helm balance w/ rig conversion

Probably already exists somewhere but here goes. I've taken ownership of a chinook 34 keel/centerboarder. She was without her rig when I obtained her and originaly was drawn as a 7/8 fractional sloop. I got a spar with 2 suits of sails for real cheap. There are some differences obviously: masthead sloop with 120% genoa. The foot and luff of the new main is 2 feet shorter.

 

The center of effort is moving forward I tad. It has a centerboard so this will help some with handling. The trouble is the mast position can't be moved aft for structural reasons given the way the coach roof was constructed. Raking aft with backstay will help, but is this enough? Is this a good candidate for a yawl conversion? The mizzen could gain back the area lost in main plus transfer some area from genoa so I might be able to approach a self tending club jib in the future as I intend to single hand the boat. I might add that I am doing a thorough restoration and adding a mizzen will be a relatively smooth addition. I was considering a free standing mizzen that cants out to windward. Any thoughts?

 

There's really only one way to insure things will balance out and this is a scale drawing of the previous sail plan, over it's appendages and the new one, superimposed. The obvious choice would be a new main with a longer, more appropriate foot length. It's a fair bit easier than making a yawl rig, which will probably move the CE too far aft without moving the main forward, or further cutting down on the new main boom and recut of the mainsail. The sloop lead and the ketch or yawl leads in relation to the currently available lateral area will be different, possably significantly so. In other words, just matching up a yawl CE with the previous one of the sloop will probably not balance the boat. Typically, you'll want to decrease the lead by a few percent on a dived rig, compared to the single stick, more so on a fractional.

Can you post the original and proposed rigs?

 

Thanks for the reply!

 

PAR-I see what you mean about the yawl. The original sailplan can be found ~@sailboatdata.com Do a search for chinook 34. The new main is exactly 2' shorter in both foot and luff dimensions. The jib I haven't gotten the chance to measure but I have seen a picture and it is a big genoa. That will move the CE aft to help compensate for the main right? Plus with the centerboard the CLR should move forward when underway and sailing on the wind...? Shooting from the hip I have a feeling it will balance just fine with the new rig...or is this naive?

 

Naive for several reasons (sorry). I'm familiar with the sail plan for your boat, but there are a number of considerations to work through. You only count the area of the fore triangle in the CE calculation, so the big genoa having more area aft, isn't as important as you might believe. Some of these were done as yawls, but I'm not sure of the proportions used.

 

No. It will continue to shift it farther forward. Instead of a 50/50 or so power split with the 75% jib, you will have about 80/20 with a 120% masthead genny.

 

Oh ok. I wasn't sure about the fore triangle. So with that knoweledge I've made some comparisons
The lead with new rig (using ted brewers add/subtract table for hull form) I've. Arrived at 18-19% lead. That is too high according to what I've read(13-17% for sloops). She's gonna be a bear to steer efficently on paper right? The idea on the yawl was really a matter of liking the utility of the rig. I like balance. Re-cutting a sail and getting out spars + rigging is not a big workload for me, especialy given what's to gain. However, CE too far aft is a problem as well. To find the split rig CE, do I calc. The combined main/fore triangle, then combine that with the CE of the mizzen? I did that and ended up with a positive lead around 5%! Might that be acceptable for a narrow/shallow full keel?

 

The math is really quite simple and purely geometric. You don't need tables. For lead, there are several mitigating factors to consider. Generally, a sloop will want about 15%, though this can vary a few degrees each way, depending on circumstances. A yawl of typical proportions is similar to a sloop, though slightly lower, say 13.5% as an average. A ketch usually wants even less, so subtract another percentage point. You also don't usually count all of the mizzen's area in the process. Simply put, there are enough variables that you'll wan to have this done professionally, or at least by someone with considerable rigging experience. In other words, you can play with drawings at the kitchen table and nail it down pretty good, if you understand the variables, but if you don't, it's can be a costly learning curve farther from shore then you can swim back to. This is a common request for an NA or designer and not terribly costly. You'd be best advised to have the alterations gone over by a pro.

 

Thank you for your honesty