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Old 21-10-2009, 09:39   #16
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the plot thickens!

a stat you might find more informative is the ballast to displacement ratio. The Tayana carries 11,800 lbs of lead. I couldn't find a ballast weight on the Cheoy Lee, despite having tracked down the original sales brochure here Goldenwave (Cheoy Lee).

I'll bet that if you contacted Robert Perry directly, you'd find him quite helpful.
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Old 21-10-2009, 09:44   #17
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I think there is too much fixation on "light air performance" and "windward performance". Look, unless you're racing around the buoys an extra couple of degrees of pointing ability won't offset being undercanvassed/unbalanced on a reach or other issues that would arise with lighter displacement in different weather conditions.
If you want to race, buy to the rule- but not if you want to go long distance.
If you look at the engine hours on a lot of cruising boats, you will see why light air performance might be important. Everyone has their own tolerance to when motoring is fine. If your boat does not have light-air performance, then you have no choice but to motor. There is a lot more light-air out there than there is storm conditions.

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Old 21-10-2009, 10:19   #18
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If you look at the engine hours on a lot of cruising boats, you will see why light air performance might be important. Everyone has their own tolerance to when motoring is fine. If your boat does not have light-air performance, then you have no choice but to motor. There is a lot more light-air out there than there is storm conditions.

Paul L
True, but by itself, having high displacement doesn't necessarily mean the boat is a dog in light airs. SA/Disp is a better gauge of that.
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Old 21-10-2009, 10:23   #19
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True, but by itself, having high displacement doesn't necessarily mean the boat is a dog in light airs. SA/Disp is a better gauge of that.
I agree with you on this. What is more important is can the boat sail well in a wide range of conditions, of which much it will be light. Is the boat easy to sail with a short-handed, Mom & Pop crew, in wide range of conditions, some of which will be heavy air.

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Old 21-10-2009, 19:37   #20
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Fixed it for you.

I think there is too much fixation on "light air performance" and "windward performance". Look, unless you're racing around the buoys an extra couple of degrees of pointing ability won't offset being undercanvassed/unbalanced on a reach or other issues that would arise with lighter displacement in different weather conditions.
If you want to race, buy to the rule- but not if you want to go long distance.

Our boat will go in the mid 8's in about 10-12 kts of breeze
Disp: 30000 lbs+
Beam 12'
Draft 8'
LWL (unheeled) 34'-long overhangs.

Shape counts more than just the raw numbers but given sufficient SA, the ratio counts more than the raw displacement. Remember, wetted area drag is significant only at the slowest speeds - and you need mass to allow you to ghost between puffs (that's more shape dependent than anuthing else IMO)
Don't take my comments wrong. I wouldn't suggest a race boat for cruising. However in the original post we are given length, sail area and displacement and asked which one goes faster.

There are a lot of other considerations for cruising.

One of them for me is that the ability to pick and track weather is so good now that surviving the perfect storm is not as critical as it once was.

Yes you can still get caught out in a very bad storm, happens frequently, but being able to pick your weather windows is a lot better.

So, I personally would give up some heavy weather capability in order to not motor as much. I think this is why we see a lot of 40 foot plastic boats that many would characterize as "coastal boats" out cruising.

Not judging one way or another. Just my humble observations.
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Old 22-10-2009, 12:29   #21
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Given that a heavier boat is going to take more to get going, if they both have the same DA/Displacement is the heavier boat really going to be all that much slower in light wind once it is moving (even though I said disregard light wind)? I just don't know and that is why I started this. Rememebr we are talking cruisers not racers so disregard hull shapes in general. I've read owner reviews of that Tayana 42', which seems like a heavy pig of a boat, and all praise the sailing of it and I haven't seen 1 complaint about light air perforamce. On the other hand you can read about the older Morgan 41 OIs that only have a 15 power ration and everyone admits it is alight wind pig.
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Old 22-10-2009, 13:10   #22
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It'll have more momentum so it'll accelerate and decelerate slower. It shouldn't have an effect on final speed if the SA/disp is the same.

Shape always matters, even in cruising boats. (it's just that the shapes you consider are different)
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Old 22-10-2009, 13:59   #23
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two boats with the same SA/D ratio will perform differently if their SA/Ballast ratio is different. For example, let's assume two boats with the same LOA and beam, where boat #1 has 40% of its weight in ballast and boat #2 has only 30% of its weight in ballast. Boat #1 will be notably stiffer, able to carry more sail in higher winds.

This gets us back to the question in the original post. A carbon fiber boat of equal displacement will necessarily have a higher SA/B ratio. All the weight that is saved in the hull and rig can be transferred into ballast.
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Old 22-10-2009, 14:44   #24
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Don,

I'm told (by third party witnesses) that in the light wind area of the Gulf Islands, Canada, the Pardeys, with their very heavy boat, regularly out sail the modern light displacent boats. The reason is because they can pile on loads of sail. They have a rig that permits them to sail more than SA/D.

Remember, SA/D is a fairly rigid computation involving the 100% fore-triangle, and even a 100% triangle for the mainsail. If the boat has the ability to add sail area above what is used in the SA/D computation, she will sail better than what her SA/D might indicate.

Think for a minute about a fractional rig versus masthead rig while going to the windward in light air. Which one can best exceed SA/D? I.e, what sails does the boat really have up, versus what is used for the computation?

Let's not confuse numbers on paper for reality.

And one more thing. Remember that 2 people are going to put a certain amount of cruising gear on the boat, regardless of displacement. As a percentage of displacement, this will be lower for the heavy displacement boat. So, for an all-up loaded-for-cruising condition, the heavier boat may well have a higher actual SA/D because the increase in weight is proportionately less, albeit in actual pounds equal.
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Old 22-10-2009, 14:52   #25
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Quote:
Originally Posted by Bash View Post
two boats with the same SA/D ratio will perform differently if their SA/Ballast ratio is different. For example, let's assume two boats with the same LOA and beam, where boat #1 has 40% of its weight in ballast and boat #2 has only 30% of its weight in ballast. Boat #1 will be notably stiffer, able to carry more sail in higher winds.
Not necessarily as a good deal of the righting moment at a given angle of heel comes from form. There's not enough info in SA/Disp or % ballast to get there. Note centerboarders for example.

Quote:
This gets us back to the question in the original post. A carbon fiber boat of equal displacement will necessarily have a higher SA/B ratio. All the weight that is saved in the hull and rig can be transferred into ballast.
This is true, with identical forms, then it works.
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Old 22-10-2009, 15:00   #26
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Don,

One more comment. Between the two boats I have owned, the mid-weight boat sails to the windward better, even though the heavy boat has a higher SA/D ratio.

They have the same draft, 6'.

Why does the mid-weight boat sail better to the windward? Because she has a larger keel.

The boats have the same draft, but the hull of the heavy boat sits lower in the water, leaving a shorter keel, i.e., less leading edge and therefore less lift. Less lift directly translates into less windward force--despite higher SA/D.

You can't look at these numbers in isolation. It's all connected.



(PS Also the heavier boat has a lower B/D ratio and that's a factor in the ability to carry sail power. I reef sooner now than in the mid-weight boat)
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Old 22-10-2009, 15:03   #27
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Don,

One more comment. Between the two boats I have owned, the mid-weight boat sails to the windward better, even though the heavy boat has a higher SA/D ratio.

They have the same draft, 6'.

Why does the mid-weight boat sail better to the windward? Because she has a larger keel.

The boats have the same draft, but the body of the heavy boat sits lower in the water, leaving a shorter keel, i.e., less leading edge and therefore less lift. Less lift directly translates into less windward force--despite higher SA/D.

You can't look at these numbers in isolation. It's all connected.
having a larger lateral plane for the keel can be pretty independent of displacement.
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Old 22-10-2009, 15:20   #28
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having a larger lateral plane for the keel can be pretty independent of displacement.
Agreed.

But, most of the lift for windward performance comes from the leading edge, so the vertical length of the keel (cord) is what really counts, not size of the lateral plan. Thus, if draft is an issue, you are going to give up windward ability with a heavier boat. If you can afford the draft, great.

Personally, I wish my keel was 6 or 9 inches deeper.

Ironically, many heavy designs have smaller keels depths. Bad news.
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Old 22-10-2009, 15:29   #29
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This is kind of like the previous discussion. Assuming similar hull shapes, moments....

Boat 1, (the heavy) will be quicker when loaded for cruising gear because it will retain more "horsepower" as the gear is added.

Boat 2, (the light) will be quicker when empty because it will have a longer waterline but will carry less "total" sail although the horsepower "SA/D" is the same for both.

Here's the data, making some assumptions:

Heavy ~ 40 loa, 34.7 lwl, d 30,000#, sa/d 17.5, sa 1055, d/l 320

Light ~ 40 loa, 35.5 lwl, d 25,000#, sa/d 17.5, sa 935, d/l 250

Beam for both is 12.

sa/d with cruising gear of5k: Heavy~15.8, Light~15.5

The more gear you carry the more starved for power the light boat will be. The heavy boat will have a bigger rig and you can proportionately increase the sa/d more.




Quote:
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When looking at 2 different boats of same length; what is the sailing performance difference for "normal" conditions (not real light air like 5 knots) between the boats with different displacement, but same displacement/sail area? Say for the 2 boats below:

Boat 1
displacement - 30,000
DLR - 320
displacement/sail area - 17.5

Boat 2
displacement - 25,000
DLR - 250
displacement/sail area - 17.5

Both have the same power ratio for their displacement so would seem to me to have same sailing performance in general. Given the heavier boat is going to be more comfortable and I think may be faster at higher wind because it may standup better at the higher wind before having to reef.

If you have any real ratios etc from your experience to go along with your answer that would be nice.

Thanks
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Old 22-10-2009, 15:33   #30
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Ironically, many heavy designs have smaller keels depths. Bad news.
That's probably where the idea that heavy= pig comes from. Production boat designers think that most people want beam, not depth. (rightly or wrongly)
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