SOG v MPH v Knots

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SolentPhill

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Guys

Last week I got a B******* from my Dep Harbour master for going too fast in the narrow channel with 4 knots of fast water against me, I was according to my Raymarine doing 11 knots, the plotter has SOG which I didnt look at. You are allowed in the outer channel to go faster than the 6knt you have to do inside.

My question is this if Im doing 11knt against 4 knt water am I not doing 7knt.

My boat has 3 gauges one connected to the wheel underneath, the sog on the plotter, and MPH gauge connected to the transom sensor.

Can someone explain in simple ways please?

Thanks
 
I loved the "don't speed cos it means more work for me" bit.

I didn't quite buy into the idea that the harbour board's launches all have their logs calibrated on a daily basis. Perhaps that remark was just a little bit tongue in cheek?
 
As someone wisely said on here not long ago, the prime factor is "No wash" the secondary is "max speed x knots".

I find that I do a good deal of looking at what's behind me in two contexts: first in restricted waters to check the wash and secondly in open waters because, with a top speed of 15 knots or thereabouts, I'm slower than a lot of big vessels.

A further thought on wash: my current boat is quite kind because of a rounded hull section. My previous boat had quite a square stern and a small difference in speed from, say, 6 to 7 knots made a huge difference in the amount of wash.
 
I don't think its the speed so much as the wash that always suprises me everytime we go out. I expect the bigger the hump behind the stern of a boat, the more fuel it must be using.
At around 6 knots we have a very small wake and no appreciable stern hump? wave? is there a term for it? We do get rocked about by othr boats in the harbour doing 10 knots though. It looks like at 10 knots, the smaller the boat, the more wake it produces
 
It's definitely the wash from mobos(and sailers under power too!) that annoys us raggies rather than the actual speed.

We noticed over the last 5 weeks cruise down to St Malo and the Rance that the French powerboaters were very good at slowing down before moorings, pontoons, and near raggies, except professional fishermen and a few idiots- friendly French berthholders taught us a few choice phrases to shout at them and demostrated correct pronounciation and volume too /forums/images/graemlins/blush.gif.

Closeness of approach doesn't impress us or anglers either.

The problem is that Planing hulls of any size in displacement mode create terrific wash for their size, and if the revs are up to counter the tide or too great for the speed and the boat is dug into it's trough-

BINGO, yer DHM is not a happy bunny, nor is anyone working on or below deck or cooking on the boats you pass. /forums/images/graemlins/mad.gif

But if you are over the "hump" and on the Plane, with minimum wash, you are exceeding the harbour 6kt limit, so you get ostracised by everyone anyway.

Perhaps a change by whole BMIF back to "gentleman's motor yacht" semi-/displacement hulls for the UK market, ready for 01 November 2008 may be in order?
Saw some lovely ones on the cruise- liveaboard Brits mostly.

Our current >12 ton 50/50 yacht has a 35hp diesel, which can create an antisocial wash given the right circs.
But having been on the receiving end too often, we tend to be aware and watch the scenery at about 1kt SOG if we get the tide flows wrong.

After all, taking your 11kt scenario in point, at that 1kt SOG, you've got 5kts flow over the rudder(s) from in front so you should have steerage way ok, and can ferry glide across the stream if you need to avoid something(weed, raggie, dinghy, other vermin, etc). Unless the wind is blowing a hooligan on the beam.

Best and cheapest way is to use the tides to advantage whenever you can and relax and watch the scenery slowqly pass if you cannot- and yes, our berth has too short a tidal window too /forums/images/graemlins/grin.gif

Oh, and you might have guessed, I used to teach RYA Power for work and safety boat crews all year round, back when most of my body worked better /forums/images/graemlins/frown.gif.
 
[ QUOTE ]
Its really time we adopted the American "No Wake" rule to avoid these silly arguments.

[/ QUOTE ]

We always have. The speed given is the maximum - not a target - and wash ahs has always been the overrding factor. Check out for example the sign on the twin posts entering the Hamble River for example. And while on the point about the speed in the Hamble, the speed restrictions apply from No1 post, not from the post as shown above.

hamble4-2.jpg


During my 3 years as Portsmouth Harbour patrol coxswain it was a regular occurence to have to ask mobos to slow down. They would gesticulate that they were doing 10 knots - that was irelevant it was the wash that was the main factot. Very few mobos are even aware of the wash they create and it is very noticable when they come up the Hamble or other enclosed waters. There was a partcular individual on Saturday in a Sailtime Bavaria mobo who went charging out from amongst the anchored boats off Bournemouth - completely unaware of the havoc he was causing and then looked aggrieved when he was shouted at by all and sundry. Plank! /forums/images/graemlins/mad.gif

You must look behind to see the effects of your wash, ploughing along watching the speed is why people will get shouted at and stopped.

Simple really, if you making wash - slow down.
 
How many prosecutions are you aware of for excessive wash? Bet its zero...

How many for speeding? Bet its 'some'...

The authorities are making it clear which they see as more important.

Rick
 
I saw you, yes you were probably doing only 7 knots, but almost all the other boats struggling against that 4.2 knot tide were only going at an average of 1-2 knots, some were practically stationary, /forums/images/graemlins/grin.gif What is worth remembering is that when the tide is bringing you out very fast you lose a lot of your steering and when you are struggling against the tide coming in you are swept to the inner section sideways, a big wake makes that worse so for safetys sake you need to moderate your speed to the speed of the boats arround you.
 
It's really funny how most boaters bother comparing SOG and SOW against speed limits, when discussing about wash in harbours, channels, etc.
There is only one thing to look at, and that's not the console, it's the water.
The amount of wash is completely different for a given speed, depending on the hull shape and size.
And aside from the obvious fact that the wash is more SOW than SOG - driven, even the depth can make a helluva difference.
If a given hull makes no wash in deep water up to say 7 kts SOW, in shallow water the same hull might create wash already at 5 kts.
And depending on the rudder position (which in side current you might need to keep constantly to port or stbd), you can create wash on just one side of your course.
Admittedly, these cases are much more relevant for heavy(ish) boats.
But the bottom line remains the same, as others already pointed out: just look at the water, and forget the instruments.

PS: on a side note, if you didn't look at SOG, how on earth can you say that the current was 4 kts?
 
[ QUOTE ]
I saw you, yes you were probably doing only 7 knots, but almost all the other boats struggling against that 4.2 knot tide were only going at an average of 1-2 knots, .

[/ QUOTE ]

Where was I then???
 
Ok as I seem to be being gunned down I will explain a bit more, where I was, was outside a harbour, There are 2 markers 300 yds out to sea, no walls, no other boats, no wildlife just me facing the outgoing tide, and a 15mph cross wind, Im in a 9 tonne boat with legs facing all this surely doing 11knt to keep in control is not un reasonable, and I was slowing and did 6knt as I got tothe wall and 3knt as I went past any boats as I usually do.
 
It wasnt you then, I was on Fort Blockhouse, and the boat I was referring to, which got a ticking off from the harbour patrol boat was smaller.
It was the reference to HMs assistant and being outside on the approach to a narrow channel with a 4knot tide against which led me incorrectly to assuming it was you, my sincere apologies, but I wasnt meaning to shoot you down in my post merely answer your question about why you would get stopped even though you were under the max speed limit.
 
I could not agree more.

Speed is a factor with a bearing on safe navigation but these speed limits are worthless if its wash you need to control.


i have a RIB on a Versadock on the tidal Thames and the damage to the pontoon and dock is considerable as boats come in close (in 8 knot limit) at speeds well in excess and sometimes displacement boats even at 8knots make a big wash as they pass.

Its all down to seamanship - or lack thereof.

This country is terrible in this regard as other places i have boated are in general much more 'switched on' when dealing with basic good seamanship issues.
 
The only thing that matters is "wake" and that is governed by water speed (ie log), not Gps SOG which is irrelevant to most harbour bylaws. Bear in mind too that wake travels a long distance, so powering off 100m before is too late!

Quite a few folk don't understand the difference. Speed read out's are a "cop out", it's easy to look behind at the wake, and folk should know what typical RPM gives them a max displacement speed with manageable wake.
 
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