Measuring Mast Rake

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Long Dogs
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Measuring Mast Rake

Post by Long Dogs » Thu Feb 10, 2011 1:56 pm

Does anyone know how to measure the mast rake?

On our dinghy it's specified as a length from the tip of the mast to the top of the transom. I've seen the displacement type specifications for the Swift, ie. a measurement of offset of the tip of the mast from centre or as a number of degrees off centre. The problem is where is centre?
Pete & Jan
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nickf
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Post by nickf » Wed Feb 16, 2011 4:30 pm

There are quite a few Swifts at Bucklers HardA few years ago I asked some of them what they did as some of the info. in the old newsletters was contradictory.They said go for about 8" offset at the black band at the bottom of the mast.
To avoid windage I pull up some very strong fishing line with a 8oz fishing weight on it using the main halyard

Just a thought, your ratcheted webbing on your photo appears to be across the sliding cabin hatch cover.I put mine across just aft of the cabin so that the boat cannot slide backwards on the rollers

I am sure that for either of the above operations there is not exactly a completely right and wrong way of doing things

A good examlple of this is that in the Swift instructions it says of the shrouds"They should be tight but not too tight"


I have waggled a few shrouds at the Yacht Club and they range from piano wire tightness to sloppy
Nick
Nicholas

Long Dogs
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Post by Long Dogs » Wed Feb 16, 2011 6:35 pm

Thanks for replying Nick. I'm probably being a bit dim but 8" offset from where?

The webbing is in an awkward position. It's only because that's where the loops are on the trailer. Actually the webbing doesn't pressure the hatch and it still slides back to the webbing easily. Maybe I should find somewhere else to attch it though.

There is some discussion apparently that tightening the boat down enough to stop it bouncing on the trailer is more likely to damage the hull. I've no idea, but we did tow it nearly 1000 miles like that and all seems ok. It'll probably have sunk when we get to the mooring this weekend.

The boats held forward by the winch to a loop on the bow which is probably a later addition?
Pete & Jan
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nickf
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Offset at black band near base of mast above boom

Post by nickf » Thu Feb 17, 2011 8:35 pm

Pull line with weight on to top of the mast using main halyard.Adjust cap shrouds and forestay (or roller reefing) to give distance between line and black band at rear of mast of~8"Ensure backstay tensioner is slack whilst doing this
Nicholas

Long Dogs
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Post by Long Dogs » Fri Feb 18, 2011 11:04 am

I think the attached guide along with the tuning guide in the technical section of this site should sort it....?
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Pete & Jan
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nickf
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MAST RAKE

Post by nickf » Fri Feb 18, 2011 4:38 pm

The Rig Tuning Guidelines indicate adjusting mast rake with 60%max Tension on backstayThe Swift18 Rig Tuning document recommends adjusting Backstay tension after adjusting Mast RakeThen obviously slackening it off when the boat is not in use
The 20cms--8" recommended is about what I set my mast to

I have found that trying to position my mast centrally by measuring side to side with the main halyard not very repeatable but gives an indication onlyI checked that the cap shrouds were exactly the same length down from the support pin.It is then possible to measure distance from 1st thread below shrto centre of pin on chain plate swivelThis gives a good start point
After I have tensioned all the shrouds I check for mast linearity by lying, with the boom off, with my head just below the mast and sighting up the mast
Any , even slight deviations can then be tuned out
Nicholas

Long Dogs
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Post by Long Dogs » Fri Feb 18, 2011 5:18 pm

I guess it's not important wether the backstay or shrouds are used to apply tension for rake measurement as long as the slack is taken out of the forestay and no pre-bend is created by the tension applied.

Here's hoping for a dry weekend!
Pete & Jan
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