Wave height and depth of water

[alec]

If it's a bit lumpy and you have to cross a bar say, how much depth do you knock off for the wave height ? Or perhaps like me, you often chicken out altogether ?

I remember reading a piece from the lovely Denny Desoutter in PBO that you will lose HALF the wave height in depth. I cannot get my head round this one.

Any ideas ?

<hr width=100% size=1>

 

[oldsaltoz]

G'day Alec,

I'm not all sure ther is an answer to this.

The waves you can see (Assuming you are not over the shoul when you do your estimates) are not the waves you will be in when crossing the bar, they will be larger as water gets shallower.

I suspect the 'half the wave' theory is based on the assumtion that part of the boat will on the wave and part in the trough, however this is not true if you fall side on or abeam to the wave.

The size of a wave over a bar is the only concern when you know you have enough water depth.

The size (Or depth) of the trough is the main concern, and any estimate should be based on this alone.

Hope this helps




<hr width=100% size=1> Old Salt Oz /forums/images/icons/cool.gif Growing old is unavoidable. However, growing up is still optional.

 

[Chris_Stannard]

I would agree that the mean wave height (half way peak to trough) is probably what the sounder measures. So to find the minimum wave height you would need to subtract half of that. As you run into the bar the wave height will increase, go and look at low tide with an onshore wind and you will see this clearly. The other part of the problem is that if you surf down a wave you gain forward velocity, which can cause you bow to dig in and temporarily increase your draft forward. similarly, I believe, as the bow lifts your stern may get a downwards velocity increasing your draft at the rudder. This is important if your rudder is nearly the depth of the keel. In either case this can lead to hitting the bottom.

The other p[roblem with surfing on waves is that as the water speed around your rudder builds up you rudder becomes less effective. In shallow water particularly, you may get a surfing situation where the water at the stern is going faster than the water at the bow making a broach a very present danger. The old Admiralty Seamanship Manuals used to recommend the use of a drogue to stabalise boats landing on a shore with surf for this reason. Going out this will be less of a problem

Obviously circumstances alter cases and the bar at the Beaulieu River, where the Solent means the sea cannot build too much, is much less dangerous than the bar at Salcombe or that at Newton Ferrus, both of which the pilot states should be treeated with extreme caution in a South Wester.

Rules as far as I am concerned are:

1) Have at least the depth of the keel x 2 unless it is a very calm day.

2) If it is rough over the bar go somewhere else

<hr width=100% size=1>Chris Stannard

 

[snowleopard]

what denny means is that the depth of water in the tide tables is the average, or half way between the crest and the trough. when you are at the bottom of the trough you are half of the wave height below that.

in fact as the waves reach shallow water they become steeper so you can't look at the wave height out at sea and assume it will be the same over the bar.

<hr width=100% size=1>

 

[seaesta]

The other year I got some duff tide info from Hartlepool Marina and went into Whitby on a strong Northerley at the wrong state of tide. This got really exciting when I surfed on a breaking wave for a long way - right through the peirheads. If I had touched bottom at theis speed then it would have been a vicious pitch-pole and grief. So thank God for bilge keelers.
In retrospect I should have seen the on shore sea spray and stood off despite the tidal predictions.
Actually yacht surfing was a massive rush - the wave was breaking amidships for ages and we seemed to be going at high speed but this not something I would do again in my own yacht!
Motto - If the waves are starting to really stand up in shallow water then keep off!

<hr width=100% size=1>