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Discussion Starter #1
I am buying a boat and near my house there is a bridge height I am concerned about. When looking I see "mast height" listed in the specs. Is the mast height the max height measured from the waterline to the top of the mast (which seems right) or is it the height of the mast from where it is stepped causing me to do math.
 

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S/V Lilo, Islander 32
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I'm no more sure then you are, but it seems it *should* be from the waterline. What type of boat, and what specs are you looking at? If you find the sail specs and the forestay goes all the way to the masthead, then the "I" dimension in the rig dimensions should be the height from the bow to the top of the mast, which could give you a clue. If that is substantially less then the listed mast height, then I would say the mast height is from the waterline. Of course,if its not a masthead rig, then a little more figuring will be in order...

EDIT: I guess that would also depend on if its deck stepped or keel stepped.
 

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Also remember that bridge clearance on charts is at high tide, which means that you can get a little more clearance (or a lot, depending on the tides in your area) at low tide.
 

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Air Draft

Mast height is almost always measured from the water. But to be sure you know the how high the top of your mast is from the water, measure the actual height of the mast from the deck to the top of the halyard (tape measure) and then add a foot to account for the sheeve typically being below the actual top of the mast. Then measure the height of the deck off the water. You should be pretty close to the height to the top of the mast. Add height for antennas. I then add another foot for "safety" for bridge clearances.

As far as bridge clearance, yes the chart data is typically reported as clearance at high tide, but high tide typically changes so I'm pretty sure it as bridge clearance/height is reported at Mean High High Water. If you have an unusally high tide, the clearance is less.

My mast height is a little under 53 feet with antennas and such. We have to go under a bridge for winter storage that is 49 feet. The tides here are about 9 feet, so we have to wait until after mid tide on a falling to tide to go the 3 miles down stream to fit under the bridge when we start the season and go upstream for haul out at the end of the season during low slack tide on the flood side of the cycle.

DrB
 

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grumpy old man
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If you look at your "I" dimension it is usually taklen as a mast height from the sheer at the mast base. But for a lot of cruising boats the "I" dimension is from the top of the cabin top at the mast. I think Otter has it right. Don't trust printed dimensions. Haul a tape measure up to the masthead on a halyard and add some fudge factor for instruments and error, then add another 12" just to be safe.
 

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Discussion Starter #11
Thanks to everyone who answered. I gleaed a good bit of info. I likely will buy a Catalina 38 or 36 and willies the link provided by for additional info. Thanks again!
 

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If you look at your "I" dimension it is usually taklen as a mast height from the sheer at the mast base. But for a lot of cruising boats the "I" dimension is from the top of the cabin top at the mast. I think Otter has it right. Don't trust printed dimensions. Haul a tape measure up to the masthead on a halyard and add some fudge factor for instruments and error, then add another 12" just to be safe.
is not the "I" dimension the length of the forestay attach point to the shear line. used by sailmakers for jib measurements. only tells mast length from the deck for mast head rigs not fractional rigs
 

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Discussion Starter #13
I think so. Based on the link pirate sent I have learned a good bit and the I measurement doesn't help that I can see. I caveat that with my rookie status so take that info with a grain of salt. To compound the issue all the yacht world adds are sketchy on mast height. I have found very few that discuss this and even fewer telling the bridge clearance. There are 2 x 50 foot bridges in my area and a couple of 65 footers. It is important when deciding and I would think others would ask too.
 

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S/V Lilo, Islander 32
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Apologies, when I suggested looking at the I dimension, I assumed you where looking a specific boat, and had a specific number, but were not sure if it was from the water line or the deck. For instance, my boat has an I dimension of 37.8 feet, which is the height from the forestay attachment, straight up to the top of the mast. The mast is up on the cabin top, perhaps a foot or so higher, making the mast in the range of 36.8 feet +/-. Of course, this doesn't tell you about the overall mast height, but if an ad listed the height as 37 feet, I could deduce they where talking about the mast form the step up. If they said the height was 41 feet, you could assume that to be from the water line. If they don;t list a number at all, you could use the I to guestimate, but it would be just that.
 

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Over Hill Sailing Club
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The only way I trust that I actually KNOW how far the mast top is off the water is to climb up and drop a tape measure down. Using some theoretical spec. is trusting that it is as it SHOULD be. Also, if close, try to go through very slowly when there is slack current so you can back off quickly.
 

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A couple of times I've had to estimate mast height of the boat I was sailing, and did not have a long tape measure to run up the mast.

This idea seemed to work, if you have a mainsail with slides on the luff:

Measure the spacing between the slide cringles you can reach. Are they equal? Then say a little prayer that the sailmaker was consistent about this, and count the total of spaces. Multiply that number by the spacing distance. Add your freeboard and measure/estimate the height from deck rail to gooseneck/ main tack fitting.

Add a few inches for the cap fitting on the masthead, and you should be within a foot or so of the 'truth'. Close enough for government work? So far it has been for me.
 

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Most of the time you can Google the manufacturer and look up the specifications for every aspect of every boat model they're ever made. Unfortunately, it's not always accurate to the inch, and sometimes even the foot. I got lucky with mine and discovered the mast height of the 33 Morgan O.I. was 46 feet. Sure looks closer than that to me, though, when passing beneath the 50 feet clearance of the Havre de Grace, MD Amtrac Bridge. :eek:

Good Luck,

Gary :cool:
 

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You could measure out a known distance from your boat, site to the top of the antenna (or highest spot), measure the angle from your observation point, and use some trig to determine the mast height. Boat would have to fairly stable for an accurate measurement.
 

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Of course, you could always use length of rope, attach it to your halyard, pull it up the mast and measure the length of the rope between your boom and top of the mast. Then measure the distance from the boom to the water line, add the two together and you have a fairly accurate measurement.

Good Luck,

Gary :cool:
 
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