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Spreader angle for marieholm26

Postat: mån 12 okt 2015 18:37
av Regatta
Hello!
This is a question that I looking for to solve my problem.
I saw in the forum the correct measures from back of the mast and the line between the spreader to calculate the spreader angle = 280mm.. But this is for If boat...But I check in my Marieholm 26 and this distance is about 245... I still have the original setup old... And the spreader has a little movement but No way to get this measure!!! I still working on it because I'm on the way to order the new mast... Anyone could help me!? Maja you know the difference?thank in advance!!'

Re: Spreader angle for marieholm26

Postat: mån 12 okt 2015 20:15
av maja
The correct spreader angle is dependant of the mast rake and the position of the mast step on deck related to the chainplates of the upper shrouds. Look at the IF-boat class drawings http://www.ifboat.se/klassritningar , especially 1324-2, and estimate the mast step and upper shroud position of your Marieholm 26. It could be different from the IF-boat. Estimate the rake by hanging some heavy item in the main halyard and measure the distance to the aft mast at deck level.

For an IF-boat, with a mast rake of 400 mm, the distance between the line between tops of spreaders to aft of mast spar will fall out to be about 260 mm, which is the minimum allowed according to the class rules. Experience has shown that the optimum is about 280 mm.

Anyhow, 245 mm is "in the order of" 260 mm... 8) ...
To get advice from the rig designer, contact Seldén headquarters in Gothenburg: http://www.seldenmast.com .

Re: Spreader angle for marieholm26

Postat: mån 12 okt 2015 20:31
av johan swe-1870
I am unfamiliar with the Seldén spreaders. Are they fixed or movable? Perhaps they can't move as much as the Proctor spreaders.

Re: Spreader angle for marieholm26

Postat: mån 12 okt 2015 20:37
av Regatta
Thanks Maja!! With the rigger we found 16 degree of angle ..little high.... we must check the old rig already take out.....the selden spreader move about 2 cm back me forward

Re: Spreader angle for marieholm26

Postat: tis 13 okt 2015 10:12
av maja
Johan SWE-1870 skrev:I am unfamiliar with the Seldén spreaders. Are they fixed or movable? Perhaps they can't move as much as the Proctor spreaders.
2 cm of movement means fixed spreders, which I think is the standard for Marieholm 26. Only IF-boats were obliged to have movable spreaders as per former class rules (not anymore).

Re: Spreader angle for marieholm26

Postat: ons 14 okt 2015 21:19
av maja
Here are some calculations:
Regatta skrev:With the rigger we found 16 degree of angle ..little high.... we must check the old rig already take out.....the selden spreader move about 2 cm back me forward
Deck geomtery for the IF-boat gives:
- half breadth at chainplate= about 1050 mm
- projected distance from chainplate to centre of mast= about 400 mm

This gives: tan(angle_d)=400/1050 = 0.38 and angle_d = 21 deg.

At spreaders:

- distance from spreader tip hole to centre of mast = about 810 mm
- projected distance from spreader tip hole to centre of mast = about (260 + 60) =320 (minimum distance as per class rules = 260 mm)

This gives: sin( angle_s)=320/810 =0.395 and angle_s = 23 deg.

As angle_d > angle_s, this setup will (with lower shrouds loose) push the mast forward a couple of centimeters at the height of spreaders. Also the rake will push forward a little more. Tensioning the lower shrouds will then straighten the mast.

Experience has given: sin (angle_si) = (280+60)/ 810 = 0.42 and angle_si = 25 degres as per North Sails trim guide. This gives an appropriate balance between mast rake, prebend and shrouds and forestay tension.

Please note that a too small spreader angle may cause, even with rather low tension on the lower shrouds, the middle of the mast to bend the wrong direction and a too great angle will bend the middle of the mast too much forward and force the lower shrouds to have unnessecary high tension or the upper shrouds to have too low tention in order to achieve the appropriate prebend.

Note those figures are theoretical from drawings and may differ slightly in "the real world" and between boats.

Also note that the old trim guide from the mid 70-ties by Olof Björland recommends even 27 degrees, but with less mast rake (about half compared to today´s recommendation).