I've gone over to the dark side (secondary tides)

[ghostlymoron]

I'm reading Practical seamenship in YM (the dark side). Its a good read actually but one statement I don't understand. It says "We confirmed our readings ..... and discovered that our berth was 2.45m below CD". Shouldn't that be 'above CD'? or have I had too much Possers?

 

[James_Calvert]

Haven't read the item concerned but from what you say:

if the berth is -

2.45m below CD. Sounds good, loads of water assuming there isn't a really low spring tide and you don't draw 8 ft or more.

2.45m above CD. You'll be aground on some tides for sure.

Think I've got that the right way round?

 

[ghostlymoron]

I thought chart datum was the lowest level you could get (or thereabouts).

 

[James_Calvert]

It's the lowest that the tide will normally go.

Depths on charts are measured below this and drying heights above it.

 

[ghostlymoron]

Ah ha I see. They mean the sea bed level is 2.45 below CD. I thought they meant the pontoon.

 

[James_Calvert]

Ah ha I see. They mean the sea bed level is 2.45 below CD. I thought they meant the pontoon.

Need very long waders for one of those!

 

[Jamesuk]

I'm reading Practical seamenship in YM (the dark side). Its a good read actually but one statement I don't understand. It says "We confirmed our readings ..... and discovered that our berth was 2.45m below CD". Shouldn't that be 'above CD'? or have I had too much Possers?

Try 'our berth we calculated to be 2.45 below CD, we only draw 1.7 so we know that at we could leave it there all year and it will be floating at all states of tide.

 

[prv]

Ah ha I see. They mean the sea bed level is 2.45 below CD. I thought they meant the pontoon.

You can't give a figure for how high the pontoon is above CD, because it goes up and down with the tide!

Pete