Tide heights and launching from trailer

[underwurlde]

Hello!

My RYA day skipper course was a waste of money because I can't remember a thing about tide hights etc. (OK, I did do it 8 years ago now...).

Anyway, my information:

I propse to lauch my motor boat from a trailer at Baiter Park slip, Poole Harbour, tomorrow (24th April) for the first time from this slip.

Now, I know I need at least 1M depth of water to do this, to get her in & out.

I have looked up the tide heights for tomorrow, the wavy line that represents tide height goes from HW=1.8M @06:46, down to about LW=0.8M @13:32 and then back up to HW=1.9M @19:41.

For the slip at Baiter's Park, I see on my (rather old) paper chart that the indicated depth here is 1.1M ABOVE chart datum.

Note: Times include +1hr for BST.

Now on to my question:
I want to be able to draw a line across the graph that indicates tidal heigt that represents a cut-off depth below which I cannot launch - this will then give me cut-off times that I have to be in / out by (plus a bit either side of course). So.... based on above info, at what height do I draw my line?

Thanks in advance,

Andy

PS. what I do remember from the RYA course was that tidal heights was damn confusing!

 

[andyc]

If tide at the launch site is 1.1 above datum and you need 1m and the lowest at datum is 0.8 then surely you can launch anytime as your lowest will be 1.1 +0.8=1.9m.

Looking at easy tide predictions below for poole pottery,Its low at mid day so maybe launch before 10 and recover after 3pm.not sure where your slipway is in comparison to this though.
Probably best to ask someone on site who knows for sure.

http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/GraphImage/GraphImage.aspx?Display=EAkInnZn%

This is what it says about baiter .No mention of tide troubles.

Large wide ramp. Very shallow entry angle, which makes it easy to launch and recover. Trailer needs to be walked out a distance to adequate depth for boat/ski launching. That means that it is very difficult to get a cabin trailer sailer off its trailer without getting the brakes wet particularly when not at the top of the tide. A long warp will also be required to recover such a boat and trailer without getting salt water on the car. This is a very popular launching site, which in high season can become very busy. It is the recommended slipway for Jetskis and close to the designated Jetski area in Poole harbour.

 

[hlb]

Well tidal heights are certainly confusing in Poole, as it has two tides per day.

I'm also not sure how you can have a tidal height on a slipway, which will obviously be dry at some point. I think local knowledge would be best.

However you seem to be on neap tides, so expect much better than your figures. I'd sort of aim to launch ASP AM. Maybe 10AM?? Coming back will be much easier, just hang around till a bit more water.

Some one will hopefully come along with local experience, which if far more reliable than my sort of guess work.

If not, this trip has to be a bit experimental for next time. Look when the chart was last surveyed, it might have been in the 1800's

 

[hlb]

Andyc. Looks like we're both saying the same thing, I think anytime will be OK, but better aiming for better. You can launch a jet ski off a wet dishcloth, no idea about OP's boat.

 

[VicS]

Maybe I have misunderstood you.

But you say

For the slip at Baiter's Park, I see on my (rather old) paper chart that the indicated depth here is 1.1M ABOVE chart datum.​

That means the slip dries 1.1 m above chart datum.

If you require a depth of 1m then the height of tide that will give you that must be 1.1 m + 1.0 m = 2.1 m

On the day in question the tide is at no time as high as 2.1m so you will not be able to launch from there

Even at the time of the evening high tide (1.9m) the water depth will only be 1.9-1.1 = 0.8m

Just to complete the picture. At low water. the tide will be 0.8m above chart datum but the slipway is 1.1 m above chart datum so the water will be 0.3 m below the slipway

OR
Did you in fact mean that the charted depth is 1.1m
That would mean it never normally dries and even at the lowest astronomical tide (LAT) there would be 1.1m water depth enabling you to launch and recover at any time .. any day. (That does not sound like that part of Poole)

FWIW Boatlaunch.co.uk says:

Large wide ramp. Very shallow entry angle, which makes it easy to launch and recover. Trailer needs to be walked out a distance to adequate depth for boat/ski launching. That means that it is very difficult to get a cabin trailer sailer off its trailer without getting the brakes wet particularly when not at the top of the tide. A long warp will also be required to recover such a boat and trailer without getting salt water on the car. This is a very popular launching site, which in high season can become very busy. It is the recommended slipway for Jetskis and close to the designated Jetski area in Poole harbour.​

 

[underwurlde]

Well, yes it does get dry, height indicated is above chart datum and at the slip is shown as 1.1M (1.1 with a line under it) - the chart is green for that location, so in my mind, the minimum tidal height required is +1.1M (at the slip) + 1.0M (for the boat & trailer) = 2.1M

Now if the max height of tide for 'Poole Harbour' is 1.9M then I'm stuffed, no? Hence me getting confused and starting this thread....

If tide at the launch site is 1.1 above datum and you need 1m and the lowest at datum is 0.8 then surely you can launch anytime as your lowest will be 1.1 +0.8=1.9m.

Oh boy, who's right here... I was thinking:
For LW: -1.1+0.8 = -0.3 (i.e. 'negative' depth, in otherwords at LW slip will be dry here)
Or
For HWL -1.1+1.9 = 0.8 (0.8M deep water, maximum for the day, but not enough for my boat that requires 1.0M)

Andy

BTW Baiter Park slip is at 050°42'67N, 001°58.30W

 

[strakeryrius]

I propse to lauch my motor boat from a trailer at Baiter Park slip, Poole Harbour, tomorrow (24th April) for the first time from this slip.

Now, I know I need at least 1M depth of water to do this, to get her in & out.

I have looked up the tide heights for tomorrow, the wavy line that represents tide height goes from HW=1.8M @06:46, down to about LW=0.8M @13:32 and then back up to HW=1.9M @19:41.

Hi and welcome to the forums

I'll be out and about from Poole tomorrow too, but not launching from Baiter.

Its a while since I was last there, but its got a long slip and the water should be sufficient for you to launch all day without having to go out too far.

Its neaps as the others have said, and you will find at well over 1m apart from about an hour either side of LW. I'd aim to be there as early as poss if I were you - the forecast is good and its going to be busy no matter what time you get there.

Edit: The top of the slip is at CD +1m, but of course as the slip goes out it goes down and the water gets deeper - you should be fine as long as you don't mind getting the trailer brakes wet.

 

[underwurlde]

Cool, thanks for that!

My chart is 2002 (cough, ahem). What I see here is:

Neap tide is the shallowest range, no? Bummer....

Andy

 

[VicS]

Now if the max height of tide for 'Poole Harbour' is 1.9M then I'm stuffed, no?

Yes you are stuffed!

Looking ahead it will be Monday evening before the tides get as high as 2.1m

 

[strakeryrius]

My 2009 Reeds shows the depth at the bottom of the slip as 0.5 instead of 0.4, but if you then add the tide the least depth even at LW will be 1.3m at the bottom of the slip. At the top of the tide the depth there will be 2.3 - 2.5 which ought to allow you plenty of depth to launch and recover.

As I said though you may have to take the trailer out a long way down the slip to get the boat off it.

If you decide to launch elsewhere Cobb's Quay have a decent slip into very calm water, and what you end up paying in fees will be offset a little against what you pay in car park at Baiter.

 

[VicS]

Looking at the chart you have posted I would estimate that the bottom of the slip is closer to "Dries 0.9m"
That means you need a tide height of 0.9 + 1.0 m = 1.9m

You would just about have enough on Saturday evening but not quite in the morning.

 

[hlb]

Yes you are stuffed!

Looking ahead it will be Monday evening before the tides get as high as 2.1m

No, tidal hight is nearly always 1Metre above chart datum. Chart datum is the theretical worst possible condition that could ever happen. So it would have to be springs and may happen once every hundred years or so. If ever.

So you have 1 metre above that. But it's neaps, not springs and hopefully it's not the worse time in a hundred years. Not got the details to hand, but theres a bit more depth to add on. But there is little tidal range at poole, plus there are two tides a day.

So It looks like any time will do, just some are better than others.

But the .08 depth, needs +1m then another bit adding to it.

 

[underwurlde]

Chart datum is the theretical worst possible condition that could ever happen

Indeed, that's how I think of it, the lowest POSSIBLE level, i.e. 'the coast line'.

No, tidal hight is nearly always 1Metre above chart datum.

Is it! I never knew that! So all tidal info I see on-line / whereever, I've got to add 1M to? Now I'm wondering why the f don't they just add that meter in, or is it intentionally confusing? Don't mean to come across as a Doubting Thomas, but can anyone else confirm / deny this? I can't remember that from my RYA course days...

Cheers guys for great input so far. What Me & mate are going to do is forget going out tomorrow. What we will do is get our butts over to Baiter's park and take some depth readings for a given GPS pos at a given time etc. etc. to see if reality tallies up with charts & tidal predictions. Then we'll know FOR SURE. If anyone's interested, I'll post my findings back into this thread.

Andy

 

[VicS]

No, tidal hight is nearly always 1Metre above chart datum. Chart datum is the theretical worst possible condition that could ever happen. So it would have to be springs and may happen once every hundred years or so. If ever.

So you have 1 metre above that. But it's neaps, not springs and hopefully it's not the worse time in a hundred years. Not got the details to hand, but theres a bit more depth to add on. But there is little tidal range at poole, plus there are two tides a day.

So It looks like any time will do, just some are better than others.

But the .08 depth, needs +1m then another bit adding to it.

Predicted tidal heights are the heights above above chart datum. Chart datum is LAT.

Drying heights are also the heights above chart datum.

So at any particular time the depth of water over a drying area or other drying object is the height of the tide minus the drying height.

In the case in question on Saturday morning HW is 1.8 metres ! The chart posted indicates that the bottom of the slipway dries about 0.9 metres that means the depth of water over the bottom of the slipway will be only 1.8 - 0.9 = 0.9 m. That is less than Underwurlde requires to float his boat.

Having revised the drying height for the slipway end from the originally stated figure of 1.1m to the estimate of 0.9m by studying the chart one can predict that there will be just enough water on Saturday evening when the tide height is predicted to be 1.9m.

With luck and a bit of pushing and shoving he might just get afloat on Saturday morning on the very top of the tide and hopefully the tide will rise to the predicted level in the evening and he'll be able to recover it. He would be better advised though to find a slipway into deeper water this weekend

Where on earth do you get this extra 1m from!

Poor fellow was not confident about calculating water depths from tidal predictions, charted depths and drying heights. Must be totally bewildered by your post now!

 

[grumpy_o_g]

As Straker says you should be fine - this is based on having stood waist deep on the slip nearer to low water than high (the water goes out much quicker than it comes in in Poole Harbour btw). You'll need to go a fair way in to get a metre of depth though - it's a not an especially steep slip.

I suspect Baiter may be fairly busy tomorrow as jet-skiers, etc. scent the first half-decent day of the season. I've never seen the end of the slip but it's possible to fall off the side so don't be tempted to go near the edge if (when) some plonker leaves their trailer in the middle of the slip for half an hour while faffing around.

 

[VicS]

As Straker says you should be fine

Given the drying heights on the chart and the predicted heights for highwater of 1.8m and 1.9m. Please explain how that can be fine for someone wanting 1m of water to get afloat.

Please show some detailed calculations of the depth of water on that slipway at HW on Sat 24 April

 

[hlb]

Predicted tidal heights are the heights above above chart datum. Chart datum is LAT.

Drying heights are also the heights above chart datum.

So at any particular time the depth of water over a drying area or other drying object is the height of the tide minus the drying height.

In the case in question on Saturday morning HW is 1.8 metres ! The chart posted indicates that the bottom of the slipway dries about 0.9 metres that means the depth of water over the bottom of the slipway will be only 1.8 - 0.9 = 0.9 m. That is less than Underwurlde requires to float his boat.

Having revised the drying height for the slipway end from the originally stated figure of 1.1m to the estimate of 0.9m by studying the chart one can predict that there will be just enough water on Saturday evening when the tide height is predicted to be 1.9m.

With luck and a bit of pushing and shoving he might just get afloat on Saturday morning on the very top of the tide and hopefully the tide will rise to the predicted level in the evening and he'll be able to recover it. He would be better advised though to find a slipway into deeper water this weekend

Where on earth do you get this extra 1m from!

Poor fellow was not confident about calculating water depths from tidal predictions, charted depths and drying heights. Must be totally bewildered by your post now!

If it says drying with a line underneath, then different. But a depth of water is always 1 metre higher than it show on the chart

 

[mlines]

If you decide to launch elsewhere Cobb's Quay have a decent slip into very calm water, and what you end up paying in fees will be offset a little against what you pay in car park at Baiter.

Baiter is descibed as being 3/4 tidal and when we popped down for a look last year seemed to prone to a lot of queueing and car park shuffling.

So we launch from Cobbs Quay instead. Expensive (approx £48!) but available across the full tide range so you do not have to think too much (apart from NOT cutting corners leaving Cobbs Quay Marina). Car Parking can be a bit of a problem on busy days at Cobbs Quay but you can usually find a place for your trailer in the boatyard.

Martin

 

[Malabar]

Don't forget that all predictions are based on a standard atmospheric pressure of 1013 mb. When pressure is high, as at present, the tides will be pushed down a little. Without looking it up, 1mb pressure difference makes a difference of 1 cm tide. converse applies when pressure is low.

 

[underwurlde]

So far, my thoughts about heights are with Vics and predict exactly as he states (even before posting my thread).

But..

If it says drying with a line underneath, then different. But a depth of water is always 1 metre higher than it show on the chart

Who am I to argue with this, I know little with even less experience. I would like to know where you get this info from or point me at something that states this.

Is this something to do with the figures for MHWS MLWS MHWN and MLWN that is printed on my chart? Does this figure into the equation?

Don't forget that all predictions are based on a standard atmospheric pressure of 1013 mb. When pressure is high, as at present, the tides will be pushed down a little. Without looking it up, 1mb pressure difference makes a difference of 1 cm tide. converse applies when pressure is low.

Oh yes, I remember that too.

So we launch from Cobbs Quay instead. Expensive (approx £48!)

Christ on a bike! Oh much! Cheeky basta....s


Cheers all,

Andy