Jib/Genoa size vs power

[Geoff K]

How much difference in forward force is added by making the headsail larger? I know everyone says big genny equals big engine etc. But what is the data to support it? is there a number such as adding x% to the foot will add y% to the forward force vector? Any such data would have to be specific to a given apparent wind angle. Does this exist?

Looking at my 130% genoa at any point of sail forward of a beam reach it appears that the perpendicular to the sail in the aft part results in a vector directed aft of the beam. If so, this means that part of the sail is actually holding the boat back and adding to the heeling force. With the sail sheeted in tightly it appears only the forward 1/3 of the sail can be producing any forward force. IF this is the case, spending a full day on a close reach would go faster and more comfortable with a working jib than with a genny.



Geoff

 

[RichH]

IMHO - The modern trend is for shorter LP jibs/genoas. Large LP jibs (to me) are only for boats that dont fly spinnakers and where a large LP is of principal benefit when going downwind. The less the LP, the higher the relative aspect ratio; hence, the better pointing ability.

Most of the 'umph' in a sail comes from the forward luff section (highest 'suction peak') ... yet the leech of the jib when timed/cut perfectly with the point of maximum draft (shaped and located by trim) of the mainsail will usually give the highest 'dumping velocity' (bootstrapping) enabling the optimum 'sail interaction'. My personal reasoning/preferences is that the 'longer' sail is NOT going to promote 'more' air to flow over the leeward side, and therefore is of not much value .... especially when the leech of jib is anywhere 'aft' of the point of max. draft on the main. I 'think' that once a jibs leech is aft of the POMD, all benefit of a large jib stops .... for going upwind; downwind you need Sail Area and that when a the larger LP can be of benefit.

For details of 'sail interaction' refer to C. A. Marchaj's "Sail Theory". etc.

Simple speak: the 'top shelf race boats' seemingly are using smaller and smaller LP jibs ... that should give a 'hint'.

 

[Bill1565]

I agree with what Rich says about downwind. Big genoas way out on a pole approach spinnaker speeds

 

[Geoff K]

Thanks Rich and Bill. I have Marchaj's book "Seaworthyness, The Forgotten Factor" but I don't see a chapter on sails. I'll search for other books by him. My boat came with a spinnaker but I don't think I want to learn how to use it. It seems like a lot of work. OTOH, my previous boat (Hunter 23) had an asymmetrical spin and that was fun to use. Perhaps a "working jib" and an asymm spin would be a good combination.

Geoff

 

[r.oril]

I don't have a spinnaker but would like to have one for my 26D. They look so cool!!
I will try the big jenoas on a pole next time out on the lake.

 

[RichH]

C. A. Marchaj's landmark book is "Sailing Theory and Practice"; Dodd Mead; 1964, ISBN 64-13694
This is a very 'technical' work involving a lot of high order mathematics to support the fluid dynamics involved. Until the publications, etc. by Arvel Gentery beginning in c1974, Marchaj's work was THE technical book on sails, hull forms, etc. Definitely not for the 'non-technical' reader.

There is(was) also a landmark 'compendium' of articles and papers by NASA/NACA 'types' on sails, hull forms, etc. by the then avid/leading sailor hydrodynamicists, aerodynamicists: "Ancient Interface" which can be found in most Technical/Engineering University libraries.

"Seaworthyness" was written in response to the infamous "Fastnet Disaster".

 

[Geoff K]

Thanks, Rich. I like technical stuff to a point but when Calculus and beyond gets involved I don't have the patience to sort it out. It's been 40 years since I "studied" that stuff. Your discussion has answered my question; I'll probably stop at this point.

I did, however, enjoy Marchaj's "Seaworthyness..." as well as Rousmenierre's book about the Fastnet disaster.

Thanks for your help.

Geoff