Boat speed

[Talbot]

not strictly true. the water on the surface of the downslope is travelling at the speed of the wave, thus in certain circumstances if the wave is large enough you can get into a very dangerous condition where the boat speed and the water that the rudder is in are at near enough the same speed, and the rudder stalls. This can cause a very nasty broach which ends up with the boat being rolled. It is also one of the reasons for towing a drogue in really bad conditions.

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[Mr Cassandra]

Sorry but I still don`t quite understand! Let say you are going at 8 kts and you are then overtaken by waves traveling at 16 kts on which you sit on top ,your GPS will then read 16kts ,but as you are sitting on top/along the wave ,your paddle wheel is still showing only 8kts as this is your boatspeed through the water I dont see how the log would show 16 kts ? cheers bob t

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[qsiv]

I'm quite willing to be corrected by your brother as a practising Naval Architect (mine hasnt been used since University days).

First, I would venture to suggest that a winged keel doesnt generate lift in the sense you imply - the winglets do improve the lift of the keel in hydrodynamic terms (by reducing tip vortices) - but that lift is in the opposite sense to leeway (i.e it reduces leeway), not in the vertical plane.

Second, the ability of a boat to plane is governed by two primary factors. The major factor is the power to weight ratio (or SA/D) - not enough power - you'll never achieve a sustained plane. The other factor is the ability of the boat to 'carry' the power of the rig - this translates to either form stabilty (wide waterplane, particularly aft) or dynamic stability (deep keel, or swing keel, or water ballast, or bodies on the rail). If you have to reduce sail to keep the boat on her feet you have taken your foot off the accelerator. Of all the options the canting keel is the best, as you are not increasing the displacement (and thus reducing SA/D). Deep keel isnt wonderful, as you only get significant righting when the boat heels (and that distorts the waterplane, compromising both lift and drag). Bodies on the rail and water ballast both serve to reduce SA/D just when you dont want to (and you have to feed crew and buy beers - bad).

The other key limiting factor is drag - both hydrodynamic (hull and appendages) and aerodynamic (rig and sails). Both rise dramatically as speed increases, so you need special foil shapes (you will notice nearly all high performance rudders have a quaint flattened profile at the bottom of the trailing edge - this originated from Dick Carters work [Ragamuffin fame] if I recall).

Minor imperfections will cause disproportionate effects, and lead to massive losses.

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[Robin]

One more try Bob!

We were making an easy 8.5kts +/- through the water, all wind powered. A big breaking wave came up astern and gave us an extra shove, down the face of the wave (gravity as well) and we broke free from the normal displacement speed barrier and accelerated to 16.7kts THROUGH the water, I guess planing. The effect is just like in a speed boat, a lot of power is used to break the barrier and get onto the plane, much less is required to maintain the plane. In our case the sudden extra power came from the breaking wave, just like it does to a surfer taking off on a board. Remember how fast a surfboard can go with no sails? We had sailpower as well.

Did I succeed?

Robin



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[Oldhand]

How about 17.4kts on an averaging log with only a 30' waterline? Its called surfing.
Ferry wake on the edge of Bembridge ledge with a prolonged gust of 29kts of wind under kite and full main is one way of doing it. We were only 2 handed on the Island Double race. Our GPS plotted VMG and when the peak downwind VMG was resolved for wind angle, it reckoned a peak SOG of 21.3kts.

Any yacht should be able to surf but the heavier and fuller the hull the more sail, more wind and more wave will be required.

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[MainlySteam]

Re: Boat speed & Stability

Seeing there have been many complaints about there not being enough real boat talk on the forum I will add a bit more on a matter of yacht performance in waves (either its own wave making or in seas) which does not seem to be commonly understood.

Most yachts, when in a sagging condition (trough of wave under amidships) suffer reduced stability and this is at the worst when the wavelength of the waves is around that of the boats waterline length. This wavelength is, of course, that which occurs from the boat's own wave making when at displacement speed.

It would be unusual for most of us to encounter waves of that wavelength and of sufficient depth to cause a significant sagging condition at sea because when conditions are hard the seas are of longer wavelength. It can occur though in bar or rip conditions (or wind against tide) where it may contribute to a loss of control, especially if the rudder is in aerated water as well and I have often wondered the extend of this contribution to loss of stability has when boats lose control under bar conditions (sometimes with fatal results).

Beamy boats, especially where that beam is carried aft, are less subject to this stability loss (one up for the AWB's) and while we do not have an AWB our boat is beamy aft (fine forward) and we have sometimes experienced strange wallowing sensations when in the troughs of steep deep seas, of around hull length wavelength, when on a flat run (flat run being the common situation where a sagging condition is likely to occur).

While tripping over the appendages is a big influence I have wondered what effect this loss of stability in a steep trough has in the histrionics powered up boats get into when broaching on a spinnaker run too (when their own wave making is likely to be of order of a boat length).

If anyone has any experiences or thoughts on this I would be interested.

John



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[Mr Cassandra]

I have tried to learn a few thing about Boatspeed Dave Gerr formula states [I Think ]Once hull speed has been reached ,to double the speed takes 8 times the energy, or 4 times the power . cheers bobt

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[Mr Cassandra]

Did I succeed! DID WE SUCCEED at least we have a thread on sailing cheers bob t

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[BrendanS]

Re: breaking wave in deep water

Summary - in a fully developed sea, waves no longer grow in size- and additional energy supplied by the wind must be balanced by energy lost by breaking waves


In deep water, for any given wind strength, there is a maximum size of wave that can be generated - the waves are 'fully developed'.

Waves can be measured as height (H) and length from crest to crest (L). Waves will break where steepness of H/L is greater than 1/7

Once fully developed, if the wind continues to impart energy to the wave, that energy must be dissipated. This occurs by waves breaking and white caps forming

That's the simple version, with a bit more background reading below

<A target="_blank" HREF=http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part2.html>http://www4.ncsu.edu/eos/users/c/ceknowle/public/chapter10/part2.html</A>

After that is starts getting complicated

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[MainlySteam]

Re: breaking wave in deep water

Thanks for that Brendan.

I had always assumed (and it was pure assumption, nothing else) that the size of waves that were generated by a particular wind were due to an equilibrium being reached between the energy available from the wind and the energy required to maintain the particular wave size. I had not thought of there being some other limit on maximum wave size for a given wind which resulted in excess energy being available from the wind (assuming constant wind speed).

Have to have a bit more of a look into it now, seems to be a bit (lot?) more complicated than that. Thanks again

Regards

John

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[BrendanS]

Re: breaking wave in deep water

The more you look into it, the more complicated it gets. The link I posted was the simplest I could find that explained from basics to why a wave cannot gain size after certain conditions have been reached. After that it descends into mathematical treatises of obscure points, all of which have to be understood to pull it all together.

If you are going searching, Stokes waves seems to be a key point.

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[BrendanS]

Re: Hurricane approaching

A little challenge. How do you tell if a hurricane is approaching from observing waves? (other than they get very big and the wind is strong!)

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[PeteMcK]

Delft Systematic Hull Tests

These tank tests of 40 models encompassed virtually the entire spectrum of yacht hulls. The tests enabled a (fairly lengthy) formula to be produced which predicts hull resistance forces over the full range of speeds, well into the surfing range. Basically, if you plug in your length, beam, displacement, prismatic coefficient, and one or two other parameters, all fairly easily obtained, the resistance for any given speed falls out.

For skinny deep heavy displacement hulls, the resistance heads off to infinity at not much above "hull speed". For lighter beamy hulls with flatter bottoms, there is, or can be, a "point of inflexion" at which the rate of increase in steepness of the gradient begins to reduce again: in other words, the curve of resistance against speed (x) takes on an 'S' shape. Once you pass through this point, it still needs more driving force to go faster but at a diminishing rate, if that makes sense. The power required to push the boat to these speeds can be obtained downwind with big winds and lots of sail: waves help in that you're using gravity to carry the boat down/across the front of a following wave (surfing).

Anyone who wants to set up a calculation in Excel can find the equations and tables of coefficients in yacht design books such as Larsson & Eliasson.

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[Mr Cassandra]

Re: Delft Systematic Hull Tests

Hi There is a book by David Gerr "The nature of boats
"WWW.cyberus.cal/~campione/alberg22/comparisons.html. He states that the 1.34 formula was originally developed for "British men- of -war" cheers bobt

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[PeteMcK]

Men-o-war

They probably didn't surf much!

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[Robin]

Re: Men-o-war

But the formulae would work quite well for a few of the non-AWBs around here! If the forecasts are right, they might get to sail with their engines off tomorrow, if they can reverse out of their berths....
Just joking peeps:0)

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[Mr Cassandra]

Re: Men-o-war

Yes good idea, tow then over to Ireland and they maybe able to passage plan their sail back at 4.9 kts cheers bobt

ps

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[Peppermint]

Re: Getting over the hump

Sailing monohulls sail along in a hole in the water. To get them speeding they have to find enough energy to climb out of the hole.

Surfing down wind on run or reach is a situation that leads to speeds greater than calculated hull speed. You borrow some energy from wind and wave add a bit for gravity and away you go.

In flatter water you can still plane a boat with flat aft sections, enough sail area and wind power. Things that will help are getting weight towards the back and trimming a kite for lift. Keeping your steering inputs small, which if your not surrounded by top trimmers is tricky, stops your rudder acting as a brake.

Planing a keelboat upwind is less likely to happen, though I'm told that some of the new high performance dayboats will do it in flat water, as all the resistant forces are from forward of the beam.

Because most production yachts are a compromise between all of the points of sailing, ease of handling and comfort they are usually to heavy or to under canvassed or without the flat run aft that promotes planning from wind power alone.





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