7m tri beams

[bushsailor]

I am building a 7m racing tri similar to a multi 23. I started with a old tri cut it up and kept modifying it till it works. (This process was slow but has avoided some design faults and was a huge learning experience) The plug is now complete and I will be making a mould next week. (easier to make a mould than fair several hulls). I now need some advise on beams. I have decided to use a grainger system similar to the new grainger 7 boat. Any advise on material to use and ideas would be greatly appreciated. My original idea was to 3D model a beam, 5axis route a plug make a split mould then layup vacuum infuse 4 carbon beams.However this is expensive.
Weight is critical to this boat. My target is 250kg so I have little margin to waste on heavy beams.(10kg each max)
I am not a boat designer so I have no idea how to calculate the strength required. Boat details are as follows:
Max sailing weight 500kg
Beam 5.2m
Beam length 1650mm
Main hull max beam 900mm
Underwater stay 800mm (supports beam nearly in the middle)
The tri will be sailed on one float upwind
Distance between underwater stay hull mount and top of beam 350mm
I worked out that breaking strain of one assembly would need to be about 1000kg including safety margin. Would this be correct?
I did post this in another thread but no response so I will try again.
Thanks

[Fanie]

Hi Bushsailor,

What you're asking is not something simple.

I suggest you find someone who specializes in glass / carbon structural strengs designs. If you make a mistake you could end up testing the PFD instead of the tri. You can get some very good ideas from someone who knows what they're doing, and he may be able to help you with some calculations and indicate how the best strength vs weight can be achieved.

It may cost you a couple of bucks too, but may be well worth it.

Don't be too skimpy on the materials where it counts. You won't notice 10kg extra, but the lack of it may.

Could also be nice if you upload some pictures... and some as you progress. Like you, we're all learning here. Some of the okes could come up with some good advice as to what should happen where.

[CTMD]

Where in Australia are you? Give me a buzz 0400628379 and I'll try and point you in the right direction.

[redreuben]

See if you can get some free engineering from a materials supplier, yhey may come to the party since you are building moulds
Cheers,
Reuben

[bushsailor]

Thank you for your replies.
I guess I do know that if I build beams out of uni carbon till I get a laminate 3mm to 4mm thick using 3mm ply as a sandwich it will be strong enough. I am also looking for some other ideas.
Cheers

[Fanie]

On 5m beams the 4 - 6 mm sounds about right. My beams were made with 9 laers of 450g glass for a 4m beam and they came out ok. They are about 80mm x 150mm OD. I used PVC gutters glued 2 together, sealed the slot where they met, wound some platic around it, then the glass and vac infused them. The PVC was pulled out afterwards. Mine is 3mm thick, but I think you should be ok with about 13 layers and a bit larger beam size, ie 100 x 200mm. That extra meter will require a bit extra.

450g glass gives a thickness of around 0.35mm per layer.

If you don't make it stiff (and strong) enough the ama's is doing to flap around the center hull.

[bushsailor]

Another question
I have observed that most small trimarans have the centreboard positioned under the mast. Generally the aerodynamic centre of the sail plan is well aft of this position. Why are they so far forward. Beach cats such as f18 run their centreboard in the corect position. I positioned the centreboard on the test boat right on the aerodynamic centre and it made the rudder too light with no feel, but it is about 1200 back from the mast step.
Cheers

[bushsailor]

A couple of photos of float.

[redreuben]

If using uni's incorporate some double bias to tie it all together and absorb tortional loads.

Cheers,
Reuben.

[sailor2]

Originally Posted by bushsailor

I am building a 7m racing tri similar to a multi 23.

Boat details are as follows:
Max sailing weight 500kg
Beam 5.2m
Beam length 1650mm
Main hull max beam 900mm
Underwater stay 800mm (supports beam nearly in the middle)
The tri will be sailed on one float upwind
Distance between underwater stay hull mount and top of beam 350mm
I worked out that breaking strain of one assembly would need to be about 1000kg including safety margin. Would this be correct?

Is that a folding beam design or demountable or solid beam case ?
On that size tri with carbon beams, the most cost effective way is propably to use self supporting beams without waterstays if foldability isn't require otherwise. Waterstays are good for increasing overall stifness for upwindwork though. But it's a lot of extra work and end fittings take all the weight advantage you might get by greater separation between tension & compression due to using waterstays.
If minimum weight is the target, then using plywood as a core is not a good idea, as it takes hardly any load with carbon skins being so much stiffer.
In what direction is "Beam length 1650mm" measured ?
If port to starboard then how does that lead to overall beam being 5.2m ?

[bruceb]

I am following this thread as I would like to replace the beams on a similar sized tri. How did you determine the "1000kg" loading? Is that load at the float, the water stay attach point or ?. Your boat weighs about 500kg plus crew, so how much of a safety factor is there? (and how much is necessary) Bruce

[bushsailor]

Originally Posted by sailor2

Is that a folding beam design or demountable or solid beam case ?
On that size tri with carbon beams, the most cost effective way is propably to use self supporting beams without waterstays if foldability isn't require otherwise. Waterstays are good for increasing overall stifness for upwindwork though. But it's a lot of extra work and end fittings take all the weight advantage you might get by greater separation between tension & compression due to using waterstays.
If minimum weight is the target, then using plywood as a core is not a good idea, as it takes hardly any load with carbon skins being so much stiffer.
In what direction is "Beam length 1650mm" measured ?
If port to starboard then how does that lead to overall beam being 5.2m ?

Thanks for the info.
It has a folding beam similar to a Grainger. Unfortunately I believe easy folding is essential for ease of use. Originally I was going for a sliding design (L7)type, using off the shelf carbon tube but binding issues have put me off. We have to reduce the beam while rigged to hardstand the boat.
I never thought about ply and carbon like that but you are absolutely right.
The beam length is from the attachment point to the hull(near gunnel) to the point where the beam attaches to the float. I am slowly modeling it up. Overall beam is main hull 900+1650+1650+500x2(distance from beam end to outside of float)
Bruce,
Picture a tri 5m wide with 4 men hiking hard. Only lee float is in the water.
We then have 300kg + 40kg for floatbeams,beer counerlevered outfrom the lee beams so this load on the lee beams is at least double giving 680kg. Max weght of main hull is say 150kg including rig plus less counerlever 100kg =930kg. This is spread over 2 beams but you need to have at least double as a safety margin for coming off waves etc. This is safe operating strength not breaking load. This is all worked out on logic not engineering principals, hence the question.

[bushsailor]

Bruce,
In race trim the boat will weigh 500kg including crew, but to work out a safe working load we need to assume more people on board.

[Gary Baigent]

hey Bushie
Your boat sounds very interesting - any chance of some drawings, more jpegs?

[bruceb]

Yes, any details/pictures of your attaching points on both the inner and outer hulls. I keep worrying about the loads on the forward crossbeam particularly. Fore and aft as well as vertical loading; falling off a wave sideways at 15+ knots has to impose a shock load several times the weight of the boat. I have broken the forward pylons out of Hobie hulls several times. Many failures on offshore tris seem to be around the forward outer beam/float area. I don't want to add to the statistics Are you planing for boards or rudders on your floats? Bruce