Max hull speed vs current

[stee]

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? Makes sense to me but just wanted to check if current flow against hull is any different to hull resistance moving forward... thanks in advance

 

[scottie]

Yes it like standing on a moving pathway

 

[pvb]

You were right first time.

 

[jbweston]

You're right.

Like walking at 4 - 6 knots along the walkway as it moves 6 knots against you. At best you'll do 2 knots towards your destination. At worst you'll stand still against the land, banks of the river, or whatever is fixed to the earth.

As I think you realise (which is why you've asked the question) usually this isn't a desirable state of affairs. The traditional thing to do was to anchor until the tide turned, assuming that the flow against you is tidal. Nowadays most larger boats and ships can travel faster than the tidal flow in many areas, but it's still a problem for sailing boats in light winds, and even for quite fast boats in places where the tide is fast.

Also bear in mind that going the other way you add your speed to the tide or current, so you can be bombing along in the right conditions.

 

[KREW2]

Go with the flow

 

[Refueler]

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?

Actually not strictly true.

Before the Forum Ex Spurts scream Idiot .... let me explain.

You are doing 4 - 6kts through the water. You are moving forward in the body of water regardless ... ie speed log will show +ve speed

But in relation to fixed land / sea bottom - you will be moving backwards if the water you are moving forward in is moving opposite direction faster than you. ie GPS will show net speed backwards. Like walking forward at a slower speed than the travelator is moving opposite way ....

Think of like you are in a 'box' - you are moving forward in the box but the box is being pushed backwards.

 

[V1701]

Depends on the type of hull - for displacement hulls yes the formula is 1.34 x sq root of waterline length, so for a displacement hull with a waterline length of 25 feet, the theoretical max hull speed would be 5 x 1.34 = 6.7 knots. I don't know what the definition of a semi-planing hull is or whether there's a formula for calculating their theoretical hull speed but it would presumably be higher, and the theoretical hull speed of a planing hull, higher again and presumably the more the hull speed would be affected by a more powerful engine. At a guess with the boat you describe with a 12hp motor you would be able to make some progress against a 6 knot current, but I'm only guessing. Give it a try...

 

[Rappey]

theoretical hull speed of a planing hull,

The semi displacement is able to climb its own wave and therefore achieve a faster speed but i do believe it too has a max hull speed. There is no limit to a planing hull speed. Its all down to how much power you can throw at it..
Whilst i totally agree with the explanations of hull through water im sure there is an odd scenario.. ie, portsmouth harbour current flowing at 4 knts, me able to do 7 knts yet flat out against the current only seem to be making 1.5 knts.. where has the other 1.5 gone? Lets also assume boat has jus been antifouled..

 

[pcatterall]

The semi displacement is able to climb its own wave and therefore achieve a faster speed but i do believe it too has a max hull speed. There is no limit to a planing hull speed. Its all down to how much power you can throw at it..
Whilst i totally agree with the explanations of hull through water im sure there is an odd scenario.. ie, portsmouth harbour current flowing at 4 knts, me able to do 7 knts yet flat out against the current only seem to be making 1.5 knts.. where has the other 1.5 gone? Lets also assume boat has jus been antifouled..

Chop? Wind?

 

[Rappey]

Chop? Wind?

I look on tidal flow charts and wonder why the current always seems stronger than max stated?
the only solution i can think is i must be stupid ?

 

[ithet]

I look on tidal flow charts and wonder why the current always seems stronger than max stated?
the only solution i can think is i must be stupid ?

No, you're not stupid. The max is mean spring tide rate. But very often spring tides are bigger than average. That's why many years ago with my father we spent several hours motoring at the same place doing 9 knots off Alderney.

 

[srm]

I look on tidal flow charts and wonder why the current always seems stronger than max stated?

No you are not stupid. Tidal stream charts give predicted tidal stream for mean spring and mean neap ranges. If the range on the day is more than the figure you are using the current will be correspondingly stronger.

Additional meteorological factors can also increase or decrease the predicted stream.

Then again the tidal stream information may be just plain wrong. It is many years since I took the passage between the Isle of Man and Calf of Man so this is from memory. Chart indicated spring rate of 2 knots so I went through against the stream entering at the start of the adverse stream. At five knots we met that magic spot where boat speed is equal and opposite to water speed and ground speed is zero. At full throttle we had a hull speed of a shade under 7 knots and very slowly pushed through the narrows.

Incidentally, when living in Orkney we often saw visiting yachts sitting in the magic spot for an hour or more when they tried to make a passage against the stream.

 

[Refueler]

Depends on the type of hull - for displacement hulls yes the formula is 1.34 x sq root of waterline length, so for a displacement hull with a waterline length of 25 feet, the theoretical max hull speed would be 5 x 1.34 = 6.7 knots. I don't know what the definition of a semi-planing hull is or whether there's a formula for calculating their theoretical hull speed but it would presumably be higher, and the theoretical hull speed of a planing hull, higher again and presumably the more the hull speed would be affected by a more powerful engine. At a guess with the boat you describe with a 12hp motor you would be able to make some progress against a 6 knot current, but I'm only guessing. Give it a try...

OP says an ex Ships Lifeboat ..... so a) its a displacement hull, b) its a workboat shape .... so it will have a lot of drag ... it sure 'aint no planing hull' !!

 

[Rappey]

Im flattered that i see posts of "im not stupid" rather than expecting to see likes
I always assumed the tidal flow charts show the max possible tidal flow..
flat out tidelss water i leave a little wake. against the harbour flow im leaving a large wash ?
there are many forces at play, sideways currents, currents risng and going down etc.
Solent sailing - whenever and wherever i go i seem to be against the currents Its not so much bad planning so much as early morning, lets go, evening- time to return... take whatever the tide is doing.
A different story if i actually want to arrive somewhere... But isnt it about the journey, not the destination ?

 

[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.

 

[Ian_Edwards]

It's all a little more complicated than that, the 1.34 is a fudge factor, based on an average hull shape and it's wave making resistance.
A long thin hull will have higher maximum non planning speed than a fat one of the same length.
Just think of a catamaran, a 47 foot Outremer catamarans will regularly exceed 20 knots, my 46 foot mono hull will make 9 knots on a very good day. Neither of these hulls plane.
The secret of the cat is very long narrow hulls with very little wave making resistance.
I'd suggest that the OP ex-lifeboat hull will struggle to make the 1.34 predictions.
When I was lad, the formula was 1.5 x square root of the water line length in feet, the change perhaps reflecting the modern trend of broader beam boats will little of no difference between the waterline length and the length overall. Older hull shapes effectively increase their waterline length when underway, especially when heeled over going to windward.

 

[scottie]

It's all a little more complicated than that, the 1.34 is a fudge factor, based on an average hull shape and it's wave making resistance.
A long thin hull will have higher maximum non planning speed than a fat one of the same length.
Just think of a catamaran, a 47 foot Outremer catamarans will regularly exceed 20 knots, my 46 foot mono hull will make 9 knots on a very good day. Neither of these hulls plane.
The secret of the cat is very long narrow hulls with very little wave making resistance.
I'd suggest that the OP ex-lifeboat hull will struggle to make the 1.34 predictions.
When I was lad, the formula was 1.5 x square root of the water line length in feet, the change perhaps reflecting the modern trend of broader beam boats will little of no difference between the waterline length and the length overall. Older hull shapes effectively increase their waterline length when underway, especially when heeled over going to windward.

you both missing the point he is just asking a basic question does tide affect speed over ground

 

[Stemar]

Im flattered that i see posts of "im not stupid" rather than expecting to see likes
I always assumed the tidal flow charts show the max possible tidal flow..
flat out tidelss water i leave a little wake. against the harbour flow im leaving a large wash ?
there are many forces at play, sideways currents, currents risng and going down etc.
Solent sailing - whenever and wherever i go i seem to be against the currents Its not so much bad planning so much as early morning, lets go, evening- time to return... take whatever the tide is doing.
A different story if i actually want to arrive somewhere... But isnt it about the journey, not the destination ?

Which is why I look at the tide table before leaving Portsmouth and the decision whether to go to Newtown Creek or Chichester Harbour depends on which one's downhill for the next few hours

 

[PCUK]

Hull type makes absolutely no difference in this instance. If the maximum speed is six knots it is six knots. Therefore a foul tide of six knots will mean zero speed over the ground and a fair tide 12 knots over the ground!

 

[Refueler]

Hull type makes absolutely no difference in this instance. If the maximum speed is six knots it is six knots. Therefore a foul tide of six knots will mean zero speed over the ground and a fair tide 12 knots over the ground!

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 ...