Boat speed under sail, what should I expect?

Sailfree said:
The true speed a boat sails at is the one used for passage planning!

On my 36 Dufour it was 5.5 kts but only 2kts against a headwind.

I then bought a 38 Dufour and learnt to sail only with the wind ( gentlemen don't beat) and passage planning was 6 kts.

On the 43 Jeanneau it is about 6.5kts.

For these speeds I am assuming the 60ml to Cherbourg etc not the spot speed you get for 15 min with favourable conditions but then I am probably a **** sailor in spite of 20 yrs racing a Wayfarer.
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In my Stella the record for Bradwell to Ostend via longsand head about 85 miles is 13.5 hours moored to tied up. ( longest was 32 hours but a lot at 20-22 hours) Making best use of tides
In my Hanse 311, I have done Bradwell to Ostend via LS Head in 14 Hours but normally take anything between 15 & 17.5 hours

I tend to use my Aeries a lot as with it i can set it & then trim the sails for best performance then leave it alone to get on with the job better than I can otherwise I find my speed drops off as i get tired or loose concentration
 
Trop Cher said:
I sail a awb of 40 feet. I usually get her up to seven knots quite easily say in a F4/5 and I'm chuffed when I get over eight knots, top speed so far is 8.7 knots, this is boat speed using the log not gps.
I was recently talking with someone who sails a 27 ft boat and said that he regularly gets over nine knots. I suggested that that must include tide, but he said no, that was from his log.
Now either I'm doing something wrong or he needs to recalibrate his log?
What sort of speeds should I expect?
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I think as has already been pointed out on here there are boats... and then there are boats! What you gain in one area you lose in another and if you want speed it usually comes at a hefty price. The best I've had out of my 27 footer is 22 knots and she regularly sails at over 15, but I can assure you that sailing at that speed is fairly full on so you don't get to go below and put the kettle on... Also I don't have anywhere near the accommodation or load carrying capability of your 27" boat, and I have to maintain three hulls. She is quite good fun though......
 
No-one, least of all me, would ever claim the A22 had been comfortable in F11, just that she always seemed likely to survive when we found ourselves in those unforecast breezes; bit of a difference...:rolleyes:
 
Salty John said:
Your figures seem pretty well spot on for your boat.

I blogged on this subject here:http://www.saltyjohntheblog.com/2015/02/displacement-hull-speed-2.html

(I defer to Grumpy o.g's hugely superior knowledge of hydrodynamics and apologise for using 'climb its own bow wave' to describe the moment the displacement hull hits the wall).
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I did apologise for a rather dry post and it certainly wasn't aimed at anybody in particular but you have to admit that a boat doesn't try and climb it's own bow wave even if it's a convenient description...
 
Dave-Shafa said:
In my esteemed opinion, anyone who wories about boat speed in a yacht, is in the wrong sport, cept racing yachts.
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I think differently - from my perspective- When ever i sail I do so to get the maximum from the boat in respect of speed .I do it because I like to think that I am sailing well.That being said i would not be slow to reef & do not carry the cruising chute that much.
But I still strive to get the best speed & consider that a part of sailing. Just sailing slowly when I could be sailing faster just does not seem right to me. That has nothing to do with racing.
When i first bought my yacht i struggled to get 6.0Kts upwind. A world champion came aboard & stuck it straight on 6.5 kts & kept it there. I want to achieve the same sort of skill- eventually
 
Bit late to this thread but my halfpence worth to OP: as others have said the SL Ratio is usually quoted as 1.34, which is then inserted into a simple base-line formula for calculating the displacement speed of an average hull. A boat’s theoretical hull speed is simply the max speed when its wave length equals its waterline length, hence the importance of the length of the wave relative to the length of the boat. However, hulls are different and depending on factors such as displacement, beam and the boat’s shape may create a larger or smaller bow wave in proportion to its length and smaller ones are easier to climb.

It is often said that at displacement speed the power to increase speed increased exponentially – this is an over simplification for many sailing boats which can push on into a semi-displacement mode without too much trouble – i.e. depending on the steepness of the wave & power of the rig..

In practice a lightish displacement 27’ boat, if pushed hard even in fairly flat water could easily have a look at 9kts on a whoosh so I wouldn’t be too quick to dismiss your friend claims and on something like a J-27 one could whack up towards 15kts on a downhill plane without too much trouble.

Edit: just seen the boat in question is a Trapper 500. Never sailed one so Googled it and suddenly full of doubt about your friends claim. :ambivalence:
 
Daydream believer said:
I think differently - from my perspective- When ever i sail I do so to get the maximum from the boat in respect of speed .I do it because I like to think that I am sailing well.That being said i would not be slow to reef & do not carry the cruising chute that much.
But I still strive to get the best speed & consider that a part of sailing. Just sailing slowly when I could be sailing faster just does not seem right to me. That has nothing to do with racing.
When i first bought my yacht i struggled to get 6.0Kts upwind. A world champion came aboard & stuck it straight on 6.5 kts & kept it there. I want to achieve the same sort of skill- eventually
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Nothing wrong with that but I am with Dave_Shafa.

A mate once asked me if I used the mainsheet traveller much. I said yes, "I move it out of the way every time I want to go downstairs"

He seemed to think this was funny.
 
Seajet said:
No-one, least of all me, would ever claim the A22 had been comfortable in F11, just that she always seemed likely to survive when we found ourselves in those unforecast breezes; bit of a difference...:rolleyes:
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http://www.ybw.com/forums/showthread.php?362370-How-big-is-your-tender&p=4218868#post4218868


In fairness Seajet actually says "prolonged squalls of F11" with the the first part of that phrase overlooked by subsequent posters, but that's obviously how legends start...
 
grumpy_o_g said:
A typical displacement hull has to go onto the plane after the wave it generates exceeds the hull length - this is the "climbing onto the bow wave" often wrongly referred to - it's not that you're trying to force the boat uphill at all - the rules of physics mean that a fluid is fairly easy to move around within certain constraints. An analogy is perhaps like trying to make something vibrate outside it's natural resonant frequency - you're making matter behave in a way nature doesn't want you to. It takes huge amounts of energy to make it move outside those constraints because you're now trying to compress the fluid as well as move it (okay, simplistic but effectively the faster you drive a displacement hull the more you compress the fluid it is travelling through).
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Don't the rules of physics state that a fluid is incompressible? As you acknowledge, fluids are fairly easy to move around, so it just gets "pushed" out of the way - or in this case built up into a larger wave. As a displacement hull hits, then exceeds 'hull speed', the stern drops into the trough and the bow pushes up against the bow-wave - at this point form drag has increased as the relatively small surface area of the bow has been supplanted by the larger bottom, as the boat is pushed essentially keel-first into the water. If the engine can be tilted this could be maintained and speed could even be increased just by throwing more power at it to overcome the drag. But most vessels produce thrust roughly parallel to the design WL, so as a lesser component of thrust is available to push through the water, more is being applied downward - acting opposite to gravity. To plane, the vessel then has to overcome the gravity and climb up over the bow-wave. As that occurs, the effective frontal area decreases, thus decreasing drag - speed increases. On plane there's bugger all form drag, and wetted surface area is minimized. What keeps a vessel on plane is that the incompressible liquid provides a lift component - essentially climbing the now tiny wave (I'd call it a bow-wave, but it's usually moved well aft of the bow).
 
Cruiser2B said:
Don't the rules of physics state that a fluid is incompressible? As you acknowledge, fluids are fairly easy to move around, so it just gets "pushed" out of the way - or in this case built up into a larger wave. As a displacement hull hits, then exceeds 'hull speed', the stern drops into the trough and the bow pushes up against the bow-wave - at this point form drag has increased as the relatively small surface area of the bow has been supplanted by the larger bottom, as the boat is pushed essentially keel-first into the water. If the engine can be tilted this could be maintained and speed could even be increased just by throwing more power at it to overcome the drag. But most vessels produce thrust roughly parallel to the design WL, so as a lesser component of thrust is available to push through the water, more is being applied downward - acting opposite to gravity. To plane, the vessel then has to overcome the gravity and climb up over the bow-wave. As that occurs, the effective frontal area decreases, thus decreasing drag - speed increases. On plane there's bugger all form drag, and wetted surface area is minimized. What keeps a vessel on plane is that the incompressible liquid provides a lift component - essentially climbing the now tiny wave (I'd call it a bow-wave, but it's usually moved well aft of the bow).
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Yes, you're trying to compress rather than compressing. I still don't agree you're trying to climb the bow wave though. Imagine you've put a trolley jack at the front of your car and raised it up but then try and drive it along (assuming the trolly jacks has wheels). The front of the car may be raised but it's not trying to go uphill. If the wave was stationary then yes you'd be trying to climb up it but it actually moves with the boat so there's no energy expended trying to overcome the force of gravity by making the boat go upwards - all that energy is used in moving the water.
 
grumpy_o_g said:
Yes, you're trying to compress rather than compressing. I still don't agree you're trying to climb the bow wave though. Imagine you've put a trolley jack at the front of your car and raised it up but then try and drive it along (assuming the trolly jacks has wheels). The front of the car may be raised but it's not trying to go uphill. If the wave was stationary then yes you'd be trying to climb up it but it actually moves with the boat so there's no energy expended trying to overcome the force of gravity by making the boat go upwards - all that energy is used in moving the water.
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I'm afraid that argument fails.

Let's considers what would happen we suddenly stop pushing your car along - it will stay on top of the trolley jack as the jack is resisting the downward gravitational force of the car.

If we did this with a boat the bow wave would disappear, the gravitational force of the bow of the yacht dispersing the water. Compression is irrelevant as we are not in a constrained volume e.g. there is nothing to resist the force moving the water.
 
birdseye said:
Dont know about what speeds you could expect but I do know you can expect a lot of wishfull thinking in replies to your post.
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I passage plan on 7 knots and generally achieve 7-7.5 average in practice. Having read many of these threads, I am at a loss to know why I am not constantly being overtaken by AWBs.

Perhaps I should take to quoting my best speed achieved (17 kts) in response.
 
Although compression is not relevant here, nothing, other than perhaps a neutron star, is incompressible. The water at the bottom of the ocean is denser than that at the surface..
 
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