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Go Back   SailNet Community > General Interest Forums > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #11  
Old 09-24-2011
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1) Why is 1knt of current = 4knts of wind ?


Water is denser that air.


I was taught a rule of thumb tHat water is 15 times denser than air- have no idea if this is accurate, but it helps me to fathom current vs. Breeze issues.


2) Laying a mark: sailing near shore the wind will have big shifts ( lifts and headers ) . tide will also change quite a bit depending on bottom, there might also be reverse eddies. Therefore, one needs to be well aware of local conditions up the course and be aware that the conditions at the make may be different than 1 mile away.

This is what make sailing such a pleasure - the playing field is a constantly changing surface
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Old 09-24-2011
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Try this once in a while: What ever tack the pack takes to the mark, go the other way. It works about half of the time. Nothing like clean air. I won my first race that way on a Hobie 18. I thought I screwed up when I rounded the upwind mark 15 boat lengths ahead of the "Guy to beat". I was shaking like a leaf on the downwind. My crew asked me what the problem was when I started nervously barking orders. I screamed WE'RE WINNING!!! I DON'T KNOW HOW BUT WE ARE. It was blowing about 15 knots to boot. The "Guy to beat" who should have been sailing in the "A" class came up to me to congratulate me when we hit the beach. A day I'll never forget. I was sailing with my GF's little brother who had only been on the boat a few times. We had to put 15 pounds in each hull to qualify. This would not be the case today. I finished the 3 day regatta with a third. Hey.
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Old 09-27-2011
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Quote:
Originally Posted by WDSchock View Post
I was taught a rule of thumb tHat water is 15 times denser than air- have no idea if this is accurate, but it helps me to fathom current vs. Breeze issues.
The density of air is about 1.22 kg/m3 at sea level at moderate temperatures. The density of pure water at precisely defined standard conditions is exactly 1000 kg/m3. The density of sea water is about 1025 kg/m3. Therefore sea water is about 840 times as dense as air!
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Old 09-27-2011
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Quote:
Originally Posted by svHyLyte View Post
the wind tends to veer or back as wind always tries to cross a water/land boundary at right angles, just as waves always wash up on a beach at right angles.
Good luck...
That sounds right. Underwater the ripples in the sand tend to be parallel to the beach.

I will try to find independent confirmation of this!
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Old 09-27-2011
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I found this from Wikipedia:

Coastal geography - Wikipedia, the free encyclopedia

Wave action and longshore drift
Port Campbell in southern Australia is a high energy shoreline.

The waves of different strengths that constantly hit against the shoreline are the primary movers and shapers of the coastline. Despite the simplicity of this process, the differences between waves and the rocks they hit result in hugely varying shapes.

The effect that waves have depends on their strength. Strong, also called destructive waves occur on high energy beaches and are typical of Winter. They reduce the quantity of sediment present on the beach by carrying it out to bars under the sea. Constructive, weak waves are typical of low energy beaches and occur most during summer. They do the opposite to destructive waves and increase the size of the beach by piling sediment up onto the berm.

One of the most important transport mechanisms results from wave refraction. Since waves rarely break onto a shore at right angles, the upward movement of water onto the beach (swash) occurs at an oblique angle. However, the return of water (backwash) is at right angles to the beach, resulting in the net movement of beach material laterally. This movement is known as beach drift (Figure 3). The endless cycle of swash and backwash and resulting beach drift can be observed on all beaches.
Rhossili in Wales is a low energy shoreline.

Probably the most important effect is longshore drift (LSD)(Also known as Littoral Drift), the process by which sediment is continuously moved along beaches by wave action. LSD occurs because waves hit the shore at an angle, pick up sediment (sand) on the shore and carry it down the beach at an angle (this is called swash). Due to gravity, the water then falls back perpendicular to the beach, dropping its sediment as it loses energy (this is called backwash). The sediment is then picked up by the next wave and pushed slightly further down the beach, resulting in a continual movement of sediment in one direction. This is the reason why long strips of coast are covered in sediment, not just the areas around river mouths, which are the main sources of beach sediment. LSD is reliant on a constant supply of sediment from rivers and if sediment supply is stopped or sediment falls into a submarine canals at any point along a beach, this can lead to bare beaches further along the shore.

LSD helps create many landforms including barriers, bay beaches and spits. In general LSD action serves to straighten the coast because the creation of barriers cuts off bays from the sea while sediment usually builds up in bays because the waves there are weaker (due to wave refraction), while sediment is carried away from the exposed headlands. The lack of sediment on headlands removes the protection of waves from them and makes them more vulnerable to weathering while the gathering of sediment in bays (where longshore drift is unable to remove it) protects the bays from further erosion and makes them pleasant recreational beaches.
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Old 09-27-2011
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This has an animation:

Geography Site: Coasts - Longshore Drift
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Old 09-28-2011
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Quote:
Originally Posted by davidpm View Post
I found this from Wikipedia:

.....Since waves rarely break onto a shore at right angles, the upward movement of water onto the beach (swash) occurs at an oblique angle.
David--

Some of that article is correct but some is erroneous, like the foregoing. Wikipedia can be a research tool but it is open source and much of what is written is simply opinion.

But, for the sake of the exercise, if you have it open Google Earth and zoom in on any shoreline. What you will observe is that the surf break is always parallel with the shore line. While waves at sea (or on a river for that matter) may approach the shore at an angle, they always curve into the shore to hit it in parallel lines. This is so, in part, because the (energy) flow that hits the more shallow water first is slowed and deflected upward, and the wave height increased, while the flow beyond continues at speed until it too hits the shallows. Surfers take advantage of this phenomena to surf at angles to the beach as the "shore break" will begin on the side nearest the shore and progress along its length but the break itself always parallels the beach.

N'any case, get yourself a hockey puck, a plastic board, and a grease pencil and you may find it's easier to lay a mark than without.
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Old 09-28-2011
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Quote:
Originally Posted by acunningham View Post
If the mark is moving downwind, you'll make the mark, and you can even bear away to gain some speed if you're confident you won't be headed by a wind shift.
Although the thresholds are tighter racing than cruising, don't give up ground to windward too early or too easily. More often than not you'll need it later.
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Last edited by SVAuspicious; 09-29-2011 at 09:44 AM. Reason: typo *sigh*
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Old 09-29-2011
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Quote:
Originally Posted by SVAuspicious View Post
Although the thresholds are tighter racing than cruising, don't give up ground to windward to early or too easily. More often than not you'll need it later.
Quite true.. unless the wind changes.
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Old 09-29-2011
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There are so many variables involved in fetching a mark that you simply cannot have a formula to achieve your goal.
As mentioned by SVAuspicious the best thing to do is give your self some wiggle room for the variables, ie; wind and current, giving right of way to other competitors. Keep your course high until you are sure you can fall off to make the mark.
And get your dog off the helm, dogs have poor depth perception.
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