First of all; I didn't want to begin with mentioning this, because I feared the discussion would quickly become more technical than I had intended and am capable of following, but since it turned out that way anyhow, here it is; I'm studying to become a mechanical engineer. So while I don't know much about the physics and mathematics involved in boat building yet, in time, the goal is that I will.
@Skyak, it may not have been your intention, but your post comes across as somewhat condescending, especially the last part. Of course trimarans are generally more complicated than monohulls, but they are nevertheless pretty well understood. As I have stated earlier, I never had the intention of building a boat without having proper calculations done.
Regarding the freestanding mast; you say that it is a bad idea, but give little explanation as to why. There are plenty of multihulls with freestanding masts, including trimarans. What is "significant size"? Without the bowsprit, my concept is only 26 feet, I'm only guessing, but I think most would classify that as small, possibly medium. I'm still a novice, but I've done some quick reading so far; jib luff tension may prove to be sub-optimal without any backstays, but that is a flaw of free standing masts in general, not this particular layout, no? The jib is a small part of the overall sail area in this configuration and its steep angle to the mast top should reduce bending of the tip, no? I may even consider tilting the mast a few degrees aft if that improves matters. As for the mainsail, it has a top haul, a peak haul and a down haul, shouldn't those be enough to generate luff tension? I don't know enough to calculate forces on a sail yet, but from studying other designs and dimensioning up until proper calculations can determine optimal sizing, I have used a 20cm/8" carbon fiber tube as reference. For another design using standard carbon fiber tubes, see "Troublemaker 9500".
My own arguments for having a freestanding mast are as follows;
- Simplicity. While the construction and installation may be more complex, maintenance and usability should improve significantly. Multiple small (but critical) failure points are traded for one large one. Most modern trimarans have a stayed rotating wing mast with spreaders and articulated boom. That is a pretty complex arrangement for a beginner designer/sailor. A freestanding mast with a luff pocket acting as a "soft wingsail" should give adequate performance with a lot fewer parts.
- Load paths. My understanding here is very basic, but the idea is this; usually a trimaran has sidestays connected to one or multiple points on the amas/outriggers. This increases bending loads on the akas/beams and requires additional calculations and reinforcements. The beams have to resist the buoyancy induced twist when the trimaran is heeling or dropping, and the stays would add additional forces acting in the same direction. In addition, when the structure in the main hull is strong enough to resist the bending loads from the beams, it should be a long way on the road to be stiff enough in the perpendicular direction, resisting the bending load from the mast. At least, that is my theory. It also eliminates chain plates and the necessary reinforcements to support them. Stays also present problems with folding geometries. How do you easily allow for adjustment between folded-out and folded-in mode without having the mast be unsupported in the transition?
- Safety. A trimaran is much worse at spilling wind in gusty or severe weather than a monohull as it resists heeling more. Having a mast that can bend somewhat mitigates this. It also makes sailing easier and more pleasant, because frequent and early reefing is less crucial.
I think I stated why I wanted the dimensions minimized where possible in the first post, but here are some more reasons:
- To fit in a 40ft container the mast can't be more than 40ft long. Rule of thumb regarding mast to hull length is about 3/2, which leaves the maximum hull length at about 26 feet unless you're willing to have a two masted rig (which I have considered, but would rather avoid).
- The hull is tall to provide ample headroom for me and still leave enough structural space for a strengthened bottom and roof. I may as well fill up the additional space with accommodation, which if unused weighs preciously little.
Bulkheads. I have never made a general sketch for a boat before, so I don't know how to properly indicate bulkheads. There IS a bulkhead at the forward crossbeams. In fact, all the partitions in the boat are meant to act as additional stiffeners/bulkheads, but that may not be obvious on my drawings. Assume at least 2,5cm/1" thick reinforced bulkheads.
As far as I know you have to keel step a freestanding mast. Your point about the floor only makes sense with a stayed mast, and since the mast in the drawing is unstayed, of course it is keel stepped.
The last paragraph I'm just going to ignore
@Skyak.
. However, multihulls are usually very weight sensitive because of the narrow hulls. The exercise of calculating weights will serve you well if you ever build a boat. It is common to see boats perform poorly because they had too much weight added during construction. A careful account of everything that goes in assures the boat will float on her lines.
