I have heard of aspects of sails and keels, and it seems that taller skinnier sails and keels have this higher aspect ratio. What is it and what does it do for your sailing?

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    "Aspect ratio" is essentially the ratio of width to height - so "A high aspect ratio indicates a long, narrow sail, whereas a low aspect ratio indicates a short, wide sail". Keel is probably similar. Jan 2, 2016 at 4:22
  • @DJClayworth would you like to convert this to an answer?
    – Escoce
    Jan 3, 2016 at 18:17
  • Not really because I know nothing about how it affects sailing. However if you're only looking for a definition I could. Jan 3, 2016 at 20:03

2 Answers 2


Aspect ratio describes the ratio between sail/keel height divided by (average) width. So a high aspect ratio is tall and skinny and a low aspect ratio is short and fat.

For sails one of the main affects of aspect ratio is on drag. High aspect ratio tend to produce lower drag (i.e. more powerful) sails for the same area. Therefore, modern racing boats tend to have fairly square sail designs (higher aspect ratio) see picture. Another minor advantage of high aspect ratio sails is for inland sailing where wind is often obscured. In this case higher aspect ratios are more likely to get clean air than a shorter sail, see Thames A raters. On the other hand higher aspect ratio gives greater truning motion and so requires a bigger keel/is more likely to capsize.

For keels the greater righting motion of high aspect ratios is one of the main advantages. Additionally, high aspect ratio keels are easier to turn although there will be some loss of sideways resistance from the decreased width.

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    Also using a longer keel allows more of its length to be in 'stable' water underneath the turbulent surface layer. This reduces drag losses.
    – Rory Alsop
    Jan 4, 2016 at 12:37
  • "modern racing boats tend to have fairly square sail designs (higher aspect ratio)". Square sails have lower aspect ratio, as you say elsewhere. I believe other factors are involved in the squareness of racing sails. Jan 7, 2016 at 17:40
  • When you say square, do you mean rectangular in the sense of squarer corners, less square in the sense of the height being at a higher ratio to the width?
    – MikeRoger
    Nov 16, 2016 at 12:33

The keel does two 'things' ... 'slice' ...it keeps the boat from turning when on a tack, in fact it is the reason you can tack at all. The other purpose is to offset the forces applied on the boat by the sails by leveraging the ballast against them. a skinny keel with a huge mass at the end will provide a better mechanical advantage. a long and deep keel will give you the best forward 'slice'.

  • That's actually kinda backward. A full keel will prevent turning better than a fin keel will which has a tendency to dance atop waves. However a fin keel is supposed to have a higher aspect ratio which allows better pointing into the wind, but I don't understand why.
    – Escoce
    Mar 7, 2016 at 19:42
  • but I don't understand why... that is very clear and then you should not be commenting and criticizing someone with 50 years exp and 35,000 miles of blue water experience
    – SkipBerne
    Mar 9, 2016 at 18:03
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    Hehe, ok full disclosure. I posed the question to get some sailing content going was hoping for a good answer that I didn't provide myself. I know full well how aspect works. I also happen to have a lot of experience on the water and I am sitting for my Masters exam on Wednesday. A full keel tracks much better, but it's drags to the side when trying to point. A fin keel with high aspect sails points really high to the wind, but when it comes to rough seas, it's much tougher to hold your heading, because there isn't much keeping you from spinning around the keel, especially atop a big wave.
    – Escoce
    Mar 9, 2016 at 18:25
  • OK got it I would like to get the same going here as well. the postings here are clearly from those who are going to get hurt sailing and that is sooo wrong. the problem with this site is requires a question to be asked and then an answer randomly answered ... and mostly by people that have seen a video and do not understand the kinematics of sailing. GOOD LUCK ON YOUR EXAM!!!! fyi I'm rated at 100 tons and have sailed the Mystic Whaler.
    – SkipBerne
    Mar 9, 2016 at 18:43
  • I know the mystic whaler very well. And after next week I will also be 100 GRT (well I still have to file the paperwork). So the question I have next is, are you still around this area and if so can we go have a beer?
    – Escoce
    Mar 9, 2016 at 18:45

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