How Much Mast Offset?
How do you decide how much to offset a stayed proa mast from the CL of the main hull? Is there any rule of thumb or guide for guesstimating?
My initial thought was to get the shroud angle to something greater than 10 degrees to lower compression on the mast should the boat be caught aback, but that would be a rare occasion and since the boat wouldn’t be driven, the forces might not be so great even at low angles.
That got me thinking about the opening of the slot between the jib and the main and that perhaps it was possible to have too little offset to take advantage of that.
Then I saw Diazo’s post on aligning hull drag with with aerodynamic drive, which is great information. But while I was thinking about that, I started thinking about how moving the mast to windward also makes the main hull take some of the rotational force of the sail rig.
What else am I missing? How do you juggle these considerations without trying to take extreme advantage of any of them to end up with a nice sailing, nice looking proa?
The further away from the big hull and the less you will be able to center the boom.
So not too good for pointing.
On the proa that my brother is designing for me, the mast will be as close to the main hull as it can.
But even then, the most I will get out of the boom is 6 degrees with some twist on the main before the back stay gets in the way.
The further away from the big hull and the less you will be able to center the boom.
So not too good for pointing.
On the proa that my brother is designing for me, the mast will be as close to the main hull as it can.
But even then, the most I will get out of the boom is 6 degrees with some twist on the main before the back stay gets in the way.
With a long enough mainsheet strut, you can move the mainsheet attachment point to center with the mast. At that point, the limit you can move the mast to windward is the main hitting the back stay, as you mentioned.
Maybe opening up the slot also gives some leeway with how tight you have to sheet in the main and a wider boom angle is okay for sailing to windward.
There’s always the option of making the sail triangle fit inside the shrouds to the ends of the Vaka. My personal favorite is a free standing mast with a junk sail (check out the James Brett designs). I do realize this structurally complicates the mast location and isn’t a favorite choice for those looking to maximize windward performance.
http://www.youtube.com/watch?v=h_ILYfWTEbk&feature=mfu_in_order&list=UL
As you can see a properly designed Junk boat can move pretty good 😉
Tom
Very nice Tom! I think I’ll stick to the stayed Marconi rig, already too many unknown just as it is.
You can certainly center the boom, but you would end up with too much twist.
I guess that’s part of the compromise.
johnp, here’s a quick-and-dirty way to roughly estimate the lateral Center of Drag so you can align it with the sail power location:
Use this convenient on-line calculator to find the drag for the ama and the hull at a low-to-moderate speed, ignoring heeling effects (because imbalances will be more apparent at low speeds) Use the same PC for both ends of the hulls:
http://boats.yyrtti.com/performance/hulldrag/hulldrag.html
Write these figures down.
Now find the lateral balance point between these two, which is to say, find the spot where a string attached to the platform will pull the boat in a straight line. Mark this on your sketch.
Now, add the drag figures together and multiply the sum by 0.35 to get a very approximate drag figure for your rudders/boards. Yeah, that much and sometimes more; leeway resistance and steering are quite draggy, and there’s a lot of skin area in the boards. Obviously this figure will change a lot depending on the side force applied to the sail, leeway, board design, retracted/extended, and pointing angle. This is just a rough estimate for preliminary sketches.
Decide where you are going to put your boards, for example, on the lee side of the main hull, through the bottom of the hull, or on the windward side of the hull. Now find the balance point between this drag and the CD of the hulls. This will give you your preliminary Center of Drag for the whole platform, and this is also the initial point you want to align your sail power to. Mark this point on your sketch as “Center of Drag.”
When you do this, you’ll notice that the overall Center of Drag location will change substantially if you move the boards from the lee side of the hull to the windward side. If you want, you can take advantage of this to move the mast closer to the main hull by placing the boards on the lee side.
When aligning the sails to the Center of Drag, keep in mind that if you are using a sloop rig that the jib, for its area, will provide comparatively more forward thrust and less side force than the main, for its area.
OK, that’s for the first-level sketch. Using this, you can check your rigging and spar arrangement to see if everything can work mechanically.
To refine your sketch, the most important changes will be to make a much more accurate estimate of board drag, and to decide which conditions you want the balance of the boat to favor. Normally this will be light and gusty air, and off-wind courses- the most problematic conditions for proas- but that won’t be true for everyone. You can use standard aero/hydro calculations to estimate board drag pretty accurately. Since skin friction is such a major drag component of boats, remember the wetted surface of your boards is going to be just over double their area (people do tend to forget that…). You should also try your estimated sail positions at different pointing angles and with different loads on the ama (heeling).
Board sizing is beyond the scope of this note, but be careful not to wishfully cut the margins too fine. The better the forces are aligned, both laterally and fore-and-aft, the less work the boards have to do, and the smaller (and less draggy) they can be. If there is a well-designed trimaran of similar rig size to your proa design, you can compare to that and your boards will definitely not be smaller. If your sketch is well aligned and the boat is of normal multihull proportions, you can initially estimate the deployed area of the boards to be about 4% of sail area. That’s conservative, has been proven to work well on deep-water multihull cruisers, and the extra area will help a lot in slow-speed, gusty conditions. The more the forces are out of alignment, the bigger the boards need to be.
Lastly, compare your preliminary sketch proportions to a Russ Brown boat, as these are positively known to balance well. If you’re close, you’re in business… 😊
johnp, here’s a quick-and-dirty way to roughly estimate the lateral Center of Drag so you can align it with the sail power location: ...
Thanks! This is fantastic information. I’ve got to digest it and then I’ll have even more questions.