Just started putting more focus on the actual sail plan. Here’s what I’ve come up with so far.
There are some issues with this. First of all the Dmin (the line from clew to sheeting point) is over the required 30 degrees to the boom (45 if sheeting point is at deck level, 35 if at the pushpit), which is all fine and dandy. However, the length of the Dmin in this case is just 2.35 meters. This translates to 1.8P (1.8 times the panel height, ~1.3 meters in this case), which means that I could only use basic sheeting systems that require a minimum of 1.25P. The sheeting systems I had been looking at require 2-2.25P. So I could either move the sail or mast forward, which in case of the former would mean a batten/jaw redesign, or in case of the latter, an unusable forepeak double berth. There would also be the issue of lead (CLR of the hull in relation to CE of the sail) A third solution would be to increase the number of battens from 7 (+ yard) to 9 (+ yard) and thus decrease panel height. This would require slightly more work and more importantly, added weight to the rig. An advantage would be more precise reefing and flexibility in sheeting arrangements. Decisions, decisions.
Let’s take a look at how 9 battens + yard sail plan would look like.
With the decreased P of 0.97m we are now at Dmin = 2.42P with the sheeting point at deck level and 2.25P at the pushpit. This feels like the best solution.
To calculate the lead I’ve used a rough CE of 35% of the chord. PJR recommends a lead of 9% of the waterline. This is however with flat sails, so I’d imagine the CE is way forwards compared to that so thus I’m using 35% of the chord which according to my playing around with XFLR5 should be about right. Given these parameters the lead is calculated at ~9% of the LWL. This should be acceptable because even assuming the sail was flat the CE would still be front of the 50% mark because of the chord narrowing upwards of the sail.
Note: The images above aren’t 100% accurate, they’re still just rough sketches. For example, one thing I just realized was that the sail will be raked a bit more forwards up the mast, because the mast is tapered while the jaws are of equal size. The sail will thus be slightly more in front of the mast upwards of the sail compared to downwards. This is only slightly relevant to the calculations though, but will still result in a slightly higher lead. The mast is also now raked 2 degrees forwards and if required it could be raked more, probably up to 4 degrees, which would further shift the lead forwards. If we need to decrease the lead, we’re in trouble. The mast step is now placed pretty much right in front of the main bulkhead and reducing lead would require the mast to be moved a whole lot aft. This might be doable and even preferred, so as to not interfere with the sleeping quarters in the forepeak. On the other hand, the Dmin might be cut too short. Let’s see how it would look with the mast aft of the main bulkhead.
The problem here is, as I assumed, that the Dmin to the deck point is 2.18P. I could at this point decrease the sail area by shortening the chord, but the SA is already at 32 m2 and I wouldn’t want to go much below that. Or I could just ignore the short Dmin. To achieve a Dmin of 2.25P I’d have to scale down the chord 3%, to decrease the SA from 32 to 31 sqm. Ok, just to be thorough, here’s drawing #4:
More to come later as I figure things out!
Edit (January 25, 2015): Stop the press! Looks like I messed up my calculations slightly. The mast has to be positioned forward of the main bulkhead after all.
The sail will more likely have to be placed even slightly further but I will have to do some research before I can finalize the design.
Edit 2 (January 25, 2015): Small tweaks made.
Nice project, Oscar… I have the same ;-)… But I must finish my boat before…
I am playing around with a junk design. And I had the same issue with the Dmin.
One work around I had was, forcing the first reef by pulling up the topping lift. This effectively reduces your drift (Dmin) value.
So if you need a Dmin of 2.25P, I will assume your sheet span for the boom and first batten must have length 2P + 0.25P (for euphroe). This length is from the clew to the sheet anchor AFTER ITS BEEN REEFED NORMALLY.
However, if your reef up (pull on the topping lift), the Dmin actually gets shorter. So just base your Dmin requirement on the unreef sail.