Max hull speed vs current

[TLouth7]

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.

This is true in tidal races, and for example around headlands there are often eddies inshore that can be exploited by knowledgeable locals.

On the other hand in many areas, for example the entire North Sea or the English Channel, the water pretty much does just move backwards and forwards in one coherent lump and so has no effect on boat handling except that the tidal flow needs to be added to the boat's velocity to find COG and SOG.

 

[Refueler]

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.

It is quite common that in event of a strong current say trying to enter a harbour - that getting as close to shore as possible will often give you a significantly reduced current or even a slight reversed.

But the OP is basically asking about SOG and STW .... as a general matter.

 

[Rappey]

I see so many boats approaching portsmouth harbour right out in the channel fighting the current , while others hug the shallows and make much better progress.
Some spend an hr in the harbour mouth not moving anywhere as they have chosen to sit in the strongest part of the current flow.
Local knowledge is a great thing. Understanding current flow for some seems to be lacking

 

[John the kiwi]

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.

Urm.. this would be true if the current was caused by a large underwater propellor in a tank with a constant water level, but its not the case.
Tidal flow is caused by a difference in height of water levels. i.e to state the obvious, water flows down hill!
So if you are going upstream against a high speed tidal stream you are actually and literally going uphill. To go uphill requires an input of work. In physical terms you are gaining gravitational potential energy. So some of your motor power is being used to push you up the hill and since your motor has a fixed power output at given revs, this subtracts from the power available to push you through the water.
In my home port i would reckon that when the ebb was at max speed of say 6 knots the height difference could be as much as 1 meter in 200.

 

[Rappey]

I like John the kiwis explanation of going uphill. I wonder what the tidal height difference is each end of Portsmouth harbour mouth as it's a lot of water through a narrow channel.
That could theoretically account for a missing 1.5 knots when going against 4knts of current, doing 7 through water but only making 1.5 over ground ?

 

[MarkCX]

The effort of ascending a ½% incline compared to pushing your boat through 6kt current is fairly inconsequential.

 

[Refueler]

The effort of ascending a ½% incline compared to pushing your boat through 6kt current is fairly inconsequential.

Especially when given water is not a straight sheet but has its crests and troughs ... even in light airs ..... AND often strong currents cause waves to be created as it transits over the seabed.

 

[srm]

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.

So Judgemental.
Some years ago I came across a very nice sailing lifeboat conversion in Orkney, the owner then spent the following winter in Shetland. It had a snug gaff rig and the owner was slowly sailing and working his way around Britain. From memory I think he may have reduced the freeboard.

 

[canvey]

But so true and worthy of consideration from a safety point of view.

Emphasis on the "slowly", and during calm weather only in their case, I'd bet. Do the maths on how far the bridge swings in a modest style swell. Tides and especially winds don't always come head on.

However, I did make the mistake of not to asking whether the original poster meant one of those big, orange, teletubby-style freefall vessels; or a simple, old fashioned rowing boat style. I was thinking of the big orange ones.


Fair enough, in India, where they break a lot of the boats they come off, they cut the tops off them and make them into river boats. I read someone suggest that if you were to replace the top with a cabin, cockpit and deck, you'd end up with something not unlike a Dutch tjalk, or sailing barge, but even they generally came with leeboards.

A couple of architect students recently got some media and sponsorship for converting one into a caravan-without-a-keel) and motored it all the way round the North Sea to at least Norway on their way to the Arctic (they could not sail it safely straight across). The promotion of the idea was beyond disingenuous architect waffle to, quite frankly, dishonesty*; least of all not saying what it cost to push such an inefficient hull all that way.

* It was promoted as eco- and "solar powdered" until viewers came down hard on their diesel engine and it turned out the only thing that was solar powered were their Apple Macs, which begged the question why, if they were running on diesel all that way, and intent on pumping fumes out into the pristine Arctic.

I'm afraid a load of people will see it and follow them.

Apart from the East London pose factor, what were they actually doing and why? What were they after. For sure, for "architects" they did not read a book on naval architecture. Seems the only advantage was two full height picture windows that, again, would limit where and when it could be taken. The hull shape is designed NOT to cut through water, so when it hits the water it bobs back up, not submarines. Which is the opposite of what you need for efficient motor/sailing.

Am I wrong? They are sails. I'd want to go straight out a long way to avoid the risk of lee shores, and have a second means of propulsion at hand.


How often do you need to do this at sea?

 

[JumbleDuck]

In case you were all wondering where the hull speed formula comes from ...

Drag increases rapidly when the stern of a boat squats down into the trough of the bow wave, so the maximum speed before this happens is when the wavelength is the same as the length of the boat. In deep water (ie the sea bed has no effect) , the speed v of a wave depends on its wavelength l:

v = sqrt (l x g / 2 pi)

All that's in metric, so putting it in knots (V = v x 1.94) and feet (L = l x 3.28),

V = 1.94 x 1.25 x sqrt (L x 9.81/2 pi x 3.28) = 1.34 sqrt(L)

 

[LittleSister]

Converting ships' lifeboats to leisure use was very popular in the 60s and 70s. (A friend lived on one.) I seem to recall people often added shallow keels, presumably to aid directional stability, as well as to reduce leeway if they were adding a sailing rig. They were never going to be high performance craft, but I imagine some got a lot of fun for relatively little money.

The ex- lifeboats in those days would presumably have been mainly wooden, and perhaps with a more conventional boat shaped hull than modern specialised lifeboat hulls.

 

[Refueler]

Converting ships' lifeboats to leisure use was very popular in the 60s and 70s. (A friend lived on one.) I seem to recall people often added shallow keels, presumably to aid directional stability, as well as to reduce leeway if they were adding a sailing rig. They were never going to be high performance craft, but I imagine some got a lot of fun for relatively little money.

The ex- lifeboats in those days would presumably have been mainly wooden, and perhaps with a more conventional boat shaped hull than modern specialised lifeboat hulls.

Wooden ??? They were out many years ago ... 60's ... 70's .. were by then GRP jobs.

Round hull double ended with bilge grab rails. The central keel was basically a GRP box with only a couple inches proud of hull.
On a ship - typically in those days one would be motor - other sail ... both with oars. Open type.

They rolled like pigs unless well weighted down.

 

[AntarcticPilot]

Converting ships' lifeboats to leisure use was very popular in the 60s and 70s. (A friend lived on one.) I seem to recall people often added shallow keels, presumably to aid directional stability, as well as to reduce leeway if they were adding a sailing rig. They were never going to be high performance craft, but I imagine some got a lot of fun for relatively little money.

The ex- lifeboats in those days would presumably have been mainly wooden, and perhaps with a more conventional boat shaped hull than modern specialised lifeboat hulls.

Indeed - my dad's first cabin boat was such a converted lifeboat, and they were very common in the late 50s and 60s. Ours had been very nicely done by the previous owner, who had made a lot of the rigging in the workshops of Blackburn Aircraft! She was cutter rigged with a gaff mainsail, and wasn't the most sparkling performer, but she got from A to B, and with bilge keels that extended more than half her length, she was reasonably weatherly. She had a load of iron ballast to stiffen her. She took the ground very comfortably too - very handy when Dad missed the channel at Trent Falls and we spent a tide aground! Actually very peaceful and beautiful clean sand for us youngsters to play on!

 

[LittleSister]

Wooden ??? They were out many years ago ... 60's ... 70's .. were by then GRP jobs..

On the ships, no doubt, but there were still a lot of older former lifeboat hulls on the market.

As I mentioned, a friend lived on one. That was wooden hulled. He tried to persuade me to buy a lifeboat hull that was for sale locally (this would have been sometime about mid-70s) to convert as a live-aboard, though that particular one was metal hulled.

 

[srm]

But so true and worthy of consideration from a safety point of view.

OK, so you have a hang up about fully enclosed/free fall ships lifeboats and architects.
The OP clearly says he has a 5m ex ship's lifeboat.
Certainly all the fully enclosed and partially enclosed modern lifeboats I have come across looked quite a bit longer than 5m / 16 ft. The free fall lifeboat I did an exercise in was a lot longer as it could hold the full complement of an offshore oilfield support platform.
Perhaps you would be so kind as to back up your analysis with an account of your personal experience(s) of these vessels?

 

[Rappey]

Since there is a fair bit of lifeboat talk here is an interesting conversion. Not sure if was mentioned earlier. I can't keep up ?
How two architects turned this cheap old lifeboat…:

 

[johnalison]

I like John the kiwis explanation of going uphill. I wonder what the tidal height difference is each end of Portsmouth harbour mouth as it's a lot of water through a narrow channel.
That could theoretically account for a missing 1.5 knots when going against 4knts of current, doing 7 through water but only making 1.5 over ground ?

If you want to try motoring uphill, then try leaving St Valery en Caux at the first gate opening, along with the local fishermen. There really is a visible gradient through the bridge passage, and wiser folk wait for a quarter of an hour before casting off.

As for turbulence, I have a very good idea of the speed my boat should do through the water for any given rpm, but some areas of turbulence can easily knock half a knot off what I expect. A particular spot locally is rounding Stone Point when returning against the ebb.

 

[Rappey]

If you want to try motoring uphill,

That is something I would have never expected to read. Quite funny ?

Ive seen videos of possibly Greece? where a large bay is connected to the sea with a stone or concrete wall cutting through a strip of land. There is a fast flowing current . Many yachts attempt to "run the rapids" and end up playing pinball as they loose control and get bashed into the walls.

 

[alan_d]

If you want to try motoring uphill, then try leaving St Valery en Caux at the first gate opening, along with the local fishermen. There really is a visible gradient through the bridge passage, and wiser folk wait for a quarter of an hour before casting off.

(I was going to say "closer to home", but for quite a lot of the UK it isn't.) You could try going under the Tay Rail Bridge against a strong ebb.

 

[Spirit (of Glenans)]

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

But with several places where you can drop the anchor to wait out a foul tide.