Subject: [harryproa] Re: Aspect ratio? |
From: Mike Crawford |
Date: 12/12/2008, 10:36 AM |
To: harryproa@yahoogroups.com.au |
Reply-to: harryproa@yahoogroups.com.au |
<<His statement that 3 is a good compromise gives me some feeling
that I am in the right ballpark. Do you agree?>>
I think everything you've proposed in terms of sail area, boat size,
beam, etc., is a good compromise.
The compromise is really how much you want to push things to the
edge. Another six feet would be even better upwind, but at the cost of
having to sail with a reef most of the time. In a really high-aspect
sail, the top reef or two would only be used in very light wind; that
section of the sail would be the equivalent of the reacher that only
gets pulled out in five knots or under.
You're not going to see too many sails go much over 3. The Reynolds
33 catamaran, which many people feel is unsafe, has a main with an
aspect ration of roughly 3.5. A stock F-31 has an aspect ratio around
3.75, and a stock Stiletto 27 around 4.3.
So you're not too far off when compared to some pretty speedy boats.
There's a lot to be said for not having to put down your cold drink
to constantly mess with sails in order to avoid capsize.
<<Without the 2 part mast, I only ended up with a 28.5' luff, so
I have gained 6.5'.>>
Nice. That will help. I like the way you've done it while keeping
the sail track whole.
<<Did you send me a link about calculating the heeling force from
sails?>>
Nope. That was nosum. Here's the link:
http://www.boatbuil
<<that adding the extra sail helps downwind sailing and doesn't
hurt
upwind?>>
It shouldn't hurt you upwind. Maybe a touch more drag, but more
importantly, you'll have a lot more downwind area.
<<It would just be "better" to have that same sail area by
increasing the luff?>>
Not better, just different. Increasing the luff will get you more
speed upwind, but will make the boat more of a handful to operate.
<<I want to make sure that I am not going to flip the
boat.>>
That's easy! You will flip the boat... if you sail it in too much
wind.
I know that's a glib answer, but it's something to consider if you
want a really high-powered boat. If you want to zoom past the others,
you'll have to have enough sail area to do so. And if you have enough
sail area to do that in low wind, you'll have too much to do it in high
wind. There's no two ways about it.
To get subjective, if it were me, I'd go for a mast height needed to
enjoy the boat in five knots of wind (not fly a hull, but move nicely),
and then put in a series of reef points and be willing to use them.
You could probably have a good roller-reefing system if you went with
bolt rope -- the fixed boom really helps in that regard.
The question is never whether you have too much sail area, but about
whether or not you're willing to reef. The less sail area you have,
the less often you'll have to put in a reef or shake one out, but the
less fun you'll have in five knots.
One thing to note: your force in gusts will be less than the
calculated force because the masts will flex. Your unstayed proa will
be more forgiving than a stayed mast. Some folks say that you should
carry the proper sail area for the current wind speed plus a 40% gust,
but the masts will let you be a little less conservative in that area.
<<Why is it with sailing that there are never SIMPLE answers?
>>
:-D
That's what I love about boat design. You're never wrong. You just
have a different set of priorities. It's fascinating how many ways
there are to try to address the same set of conflicting priorities.
Again, your boat looks great. Any suggestions about a longer luff
being faster in some points of sail are just discussions, not
criticisms of the design. Fitting all this inside a container is a
lofty and admirable goal.
- Mike
Gardner Pomper wrote:
Hi Mike,
I actually have gone with the idea of the 2 part masts. I will upload new drawings as soon as I get the rudders redrawn. The way I implemented the 2 part masts is with a 12' base section, that goes from the "keel" up through the hull and past the boom level about 1.5'. This way I can slide both the upper part of the mast and the boom into reinforced areas of that base section. This lets me have a bury for the upper part of the mast of 5', which brings the overlap down to the cabintop, and I end up with a 35' luff on a 39' mast section (1' of the luff is in the overlap). This way the mainsail track is just on the upper mast section and I don't have to worry about trying to mesh the track. Without the 2 part mast, I only ended up with a 28.5' luff, so I have gained 6.5'.
I am hesitant to take the rig any higher than the 45' off the water it will be, in that I only have a 16' beam. Did you send me a link about calculating the heeling force from sails? Because I thought I saw one, but now I can't find it. I want to make sure that I am not going to flip the boat.
Anyway, I appreciate the explanation on aspect ratio, but it is alot like many other discussions on the web. High aspect is better upwind. But what is "high aspect"? For a give sail area (say 500 sq feet), how much better is an aspect ratio of 5 (is that "high") versus 1 (I know that is low). Arto's correction of my aspect ratio calc gives me 3.125 for a 35' luff and a 14' foot for an 80% roach. His statement that 3 is a good compromise gives me some feeling that I am in the right ballpark. Do you agree?
If we say that we keep the luff constant, and just increase the foot and therefore the sail area, are we saying that the boat will continue to go upwind just as fast with the lower aspect ratio? that adding the extra sail helps downwind sailing and doesn't hurt upwind? It would just be "better" to have that same sail area by increasing the luff? So, if it is a choice between a 35' luff and a 10' foot, or a 35' luff and a 14' foot, I might as well go with the 14' foot because my upwind performance stayed the same, and my downwind performance increased?
Why is it with sailing that there are never SIMPLE answers? <grin>
- Gardner