Max hull speed vs current

[Sandro]

Due to my location, I know very little (or nothing at all) about tides but I have the impression that a current is not just a vast mass of water moving rigidly all together, but there is something inside the water body, like different speeds in different spots or eddies or whatever else, that makes the matter more complex than we think.

Looking at a tide race one can “see“ that there is current, from wavelets or the like. One could not tell the difference if the water were flowing solidly, whatever the speed.

This could explain the apparent erratic behaviour of hulls and the non compliance to the simple algebrical (really vectorial) sum: speed over ground = water speed on ground plus boat speed on water.

This could also affect the maximum hull speed.

Sandro

 

[tillergirl]

Don't forget hydrodynamic drag. If the OP's boat can only achieve 6 knots boat speed with WOT WITH the tide, he won't be able to achieve 6 knots boat speed with WOT against the tide. The extent of the drag depends on shape of the boat.

 

[johnalison]

Adverse conditions are invariably worse than predicted. It's either sod's law or my paranoia. In nearly half a century of sailing, I don't think that I have ever said to myself "That's good; the tide is less than I thought!". Even 2.5kn against can seem like an eternity when you imagine that you are in a hurry.

 

[srm]

Yes, but a current as fast as six knots is rare, and can almost always be avoided by going a different way or at a different time.

Depends where you are - Fairly normal with spring tides around the Orkney Islands, and the Pentland Firth hence the islands being a test site for tidal energy generation projects. Comes as a surprise to most first time visiting yachts. They soon learn that the only way to make progress is to go with the tide, which can give 'brag about' GPS/ ground speeds.
However, a vessels hull speed is usually taken as its maximum achievable speed through the water and does not change between still or moving water. Wave motion may well reduce a vessel's water speed.
Lots of variables that seem to be getting a bit confused in this thread. The OP was a simple question that should be answerable by anyone who has done a few basic chart plotting exercises to find an estimated position allowing for the effect of tide.

 

[KREW2]

Yes, but a current as fast as six knots is rare, and can almost always be avoided by going a different way or at a different time.

Not really, from the Solent down to Lands end you have several strong tidal gates.

 

[LittleSister]

Yes, but a current as fast as six knots is rare, and can almost always be avoided by going a different way or at a different time.

Not really, from the Solent down to Lands end you have several strong tidal gates.

Try reading what I wrote again!

 

[Refueler]

Try reading what I wrote again!

Fine ... but many in Solent have to live with such currents ..... have a berth in any of the main harbours such as Portsmouth ... Langstone ... Chichester ... the size of the harbours can mean that the distance to your mooring dictates the time of entry to avoid next tide etc.

 

[srm]

Don't get hung up on currents of six knots. The bottom line is how long will it take to get there?
Boat speed at 6 kn with 2 knot of tide in the same direction = ground speed of 8 knots. - That's good.
Boat speed at 6 kn with 2 knot of tide against it = ground speed of 4 knots. - Not so good, in fact twice the time to get there.
Now put the tide across the direction you want to go - it gets interesting, especially if you finish up down tide of your destination.

 

[PCUK]

Don't forget hydrodynamic drag. If the OP's boat can only achieve 6 knots boat speed with WOT WITH the tide, he won't be able to achieve 6 knots boat speed with WOT against the tide. The extent of the drag depends on shape of the boat.

Not true, As far as the boat is concerned it is running through still water no matter which way the tide is running. Only the speed over the ground is affected.

 

[Refueler]

Don't get hung up on currents of six knots. The bottom line is how long will it take to get there?
Boat speed at 6 kn with 2 knot of tide in the same direction = ground speed of 8 knots. - That's good.
Boat speed at 6 kn with 2 knot of tide against it = ground speed of 4 knots. - Not so good, in fact twice the time to get there.
Now put the tide across the direction you want to go - it gets interesting, especially if you finish up down tide of your destination.

OK ... getting away from the OP's original question.

Yes the Vector question.

The Spinnaker pub opposite Moody's .... pal and I were in there one day .... next to us were a couple of boaters and discussing crossing channel ... we could not avoid hearing them - they were quite loud.
After hearing them getting tied up in knots over course allowance for current ... I politely spoke with them and we had a round together ... then I offered the blank side of a beer mat and just drew a simple vector triangle ... to which I was then subjected to most absurd being told it was not so simple and I didn't know what I was talking about ... to which I just mentioned that years navigating ships may just mean I might know something about it ...

Needless to say our next round was only my pal and I !!

 

[KREW2]

Try reading what I wrote again!

Not quite sure what to look for.
I agree, and said in an earlier post go with the flow, but 6 knot tides are not that uncommon.

 

[srm]

to which I was then subjected to most absurd being told it was not so simple and I didn't know what I was talking about

Refueler, Yes, the question was simple and the first couple of answers were to the point. Then all sorts of red herrings got thrown in.
But after many years teaching navigation and met to RYA, fishing and MN deck officer candidates I am used to people coming up with all sorts of weird and wonderful complications.
Had one MN candidate who objected to having to make navigation checks on the ENRAST simulator course as his ship had two GPS sets. I quietly took him aside and pointed out that I was the person signing his completion certificate and did he want it endorsed "only valid for vessels with two operational GPS systems". I think the problem was that he thought I was only a WAFI.

 

[[163233]]

The important thing is to remember to untie it.

 

[Refueler]

Refueler, Yes, the question was simple and the first couple of answers were to the point. Then all sorts of red herrings got thrown in.
But after many years teaching navigation and met to RYA, fishing and MN deck officer candidates I am used to people coming up with all sorts of weird and wonderful complications.
Had one MN candidate who objected to having to make navigation checks on the ENRAST simulator course as his ship had two GPS sets. I quietly took him aside and pointed out that I was the person signing his completion certificate and did he want it endorsed "only valid for vessels with two operational GPS systems". I think the problem was that he thought I was only a WAFI.

My Father used to give talks to clubs about Navigation ... but after a while - he declined. I remember what he said then ... you face a group that there will always be one who thinks he's smarter and knows absolutely zero - but it will spoil the whole exercise.

Later I was to find the same and I stopped giving talks.

 

[john_morris_uk]

Adverse conditions are invariably worse than predicted. It's either sod's law or my paranoia. In nearly half a century of sailing, I don't think that I have ever said to myself "That's good; the tide is less than I thought!".

Ain’t that the truth.

 

[alan_d]

My Father used to give talks to clubs about Navigation ... but after a while - he declined. I remember what he said then ... you face a group that there will always be one who thinks he's smarter and knows absolutely zero - but it will spoil the whole exercise.

Later I was to find the same and I stopped giving talks.

Yes, there is very often a smartarse know-all in the group, whatever the subject being covered, but if all teachers reacted like you and your father nobody would get taught anything. Most people who teach groups have developed their own technique for dealing with them, just as stand-up comedians learn to cope with hecklers.

 

[canvey]

I’ve got a 5m ex ships lifeboat, guessing I have a maximum hull speed of 4-6 knots with a 12hp inboard... if I try and go against a 6 knot current, am I going to go backwards?

No one has factored in

a) the ridiculous amount of freeboard he/you have, equivalent to a sail almost, and
b) the effect of the roll sitting 3m high in the air of one of those things.

Can I ask a polite question or two, Stee?

What are you actually considering doing and why, and how far is your dream/project developed?

Hull speeds are only a theoretical estimate, some boats will go faster, I'd put a bet on this one actually going slower. Add a wind into the equation and you'll also be going sideways and rolling about 6' back and forward at cabin height. Those things really aren't coastal sailing or motoring boats and they suck at those jobs. They are what they were designed for which, worryingly, is being rescued from, not going anywhere.

I know they are fashionable right now among liveaboard converters, and they may well be great caravans conversions for canals on the inside, albeit eyesores on the outside, but they are not designed to be great sailing boats. Where they're intended to be use, there's not a lot of things to hit, lees shores, and so on.

If it's just a rush from one marina to the next to live, then pick your times and perfectly calm days to make the dash. I imagine it's not going to be long until we read of one being blown into some rocky shore. Any more than a few hours (tide changes), and I've be wanting a tug escort just in case.

 

[Mudisox]

As usual - the expert technical members like to churn out the data !!

Nice to see another who reads the OP post for what it is ...

Agree.
But why let the truth interfere with uninformed discussion?

 

[TSB240]

We can regularly experience a situation where our log shows 5 knots and our GPS sog shows 13. After 56 years in this area we have also found staying in the tide when going against it will not give you a different view for a long period of time. This is fine if you enjoy observing one of the largest colonies of Herons in the UK. 10 hp engine in a 9m yacht.

 

[Refueler]

Yes, there is very often a smartarse know-all in the group, whatever the subject being covered, but if all teachers reacted like you and your father nobody would get taught anything. Most people who teach groups have developed their own technique for dealing with them, just as stand-up comedians learn to cope with hecklers.

True .. but this was voluntary talks in clubs as part of the clubs social and activities.