Help needed to design a curved U section beam

[Westcountryboy]

Hello everyone

I am building a mini transat yacht and have nearly finnished, the last part is to build is a curved beam which supports the main sail track.

The track has a radius of 3.2 meters with a overall width of 2.7 meters. The track is 32 mm wide or 1" 7/32. The unserported part of the track is 1.7 meters.

I would like to build the beam out of plywood and then vacuum bag carbon fibre to the beam.

I have been told the load on the track is 500kg. The mainsheet block has a breaking load of 1100 kg. I would like to make the beam stronger then the block so should be able to withstand excess 1100 kg.

It would be great if I could get some help to determine the size of beam needed, what amount of carbon needed and advice on the fibre direction.

Thankyou in advance

Tony

[Olav]

Hey Tony,

very interesting project! Would you mind to show us some pictures of your progress?

Your mainsheet track issue doesn't sound too complicated to me. Basically you have a bending beam with 1700 mm length and the ends may be regarded as fixed supports. The maximum bending load will occur when the traveller car is locked in the middle and the sheet load acts at 90° relative to the beam (i. e. straight upwards). Load components within the plane of the track can be assumed to be taken by the tackles that control the traveller car.

Now there's the curvature of the track which give an offset of 150 mm compared to a straight beam, resulting in torsional loads at the ends of the unsupported part of the track.

A laminate schedule would therefor call for unidirectional fibres along the track to withstand the bending loads, and some additional fibres at +/-45° to take the torsional stress. The shear force will be taken by the plywood core.

If you can provide me with some more detail on that track and the boat's structure around (a scaled drawing would be best), I could try and figure out some numbers. PM me if you're interested.

[Westcountryboy]

Here are some photos to give you an idea of the boat

Tony

[Petros]

If it were me, I would not use plywood. It adds a lot of weight and half of the grain direction is useless to add strength. I would rip my own veneers of some light weight wood and laminate with all the grain running the same direction. Most of your strength will come from the carbon fiber wrap anyway.

I have a lot of experience with structural design, it would not be difficult to calculate the loads on it, but I am not familiar with carbon composites, So perhaps Olav or someone else would be better for specifying a layup schedule.

Good luck.

[waikikin]

Originally Posted by Olav

Hey Tony,

very interesting project! Would you mind to show us some pictures of your progress?

Your mainsheet track issue doesn't sound too complicated to me. Basically you have a bending beam with 1700 mm length and the ends may be regarded as fixed supports. The maximum bending load will occur when the traveller car is locked in the middle and the sheet load acts at 90° relative to the beam (i. e. straight upwards). Load components within the plane of the track can be assumed to be taken by the tackles that control the traveller car.

Now there's the curvature of the track which give an offset of 150 mm compared to a straight beam, resulting in torsional loads at the ends of the unsupported part of the track.

A laminate schedule would therefor call for unidirectional fibres along the track to withstand the bending loads, and some additional fibres at +/-45° to take the torsional stress. The shear force will be taken by the plywood core.

If you can provide me with some more detail on that track and the boat's structure around (a scaled drawing would be best), I could try and figure out some numbers. PM me if you're interested.

Just a passing thought............... would a central post or tension member to cockpit sole/whatever cost much less in structure & weight in the scheme of breaking the span on the curved track or the use of a strait track with a short track to underside of boom to keep main sheet load aligned , maybe not as "cool" looking though?

[Olav]

Originally Posted by Petros

If it were me, I would not use plywood. It adds a lot of weight and half of the grain direction is useless to add strength. I would rip my own veneers of some light weight wood and laminate with all the grain running the same direction. Most of your strength will come from the carbon fiber wrap anyway.

In my opinion, plywood as a core material has quite a lot of merits in this case: It's fairly cheap, it's quite easy to work with, can take enough shear stress and don't forget the numerous bolts that are used to attach the aluminium track for the traveller car to the beam. Bending and torsional strength and stiffness come from the carbon laminate, of course.

Originally Posted by waikikin

Just a passing thought............... would a central post or tension member to cockpit sole/whatever cost much less in structure & weight in the scheme of breaking the span on the curved track or the use of a strait track with a short track to underside of boom to keep main sheet load aligned , maybe not as "cool" looking though?

Yeah, that was my first thought too, but from the pictures I see that the boat will have a central tiller that would otherwise interfere with such a pillar.

[Westcountryboy]

Hello again

Just wondering if there is someone who could help with the carbon fibre ? I'm not sure what amount I will need?

At a guess I would put 3 mm of uni on the top
1.5 mm of bi axle on the sides
Not too sure what to put on the bottom as would be in compression ?
Also not too sure what to use where the beam meets the deck ?

Thanks in advance

Tony

[Stumble]

West,

I would honestly just use a Harken high rise track, or the equivalent. There are a lot of weird loads on main travelers, and they also need to be capable of handling shock loads, and act as bearing surfaces for the traveler races. In short, the guys I know who are racing minies competitively, with large budgets, and sponsors from composite shops are still using aluminium here.

Unless you have a in house engineer, and are ok breaking multiple iterations as part of a RnD budget I think you are begging for a expensive mess. And honestly using CF and plywood seems ridiculous. If you are willing to spend the money on CF there are better and cheaper core materials, though they are more expensive.

[Westcountryboy]

I am using a harken Hi track, but I need a mounting surface for the track. I am asking for help to build a beam which supports the track.

I have had a look on the harken website and with the length of track I am using and the size of main sail, I can have a unsupported section of 30 inches, unfortunally my span is around 70 inches, so I will need to make a beam to support the track.

Tony

[Stumble]

Ahh, I see. In that case go From the small boat traveler to the mid, and you should be able to span the additional distance. I would call harken for advice however. They are pretty good about recommending things like this as part of their customer support. It might cost you a little in weight, but probably ablout the same as the CF support beam.

Alternatively Harken might have some experience using CF C challels or I-beams to support their tracks.

[RThompson]

As was mentioned above -

You can have up to 30" unsupported span with the Harken Hi track, but you must actually span 70".

It seems the simplest, and (probably) cheapest and lightest would be to put a post in the middle to reduce the span to 2 x 35". You could then put a short 10" beam on top of the post to reduce the span down to the required 30". ie the post would look like a 'T' .

Aluminium seems ideal for the job. You could do a composite one, but think lots of work with a possibly uncertain outcome unless you happen to have a tame (and experienced) composite engineer/ at your disposal.
Another possible benefit of using a post is that you should be able to transfer the loads into the keel floor structure efficiently-although I dont know if thats suitable for your boat.

OTOH - building a composite beam is cool. would be an interesting project to get it done efficiently (adequately strong but cheap and minimum weight). In that case carbon over ply seems a good place to start. What about building a mold and use just carbon (no core)? it would be a simple mold.
Rob

[Westcountryboy]

Here are some more photos to show whats going on.