finding the ceo of a sailplan

[matoi]

What is the proper procedure for finding the resultant centre of effort of a sloop sail-plan?

More specifically:

Is it enough to find the individual geometric centers of the genoa and the mainsail, then draw the line between those two and then split it in ratio which corresponds to the ratio of foresail and mainsail areas - or is there something more that one should know?

How does genoa overlap influence the resultant centre of effort?

Thank you

Mato

[TeddyDiver]

Originally Posted by matoi

Is it enough to find the individual geometric centers of the genoa and the mainsail, then draw the line between those two and then split it in ratio which corresponds to the ratio of foresail and mainsail areas

Yes!
Generally with foresails only the fore triangel counts (wheather you hoist a jib or genoa)

[matoi]

Just to check if I got it correctly (sorry but English is not my first language):

The overlapping portion of the genoa does not influence the position of the resultant centre of effort of the whole sail-plan on a sloop?

What about the mainsail roach? I heard somewhere that this is disregarded too, but I am sceptical...

Thanks!

Mato

[jehardiman]

Roach is sometimes ignored, and often only the working jib area is used. It all depends of how you measured the "lead" (the difference between CLR and CE) on other, well behaving, simliar hullforms.

[marshmat]

Hi Mato,

Probably the most common method, at least among the relatively limited number of sailplans I've studied, seems to be to consider only the triangles and not the roach or overlap. So the headsail area and CE are found from the foretriangle area and centroid, not the actual sail (which may overlap the main). The mainsail area and CE, at least if the roach is small, are found from the area and centroid of the mainsail triangle, excluding the roach.

I've seen a couple of sail plans with big-roach full batten mains where the roach is included in the area and CE.

But here's the catch- when you find all of these points and areas from the sail plan, you're looking at geometric centroids. The actual centre of effort could be radically different in different conditions.

So really, what you're doing when you find the CE of the sail plan, is to compute a set of geometric centroids for different sail configurations, which can be compared to a similar set of points calculated in the same way from a similar boat. Thus you have a common method of comparison, making it easier to estimate the new boat's behaviour compared to existing boats. As Jehardiman points out, what's really interesting is the lead between CE and CLR, compared to other boats, so you have to calculate sail area and centroids the same way as for the boats you're comparing against.

Only with lots of money, lots of time, and access to CFD and wind tunnels can you find the actual centre of effort for each set of sailing conditions. Few bother with this, so geometric centroids are used and compared against existing boats to get "close enough for engineering purposes".

[TeddyDiver]

sorry but English is not my first language
nor mine

The overlapping portion of the genoa does not influence the position of the resultant centre of effort of the whole sail-plan on a sloop?
excactly

What about the mainsail roach? I heard somewhere that this is disregarded too, but I am sceptical...
You heard right.. but you can if you choose to. When it's intended to make estimates of the CE vs CLP influence to helm balance etc it's essential to remember that any calculation is just an educated quess. Of course when a boat has a precessors, having more or less same characteristics, their data and behaviour can be used to make further judgement on the new one and better reliability can be expected in this regard..
Remember thou that some "modern" racer mainsails however just can't anymore considered triangular with roach.

[matoi]

Thank you all for helpful comments!

Best wishes,

Mato

[matoi]

I have just done the geometrical analysis of the Wayfarer dinghy sailplan, and it is surprising that both methods (main roach and genoa overlap exluding vs. including) give almost perfectly the same result - the difference is noticable in that 'inclusive' method puts the COE little lower than the 'exclusive' method, but otherwise they are aligned.

Best wishes,

Mato