How can i calculate forces mast applies on hull at his base?

[kistinie]

I need to move the mast forward on a trimaran to correct much too ardent behaviour
To calculate the beam to create, i need to know the forces vectors i will have to support.

Boat is 31' beam, loa 40', under 3 tons, possibly 2.6/2.8 ready to sail
Mast 14 metres; 1.4 metres above WL
Main is 40m², jib 20, code zero 70, spinnaker 90
Rigging designed to break at 6.5 tons

To have an approximate evaluation is this enough and what would be the formula ?

Thanks !

[daiquiri]

Hello, you can find some info in this thread:
http://www.boatdesign.net/forums/sailboats/vertical-down-forces-stepped-mast-please-help-24716.html
Have you tried to contact the manufacturer of your boat?
Cheers!

[kistinie]

Hello, you can find some info in this thread:
http://www.boatdesign.net/forums/sailboats/vertical-down-forces-stepped-mast-please-help-24716.html
Have you tried to contact the manufacturer of your boat?
Cheers!

Thanks for this

Manufacturer
No, he is dead. It was Lock Crowther.

The method used in this other post is wrong for me as my spectra rigging with 6.5 T break is much oversized i think.

[daiquiri]

Do you have a photo of your trimaran, view from ahead of the bow (or from aft of the stern)? I would need to see the geometry of the hulls.

[kistinie]

Since this picture, 1200Kg was taken off so water line is up 12 cm

[TeddyDiver]

I'm not sure, maybe there's some reasons not to, but would it be easier to move the daggerboards instead?

[daiquiri]

Oooops, I don't see the displacement of the boat... :)

[kistinie]

First because mast is notoriously to much backward

Also because daggerboad tunnel is structural and in perfect condition
Your are right, sorry i forgot weight !
Corrected in initial post too
Displacement is subject to change, but will be under 3 tons with wine, book, food, woman and cat inside, dreaming of 2.6/2.8

[daiquiri]

Ok, have to go now. I'll post the calc tonight. ;)

[daiquiri]

Hello,
you didn't reply to my PP, so I'm posting the results here anyways - could be a theme for discussion.
The first figure in attachment explains the symbology used.
The next two are the gross measures, deduced from your photos
The last two are the spreadsheet I have created for this calculation and the relative pdf print.

From the photos I have grossly calculated that your amas displace about 2.8 tonnes each. That coincides with your weight estimate.
Now, someone here said that assumption is the mother of all fuckups, but I need to make some...
So here is the first assumption:
- all the weight is supported by the leeward ama. It means that your trimaran is running at max heel before capsizing, which should be the worst case.
From the front-view photo I can quantify that heel at about 8°.

Now I need the sail forces. So the second assumption is that this calculator gives correct values:
http://www.wb-sails.fi/news/SailPowerCalc/SailPowerCalc.htm
I have made several runs, with sail angles from 10 to 85 degrees and apparent wind speed of 25 kts and created the sail forces vs. AoA table (you can find it in the excel spreadsheet below).
This is the weak point of the calculus, as I really ignore the precision of this on-line calculator - though it seems done pretty well.

The third assumption, if I have interpreted correctly your photos, is that your halyards are at the mast and are not attached to the hull. So their loads are not transmitted to the mast step. Also, I don't see any spreader over there.

I don't remember the fourth assumption, but well... one more, one less. :D

The pdf file gives you the results, so if you need to ask me something about it, just do it. The maximum values of each quantity is marked with yellow band.
I'm pretty confident that the vertical load is not far from reality (in spite of so many assumptions) because it coincides reasonably well with values given in Larsson & Eliasso's book (they estimate it as 85% of boat's weight).
The horizontal force is not taking into account the mast partner. I have made these calcs before seeing your last photo of boat interior framing.

And just one final remark: The numbers you see should be multiplied by a robust safety factor (at least 2.0 ) in order to obtain design loads for structural scantling of your mast step.

Now, since I've noticed that there doesn't appear to exist an uniform approach to this design problem, I would really like to read some comments from other people in the forum. If there are other, more reliable methods for estimating mast step loads I'm always ready to learn.

[daiquiri]

By the way, your shrouds appear to be dimensioned correctly, imho.
6.5 tonnes of breaking load might sound like too much but it is not, as I have argumented in this discusion:
http://www.boatdesign.net/forums/sailboats/vertical-down-forces-stepped-mast-please-help-24716.html
The correct pretension value should not exceed 25% of shroud breaking load, and that appears to be compatible with shroud tensions in the spreadsheet.

[yades]

I need to move the mast forward on a trimaran to correct much too ardent behaviour
To calculate the beam to create, i need to know the forces vectors i will have to support.

Boat is 31' beam, loa 40', under 3 tons, possibly 2.6/2.8 ready to sail
Mast 14 metres; 1.4 metres above WL
Main is 40m², jib 20, code zero 70, spinnaker 90
Rigging designed to break at 6.5 tons

To have an approximate evaluation is this enough and what would be the formula ?

Thanks !

:idea: :cool:

Have you tried "MAST & RIG EVALUATION example (329.5 KB)" in the spreadsheet library? It could perhaps be of some help....it's pretty complete though......