Boat speed

[Mr Cassandra]

Och .I am not saying people on this forum are exaggerating ,it when I hear the charteres talking in the tavenas, about going 12kts in a 36ft ben /jen /bav it makes me wonder how? cheers bobt

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[AuntyRinum]

No, you are incorrect. Regardless of hull speed, if the boat is travelling through the water at the same speed as the wave which is 10kts then the log will read 10kts. The boat is exceeding its hull speed because the trough which the boat is sitting in is now being carried by a faster wave with a longer wavelength. The log impeller will increase speed accordingly.

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[Robin]

This is frustrating, someone help me out please!

Firstly, what most people call 'hull speed' is the maximum speed a displacement boat can normally attain, hence the formulae.

Speed over the ground (SOG) is the speed resulting from a combination of speed through the water and any water movement such as tide or current, plus leeway.

Nothing the tide does has any effect at all on speed through the water, only on SOG. We ALL can only achieve boatspeed by use of the wind or the engine. But if you have seen 13.9kts on the GPS then you were breaking the 'hull speed' barrier and were almost surely surfing, unless there was a local current.

Cheers :0)



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[Robin]

Several options:-

1. The log is wrongly calibrated

2. The boats got up on a surf

3. Someone is telling porkies

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[Robin]

It has been known! Unfortunately most people know the rule of 'hull speed' which gives the lie very quickly. Surfing though is something else, watch the Open 60s on a wave to see what is possible, albeit for short periods.

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[zephyrsailor]

some charter companys crank up the speedo to make it appear they are going faster.

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[Neraida]

I am prepared to be shunned and shotdown about this, but I am fully aware of the hull speed theory, as I have sailed in wooden yachts all my life until recently, which get to their hull speed then dig a hole in the water and sit there..

HOWEVER!!!
I have sailed in a 35' MONOHULL racing yacht at 21.6knots (registered through the water, about 23ish otg) downwind in a 7, relatively flat sea (no surfing). There were eight of us onboard, so I didn't imagine it. There are a few yachts capable of these speeds, Kerr, Corby, MaxFun, etc.

Little Neraida has achived a 15 minute average of 14 knots ttw and she is only 8m ish.

I put this down to a "planing" hull, an overpowered rig to impliment the collosal force required, and also the extra lift of a winged keel on some boats. My big brother is a CEng Naval Architect, who I have sent a mail to asking him to explain this further...

Without planing this saturday, with the boom in the water and hearts somewhat in our mouths, we averaged 9knots on a beam reach on the short hop from Calshot to Cowes. now I am not sure of a First Class 8's hull speed, but 9 knots must have been well over it, so i guess this was due to "overpowering"???

I'll let you know if I hear from bro..

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[Robin]

It's not restricted to wooden boats unfortunately!

IMHO:-

The race yacht downwind in F7 was planing like a dinghy, very light, loads of sail. These boats are little more than very big dinghies and dinghy rules apply.

Neraida was also planing/semiplaning, even last weekend Those things are QUICK!

The rest of us have to obey displacement speed rules EXCEPT when we can hitch a ride and surf like a Hawaian in the tube... I hope your bro can help, did you ask him about the wavespeed/boatspeed thing?

Hang Ten

Robin




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[Strathglass]

Quite a few boats now can exceed the 'theoretical' maximum speed.
I have seen 10 knots on the log of an E-boat in windy conditions and a relatively flat sea. That was just before we has to dump everything (including the kite) to try and gain some form of control.

Many modern racing yachts will surf and plane much easier then the E-boat even without the effects of waves.

I would not however expect a heavier craft like an Elezabethan 31 to surf too often.

Iain

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[tome]

Hi Robin

There's some good stuff here: waves travelling backwards to increase water speed included. Surely it's more simple than this. I believe everyone is correct about hull speed, but some hulls are easily lifted beyond hull speed onto the plane. I wouldn't expect Amaya to plane for more than the odd brief surf, but I would expect any light displacement easily driven fin & skeg to regularly exceed theoretical hull speed if driven in suitable conditions.

Tom

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[Robin]

Yes absolutely agree, but by light displacement it does mean just that. To some people anything that weighs less than a Colin Archer in Ferrocement is lightweight, your Oyster and my Sun Legende included, but sadly (well no not really) they are actually 'moderate' displacement and unlikely to plane except under exceptional conditions surfing on a wave, but when they do, WOW!

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[charles_reed]

Possible explanations

1. Wrongly calibrated log
2. Max speed being quoted which is often due to anomalous effect of a breaking wave.
3. Surfing
4. Human overoptimism

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[MainlySteam]

Andrew Sheahan's Leading Edge column in September Yachting World covered planing keelboats quite well in a chatty way and is worth a read if not done so already.

I am not a naval architect so the following may be a little approximate, but is in general correct I believe. I think the general view is that providing appropriate hull shape, stability and power, a yacht is capable of planing without wave assistance if its length/displacement ratio in metric units (LWL/Displacement**0.333) is greater than around 5.7. So a 10m waterline boat needs to be under about 5.5 tonnes. Obviously the power available depends on the point of sailing and to get high power close to the wind one needs greater stability (which usually means weight putting the length/displacement ratio down) - so keel yachts tend to be able to only plane on the more powerful points of sail and if they are light enough.

For my own boat, heavy displacement cruising (LWL 10.7m, fully laden 12 tonne) the length/displacement ratio is around 4.7 when fully laden so as expected has no chance of planing. In fact even on waves it is quite hard to get it over displacement speed. However, until modern light boats can plane on the wind in seas (unlikely for a long time) my observation is that heavier boats are likely to perform better in hard going when close to the wind in seas (talking about cruising and AWB's here, not race boats).

Andrew Sheehan's article talks about an attempt to design a boat to plane on the wind in the YW article, and for a 35 foot boat the target weight is stated as being around 1.25 tonnes.

John

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[AlanPound]

"waves travelling backwards to increase water speed "

.... Don't mock! I was being serious (however, I wasn't suggesting that it would produce any real or apparent effect on boatspeed).

I think I can rationalise it (for what it is worth...). If you imagine a vertical slice or column through a mass of water - as a wave passes, the total height of the column changes. Clearly, it is not as a result of compression of that water - it can only be a result of some of the water having moved out of the way horizontally (in or against the direction of wave travel). That water movement being exclusively in the top few feet/metres of the water mass...

As there is no gross migration of the water mass exclusively in the direction of wave travel, I would suggest that tracking the odd molecule on the surface would show it to move forwards during part of the passing of the wave, then back again.

.... well, IMHO, of course...

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[MainlySteam]

At the risk after my previous post of seeming pedantic (I was originally trained as a physicist so please excuse me) a particle on the surface of the water (or a molecule of water at the surface if you like) moves in a vertical circular motion as the wave passes just as you say. The circular speed of the particle is slightly higher at the top of the circle (at which point the particle is moving in the direction of the wave travel) compared to at the bottom (at which point it is moving in opposition to the wave travel).

So the end effect is that a particle on the surface of the water (or a molecule of water at the surface) does travel backwards and forwards, and the sum of those means it actually travels very slowly in the direction of the waves. This effect is very small and has no effect on the speed of a yacht.

John

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[NigeCh]

Deane\'s formula doesn\'t work.

Deane's Foumula is Hull Speed = 1.34 * (LWL in feet)^0.5.

A better formula is Gerr's formula: S/L = D/L from which you can derive that Hull Speed = ((LWL^0.5)*8.26)/(D/L)^0.311 where D/L = Displacement in long tons/(0.01*LWL)^3 (Note: all units are Imperial)

Both these formula are for displacement vessels. I don't think that there are any easy formulae for semi-displacement vessels.

Let's take 2 boats that I've sailed on that seem to have exceeded their hull speed on passages: a Farr 50 Pilothouse and a Beneteau 473 Oceanis Clipper. According to Deane, their hull speeds are both 8.87 knots. According to Gerr, the Farr 50's hull speed is 10.52 knots and the Beneteau 473's hull speed is 11.97 knots.

Now for the reality, in August 2002 in the Farr 50 from Cabo Villano to Cape St Vincent we maintained an average SOG of 13.2 knots with a maximum of 15.6 knots. The current with us averaged 1.4 knots. This year in June on the Beneteau 473 sailing to Bermuda we had a maximum run of 236 miles in 24 hours ie an average of 9.83 knots with a maximum SOG of 13.8 knots which was recorded as we went round a cold eddy and for about 8 hours we had a 1.6 knot current with us.

Both of these boats with their fine bows, shallow bilges and long flat sections can get up and become semi-displacement boats, but if you look at Gerr's data the Farr 50 achieved a planing status but the Beneteau didn't quite make it.

Gerr's formula is not straightforward, but it is more accurate than Deane's formula. Can I suggest that you enter the data for Gerr's formula for your own boat adding, of course, the weight of anything extra that adds anything above the manufacturer's working/loaded displacement.

You will be surprised how the hull speed closely fits in with what you achieve on the water.

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[qsiv]

Come, come. If you were going to be pedantic you would explain that the circular motion exists only for deep water waves that are not breaking. In shallow water the breakdown of the circular motion leads to the breaking wave, as the retrograde motion at the 'bottom' of the circle is retarded due to frictional effects, leaving the top to accelerate away, and therefore break.

Fortunately I dont recall the physics of deep water breaking waves!

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[MainlySteam]

Hi QSIV, but as we sail proper boats I thought real deep water was sufficient for us.

Your entry reminds me of the couple of months we spent rafted up at a berth with a retired naval officer cruising the world. Every morning he asked me what I thought the weather was going to be like and general seasonal weather patterns for planning their various voyages around NZ. It wasn't until very late in the piece that I discovered he had a PhD in Oceanography. I didn't know whether to feel honoured that he had asked me or feel a right dick (shhhhhhhh everyone) for having given my comparatively meagre opinion!

Anyway. Yes, to extend the pain, as the water gets shallower the circle becomes more elliptical (with the long axis horizontal) until when the water is very shallow the motion is only backwards and forwards (but I suspect we would all be aground at that stage). As you say when the wave reaches shoal water the tops overtake bottoms.

I too do not know why waves break at sea (sigh of relief from all?)? Unless it is just a random event caused by random failures in the theory (sic) or random disorganisation I do not know - too far away from discussions about parrots for me. I wonder if anyone else knows?

Regards

John

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[tome]

Alan

I wasn't really mocking, just trying to simplify what seemed a rather complicated discussion. I'm aware of wave particle theory but don't think it will have any significant effect on boatspeed.

Cheers
Tom

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[Robin]

Our 16.7kt wave was on a shallow patch approaching Poole entry channel, between No1 (Bar) buoy and No3 buoy, depth varies from 12ft to 16ft, we were close to 2nd HW (Poole has double tides - don't ask!) and had just seen the sounder reading around these values. The wave was steep enough for me to ponder how much (or how little) water was at the bottom of the hill, we draw 2.08m or 6'10" in old money. The wave was breaking and until we were slowed by hitting the wave in front we stayed ahead of the breaking crest, then it fell into the cockpit. Fortunately we have huge cockpit drains (about 100mm/4" dia) and a good sized bridgedeck so only my feet got wet - SWMBO lifted her's clear in time! After this wave we had a few more short surfs but no more than 9.5/10kts as we entered the channel proper. Poole is exposed to winds from the east, we had east F6 gusting F7 and had seen 34kts of wind. for Scuttlebutt Cherbourg trippers, this was the Sunday you came back.

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