Tide heights and launching from trailer

[strakeryrius]

Photos to prove it

After all the controversy in the thread I thought I'd swing by Baiter on the way to the boat yesterday. The following pics were taken about 12:30 so about 1 hours before Low Water.

This was the view of the slip

Loads of people seemed to have launched OK judging by the car park.

The yellow poles in the foreground are where I would judge that it is 1m or so above CD, and the slip goes out a long way. I think you would have been OK to launch if you'd been there a couple of hours either side of LW.

 

[timbartlett]

(2) Mean High Water Springs at Poole is 2.2m above CD

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

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.

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.

Exactly so!

The yellow poles in the foreground are where I would judge that it is 1m or so above CD

How do you "judge" that? If they are only 1m above CD, then the car park will be under the best part of a metre of water at high water springs!!

 

[VicS]

After all the controversy in the thread I thought I'd swing by Baiter on the way to the boat yesterday.
The following pics were taken about 12:30 so about 1 hours before Low Water.

This was the view of the slip

_____________________________________________________________________________________________________


I assume the two stbd hand marks are at the end of the slipway. It looks as
though there is about 0.5 metre there. That is at approx 1 hour before LW
when, according to the tidal curve posted, the height would have been 1.0m (above CD)
Underwurlde said he needed 1.0m depth. He would have had that when the
tide height was 0.5m higher than it is in the photo. Ie when it was 1.5m
(above CD)

From the tidal curve that condition would have existed from 0300 hr to 1030
hrs.

We now know that he would in fact have been able to launch on Saturday
morning and at the top of the tide he would have had approx 0.3m to spare.**

Unfortunately the information to make that prediction was not available on
the chart.
It appears that the drying height of the slipway end is about 0.5 metres not
the 0.9 metres I estimated from the chart


**Some note of caution though. Adverse meteorological conditions , a high
barometer ( it was only 3mb above average on Saturday morning) and a
strong wind from "the wrong" direction, could easily have depressed the tide
sufficiently to have made launching impossible.

I would think the two yellow poles mentioned are probably close to or
even above MHWS so at least 2.2 m above chart datum. Maybe even above
HAT which would put them over 3m above chart datum. They ceratinly appear to be above the high water line.

 

[AndieMac]

Are there any other options of a more suitable ramp in the area, that arrangement has got drama written all over it. Trailer boating isn't meant to be that hard.

 

[Kawasaki]

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,

Tim.
We need to remind peeps that Admiralty, Imray and Stanforth have different colours for various depths.
So do to the Publishers of Chart Plotters!
Like
Imray have land coloured as green
Drying bits (sand!) is coloured like -- erm sand!
Shallow bits, erm-- is coloured, like a bit pale and deeper bits get a bit more blue ish!!

Peeps brought up on 'Imray' get a bit confused with 'Admiralty'.
Where Sandy is Land and Green is Mud and Blue is shallow!!!

Makes yer think if Nelson and Co were colour blind

 

[VicS]

Are there any other options of a more suitable ramp in the area

Boatlaunch.co.uk has been mentioned a couple of times during the course of the discussion.

 

[maverickofpoole]

Boatlaunch.co.uk has been mentioned a couple of times during the course of the discussion.

So has Cobbs Quay and that is the only other suitable slipway useable by the general public in Poole Harbour. There are small slips suitable for dinghy's and some good slips in yacht clubs but nothing much else for Joe public.

Martin

 

[underwurlde]

I assume the two stbd hand marks are at the end of the slipway.

For what you've based your assumptions on yes, but in fact the step is a bit beyond those two posts - those guys on Saturday launching their cabin cruiser went quite a bit past the posts before their boat suddenly dropped.

Are there any other options of a more suitable ramp in the area, that arrangement has got drama written all over it. Trailer boating isn't meant to be that hard.

I would now actually say that this slip at Baiter's park is an excellent place to lauch based on the fact that people were there all day launching & retrieving, EVEN AT LOW TIDE (all be it low tide, NEAPS).

---------

Looks like my orignal question has moved on to another, more basic question then, aka: "what is the drying height indicated on a chart in relation to tidal heights".... or something like that.

Andy

 

[ancientsailor]

I'll try to answer the point in the last post

"Looks like my orignal question has moved on to another, more basic question then, aka: "what is the drying height indicated on a chart in relation to tidal heights".... or something like that."



A picture (would) tell a thousand words ... but I'll try with fewer than a thousand:

CD = Chart datum. Both charted depths and DRYING HEIGHTS are measured from it.

Height of tide = height of tide at any moment above (or sometimes below) CD. Obtained from curves or software. Note this doesn't give depth of water!




Depth of water at any given time therefore is:

Height of tide + charted depth

or

Height of tide - DRYING HEIGHT

 

[timbartlett]

Looks like my orignal question has moved on to another, more basic question then, aka: "what is the drying height indicated on a chart in relation to tidal heights".... or something like that.

Wouldn't it be easier, quicker, and in the long run cheaper to buy a book? Learning such basic navigation by asking piecemeal questions on a web forum is likely to be a very long drawn-out process!

 

[hlb]

Wouldn't it be easier, quicker, and in the long run cheaper to buy a book? Learning such basic navigation by asking piecemeal questions on a web forum is likely to be a very long drawn-out process!

I't's like asking. How many forumites does it take to change a light bulb.

Answer one.

But it takes ten, to say your half an inch out.

One to say they have found one place which is different.

Four to say, they have found some fault in your grammar or spelliing.

 

[maverickofpoole]

Or listen to the local info given that has been proved correct and avoid all ensuing what ifs.

Martin

BTW what boat are you launching?

 

[maverickofpoole]

Just thought, had you took local advice you could have been boating on saturday instead of measuring how deep the slip is

Martin

 

[ancientsailor]

Wouldn't it be easier, quicker, and in the long run cheaper to buy a book? Learning such basic navigation by asking piecemeal questions on a web forum is likely to be a very long drawn-out process!

Or a disc http://www.sailskills.co.uk/Webdemo_Tides/index.html

 

[underwurlde]

Wouldn't it be easier, quicker, and in the long run cheaper to buy a book? Learning such basic navigation by asking piecemeal questions on a web forum is likely to be a very long drawn-out process!

I did - I attended a RYA day skipper course and it was very good, but like I've mentioned above, I did find tide heights confusing (espcially those adjustment graphs for nearest major port). However I attended that course 7 years ago - I still have the books and my cousre notes but (Again) I'm getting confused, HENCE my long drawn-out process of asking questions on a forum. I just want it clarifying in my mind: I'm an engineer and therefore looking for an exacting answer that I can apply.

Also, people keep mentioning 'local advice'; a very good point, but where would one get 'local advice' for a slip that is many miles away if not on a forum?

@hlb - you made me laugh there. (and surprisingly, no-one's mentioned spelling or gammar yet).

BTW what boat are you launching?

My pride & joy - A 21ft Bayliner Capri.

@ancientsailor
Can you therefore get a NEGATIVE tidal height, based on what you said in your post? This leads me onto another question: what is 'Chart Datum' based on, what exactly does this reference point represent? I thought (as others do) that it indicates the loweset of the possible low point of the tide for the last 100 years (I'm being facetious).

Cheers again,

Andy

 

[jimmy_the_builder]

A 21ft Bayliner Capri.

Hi Andy

I guess my question then is - do you really need 1m to launch? With the drive trimmed up you're only going to be drawing about half that, no?

Cheers
Jimmy

 

[gjgm]

Hi Andy

I guess my question then is - do you really need 1m to launch? With the drive trimmed up you're only going to be drawing about half that, no?

Cheers
Jimmy

But the trailer cradle,rollers are probably 50cm out of the water, Jimmy?

I'd pay the money and go to Cobbs. At least you can go boating without wondering if you will,
1.get stuck
2.block all your water intake filters
3. ruin your car
4.worry all day while out on the boat
5.Loos,showers,cafe etc.

 

[ancientsailor]

@ancientsailor
Can you therefore get a NEGATIVE tidal height, based on what you said in your post? This leads me onto another question: what is 'Chart Datum' based on, what exactly does this reference point represent? I thought (as others do) that it indicates the loweset of the possible low point of the tide for the last 100 years (I'm being facetious).

Cheers again,

Andy

Ouch! I set myself up for that. I’m at home, so to answer your question about negative heights with a concrete example, I had to go up to the attic to get out my 2008 Reeds (you never know when things come in handy!).

Chart datum (CD) is the “level” from which depths and drying heights are measured on charts.

CD is usually Lowest Astronomical Tide (LAT).

LAT is the lowest tide that can be expected from the movement of the planets (in “normal” weather conditions).

This does means that you can expect to find more water than is shown on the chart.

Tidal height predictions and charts are usually referenced to the same datum.



Now to answer the negative heights point. Usually trainers steer clear of it.

Consider the secondary port Burnham on Sea (BoS) in the Bristol Channel (I chose this port because there are big tides which made it easy to find a suitable tide).

MLWS = Mean Low Water Springs, that is, the average height of LW at springs (sometimes more, sometimes less, after all, it’s an average)

BoS MLWS differences from Avonmouth: - 1.1

Avonmouth 23 March (Springs): LW = 0.9m

Therefore Burnham on Sea LW is -0.2m, that is:
LW Avonmouth 0.9 m - difference BoS 1.1 = -0.2m

Naturally I’d encourage you to buy the disc I wrote!
http://www.sailskills.co.uk/Webdemo_Tides/index.html

Phil

 

[underwurlde]

But the trailer cradle,rollers are probably 50cm out of the water, Jimmy?

Correct! Have a gold star. And yes, draught of boat when in water is about 1/2 meter (gunna have to check that next time I'm out - no excuses, I should know that).

Tidal height predictions and charts are usually referenced to the same datum.

Why does that statement worry me! When wouldn't they be? Yikes.

I'd pay the money and go to Cobbs.

Hmm, be nice, but the cost!!!! £48 is it worth it? Now looking at the thick end of £100 for a day's worth of pottering around Poole Harbour incl' go-go juice (but money WELL spent, so don't get me wrong). LOL - I was shocked at £8 for Baiter's park - that's starting to look cheap. At Cobbs Key, is that the place where a forked lift drops the boat into the water for you?

Cheers for brilliant & considerate replies guys.

Andy

 

[ancientsailor]

Tidal height predictions and charts are usually referenced to the same datum.

Why does that statement worry me! When wouldn't they be? Yikes.

Andy

I had to put “usually” in to keep the post accurate (but didn’t want to). The reason why the datums differ has never been explained to me entirely convincingly – so I’m open to learn on this!

The best and most likely explanation I’ve heard is that in some places – particularly DRYING ports – there is not the information for LAT.



I’ll quote from Admiralty Easytide here, “Chart Datum is the plane below which all depths are published on a navigational chart. It is also the plane to which all tidal heights are referred, so by adding the tidal height to the charted depth, the true depth of water is determined ... In the United Kingdom, this level is normally approximately the level of Lowest Astronomical Tide.”
http://easytide.ukho.gov.uk/EASYTIDE/EasyTide/Support/faq.aspx

... and now from my (2008) Reeds (still on my desk after getting it out of the attic, and I’m guessing the entry’s still in the current Reeds) “... where tidal predictions and charted depths are not referenced to the same datum (eg LAT), errors can occur resulting in overestimation of depth by as much as 0.5m”


When doing your tidal height predictions manually, just check that both are using the same datum (they nearly always are!)


It’s really rewarding to have done your secondary port calculation and go across all the drying banks while everyone else follows the channel (though I must admit – I still watch the echo sounder!)