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Old 14-09-2019, 15:04   #1
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Calculating tides at locations without stations

I have a need to know the tides in locations where there are no tide stations. Several years ago I seem to remember reading that it is possible to predict tides mathematically. If so I'd be interested in learning about this and knowing if there are any apps that can do it. I suspect that these calculations would be very complicated because it depends on the relationship of the planets and land masses.

The alternative is to wait until I'm at a town near these locations and hope that I can get tide information there. But that does not allow me to plan ahead. For example, I want to do a 50 nm passage and in the middle there is a section that I must transit at high tide. If high tide were in the early morning and late afternoon, then the 50 nm in daylight would be difficult in the winter.

Please let me know if you know anything about this.

Thanks.
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Old 14-09-2019, 15:30   #2
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Re: Calculating tides at locations without stations

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Originally Posted by mlibkind View Post
>>>>>>>>>>>>>
>>>>>>>>>>>>>>>>>
For example, I want to do a 50 nm passage and in the middle there is a section that I must transit at high tide. If high tide were in the early morning and late afternoon, then the 50 nm in daylight would be difficult in the winter.

Please let me know if you know anything about this.

Thanks.

Where are you going to be sailing that 50 nm would make this kind of difference?


Real question.


Here's why: When I brought my boat up the coast from SF to BC in 2016, we harbor-hopped and had to make a bunch of bar crossings. Many were 6 hours apart but some were 8 which made finding slack really hard at the next one. 50 nm would be 10 hours.


But we found that the times within those 30-60 nm runs were pretty close enough for bar crossings.


The east coast or other places may be different.


IIRC, those places without stations usually had correction adjustments in the tidal literature.
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Old 14-09-2019, 16:38   #3
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Re: Calculating tides at locations without stations

Stu,

I don't think that I made myself clear and I'm sorry for that.

At least twice while I'm in the Bahamas this January (shot days) I will need to time my passage over a shallow area (or I run aground) and at the same time hopefully do it while still allowing me to complete the entire passage in daylight.

In truth, the daylight is not a absolute necessity in both of these passages but when coming into an unfamiliar anchorage having light is always wise if possible.

Here's one example. I have a passage that is 45 nm. Almost right in the center the depth is too shallow for my boat, but with 2 feet of tide I should be able to make it. Actually the depth is shallow for about 10 nm and this section is in the center of the 45 nm.

The closest tide station is Nassau or at least the one that people recommend using for the Comer Channel is Nassau. But the couple of online posts conflict when the high tide at Comer Channel is relative to Nassau.

This is important because if the high tides at Comer Channel were the same as Nassau and those are say 0500 and 1700, I would have to depart my anchorage at about 0300 to go through the channel during the high tide. The 1700 high tide is no good because I would be arriving in a shallow anchorage in the middle of the night.

But what if the high tide at the channel is 2 hours after Nassau. Then the high tide is at 0700 and I could leave at dawn from my anchorage.

So knowing the tide at Comer Channel (a specific point) can help me plan. Or knowing the correct offset from Nassau would be as good.

I've read very differing accounts of the offset. I posted a thread asking about the offset at Comer Channel on this forum and got no replies. Its not a popular passage.

Obviously I can arrive in Thompson Bay and hopefully someone in town will know the offset. I've even emailed one business in Thompson Bay asking if they could help me, but have not gotten a reply (they might be closed because it is the very low season).

The other location where I have a similar problem can be solved by passing through the shallow area and anchoring before getting out into the unprotected Atlantic Ocean. That would require another day and I'm trying to plan a trip so that while my wife is with me we can get to a couple places before she has to fly home. Otherwise an extra day here or there would not matter. I do have a few bad weather days built in, but I'd hate to use one up just because of the tide.
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Old 14-09-2019, 16:39   #4
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Re: Calculating tides at locations without stations

In most places with simple geography you use the nearest tide station, or an interpolational between tide stations on either side.
In the more interesting case where the geography is complex and you have no experience in the region, getting local knowledge is the best you can do. It isn't possible to look at say a strait that might ebb from the top north and ebb from the bottom south, and determine the tide in the middle without local knowledge. Ask a fisherman or charter skipper.
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Old 14-09-2019, 16:45   #5
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Re: Calculating tides at locations without stations

There's no possibility of calculating the theoretical tide at any place that it might be interesting. Local geography, tidal streams, and atmospheric pressure are all going to vary way too much.

You can extrapolate a secondary station from a primary, but observation is the only way to work these things out. All the tidal stations are only the result of previous observations anyway. If things are too tight for you to be sure between two places 45nm apart then you aren't leaving enough under the keel for the local weather or waves to allow you through anyway.

If you need to be at high tide between two points 50nm apart, then what is the difference between the two high tide times at those places? It's unlikely to be more than an hour different and even that would be a surprise. Splitting the difference is the best you can do there. Remember that the precise time of high tide is a little irrelevant as you have slack water and almost no tidal difference around that time. Mid-tide is the more important one to get right as that's when things are changing fast.
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Old 14-09-2019, 16:51   #6
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Re: Calculating tides at locations without stations

Thank you for the last two replies. I now know that my memory must have been faulty in that there is no way to calculate tides.

As for rough seas, I would never try to cross an area like this in rough seas. I've sailed big boats more than 20,000 nm in the last 10 years and have respect for the weather.

Thank you again.
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Old 15-09-2019, 18:03   #7
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Re: Calculating tides at locations without stations

You might want to check out this software, https://flaterco.com/xtide/

But it is going to entail some onsite observations to setup.
I was thinking about setting this up fo some places in the Bahamas but never found the time, always working on boat projects.......

Good luck.

Joe
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Old 15-09-2019, 18:11   #8
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Re: Calculating tides at locations without stations

Since you are asking w.r.t. a trip to the Bahamas please consider that if the location you are concerned about is in the Sea of Abaco, Bight of Abaco or Little Bahama Bank then depths of bars may have changed due to hurricane Dorian.

+1 to Tillsbury's answer.
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Old 15-09-2019, 18:39   #9
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Re: Calculating tides at locations without stations

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Originally Posted by Avionics_Joe View Post
You might want to check out this software, https://flaterco.com/xtide/

But it is going to entail some onsite observations to setup.
I was thinking about setting this up fo some places in the Bahamas but never found the time, always working on boat projects.......

Good luck.

Joe
Thanks for the link. I think that this is related to what I read about several years ago but could not remember. It might be too complicated for me, but I will take a look since it could come in useful. Of course, if it takes onsite observations that I can't get online then it is impossible for me to use.

Thank you again.
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Old 15-09-2019, 18:51   #10
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Re: Calculating tides at locations without stations

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Since you are asking w.r.t. a trip to the Bahamas please consider that if the location you are concerned about is in the Sea of Abaco, Bight of Abaco or Little Bahama Bank then depths of bars may have changed due to hurricane Dorian.

+1 to Tillsbury's answer.
Of course the shallows might be rearranged. But worse than that might be sunken boats!

Several years ago a large ship ran aground on the rocks at the south side of either Little Harbor Cut or North Bar Inlet (can't remember which one). They unloaded the ship and I had heard that they were getting tugs to come pull it off. I don't think that ever happened. I suspect that there could have been holes in it that would have meant it would not float.

I wonder if the hurricane with its high waters and winds were enough to push the ship off the rocks and maybe into the channel. That would be bad. It's things like that could be real problems until things are surveyed.

Normally I would spend time in the Abaco before heading to NC, but I have no desire to go there given the current conditions. It might not be a good destination for several years unless you don't need any onshore amenities. But the snorkeling will probably be good and the supply of conch and lobster might increase due to less people.

But I'm going to spend months this season in the rest of the Bahamas. One of their fears is that the destruction on Abaco and Grand Bahama will drive tourists away from other Bahamian destinations.
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Old 15-09-2019, 19:15   #11
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Re: Calculating tides at locations without stations

We use Navionics on an IPAD that has a GPS built in. The app has a lit of interactive tide and current information. This is quick and accurate. You can plan your approach as needed days in advance.
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Old 15-09-2019, 19:40   #12
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Re: Calculating tides at locations without stations

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We use Navionics on an IPAD that has a GPS built in. The app has a lit of interactive tide and current information. This is quick and accurate. You can plan your approach as needed days in advance.
I have Garmin Blue Chart Mobile (the old version that includes the Explorer Charts ... they are no longer part of the software) and it has tides and currents too. But it gives the values only at stations.

Are you telling me that your system will give the tide and current at an arbitrary point? If so, then I may want to get a copy of Navionics.
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Old 15-09-2019, 19:51   #13
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Re: Calculating tides at locations without stations

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I have Garmin Blue Chart Mobile (the old version that includes the Explorer Charts ... they are no longer part of the software) and it has tides and currents too. But it gives the values only at stations.

Are you telling me that your system will give the tide and current at an arbitrary point? If so, then I may want to get a copy of Navionics.
Tides and currents are only at points indicated on the chart. You can infer in-between if points are not too far apart. The data, charts are downloaded and stored on your IPAD. We keep the Caribbean and east coast loaded. Update anytime you have WIFI. The app lets you select a point and then the graphed data becomes live at the bottom of the screen. Use your finger to slide the graphical chart to any date and time needed.
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Old 15-09-2019, 20:05   #14
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Re: Calculating tides at locations without stations

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Tides and currents are only at points indicated on the chart. You can infer in-between if points are not too far apart. The data, charts are downloaded and stored on your IPAD. We keep the Caribbean and east coast loaded. Update anytime you have WIFI. The app lets you select a point and then the graphed data becomes live at the bottom of the screen. Use your finger to slide the graphical chart to any date and time needed.
Thank you for the reply.

Your Navionics software is the same as my Garmin software. The problem is that I am interested in the tide at Comer Channel and it might be 1.5 hours different than Nassau. There are some closer stations but there might be issues with land interference. Everyone says to use Nassau as the base. But the time offset is not clear from their blogs.

This is what makes cruising a thinking person's sport. Too bad it is so sedentary.
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Old 15-09-2019, 23:34   #15
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Re: Calculating tides at locations without stations

See if you can find one of the old pilot books for the area, they gave a lot of information regarding things like tidal ratios and time differences for lots of places along coasts.
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