measuring J and I

[dave777]

What is the proper way to measure J.

"J is the base of the foretriangle measured along the deck from the headstay pin to the front of the mast."

The issue is, what do you do about the deckhouse? Do you try to make the tightest possible line in the measurement - or do you press the measuring tape flat around the deck, the front of the deckhouse, the roof, etc. up to the base of the mast?

"I is measured along the front of mast from the genoa halyard to the main deck. The main deck is where the deck would be if there were no deckhouse."

Without drilling holes in the boat ;-) are we supposed to eyeball how much farther down the base of the front of the mast would be - or do we actually measure to the deck instead of the front of the mast?

[D.D.]

Could measure from the inside, could google specs for boat, could measure foot of jib....just say'in....

[Jim Cate]

For the J measurement, a tight tape from forestay pin to the lowest point on the mast where there is no deflection in the tape will give you a pretty close approximation.

I is a bit trickier, depending on the shape of deck and house. If you are concerned for meeting racing rules, let the official measurers do it. If it is simply to understand your sailplan, yes, eyeballing the difference from the real deck to the "virtual" deck should be close enough. If you have access to a laser level you can do a better job by projecting the level at the mast forward on to the forestay and then measuring the vertical drop to the deck, but that sort of accuracy is pretty meaningless in the real world.

All of the above is opinion, not supported by anything important!

Cheers,

Jim

[Stumble]

Dave,

You really can't measure either J or I. I guess it is technically possible, but it would take a precise tape, a good understanding of trig, and plumb bobs. Plus there is very little reason too. Unless the boat is a home design, the specs should be available from the designer, and there are a number of places online that have databases with this information.

That being said, to measure the J the easiest way is to first remove any pre-bend in the mast and ensure it is perfectly verticle. Then ensure the boat is floating directly on its lines, and is balanced for and aft. Then draw a line vertically from the bow chainplate, parallel to the mast. Finally measure the distance between the two parallel lines.

Of course you have to ensure that the mast is in the reference position both on the mast step and at the partners, which requires a call to the designer to figure out exacally where they are. And while you have him on the phone, you might as well ask about the designed J leingth too.



To get the I, once you have the J, just draw a triangle. Then measure from the bow chainplate to the genoa halyard. This is the hypotenuse of the triangle. The J is one leg, the right angle is the one between the I and the J, then a little trig will work it out.


Or go to sailboatdata.com and look it up. It's a lot faster.

[daddle]

The above posts are correct. If it really matter then the official measures will know how. If one is measuring for sails that is an entirely different matter.

[flink]

tie a piece of string to the base of your mast where it meets the cabin top. Pull the string forward until it is above your headstay pin.
Take another short string with a wieght on it and hold it directly over the pin.
Mark the string tied to the mast where the two strings intersect.
Measure the string from the base of the mast to the mark. You have found J.

Finding I

If you can go up your mast, tie a string from your spreader and drop it to you deck. Measure it. now measure the distance from the gennny halyard block to your spreader. be sure to take both measurements from the same side (top or bottom) of your spreader. You have found I.

[dave777]

Originally Posted by flink

tie a piece of string to the base of your mast where it meets the cabin top. Pull the string forward until it is above your headstay pin.
Take another short string with a wieght on it and hold it directly over the pin.
Mark the string tied to the mast where the two strings intersect.
Measure the string from the base of the mast to the mark. You have found J.

Finding I

If you can go up your mast, tie a string from your spreader and drop it to you deck. Measure it. now measure the distance from the gennny halyard block to your spreader. be sure to take both measurements from the same side (top or bottom) of your spreader. You have found I.

We have a winner!!

I could have a second person hold a level against the string to make sure it is flat.

sailboatdata.com is pretty spotty in its boat and data coverage

[flink]

Cool. I like winning. Let's play again. Give us another one.

[flink]

err...you were joking about the level right?

You do know that a level will do you NO good on a boat.

Tie your string at the base of the mast and keep it equadistant from the cabin top from the base to the fore-most part of the cabin top.

A level will do you no good.

Boats rock dude. At least mine does.

[dave777]

Well, the boats on the hard right now. But I should have said T square against the mast.

A fairly easy method to get a good measurement.

But it also answers my question of whether the "J" measurement is supposed to be increased by the additional distance from the base of the mast to the deck level assuming there was no coach roof, such as the hypotenuse straightline
from the tack to the base of the mast. Seems a safer measurement if one is ever going to do a race.