Steel Lattice Masts

[MikeJohns]

I have a design request for steel lattice masts, 3 verticals with horizontal ties (no diagonals) for a 60 foot ketch. We are doing some FEA on it now. Using mild steel galv pipe, no heavier than the same Aluminium job and suprisingly stiff (much stiffer than the alloy for equiv Ix Iy). Cutter rigg, no runners no jumpers, single spreader. Cheap to build easy to attach to.
The model even looks good.
Anyone had any experience of these types of mast.

[DaveB]

Originally Posted by MikeJohns

I have a design request for steel lattice masts, 3 verticals with horizontal ties (no diagonals) for a 60 foot ketch. We are doing some FEA on it now. Using mild steel galv pipe, no heavier than the same Aluminium job and suprisingly stiff (much stiffer than the alloy for equiv Ix Iy). Cutter rigg, no runners no jumpers, single spreader. Cheap to build easy to attach to.
The model even looks good.
Anyone had any experience of these types of mast.

Wow... that's really surprising... but it's great to hear that people are trying different things.

I can imagine that the truss structure would be relatively light and stiff if braced with diagonals, but without them I would think that the square sections would shear into parallelograms. Why don't you want to include diagonal bracing? I would have thought diagonals would have been more beneficial than horizontal members

Are you modelling it with truss elements or beam elements?

Why mild steel? Isn't high tensile steel much stiffer?

What do you think about the windage issues?

What kind of loads have you applied? Have you included tension from the sail in the aft direction?

How are you attaching the sail to the mast? I'd assume that it would attach to the aft vertical member. If you model the tension from the sail as a uniform load (even though it likely isn't... good start though), I'd think that it would cause bending loads in the member, which typically aren't desirable on simple truss structures.

I'd be very curious to see some drawings or results... I'm obviously misunderstanding something...

Regards,

Dave

[MikeJohns]

All trad engineering design screams for those diags.. The client has several photos of existing rigs with no diags and they look much nicer than a radio mast.

Higher carbon steels have problems ( we considered it ) . Mild works well for this design.

The FEA is done with separate beam elements for each member. Shear is modelled with a separate FEA model, the cross members are pipe too and the pipe end is formed to intersect with the upright pipe element. The resulting intersect spreads the load nicely.
We model all the loads worst case. Windage tests on radio masts were compared. Vibration models the works.
The sail; is attached with a std sail track on the aft vertical member. Mast is oriented with two vert members fwd one aft.
We followed one mast up which was used the other wary around and used a wire with sail hanks but it chipped the paint, added another load with the tensioned wire and increased turbulance prior to the sail luff.

I'll rustle up a picture soon.

[Dutch Peter]

I might be thinking a bit to simple, but reading the layout of the mast I believe diagonals would make the mast too stiff. What your doing is taking the section modulus from one profile and divide it over three (or more, if you want) profiles. The horizontal ties are keeping them together, but the strenght is in the profiles itself.

Peter

[Dutch Peter]

Mike,

Found an other thread on this subject: http://boatdesign.net/forums/showthr...=&threadid=445

Regards,


Peter

[DaveB]

Originally Posted by MikeJohns

All trad engineering design screams for those diags.. The client has several photos of existing rigs with no diags and they look much nicer than a radio mast. .

I thought that they were there to prevent the square sections from turning into parallelograms... I thought that the idea of a truss structure was that it wasn't intended to support moments at its connections... part of what allows them to be so light. I still don't understand the benefit of only having horizontal bracing?

Originally Posted by MikeJohns

Higher carbon steels have problems ( we considered it ) . Mild works well for this design. .

What problems? Not to say that it's ideal, but I thought it was much stiffer and often lighter... It might be trickier to weld and not as tough, but if you're trying to make a light stiff mast...

Originally Posted by MikeJohns

The FEA is done with separate beam elements for each member. Shear is modelled with a separate FEA model, the cross members are pipe too and the pipe end is formed to intersect with the upright pipe element. The resulting intersect spreads the load nicely. .

Hmm... have you tried modelling the joints themselves? Sounds like you're only modelling the beams with one element each which could be dangerous... To model the beams properly I thought you'd need at least 5 elements per beam, and to model the joints I thought you'd need to make a solid (or at least shell) model to show you all of the stresses mapped onto the surface

Originally Posted by MikeJohns

We model all the loads worst case. Windage tests on radio masts were compared. Vibration models the works. .

How did you find it compared with a conventional mast? What are the main benefits/disadvantages?

Originally Posted by MikeJohns

The sail; is attached with a std sail track on the aft vertical member. Mast is oriented with two vert members fwd one aft.
We followed one mast up which was used the other wary around and used a wire with sail hanks but it chipped the paint, added another load with the tensioned wire and increased turbulance prior to the sail luff..

You likely know this, but if it's for a smaller high performance boat, which I don't think this is (60' ketch), sometimes skyhooks can save on weight as they reduce the compression that the mast has to support from the halyard.

Originally Posted by MikeJohns

I'll rustle up a picture soon.

That'd be great, sorry to bug you about this so much, it just seems like an interesting solution to a common problem.

Regards,

Dave

[MikeJohns]

The 3 verticals on some masts are common connected at the top and bottom and varying length spacers are inserted at intervals so the mast tapers out to the middle then back in at the top.

The design we have is parallel with a taper at the upper 1/3.

The structure can only be analysed with FEA software..... Add the loads see what buckles.
Yes we generated a mesh for one intersection and got the numbers. The joint loadings are not high.

High carbon steels are more prone to fatigue failure (exacerbated by using thinner walled tubing).


The cross members are spacers but they add a lot of support from the welded intersects.

A structure with diagonals adds a lot of wind resistance, looks poor, is hard to climb, adds acute corners which are hard to weld and to maintain paint-wise. But really with diagonals it looks like a messy radio mast, with only horizontals it looks like a ladder.

I put a couple of pics in my members gallery.

[tspeer]

Have you added up the aerodynamic drag from such a mast? Compared it to a conventional mast w/ sail?

[MikeJohns]

Tom
Anecdotal evidence from existing lattice mast owners suggests good performance, and no noticable difference in performance.

I havn't done and wind tunnel tests, the client wouldn't wear it, this rig is for a 60 foot medium heavy displacement ketch (chined steel) the brief is for a stiff single spreader mast. Best performance is not an issue. Price is.

I figure the low pressure problem at the mainsl windward luff from an existing large alloy mast will be somwewhat alleviated by air venting directly through the mast. Certainly worth investigating.
Any students out there want an honours project?