Estimating head sail size

[jskamer]

Hi,

I’m ordering new sails for my Jenneau Sun Odyssey 43. The sailmaker suggested taking a 120% genoa as we’re planning on a long trip as a couple and that should make the headsail easier to handle. Currently the boat sails really well, but I did not buy the current sails, they came with the boat. As a result I would like to get an estimate of the % of the current headsail, does anyone have an idea, or trained enough eye, to do this?

Thanks in advance for any help,

[GordMay]

Greetings and welcome aboard the CF, jskamer.

The “size” [percentage overlap] of a genoa is determined by taking the sail’s “LP” length, and dividing by the boat’s “J” measurement.
% = LP ÷ J
Where:
“J” is the distance from “the forward face of the mast, at the deck, to the intersection of the deck, and the head stay pin, at the stem head”.
“LP” [or LPG Luff Perpendicular Genoa] is: “A line drawn at right angles to the luff, and that passes through the clew of the sail.”

In a perfect world, yachts would have multiple rigs, and these rigs would have sails that are appropriately sized, with depths for optimum performance in a given wind speed, and direction [upwind v downwind].

In the real world, the most versatile size furling/reefing headsail, for cruising boats, is generally considered to be in the 130 - 135% range.
I'd consider a 120% Genoa slightly 'conservative', on a furling/reefing rig.




Choosing the Right Headsail for your Sailboat
➥ https://www.sailmagazine.com/gear/ch...-your-sailboat

[psk125]

It is hard to tell from your picture, but the first shot showing the clew of the jib with the mast in the foreground makes it look like you're flying a 110% or 120% jib. The other pictures don't show enough of the overlap between the mast and the sail. Gord's equation would provide a more exact result. If you told your sailmaker the dimensions of your current sail he'd also be able to tell you.
Our boat came to us with a 135% Genoa which is also easy to handle. It did not provide enough power in the light breezes of Long Island Sound, so we got a new 150% jib that we now use instead. Depending upon where you're going on your long trip, you may want a larger sail if you don't want to motor much of the way. If you can roll it up, it shouldn't be too difficult to handle.

[TrentePieds]

jskamer:

You say: "I would like to get an estimate of the % of the current headsail..."

Don't settle for an estimate! Just measure the two dimensions that go into making an accurate calculation. GordMay, in post #2, told you all you need to know to make that calculation.

In your first pic, with the genny, completely "unfurled", it doesn't look far off the 120% to me, but do go measure it.

All the best

TrentePieds

[Lee Jerry]

Agree with previous advise to just measure it - isn't too hard to at least get close.

My new advise is to make sure you have the range in sheet track length (and car position adjustment) if you end up going to a larger size genoa. (It can't be seen in our posted photos.)

[Orion Jim]

Drop the headsail off at the sailmaker so he can copy the dimensions or, better yet, have the sailmaker visit the boat to take all the measurements himself and offer an informed proposal.