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Chikokishi 10-24-2013 12:33 AM

Question about sail area
Designing/building a small sailboat. I need 100-110 square feet of sail for her. I have chosen a Gaff cutter rig for the sail configuration. My question is that if i intend to use the main sail along with only the nearest headsail for standard sailing, do i make these two the 100 square feet, or should the 100 square feet also include the topsail and the fore headsail, which i dont intend to use all the time so that i have backup for lower wind days? that would leave me flying with way fewer area on days that i dont extend the other two sails.


zedboy 10-24-2013 06:11 AM

Re: Question about sail area

Originally Posted by Chikokishi (Post 1109797)
Designing/building a small sailboat. I need 100-110 square feet of sail for her.

Ok I'll bite, but I detect a serious lack of experience in sailing.

Go read this: SA / D Ratio

Now, how did you come up with 100 sq. ft.? Better to start with a good figure for displacement (yes, this is tricky) and then work out how much sail you want. Err on the side of more (up to a SA/D of 20 or so) but design to be able to shorten sail. Also depends on intended use - windy places or light wind places.

bobperry 10-24-2013 10:49 AM

Re: Question about sail area
Sounds to me like you need quite a bit of help with this. How do you know where to put the mast? Did you guess? There is no hard answer foir your question. How did you come up with 100 sq. ft? Did you guess? Normally the topsail and the flying jib would not be included in the "base" sail area.

Chikokishi 10-25-2013 03:12 AM

Re: Question about sail area
How did i come up with it, How did i know where to put the mast, etc:

I figured those out after about 10 pages of calculations based on a yacht design textbook. Which included everything about the boat i could possibly hope to calculate. This is also based on building 5 previous amateur grade boats and after studying scale diagrams of "classic yachts" for countless hours. =)

Im also a senior in mechanical engineering... Which helps a little with calculations and equations.

As for sailing experience, no, i do not have much. But i am trying my best considering the largest lake around here is toxic and kills everything that enters it. (I dont do much sailing there... =P)

In response, thanks for answering my question! My boat's sail to volume displacement puts it just above "cruiser class" with my main sail and first headsail. With the topsail and the fore headsail it boosts the sail area up to above "racing" class. But it also changes my centre of sail effort about 1.5 feet forward. Iv decided on a longer, shallower keel to minimize having to fight the boat with steering. I asked the original question because i was deciding if i should increase my main sail area or not to get closer to my goal.

This is my current sailplan, the rudder and keel are just sketched, not actually calculated and finalized yet:

Here is a screenshot of my hull design, although it has changed a little bit since this picture (One major difference being that i narrowed the hull by 10% which changed it considerably):
Thanks again for the replies

zedboy 10-25-2013 03:37 AM

Re: Question about sail area
Chiko you are on the right track :-) I don't know if you realize how lucky you are to have gotten Bob interested, but I think he will have some very constructive suggestions.

I'll just give you my opinion on what you specifically mentioned: the boat in my avatar is my old boat, SA/D around 12-13, long shallow full keel. She was fun, and tracked great, but slow and hard to maneuver especially astern. I sold her and got a much more modern centerboarder with a planing hull and SA/D in the low 20s (depending on crew) - going fast is a blast. In a second I would tell you make the main/fore area as big as possible (and plan for reef points in the main), and go with as high-aspect-ratio and deep a keel (and rudder) as you possibly can.

Other notes: first, get your transom above the waterline a bit. You want a clean exit; what you have now will create a lot of drag. Second, it looks like your LOA is 12 feet, but you're adding a sprit. I don't know about your budget or planned construction methods, but if you can skip the sprit and bring the LOA up, there is no substitute for waterline.

bobperry 10-25-2013 08:42 AM

Re: Question about sail area
Your rig looks OK to mme.

But I think your keel is too far forward and may result in too much weather helm. I would move the keel as far aft as possible.

Why do you have so much hollow in the bow? On a small boat like this I would give it a lot more deck beam forward. With that rig and keel the boat will not be close winded so why have such a fine entry? I'd push more volume into the bow.

Chikokishi 10-30-2013 01:29 AM

Re: Question about sail area
Hey everyone, sorry for the late reply. I never got a notification of replies! Ive been doing some refining on the boat, and we want to build the keel as more of a full keel than a fin. So i will be extending that keel to the full back of the boat and making it a bit deeper. I know that fins thin and deep are much better for maneuverability and such, but i want a nice sturdy boat. The LOA is 11.5 due to laws in my area (we are striving to miss the criteria to register boats) also the boat is this small because i drive a 4cyl toyota truck. Much bigger and i doubt i can get over the mountain passes here!

The position of the keel in the above design is based on my understanding of the need to centre the keel on the centre of sail effort. This was a large push for my original post involving sail area. However, now that i am considering running the keel full back my large question now is... What is the difference between a keel that runs fully from front to back, and a keel that starts at about 1/3 back and continues all the way back? If i understand right, if all your sail area is forward of your keel resistance your boat will tend to steer downwind - not good in an overboard situation!
If you are doing a full keel does centre of keel really matter?

I cant submit images or links anymore, unsure why.

Again, sorry for late reply!


Chikokishi 10-31-2013 04:06 PM

Re: Question about sail area
Been doing lots of looking around and thinking about what i want the boat to be like. Ive decided to go with a much more classic looking hull shape and dropped the fin notion completely. After about 8 hours of work i ended up with this:

I am debating on whether or not i want to flair the waterline out wider or not.

But that aside i am now reworking where i should have my sails.. and how much i want. I am basing a lot of knowledge on existing boats which is leading me to many hours of looking at pictures.

Anyway, this is just my current progress. Ill chime in when ive made it further.


Chikokishi 10-31-2013 04:08 PM

Re: Question about sail area
Wow, that picture came in much bigger than expected. If you right click and click "view image" itll make it fit your screen better. Also, if you notice i followed some earlier advice and got the transom out of the water. And i smoothed the nose up a lot.


zedboy 10-31-2013 04:14 PM

Re: Question about sail area
How are you planning to build?

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