Crossing the Spitway

MagicalArmchair

MagicalArmchair

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I want to cross the Spitway tomorrow (I draw 2 meters) at about LW plus 1 hour. According to this:

https://www.crossingthethamesestuary.com/Web Site Spitway download 2019 V1.pdf

the minimum I should find at MLWSs is about 1.5. At LW plus one I should have 1.4m over datum, 0.9 at LW (8:40), so even at LW I should still have enough wet stuff to get me comfortably over.

Do local observations back this up?
 
Crossed one hour after LW Springs a few weeks ago, would concur with the chartlet that the slighty deeper water is to the East of the 'Rhumb Line' between the marks.
 
Tidal Heights at the Spitway

Mean HW Springs = Approx. 42 minutes before HW Sheerness, approx. 18 minutes after HW Walton
Mean HW Neaps = Approx. 32 minutes before HW Sheerness, approx. 19 minutes after HW Walton
Mean LW Springs = Approx. 42 minutes before LW Sheerness, approx. 30 minutes after LW Walton
Mean LW Neaps = Approx. 22 minutes before LW Sheerness, approx. 21 minutes after LW Walton

Typically at the Mean Spring Range, low water at Walton can be expected to have a height of tide of about 0.4m: i.e. 0.4m above charted depths. The Mean Range Factor at the Spitway is about 1.13 on Walton. So theoretically at 30 minutes after the time of LW at Walton, the height of tide at the Spitway should be 0.4m x 1.13 = 0.45m for Springs. That of course is based upon weather conditions and the progress of a uniform tidal wave into the Estuary which can never be certain. At Mean Neaps Range, low water at Walton can be expected to have a height of tide of about 1.0m. So theoretically at 21 minutes after the time of LW at Walton, the height of tide would be 1.0m x 1.13 = 1.13m for Neaps.

There is one excellent facility. The Port of London VTS broadcasts live tidal heights at four tide gauges 15 minutes before the hour and 15 minutes after on VHF Channel 69. The 4 tide gauges reported are Walton (always first), Margate, Shivering Sands and Southend. Receiving the broadcast on Ch 69. can be difficult at the Spitway but since for the majority, LW at the Spitway is likely to be for those northbound, it is a good idea to listen into Ch. 69 from the beginning of Fisherman’s Gat.

Where are you bound for? Blackwater, Colne or Harwich? Mean STW?
 
I am bound for Chatham on the Medway, leaving SYH on the Orwell. I could go outside via the Kings Channel, however the kids like looking at the beach huts! (Not a very nautical reason I’m afraid). I went outside on the way in, so may do the same on the way out if there is any doubt.
 
Yes of course SOG will depend on the tide. My tables take into account of the tide in the plan.

Assuming a wpt close to the Cliff Foot PHB to a wpt midway between 6A and 8A of the Medway Secondary Channel via the Wallet, Spitway and West Swin on springs tides at 5kts = 36.22nm - quickest passage is 5hrs 59 minutes if you depart from the wpt close to the Cliff Foot PHB at 5:30 hrs after the first HW Sheerness (0311 tomorrow BST). But the quickest plan would bring you to that wpt outside the Medway approach channel only 30 minutes before HW Sheerness and therefore lose you the flood to carry up to Chatham. So starting earlier will allow you more flood tide: e.g starting at the Cliff Foot wpt at HW Sheerness + 3:30 would give you a duration of 6hrs 30 minutes and arrive at the Medway approach channel wpt two hours before HW Sheerness. Or +4 is duration of 6hrs 22minutes arriving at -1.38 before HW Sheerness or +4.30 is duration 6hrs 18minutes arriving at -1.12 before HW Sheerness. At any of those start times you are clear of LW at the Spitway; closest is the +3.30 start time which going to get you to the Spitway about 1hr 20mins after LW. Of course you need to allow time from the SYH to outside the Cliff Foot.
 
It just so happens that we are planning to sail from Blackwater to Queenborough on Wednesday when I read this. I planned to leave with the last of the ebb to catch slack water across the spitway and then the flood for the rest of the way. Walton is 1.4 m then so am I right in thinking that the spitway prediction would be about 1.54 ish? !!! That would give us a minimum of about 3m.

Given the last fortnight of NE and the high pressure, will this be changed? Would the swell be a deterrent? We draw 1.5m.

My thinking is that we will be fine; just checking.

Thanks from an EC visitor more used to the Bristol Channel tides?
 
5kts STW average?

We certainly would expect the persistent High to depress heights but the last HW at Walton was 0.14m higher than expected and at LW 0.11m also higher than expected! I reckon you could expect 1.5m height of water at LW at the Spitway so you would get 3m of water. LW at the Spitway is approx 22 minutes early than LW at Sheerness, best water is to the east of the line of the SWMs. But that is a matter of only 0.2/0.3m. I think the swell will be ok. The worst in my view is from the SE and we have had nothing but N and NE for ages. If at 5kts you could delay an hour to time arrival at the Spitway at the end of the first hour of the flood and the overall passage duration down to the Medway would (mathematically) only delay you 2 minutes! Off Sales Point starting at Sheerness + 5.00 at 5 kts would bring you down to the Spitway at -5.19 Sheerness (i.e. approx -5.00 Spitway) and take you down the entrance of the Medway in 5hrs 11mins. You would still have an hour and three quarters of flood down to Queenborough. Have you got the Spitway download?
 
tillergirl said:
5kts STW average?

We certainly would expect the persistent High to depress heights but the last HW at Walton was 0.14m higher than expected and at LW 0.11m also higher than expected! I reckon you could expect 1.5m height of water at LW at the Spitway so you would get 3m of water. LW at the Spitway is approx 22 minutes early than LW at Sheerness, best water is to the east of the line of the SWMs. But that is a matter of only 0.2/0.3m. I think the swell will be ok. The worst in my view is from the SE and we have had nothing but N and NE for ages. If at 5kts you could delay an hour to time arrival at the Spitway at the end of the first hour of the flood and the overall passage duration down to the Medway would (mathematically) only delay you 2 minutes! Off Sales Point starting at Sheerness + 5.00 at 5 kts would bring you down to the Spitway at -5.19 Sheerness (i.e. approx -5.00 Spitway) and take you down the entrance of the Medway in 5hrs 11mins. You would still have an hour and three quarters of flood down to Queenborough. Have you got the Spitway download?
Click to expand...
Thanks @tillergirl
Yes 5kts with engine or, as the wind looks good, probably 5.5 dead run. And yes I've got the spitway download ?

Not often it's worth delaying a departure, especially in my part of the world, but will gladly take your advice.

Thanks again

Matt
 
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