how much water will i need

[solo 32]

Newbie here,so please be gentle
Not sure if this the right place to post this question how ever here i go

My boat has a draft of 2.meters and i want to clear an object of 1.5 meters and i want a clearance of 1 meter
the way i calculate is this have i got it right ?

draft 2.5 m or draft 2.5 m
clearance +1.0 m clearance 1.0 m
object -1.5 m object 1.5 m
= 2 meters of tide/water needed clear object = 5 meters of tide/water

or would i need 5 meters of water to clear the object ?
Have i got the correct formula if not please can you give the correct way

thanks in advance

 

[Portland Billy]

I gather you mean an obstruction with a reported height from the seabed of 1.5 m. What is the chart depth at MLWS? Subtract 1.5 m from this to give you minimum water depth, then allow your 1m clearance.

 

[solo 32]

I gather you mean an obstruction with a reported height from the seabed of 1.5 m. What is the chart depth at MLWS? Subtract 1.5 m from this to give you minimum water depth, then allow your 1m clearance.

Thank you very much and yes that is what i mean,i mean a marina sil,i am doing a day skipper course and not sure if i had the correct formula

oh my question that i wrote early doors did not come out with the full content .

so it draft + clearance- sil making my answer 2.0 meters?
2.5 draft
. +1.0 clearance
- 1.5 sil

=2.0 meters required
high water is 4 meters

regards Neil

 

[duncan99210]

OK, the sill height “should be shown on the chart as a height above chart datum. So a local tide table will show you the tidal height above this level and you can work out how much water there’ll be by using the rule of twelfths. What you’re looking for in order to cross the sill safely is height of sill + your boat’s draft + a safety margin ( say 50 cm). Given the example you’re looking at, that 1.5m sill, 1.5m draft, .5m safety = tidal height needs to be 3.5m above chart datum.
If you think about it, the sill is a fixed object 1.5m above the floor (= sea bed). Your boat has a keel that goes down 1.5m, so it needs at least that much water over the top of the sill before you can cross it. Then you need to add a safety margin, say .5m.

 

[Outinthedinghy]

If it was a marina I would ask them what their tidal window was for locking in for a boat drawing 1.5 metres.

ETA reading back it seems this is an exam question so possibly not the right answer. Sorry about that.

 

[solo 32]

Hi
the sill on the chart is 1.5 at chart datum

OK, the sill height “should be shown on the chart as a height above chart datum. So a local tide table will show you the tidal height above this level and you can work out how much water there’ll be by using the rule of twelfths. What you’re looking for in order to cross the sill safely is height of sill + your boat’s draft + a safety margin ( say 50 cm). Given the example you’re looking at, that 1.5m sill, 1.5m draft, .5m safety = tidal height needs to be 3.5m above chart datum.
If you think about it, the sill is a fixed object 1.5m above the floor (= sea bed). Your boat has a keel that goes down 1.5m, so it needs at least that much water over the top of the sill before you can cross it. Then you need to add a safety margin, say .5m.

Thanks for the reply draft is 2.5 meter
clearance required is 1 meter
sill is 1.5 meters

The navigation chart reads 1.5 meters chart datum

 

[Portland Billy]

I see now it's a sounding rather than a height.
Pretty straight forward to consult the tables for enough depth to cross with your margin.
If your draught is 2.5m you need 2m of tide. To give 1m clearance.

 

[solo 32]

I see now it's a sounding rather than a height.
Pretty straight forward to consult the tables for enough depth to cross with your margin.
If your draught is 2.5m you need 2m of tide. To give 1m clearance.

Thanks Billy i just wanted to make sure.
I made it the same as you but i also did it with the formula below


2.5 + draft
1.0+ clearance
1.5 + sill
= 5 meters

would make 5 meters but the tide height was 4 meters and it confused me (thats not too hard)
once again thanks for the help.