Monday, April 14, 2008

Centerboard and masts

Second side of the centerboard glassed. Moving towards F-boat building...


I really appreciated the comments on the last posting, Silas and Martin! I think the language have made it difficult for me to express my self precise regarding the possible mast building plans, here are answers to your comments. Feel free to argue with me, I would appreciate it!

I do not intend to make the rig in other dimensions than specified, i.e. 10,7m mast (35'1") except for the dimension of the spar that is. Thus I will not need the middle section of the moulds and the thickest part will be somewhat smaller than the dimensions of the mould, but still considerably larger than the Marstöm spar (or the guidelines from the designer) and it will have to be as there are no spreaders.
I guess, however, that the spar should not be symmetrical but rather with the thickest part at the specified position for the spreaders, and thus the mast top will be the dimensions at the smallest end of the mould and the bottom somewhat larger.
As I am not an engineer I will have to pay someone to do some calculations on the required laminate thickness in order to achieve adequate strength. If it turns out that this makes the spar considerably heavier than the specified section it's a no go, I totally agree that weight aloft is not a good solution. I will save weight and drag in not having spreaders and diamond wires with corresponding attachment points for these, however.
It is quite a stick, but I don't see how it is too big if it is not too heavy? It will contribute to about 4 m^2 of sail plan though. And it will be more of a true "wing mast" rather than a low drag mast.
The Marström mast is certainly a nice one and costs probably around Euro 8.900,- plus taxes (in my case at least 25%) everything but halyards and forestay included.

A few more pictures of the moulds, made from GRP with ply reinforcements:

And gelcoat on the inside which needs some minor repairs


It is designed to have a center web lying across the section to pick up the forces trying to bend the section sideways.

5 comments:

silas said...

Ah hah, I get it now. With a bit of careful engineering it should work well. It's actually an interesting set of design constraints. I wish I had some experience with mast design!

I think you are right about not having the mast symmetric. The thickest part should be where the bending moment is greatest. If the shrouds went to the top of the mast, this would be somewhere around the middle. Since they don't, the point of maximum bending moves. If I get a chance I'll dig out an old text book and try to figure out which way it moves.

While their reputation is outstanding, at 8900 Euro the Marstrom spar is a bit of a kick in the nuts. Thats almost what I am estimating hull materials in corecell, glass, and epoxy to cost.

Rolf Nilsen said...

Ta nå endelig godt vare på formene. Det er flere som går rundt med drømmer hvor de er ganske sentrale i realiseringen..

MartinF said...

Hi again

Good idea, remove the biggest part, and you should be ok, although 4 m2 of "sail" you cannot reefe or take down is quite something, in windy conditions. Are you going to hard-race you boat??

I would like to know what the dimensions of the Marström is, compared to what you want to do.

I helped a friend building DN iceboat carbon-masts, some time ago.
Layers was to go all around, overlap aft. After placing different layers in the forward mold,(extra glasslayers was placed at the shroud and forestay fitting),a plastic tube was placed, then layers was folded over the tube, and the aft mold placed on top, carefully bolted to the lower forward mold. Plastic tube was then sealed (a knot was tied) in one end and the other end was connected to an air compresser. I think we inflated the tube to 3 bars. Not quite sure.

Martin

MartinF said...

I forgot to mention that the plastic tube is in fact a platic bag tube to press the laminate out in contact with the mold.

another real plastic tube (14 mm diameter) is used for track( for boltrobe) layed inside the aft part of mast and cut after cure.

Martin

Tor Rabe said...

Thank you all for your inputs. Martin, the method sounds great, except I would not be able to install the shear web this way. The bolt rope tube was used making the Mirage mast as well as I understand.

I have ordered the book "Aero - hydrodynamics in sailing" by CJ Marchaj on Amazone.com in order to find further theoretical considerations on the subject.

The discussion has also been running on on the discussion group of the Norwegian multihull association. Seems to be more good reasons for using this profile. It can be found on www.flerskrog.org, but unfortunately for those not familiar with Scandinavian languages it will be hard to understand. I'll get back with an extensive report.