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Tidal fill as a math function

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JimGo
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Initially Posted - 02/09/2012 : 08:11:39

My wife and I have decided to keep our C25 swing keel on Great Egg Bay in the Ocean City/Somers Point area of NJ. From what I can see on the charts, the bay has an average low tide depth of about 1-2', and according to the tide charts high tide is approximately 3-4' above those levels. So, at high tide the 2'8" (or, for comfort purposes, the 3') draft of the boat with the keel up should be sufficiently shallow to get me out on the bay. My next challenge is trying to figure out my "sailing window" around the high tide. My instincts are that if I were to plot the depth of any given location as a function of time, the tidal patterns would be roughly sinusoidal in shape (that is, more like waves), rather than a linear (i.e., triangular, or "sawtooth") pattern. I've done some searching online, but this is a tough one to find because there are a TON of false positives. Does anyone know if my hunch is correct? Obviously, the peaks and valleys (the amplitude of the plotted waveform) will vary from day to day.

Any feedback is appreciated! Once I get a feel for the function that defines the tidal fill patterns, I may see if I can't write a program that pulls the tide information for a given location, lets you put in the average depth for the location and the draft of your boat, then generates a "sailing window" for your boat around each high tide. I thought about trying to write something that lets you pick between linear and sinusoidal patterns, depending on your preference (linear being the "safer" of the two since the depth will approach your draft much closer to high tide than sinusoidal), and may just go that route if I can't find a more accurate tidal fill function.

- Jim
Formerly of 1984 C25 named Dragon Wing

NOTE: In my case, PLEASE don't confuse stars/number of posts with actual knowledge. On any topic.

Edited by - on

delliottg
Former Mainsheet C250 Tech Editor

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Response Posted - 02/09/2012 : 08:57:11

Jim,
Google the rule of twelveths. It gives you a good approximation of the height of the tide based on the high and low tide for the specific area. However there are a number of websites that do the work for you, although I haven't seen one that calculates your window for you. At least you can get the data you need to do your calculations.

Edited by - on

JimGo
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Response Posted - 02/09/2012 : 08:59:22

AHA!!! Thanks David! That's exactly what I needed!

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Dave5041
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Response Posted - 02/09/2012 : 10:42:14

Yep, the Rule of 12s is a great, easy tool. You might check "Mr. Tides", it gives a graphical display of levels/time at a huge number of stations.

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JimGo
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Response Posted - 02/09/2012 : 11:33:18

Dave,
Unfortunately, that's only for the Mac, but yes, from the screen captures that's essentially what I was envisioning.

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JimGo
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Response Posted - 02/09/2012 : 13:48:52

How cool is THIS?!?! NOAA just started publishing the kinds of info I wanted! It doesn't include an offset for MLLW, nor does it take my boat's draft into account, but this is great! http://tidesandcurrents.noaa.gov/noaatidepredictions/viewDailyPredictions.jsp?bmon=02&bday=09&byear=2012&timelength=weekly&timeZone=2&dataUnits=1&datum=MLLW&timeUnits=2&interval=highlow&format=Submit&Stationid=8534975

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John Russell
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Response Posted - 02/09/2012 : 14:06:43

I understood the general consensus around here is that the swing keel should be down while under sail. That makes the draft a whole lot more than 2'8". Is that not correct?

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JimGo
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Response Posted - 02/09/2012 : 14:15:11

It makes it 5'. As I understand it, the consensus is that it should be down under sail because if the winch/cable holding the keel up should fail, it could smash into the hull and (ultimately) sink the boat. VERY valid concern, and if I were sailing in 6-10'+ of water, I'd be inclined to heed that advice. However, in my case, the water in the bay will likely never be deep enough to allow the keel to drop all the way. Put another way, if there's only 1' clearance under the fully-retracted keel, in the event of a failure of the winch, the keel drops and gets stuck in the mud at the bottom. That sucks, and complicates the tow, but is a very different problem than having a gaping hole in the hull.

I'll still likely sail with the keel partly extended, since, as someone pointed out, that will help keep my rudder from running aground (by beaching me before the rudder catches), and it will help me know when I'm getting too close to the shallows. But, as a practical matter, most days I don't think I'll be able to sail in the bay with the keel fully extended.

In hindsight, I should probably have tried to find a wing keel instead, but I'll make due with what I have!

Edited by - JimGo on 02/09/2012 14:17:48

JimGo
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Response Posted - 02/09/2012 : 14:29:21

For reference, here's a chart of my new "playground". I circled the marina area in red, near the bottom left-hand corner of the chart. Given the data on the chart, for planning purposes, I'm going to assume that my boat has a 3' draft, and that most of the bay is about 1' above MLLW (the basis for the depth numbers in the tide charts). So, basically I think I'll need the tide graphs I linked to above to show +2' for me to be "OK" sailing through the bay.

Edited by - JimGo on 02/09/2012 14:32:55

John Russell
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Response Posted - 02/09/2012 : 14:37:50

I just don't know if that's the only question. I imagine others will chime in. I don't have that kind of boat and don't really know. That's why I raised the question. In any event, I'd imagine that you'll have real issues with the boat slipping to leeward if you sail with the keel up. I'd hate to see you obligated to that marina if you can't safely sail there.

If it is the case that the safety concerns are only as you stated, then I'd be engineering some kind of redundancy to further protect against sudden keel drop. I would also imagine that this kind of thing would have come up in this forum before and I haven't seen it.

If those kind of depths were on the charts in my area, I'd be looking for a different area to sail in. Have you seen many sails there during the season on anything bigger than a Laser?

Edited by - John Russell on 02/09/2012 14:40:38

JimGo
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Response Posted - 02/09/2012 : 14:46:44

John,
Sorry if I sounded defensive - your questions were taken in the spirit you described. That is, you were looking out for me, and I SINCERELY appreciate that! I'd MUCH rather be aware of the potential issues (even if I ultimately accept them) than to have missed something and regret it later.

As for the redundency issue, I've had that same thought. Unfortunately, I haven't been able to think of anything.

Edited by - on

John Russell
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Response Posted - 02/09/2012 : 14:56:15

Jim, I took no offense and meant none. The value of this forum is the exchange of ideas. The problem is the nuance of language sometimes comes across a little harsh.

I don't know how much overall sailing experience you have but I know that as a northern waters sailor, my days for sailing are limited by the weather and seasons already. Add the further complications of having to calculate tides to the extent you're planning would quickly make sailing more of a chore for me than a recreation.

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JimGo
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Response Posted - 02/09/2012 : 15:13:17

John,
This is our first season with the boat, and I have only limited sailing experience. However, my in-laws have a condo about 30 minutes South of the intended marina, and we've already been told that we're "expected" to be there most weekends this summer (not that I'm complaining). In discussing it with my wife, we've decided that the tide issue just means that if it's a good day for sailing, we decide in the morning whether we're going sailing before lunch or after, based on the tide. Realistically, from what I can see on the tide charts, I'm looking at a 4-5 hour sailing "window" (high tide +- 2-2.5 hours, depending on the height of the tide). Considering that many of my sails will be with a 6 year old and a 4 year old, I'm going to be LUCKY to spend 3-4 hours on the water most days. So, I hope it will mean that, in the end, the tide limitations shouldn't be too big of a problem.

Unfortunately, my other marina choices in that area aren't great. There are two that would appear off the the upper left-hand edge of the chart above, and those are in deeper water. However, both of them are in the middle of power boat alley, and I'm a little leery of being over there during my first season with the boat. Plus, one of the marinas is about twice the price of our proposed marina, and the other one has just rubbed me the wrong way from the beginning. There's another just off the chart to the right, at the end of Peck Bay that isn't bad (my parents kept our power boat there when I was younger), but it's a few hundred more than this place, and I'm not sure how Peck Bay is for sailing. The impression I had when I was a kid was that Peck was, generally, shallower than the portion of the bay near the marina. So, I'd have a 30+ minute motor out to the more "sailable" portion of the bay, and that's not really a great prospect given the attention span/patience of my kids.

I also do realize that this bay isn't an ideal spot. But, balancing all of the various factors, it's about as good as I'm going to get if I want to keep the boat and my marriage!

I'll make due for now, for a few seasons, and see how things go. Who knows, maybe my wife and kids will like it enough that we'll decide to move up to a larger, fixed keel boat that we can take out in the ocean, and then we'll move the boat closer to the condo.

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Stinkpotter
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Djibouti
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Response Posted - 02/09/2012 : 16:33:41

Jim--do you have a chartplotting GPS? If not, I think you'd benefit greatly from one, for two reasons:

1. You'll be able to see exactly where you are relative to the navigable waters in your bay, with depths (MLW) like you see on a paper chart.

2. Most chartplotters have graphic displays of local tide data, showing the "expected" high and low levels as well as times for each day, and where the tide is at the moment on that graph. I can't say there will be data precisely specific to your bay, but you can generally find out the "offset" from another area for which there is data.

If you don't have one, I think you might want to put it at the top of your Christmas (Easter?) list.

Regarding sailing with the keel up, or partway up, its position has three separate effects:

A. The deeper the keel, the more lever arm of the ballast, and thereby the "righting moment" that counteracts the heeling forces on the sails.

B. The more area is exposed, the greater the "lateral resistance" that translates the force on the sails into forward motion. Fully up, your keel is partially inside the trunk in the hull, while the exposed part has a longer horizontal dimension that resists turning (reducing maneuverability) more than when the keel is down.

C. The "center of lateral resistance" will shift aft as you raise the keel, and forward as you lower it. This CLR affects your helm--too far aft and you might have "lee helm", where if you release the tiller, the boat will turn to leeward and possibly jibe--not a good thing. The boat is designed to provide some "weather helm", which tends to turn the boat to windward, with the keel fully down. Somewhere in a partly-up position, I suspect the helm might be neutral--let go and nothing changes. Most sailors don't prefer that characteristic.

So just be aware that the keel position will affect the sailing characteristics of your boat. I don't actually know how the swinger sails with the keel up (I'm sure somebody here does), but I'd be willing to virtually guarantee it's different.

Edited by - Stinkpotter on 02/09/2012 16:38:04

John Russell
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Response Posted - 02/09/2012 : 17:40:51

You need to have unlimited towing insurance. BoatUS has a pretty good deal. The cost of a single tow could cost many hundreds of dollars and perhaps more. The cost of a single tow would probably pay for the difference in cost of a deep water marina in a deeper sailing venue and leave you funds to spare.

I'm surprised we haven't heard form a swing keel owner on the subject of sailing with the keel up yet.

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Dave5041
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Response Posted - 02/09/2012 : 18:27:16

John, I was just getting ready. I've been gone all day, and I initially thought Jim would just be transiting the shallows. The longer chord of the retracted keel probably partially offsets the wings on the wing keel, but you will still make significantly more leeway. There is noticeably less stability without the wing's increased deep mass, so light wind only, and very light until you are familiar with it. The center of mass of the lowered heel is about 2/3 down the length, possibly deeper than the fin, but it will be only a foot or less for you. I have never sailed with the keel retracted, but response to the rudder is definitely more sluggish when motoring. Keep in mind that the keel's center of resistance will be farther aft with a consequent increase of weather helm. Be very cautious as you experiment and progress slowly before taking the 6 y/o and 4 y/o out.

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Stinkpotter
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Djibouti
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Response Posted - 02/09/2012 : 18:59:07

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Originally posted by Dave5041
 Keep in mind that the keel's center of resistance will be farther aft with a consequent increase of weather helm.<hr height="1" noshade id="quote"></blockquote id="quote">Actually, reduction in weather helm, to the point of possibly being lee helm.

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JimGo
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Response Posted - 02/09/2012 : 19:04:10

Thanks guys. So far, we've only sailed her with the keel up, and I have already noticed the distinct lack of pointing ability. I'll be curious to try her out in the deeper waters in the eastern portion of the bay (the part near the top of the map above) where I can drop the keel with more confidence.

Dave, I do have a chart plotter. I need to actually learn how to use it, but that should be a fairly straightforward process.

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Dave5041
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Response Posted - 02/09/2012 : 20:55:26

<blockquote id="quote">quote:<hr height="1" noshade id="quote">Actually, reduction in weather helm, to the point of possibly being lee helm.<hr height="1" noshade id="quote"></blockquote id="quote">
Oops!

You're right, of course. A much riskier situation.

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TakeFive
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Response Posted - 02/10/2012 : 07:52:48

Jim,

I've been following this with great interest, and looking over the charts on my computer using SeaClear. I have a couple of concerns, and only a few minutes to make them, so if I sound blunt, that's why.

I think you need deeper water. Recognize that the charts are only an approximation of the depth. Sandy bottoms are notorious for moving around, so even with a chart plotter you're likely to run aground in that area. Tidal predictions are not an exact science either, and are affected by runoff from rain and winds that can blow the water into or out of the bay.

If there is a marina available with deeper water, you need to consider it, even if you have to contend with a lot of power boats. A few hundred dollars more is peanuts when you're talking boat bucks (and safety). One bad day on the water - especially with kids aboard - and you'll wish you spent the extra money.

I've never had a swing keel, so I'm commenting from what I've read and what I know about engineering. You don't want to sail with the keel up. Period. You need it down to have a decent righting moment, and for stability against excessive rocking when those powerboat wakes hit you. To me it's almost like sailing a water ballast boat with no water ballast. Google search "macgregor 26 capsize" and read some of the reports. I've heard of some racers fine-tuning their boats by canting their keels up slightly, but not enough to reduce their draft (cosine of 15 degrees is 0.96). If you run aground and need to pull the keel up, then drop the sails and motor out of trouble (hoping your rudder doesn't get stuck). (Do they make a kick-up rudder for this boat? It would be a great upgrade for your area.)

Some here have said you can sail in light winds with the keel up. One of the nice things about your area is you're going to have lots of wind, and one good blow could knock you down without warning.

I think you'll be happier at a deeper marina. Think about it before you leave a deposit.

Edited by - TakeFive on 02/10/2012 09:26:05

Davy J
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Response Posted - 02/10/2012 : 08:43:13

I don't have any experience with that area so take this for what it's worth.

After looking at that bit of the chart, I would be concerned to take my power boat out of the channel much less my swing-keel. In addition to having to worry about the tide, you would also be constrained by the wind if you could not lower your keel all the way.

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DaveR
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Response Posted - 02/10/2012 : 08:59:37

There's no doubt you've got a tough venue for sailing. I'm looking at the area from Google Earth and NOAA Charts and noticed that conditions get better towards the inlet which is only a few miles away, so when you get comfortable ..........
As far as the swing keel goes, it's much better down but in my opinion you're not in a "macgregor 26 capsize" situation. I had a 22 years ago and I never dropped the keel!

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Stinkpotter
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Djibouti
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Response Posted - 02/10/2012 : 08:59:39

I, too, don't like the look of that chart. Some day (around full-moon phase) you'll be coming home "just a little late" and you'll see mud-flats between the channel and the marina. Or worse, there will be a foot of water, you'll get stuck, and the tide will still be going out. SeaTow won't be any help. If you decide to risk it, you'll really, really need the $1000 kick-up rudder from Catalina Direct, so your rudder blade doesn't get torn off the pintles (or the gudgeons off the transom).

The keel up probably isn't that risky regarding getting knocked down... The center of mass of the raised swinger is probably only inches higher than that of the wing, with equivalent weights.

Edited by - Stinkpotter on 02/10/2012 09:03:11

JimGo
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Response Posted - 02/10/2012 : 09:48:18

FWIW, here's the northern end of the bay (it's a continuation of the left-hand portion of the map, above)

The other two marinas I mentioned before (the ones with the deeper water) are in the area circled in red near the bottom right-hand corner of the chart.

And here's a satellite view of the bay:

I've circled the marinas that I've found and that take sailboats. The one with a 1 is the marina in consideration, 2 has both of the "deep water" marinas, and 3 is the one that's farther away from the bay.

Thanks again to everyone for the feedback. And, again, I'm not trying to be argumentative, just trying to make sure I understand all of the various feedback so I can process it properly. To my read, one of the concerns is that the marina's location in the shallower areas is a problem, but that the real problem is the overall average depth of the bay, since it won't allow me to drop the keel all the way for a good bit of the sailing I'd likely do. Is that a fair interpretation? On the charts above, you'll note that while there's a channel over near the deeper water marinas, most of that deep water is only in their immediate vicinity. Having a marina that has deep water is great for access to the marina itself (and that's certainly not to be discounted), but if I'm stuck out on the bay, the fact that my slip has deep water doesn't really help me much.

So, what are my other options? I need to keep her within a 30-45 minute drive of Avalon, NJ. The southern end of Barnegat Bay is too far away, and most of the other bays in the area are at least as shallow as Great Egg Bay. I could move the boat down to Avalon and sail her in the ocean (the inlet down there is supposedly pretty good according to some of the folks with whom I chatted at the boat show), but I really had hoped for at least SOME more experience before moving to the ocean.

Edited by - on

JimGo
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Response Posted - 02/10/2012 : 09:57:20

I keep forgetting to add I ABSOLUTELY agree...not getting towing insurance isn't an option!!! I didn't see the BoatUS booth in Atlantic City, otherwise I was going to get it while there.

Also, the cost differential between the marina we're considering and the other, "deep water" marina is more than a few hundred bucks; we're talking almost two grand.

Edited by - JimGo on 02/10/2012 10:14:17

John Russell
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Response Posted - 02/10/2012 : 11:00:32

I'm afraid that, should you stay in all that skinny water, at the end of the season you'll look back and think that sailing isn't nearly as much fun as you thought it would be. In fact, I wouldn't be terribly surprised to find your boat listed in the Swap Meet forum at the end of the season -- or sooner. Whenever you go out sailing, you'll be too busy worrying about water depth to enjoy the day. I think good sailing days will far more limited than you think.

Sailing the boat with the keel up may make you a better sailor in the long run (MAYBE) since, IMO, the boat will be much more difficult to sail successfully. If the wind picks up, you'll likely feel overwhelmed much faster.

As far as wanting to get more experience before sailing in the ocean, I suggest that the skinny water conditions will be far less forgiving not inclduing sever weather issues at sea, etc.

Sorry to be so blunt but I see you headed for disappointment.

Edited by - John Russell on 02/10/2012 11:03:12

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