My Little Mule (MLM)

In real life conditions wind is not stable, it changes directions and speed. If and when you get an occasional shift how will the rig stand?
Your back stay in the drawing is obviously wrong but unnecessary, the main sheet will handle that load.
The single hulls are easy with gusts and shifts as the rigging is basically just enough to exceed the righting moment of the boat. Cats and tris don't have that luxury of small righting moments and need heavier bracing, therefore weight. A free standing mast for this boat, given the buoyancy of the windward hull would have to be massive. On the other hand the buoyancy of the leeward hull seems insufficient, the hull at critical speed might submerge and cause a capsize.
Consider you installed the tube mast of a Laser I on the tack you are designing this boat for the mast may break as your righting moment exceeds by far that of the laser and I assume the beams are of appropriate size to lift you and your windward hull up in the air. At that instant the leeward hull will need enough buoyancy not to submerge itself and slam on the brakes.
Now let's say you are going up the wind and either a quick shift and gust comes on the other side or a bad manouvre and a wave does it for you. Will the bracing at the mast base be sufficient to handle the load? Both the bracing at the foot of the mast and the beam should be capable of handling this, meaning they will be heavy.

 

I miss-spoke. Gary, I forgot… “traditional” does imply historically the Polynesian originals. I’m certainly not a historian, but I did read somewhere that someone (recently) did a study and it did confirm that the Polynesians had “evolved” to an optimum design. Kind of cool!
When I mentioned falling down masts, I was thinking of several non-HarryProa designs of the last 50 years. In some of those Proa’s the mast had NO resistance to wind coming from the opposite beam and would instantly fall down. I do have a bury that will be strong enough for an eventual free standing mast so the weak link while using the Hobie mast will be bending in the mast at the top of the bury if the boat does go aback. I haven’t had an end cap off the Hobie mast to get its cross section properties… but I will be doing that analysis and adding reinforcement at the critical section to “improve” that strength.

 

Zerogara,

I do appreciate you taking the time for the evaluation. I do not want to overlook something.

Most of my sailing is on a lake and I’m quite familiar with 90° wind shifts. It is a concern and one of the primary reasons why I wanted to experiment with this small test mule before I commit to something larger.

Back stay - I did mention above that it was a little out of place. It needs to be outside the boom’s sweep. However both stays are critical... It might not be obvious from the drawing, but I’m using an EasyRig design. The main down tensioning of the main is created by the boom (in bending) in conjunction with the jib’s leech cable. The sheets from the back of the EasyRig boom are relatively lightly loaded since no down-tension is used. The sheets only restrict the sail from weather vaning and since the jib partially balances the main, even that force is greatly reduced. Because of that, the back stays take the driving force and since it’s a Proa, both stays are back-stays at one time or another.

Freestanding mast - The analysis is far easier for it than the stayed mast. As it is, I’m having to beef up areas that would not need it if I went with a freestanding mast from the beginning. The freestanding mast needs to take the maximum righting moment and some allowance for transient loads. So with the current design and the expected max weight of the crew on the windward hull, that comes to 6000 ft-lbs. It doesn’t take that much carbon fiber to handle that and it’ll be far lighter than the aluminum mast and stainless steel rigging of the Hobie. Besides… I feel safe in knowing that Rob builds free standing carbon masts, like I breath. As he puts it… “Dennis the sooner your Hobie mast breaks, the sooner you can put on a proper mast.”

Insufficient leeward hull – You may be right. But it’s a Hobie replacement and I’m quite used to the pitch pole maneuver. However, I think the analysis paints a different story… the two main things it has going for it is… its twice as long as the Hobie and it has a high prismatic coefficient. The attached Flotsam analysis shows it at gross weight, with the crew hanging out beyond the windward hull. The leeward hull is still in a healthy location. Also note… the Flotsam analysis is highly conservative for this hull. It only takes into account hydrostatics. The hydrodynamics analysis of Flotilla indicates that this type of hull should have a nose-up tendency and it even doesn’t take into account any planning forces. In short, it shouldn’t be an issue.

BTW – this Flotsam loading condition only occurs if the boat is doing 30 knots! So even in my wildest dreams, can I imagine the submerging the hull in anything except waves.

 

"Would you be able to point me to your thread? I would be interested."

It got zero play, so it probably isn't worth tracking down. I proposed making the mast and beams unified on the LW side, and for the hull to rotate around them. The LW hull without mast loads, or rudder loads would then be just a float, which would have helped a little where making it demountable for ease of launching or trailering is concerned. I've always like Adrenaline, and was inspired again by some recent youtube about it.

 

Maybe I read you're message. Sounds a lot like what I've got designed. I plan to react the mast loads directly into the main (larger) center beam and the rudder loads will mostly be reacted by the diagonal beams. In this way the lee hull only has to take bending loads... very little torsion. I also rationalized the ease of assembly.

1. attach lee stay to cross beam
2. slide cross beam into lee hull
3. rotate lee hull on side with beam stait up
4. slide mast in horizontally
5. attach all three stays
6. pull down cross beam to right hull and raise mast.
7. attach diagonal beams
8. slide on windward hull

Think I could do it solo if I haven't botched the weights too much.