Wood-fiberglass Mast (ideas to improve it in 2022?)

I have been looking for ways to build a mast for a 50' trimaran, I do not have manufactures down here and the price to import one is really a big deal ($$$). So for the carbon-fan boys that is out of chance.

At first I was thinking to do a solid mast, but I found a couple that build a 20 meters wood-fiberglass mast that initially it was a Kurt Hughes project. They said that did not work very well (too much flex, even with the rig really tide, the shrouds are escaping the spreaders) , but they build another one (with their improvements), they increase the wood thickness and reduce the distance between sections, so this mast worked form them (no flex), they circumnavigate the globe with it and had it working for 10 years without problems. They sold the catamaran after 10 years.

These Pictures are from the first mast (Project Kurt Hughes).







These next pictures are from the second Mast with their modifications:













Picture above with both masts, the painted white one is the first one that they ditch it.

The new one they glassed it with triaxial fiberglass, total thickness: 7mm

I was thinking that it was a great idea, just wondering how I could improve it, open to suggestions! Maybe using foam for the internal sections to reduce weight, or maybe glass the inside (they just epoxy it the inside).
There is not much information how to calculate this type of mast, if anyone has any idea where I can find information how to calculate the distance between sections etc, of course depending of your righting moment... sails .. etc...
I could not find any other example with this type of mast, only solid wood or hollow bird mouth ones.

I know the best option is to buy a aluminium mast, not really a option here, hard to get one and it´s pricey. I can live with more weight and build it with wood and fiberglass, life is full of compromises.

thanks!
Dick

 

A belated welcome to the Forum Richard.

You mention that you are 'down south' - how far south?
Somewhere in South America?

I saw on your previous post here that you have posted a construction drawing by SP Systems for a 50' trimaran - is this the boat that you are currently building?
Strip-planking thickness for trimaran https://www.boatdesign.net/threads/strip-planking-thickness-for-trimaran.67207/#post-933823

SP Systems is now Gurit - it could be worthwhile getting in contact with them and asking for their advice re building a timber / epoxy mast for this trimaran?
You might be pleasantly surprised to find that their fee for advising is less than what you might expect (at least, I would hope that this is the case).
Composite Engineering https://www.gurit.com/de-de/our-business/composite-engineering

And you really should have this mast 'properly' designed - certainly not something that is designed on the proverbial 'back of a fag packet', or beefed up a bit in places until 'it looks about right'.
I saw the remains of a timber / epoxy composite fabricated mast after it failed spectacularly while in service carrying passengers - they were very fortunate that nobody was killed when the mast came down.

 

Is that a tapered mast? Can you post some dimensions and expected stiffness (EI). Or how much does the old mast bend? The one you did not like.

 

Hi Bajansailor, yes south America - Brazil

I haven´t started yet, I bought plans some years ago for a 49' Trimaran from Dick Newick.
The SP system it´s just a curious information that I found online, how they did the layup, that is not what I´m going to build. Thanks for the input I may contact them to see if it´s possible.

Yeap I´m looking for to make sure to 'over' design a little to make sure we will be safe for any extra conditions it might happen. Thanks for the input!

 

Hi rxcomposite, the first mast, project from Kurt it was tapered in the end, the second one it´s not. The cross section of the mast it was 750mm x 250mm. The sections from the first mast it was spaced differently. The second mast he was not willing to taking risks so he over strength it with sections from 40cm to 40cm, 10mm ply.

The old mast, they said, bent so much that the shrouds escape from the spreaders. I haven´t seen it, but this guy is a very experienced guy. He already built another 60ft catamaran before, the catamaran that they had at that time it was 53ft, and now he is building a new one 63ft. I mean he was not joking about the bent problem, it was critical, that´s why he ditched the first project.

 

If you can get the bendy mast for free, it can be stiffened with more laminates.

 

Interesting. The mast and sail follows the physics of an aircraft wing.

My scanner is down and cannot copy the book so I drew it.

Here is how the modern composite wing is made. The skin and ribs are made of sandwich laminate for stiffness. This gives max stiffness and requires only minimum ribs spacing. The main spar carries most of the load as it is in the centroid. The caps on the spars are unis and carries bending loads while the web carries the shear load. Main spar can be box or C spar. Web is mostly biax. The aft spar handles only the torsional load so it is not as robust as the main spar. The aft spar and the leading edge are solid laminate. This configuration gives the highest stiffness and lowest weight. The spar is tapered to save weight and to carry the diminishing load towards the end. The spar and skin layup diminishes in thickness towards the tip so expect a complicated layup. The caveat is that it must be prepreg or resin infused. Hand layup has too low a modulus and will flex like hell.

For an elliptical mast and sail, it is the induced torque from the sail that will be dominant in the design. The highest torsional shear will occur on the skin.

Commercial aircraft uses aluminum for the main spar and carbon fiber for the skin.

 

well I think it´s too late to look for it... 10 years has passed... but I would doubt that it would be a good option... the project from Kurt asked 5cm of carbon fiber ... but they put much more (they were afraid that 5cm it was too little) .... as you can see in the picture. I´m guessing one big problem it was internal structure. The top of the mast the last section the spacing was 2 meters from the project... and the vertical section (main spar) was also only ply..(not laminate).... if you compare with the new one there is a huge difference... but of course a lot of extra weight... but hey no bend

 

Wow... you are good on Excel drawing!...

Now that you explain this, it makes sense how they modified the mast. By the way they laminate with triaxial glass with 7mm on the sides and on the front and back spar it was laminated one over the other ... so.. double 14mm.

Could it be feasible to change something for divinycell H80 for example? (skin and ribs maybe) I´m asking because I haven´t run any simulation because I haven´t seen anything for mast like that. Actually I could not find any other wood mast built in this way. Haven´t look for wings by the way... but it´s a good idea..

To resin infusion each half of the mast and then glue both sides I think it´s feasible..... but I´m trying to avoid carbon ($$$).

 

Borrowing the drawing from rxcomposite to show how they built their second mast:

 

I messed up my excell drawing that time. Most time it is better.

You could use other than H80. Depends on the design (spacing of ribs). Can even be a light foam core, no ribs. Just relying on biax layers and unis. The core takes up the compression stress.

For a nodeless (no rib design, tapered, differing cross section), lay up can be 90/0/+45-45 with the uni 0 degree taking up most of the bulk. The ratio is about 15% biax on the outside to take care of shear, 70% 0 degree uni for bending strength, and 15% 90 degree fibers to prevent collapse/buckling. This is filament winding method.

Carbon uni is a necessary evil if you want bending strength approaching that of alloy. Eglass can be used for the 90 and +45-45 degree. The author cautions about using Eglass uni with +45-45 because of differing strain. Apparently the +45-45 Eglass has low strain compared to Eglass Uni. I haven't found any reliable data yet to confirm. Anyway, design is always a compromise. Bulk of 1 direction can be increased, thus Eglass uni can be used. For comparison, Carbon uni and certain species of wood has similar strain.

 

I was just searching more and found one similar project:



https://www.ericwsponberg.com/wp-content/uploads/wing-masts.pdf

 

Looks like an interesting project.I hope the earlier note of 5cm of carbon was a typo!The aircraft wing analogy is perhaps appropriate if we include biplane or small plane construction with struts as the rigging is more closely related to those types than to the cantilevered wing we see on modern airliners.I have no idea of the cost and availability of carbon and epoxy in Brazil but i fear that it could be a very expensive mast by the time it is sailing.

 

I also think that if you start to use carbon to try to make this mast stiff the price will skyrocket, need to use geometry for that. The 5cm it was not a typo . My initial idea is to start from this one, that I know that worked, at least for 10 years (I do not have any other example for this type of mast)

It has a "low" cost compared, weight is not the major problem, it´s for cruising. If I can improve it a little, maybe some weight, reduce the number of wood over divinycell, infuse to reduce the resin content, work on the layout with different layout for the fiberglass. I mean something that it´s feasible and will not increase the cost too much, where is possible.

 

I forgot to post the pictures finished.. here we go..