, in engineering circles, as one of the toughest substances known to man. UHMW has an abrasion resistance 10X that of carbon steel, a very high dielectric strength, and awesome impact strength. In fact UHMW is used as a key ingredient in bullet proof vests and body armor. You can see why it is desirable to use in the mast partners application on a J/24! UHMW has a molecular weight of 4,500,000 to 6,000,000 as compared to high density polyethylene which has a molecular weight of 500,000 or less, or when compared to low density polyethylene with a molecular weight of 50,000 or less. UHMW is inexpensive as a raw material and is generally available in standard sheet and rod stock. It is (erroneously) assumed by many engineers that UHMW-PE can not be molded into complicated shapes, and until recently they were right. This assumption seems logical since UHMW does not melt (that takes away another part of your comical Acronym). Under conventional molding technology, UHMW is impossible to mold, and until recently the only way to obtain UHMW in complex shapes was to machine a part from sheet or rod stock. Which is what I described in my previous posting. Best regards, and keep up the funny acronyms!

What is UHMW?
Ultra hard melted wax... :)

Tom,
Spartite is generally a fine product. Initially it was offered in a softer compound, which has since been replaced by a more hard and stiffer compound for use on a J/24. It can however be a bit tricky to install. Make sure you follow the instructions (they're very well written) to the letter or the block may be hard to remove from your rig. If you're not a do-it-your-selfer, see a speed shop or rigger about installing it. Spartite has a somewhat narrow temperature range to install (which may be a consideration if your J/24 is in a cold climate). In my previous posting I mentioned that UHMW can be susceptible UV, Spartite is not as prone as UHMW, but can degrade. Cover it or (as the Spartite folks suggest) paint it. Best regards, and good luck!

Would someone comment on the disadvantages of Spartite? I've read here about the low UV resistance but couldn't that be solved with some enamel paint? Other than that it seems that many of the posters here have tried it and moved on with little explanation. What else is less than great about it?

Hi guys, I have tried most of the suggested methods (Including the 2 piece wood blocks, the Spartite Kit, and Mickey's Allis blocks). The setup I liked best for racing was to use scrap UHMW stock. You may have seen "mast stairstepped blocks" on Etchells made of the same material. After careful measurement (and realizing that my partners were offset by about 7mm to Starboard),a local machine shop cut out the tapered one piece mast block. To finish it off they added a 1/4 inch piece (with finished rounded edges to complement the outboard curvature of the partners opening on deck) of the same material to the top of the mast block which was attached by countersunk screws. The machine shop used a bandsaw and Drillpress to achieve the result. The mitre was wonderfully tight (akin to Spartite), extremely light (we later crossdrilled it horizontally) much stiffer than other solutions, and easy to put on and take off. The result was in short fantastic! UHMW (like Spartite) however has somewhat low UV resistance, so we would duct tape over the top of it (but you could use any cover. The overall cost involved was less then that spent on the other solutions I have seen.

Blocking the mast has always been a pain. I have used every method imaginable, Spartight, wood blocks, shoes, wood shingles etc. They are all fairly heavy and not very good at keeping water out. You still have to use some kind of caulk and a lot of duct tape. I once hired a friend of mine, Paul Grimes, of Fusion Sailcraft grimesmc@aol.com to come up with a better solution.

He wrapped a section of my mast with Mylar packing tape. He then make a two inch fiberglass sleeve about a ¼” thick over the Mylar tape. The Tape allowed the fiberglass to release from the mast when it hardened. He then slid the sleeve off the mast and removed the Mylar tape. Then the sleeve edges were sanded. The finished sleeve was then placed on top of a 1/8” piece of G10 fiberglass board. The shape of the sleeve was then traced onto the fiberglass board and the hole cut out. The sleeve was then pushed half way through the board. It acted as a bushing to spread the load from the 1/8” fiberglass board onto the mast. The mast was then stepped and we carefully tuned the rig. We then held the mast in the correct position while Paul drilled right through the fiberglass board and the raised deck area around the mast partner. Bolts were dropped through the holes and the mast was held firmly in place. Paul then traced the outside of the raised deck area onto the fiberglass board. The mast was taken down and the excess material was cut off the board. He then made the holes through the deck much larger, and he filled them with epoxy. They were later re-drilled with the exact sized of our bolts. This prevented water from getting into the deck. To finish the job Paul glued a soft rubber material to the underside of the fiberglass board and the excess was trimmed off. He even found some rubber washers to go under the bolt heads. I filled the mast grove at the deck with a dab of silicone. The last thing he did was to make and exact duplicate without the mast hole so we could seal the hole when the mast was down.

The system worked extremely well. It was very light and perfectly watertight. No ugly duct tape was needed. I haven’t found the time to do this project on our current boat. It isn’t on top of the priority list, but I will get to it someday.

Peter, I bought the Spartite kit, but tried a quick solution first which will probably be my permanent fix. Make a cardboard template of the deck opening and your mast position in it. Get a piece of pressure treated 2x6 and cut a block that fits the opening tightly. Lay the template on the block and transfer the mast position. Cut this out with a sabre saw. Now just slide the block on from the mast butt when stepping. This holds the mast rock solid, and will not rot. Plus it's cheap!
I did not like the white blocks because they held the mast boot up too high. Try it. John

I made mine from a block of plastic cutting board that you get in the grocery store. I was in a rush, and the thing could be shaped a bit better; will probably re-do it for the spring. I don't know what the stuff is, but it may be nylon. It cuts well with a scroll saw, if you use a low speed so that it doesn't melt. Probably Delrin is the right material to use, and it should be available from a plastics supplier that you find in the yellow pages. The slices are screwed together with SS screws, and I made a top plate that covers the opening in the deck. If you don't move your boat around all the time, you can silicone it at the mast and around the edges of the top plate, and it's pretty waterproof. Looks tidy, and it certainly won't rot. The silicone comes off easily if you want to take the mast down. <

If for whatever reason you choose not to use Spartite, you can take a set of the factory TPI std wood blocks (your 101st set of course), cut them and rehab them to your desired position (max J, as near to center as your partners allow) make a groove in it for your Sailcomp wire (or not if you use a TackTick) and then mix up a batch of epoxy (West or otherwise) and coat the entire set, all surfaces, in resin. They will last just about forever.

See the thread under Mast Position. I highly recommend the Spartite system. It provides 360 degree support.

My old wooden mast partner blocks have just disintegrated for the 100th but last time. I had cut them down to max the J with the mast aft but I still liked them because they were shaped to fit the mast. Does anyone have suggestions on what to use to replace the old partner blocks?

---Commercially Available Blocks
Also if anyone is using the white (Layline?) blocks please give us your comments. What do you put in the side since these blocks are just for the fore and aft section and are not formed to the mast?