Tide heights and launching from trailer

I used to launch a Warrior 195 (20') boat at Baiter slip. The boat weighed in around 1500k plus the trailer so not exactly a light weight.

I found the slip useable 1 - 1.5 hours either side of low water springs. You could if experienced enough launch at low water neaps by utilising the drop off at the end of the slip.
The only downside doing this was getting the trailer back over the drop off :eek:.

As baiter is the only decent public slip useable by larger trailerable boats in Poole it is very popular. Also the pwc brigade use this slip and can make life shall we say interesting.

Have fun

Martin
 
Regarding Cobbs Quay you can buy a season ticket for around £500. Sounds expensive but if you use the boat a lot can work out kind of ok and its a brilliant slip.

When I launched my boat there never even needed to unhitch the trailer from the car and could drive the boat back on the trailer :)

Martin
 
As you see, the proof of the pudding is in the eating. Folk are launching boats when the chart says there is no water.

Conversely a bridge will have more hight under it, than the chart says, because in that case, the chart measures from the highest possible tide.
 
The trouble is lack of information about the slip, rather than lack of information about the tide.

Even if you don't want to/can't be bothered to/don't know how to read a tide table http://easytide.ukho.gov.uk/EasyTide/EasyTide/index.aspx will do it for you for free: you don't even have to register.

HLB is broadly right when he says that there is always more water (and more headroom) than the chart shows, because the charted depths and drying heights are shown above Lowest Astronomical Tide, while bridge clearances are measured above Highest Astronomical Tide. The logic is that they are as pessimistic as possible.

But it is a bit of an oversimplification to say that there is "always at least a metre more water than on the chart." At times (around March and September) low water will fall very close to LAT. In December and June, it will generally get nowhere near it.

At Town Quay Poole, for instance, Mean Low Water Springs is 0.6m -- which means that although some Low waters will be more than 0.6m above Chart Datum, there will be roughly the same number that are less than 0.6m above CD.

We can see from the chart that the top of the slip is above MHWS, (because it is coloured yellow) and we can see that the bottom of the slip is above CD (because it is coloured green) Personally, I would guess at about 1m above CD ... but without local knowledge (which I don't have) I wouldn't argue with anyone who suggested any other figure between 0 and 2m !!
 
VicS said:
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
Click to expand...

On the figures given I'm with you Vic. Drying height of 1.1m is rather important methinks.
However, local knowledge would appear to suggest that the drying height is not as high as we assumed.
 
underwurlde said:
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
Click to expand...

Andy

If your going to measure the depth of the slip at high water tomorrow take your arm bands as you will need them before you reach the end of the slip :D

Martin
 
whisper said:
On the figures given I'm with you Vic. Drying height of 1.1m is rather important methinks.
However, local knowledge would appear to suggest that the drying height is not as high as we assumed.
Click to expand...
The drying height of 1.1 metres is not actually on the slip itself: it is certainly not at the deepest part of the slip. It is just a spot on the seabed somewhere nearby. So it is almost entirely irrelevant. Without local knowledge, we have no way of knowing whether the slip is above the level of the surrounding seabed or by how much.

And unfortunately, this important piece of information is not usually given in books/websites.

Perhaps the reason may be something to do with the myth that tidal "calculations" (i.e. the ability to read a tide table) are complicated or confusing! Rather than presenting their readers with hard facts such as "the end of the slipway is X.Xm above/below Chart Datum, their authors feel it necessary to fall back onto approximations such as "the slipway is accessible at LW+/- Y hours" that are so woolly as to be almost meaningless.
 
Tim Bartlett.

I think the problem is, jet ski's or little boats, do not have charts, or any knowledge of the sea.

They have told me on more than one occasion, that the sea is a bowl, like a bath tub.

Trying to tell them different is difficult.
 
Tide observations and calcs

OK, gunna be a bit anal here, but its for my piece of mind, so what the hell! :D

Tide calcs & observations for Baiter's Park Slipway, 24th April 2010

Positional data was either taken from a chart that I scanned into SeaClear II or from GPS readings taken on the day using a Garmin GPS. The Chart I scanned into SeaClear was 'Poole Harbour – East 5601-9, Jan 2002'

------

First off a sanity check for the scanned-in SeaClear Chart. Using my garmin I got a fix on the slipway for the high tide-line mark - I know this won't be the maximum height of tide shown on the chart but it should be close. Here's '+' marks the spot - pretty damn good IMO.

BaitersPartkSlip-TideMark.jpg


-------

I used a tidal predtion table from this website for 'Poole Harbour':
http://www.parkstonebay.com/marina/information/tides/

BaiterParkTideTablefor24thApril2010.jpg


So, at 13:32 (BST, not GMT) I expect low tide and that to be 0.8M above chart datum

-------

On the day, I took a GPS fix on the slip at 13:20 where the water was lapping my heels. Again '+' marks the spot:

BaitersParkSlip-1320.jpg


------

So, now for a spot of Trig. I extended the slipway until it hit the 0M chart datum line on my chart, i.e. at the position: 050°42'583N, 001°58'143W

Top of the slipway is at: 050°42'667N, 001°58'304W and my low tide position on the slipway was at: 050°42'647N, 001°58'267W

I used an on-line calculator found here: http://boulter.com/gps/distance/ to calculate the distances between two GPS points.

This is the resulting Trig:
BaitersParkSlipTrigCalcs.jpg


------------

NOT BAD! you see the height above chart datum using my simple math was 0.85M and the tidal graph predicted that at this time low water would be 0.8M. :cool:

I hope that's 'accurate' - I have made many general assumptions.
 
But where does this 1.1m come from?

The only 1.1m I can see on the chart is a drying height given for a piece of mud, about 50m or so to the west of the top of the slip.

Is it really so difficult to believe that a slab of concrete (the slipway)might be a metre or so higher than this arbitrary patch of mud? That is rather like saying that my neighbour's patio cannot possibly be higher than my garden!

The top of the slip is coloured yellow on the chart, so unless the cartographers have got it wrong it must be at MHWS or higher. And MHWS Poole is 2.2m above chart Datum.
 
Like I said, I made many assumptions of which I claim no accuracy. It was only an exercise (for me) such that I can have some kind of faith in the data that I gather from charts and tidal predictions and how to apply that information.

I just want to get it *somewhat* right, that's all - to give myself some confidence in what I do in the future. OK, so the top of the slip may indeed be 2.2M high. That would probably mean I am now about 1/2 meter out. I cannot see 2.2M on my chart, so I guess that is knowledge people are born with (not me though). How would I know that BTW? 'local knowledge' again?

I have been in the situation where I got it wrong before - I whacked my damn prop and to top it all off I then had to wait 6 odd hours for the poxy tide to come back in before I could get my damn boat back out of the water onto my trailer.

So pardon me while I faff about here trying to get it sorted in my mind, OK? There are somewhat conflicting views & opinions coming back to me here and all the while i'm thinking 'it can't be that difficult, surely'.:eek:

Anyway, that aside, I did see people walking thier boats into the water at low tide (13:30) yesterday! So I guess i'm not the only bloke around who doesn't know what he's doing :rolleyes:.....

LaunchingatLowTide.jpg


^ Guys on the left there shoved their boat out (way past the two triangular tipped posts), until *plop* the boat fell off the end of the slip (dropped down a step in other words). Even then they had to huff & puff and shove about for a good 15 mins before she finally floated free.

Another interesting thing to note was that the chap in the shed positioned next to the slip there had written up the HW & LW times in GMT (as printed in his tide book). He did not know if the time he'd written up was GMT or BST! "I just copy the times up" he said.

So, it pays to bl00dy well know what's what in my opinion:p:

This chap I saw coming in at 11:30, found the water getting a bit shallow at 12:30
Strandedat1230.jpg


Then found himself marooned at 13:30:
Strandedat1330.jpg


I could only guess though that he had to of done that on purpose - but God knows why.
 
underwurlde said:
So pardon me while I faff about here trying to get it sorted in my mind, OK? There are somewhat conflicting views & opinions coming back to me here and all the while I'm thinking 'it can't be that difficult, surely'.:eek:
Click to expand...
I'm sorry: I was only trying to help.
It really isn't difficult, and it certainly isn't a matter of a "point of view" or an "opinion"

(1) On a UK chart, yellow bits are above Mean High Water Springs, green bits are below MHWS but above Chart Datum, and blue or white bits are below chart datum.
That's just the way things are. See Chart 5011, or almost any reputable navigation textbook
(2) Mean High Water Springs at Poole is 2.2m above CD
Again, it's not a matter of opinion, or local knowledge: it's published on the chart, in Reeds, in the Cruising Association Handbook, in Admiralty Tide tables, and in any decent pilot book.
 
(2) Mean High Water Springs at Poole is 2.2m above CD
Click to expand...

I ought to know the answer but where would I actually find that info?

Big trouble with sorting out the OP's original question is that you do not know (from the chart) the exact height (above CD) of the useable end of the slip way.
Not a lot of help to you if it goes deep into the mud and a real PITA if as it seems from the comments that it stops short and ends with a drop! Dodgy when launching and darned near impossible to lift boat and trailer back up if it happens when recovering!
 
hlb said:
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.
Click to expand...

I've launched at least twice in the past eighteen months into water below chart datum. It isn't quite a "once every hundred years" event
 
I've launched at least twice in the past eighteen months into water below chart datum. It isn't quite a "once every hundred years" event
Click to expand...
Chart datum on British Admiralty charts is normally LAT

LAT is the lowest level to which the the tide can be predicted to fall under average meteorological conditions. It is NOT the extreme condition produced by abnormal meteorological conditions such as strong wind, high pressure or (negative) storm surges. (Ave baro pressure BTW is 1013 mb).

If you study the tide tables you will find negative tide height predictions at some ports ... now that I do not understand!
 
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