Wake Wave Height

[Kate]

Is there any way of numerically calculating wake wave height and period using inputs such as boat length, displacement, speed etc.?

I've found general wake wave databases where wake characteristics are dependent (only) on hull shape, but would like to know if any further research has been carried out - particularly considering ski and wake-boarding boats.

Thanks!

[wdnboatbuilder]

so your looking for a craft to create a wake to jump on? If so, are trying to make the wake long also?

[Kate]

More just trying to gather data/ find out what research has been carried out in this area. The aim is to consider the impact how wake waves affect bank erosion and if this can be minimised by the boat type, length, width, load and speed it travels at.

Thanks for your interest!

[Stephen Ditmore]

Somewhere in these forums Leo <http://boatdesign.net/forums/member.php?u=182> posted the Froude number at which wave making resistance (and presumably wake height) is maximized. Especially where you're in Australia, I'd suggest asking Leo.

Froude number, of course, gives the relationship between wave velocity and wavelength, from which you should also be able to get wave period.

BTW the 25 foot (7.6M) launch pictured by my name is supposed to be "low wash" at exactly those semi-displacement speeds where most boats produce their largest wakes.

I will watch this thread unfold with some interest.

As an environmental consultant (Marine) who happens to live in Aus - on a canal system with low wake zones etc - I will find the answers provided here most interesting, I would hope.

Perhaps something that also might be of interest to you (in regards to your question and how you've phrased it) - would be wake speed - or the speed at which the wake wave intercepts the banks it operates against - as well as the angle of interception (incidence or reflection)!

I would wager - that in any serious study of such phenomena (and I believe this could be modelled well either in tank tests or computer simulation), that in the end it will be a function of the total wake wave energy that determines the amount of damage (however significant wave height will also factor in there as well), that occurs to banks, combined with angle of interception.

Sadly - the relevant marine authorities here tend to concentrate almost entirely on vessel speeds to attempt to "control" (minimise) bank erosion, and that on it's own - as a method of controlling bank erosion, is often not very effective, if at all - and in some cases it could probably be successfully argued, that it exacerbates rather than helps cure the problem.

Another twist to where your headed - (than just bankside erosion control studies) is probably "penned / moored vessel damage" from passing vessel wash - specifically in canals developments - which are all the rage downunder at the present time. You could probably garner research funds for studying this phenomena - simply as a result of the strict environmental controls being imposed on canal / marina developments - not to mention the vast sums of investor $ pool funds avaiilable for such research from such real estate development proponents....whereas - gaining access to ënvironmental research funds for bankside erosion stdies thru traditional channels (uni research grants etc) might well prove an excercise in frustration! (in my humble opinion!). I would imagine marine insurers (like Club Marine for example) might well also be found to be willing contributors - if they thought the info might help them minimise their liabilities, with respect to vessel wash damages claims!

In more direct answer to your question (with respect to vessel design...) you might find one particular hull design interesting in this respect - principally the old english "Vortex"" hull design - which is a tri hull concept...where the central hullform bow creates a bow wave, which the two external wings wrap over, to catch and ride - thus harnessing the bow wave energy and turning it into lift for reduiced wetted surface area, increased speed / reduced fuel consumption etc benefits.

The wake waves you speak of are in all likelihood - bow waves that the vessel never caught or controlled in any way, which once passed - become referred to as vessel wake....(roughly speaking).

A google search on Australian Naval architect Gavin Mair's web site for his "air rider" hullform might demonstrate what I'm speaking of.

Perhaps he might also have the detail you seek with regard to wake and vessle design.

The issue of displacement versus planng hulls will also have a large bearing upon the results most likely!

Cheers & good luck!

[Leo Lazauskas]

Originally Posted by Kate

Is there any way of numerically calculating wake wave height and period using inputs such as boat length, displacement, speed etc.?

I've found general wake wave databases where wake characteristics are dependent (only) on hull shape, but would like to know if any further research has been carried out - particularly considering ski and wake-boarding boats.

Thanks!

No, there are no simple reliable formulae. There are several images I made using Michlet and some other codes I have at:

http://www.cyberiad.net/wakeimages.htm

Good luck,
Leo.

[Leo Lazauskas]

Originally Posted by trouty

I will watch this thread unfold with some interest.

As an environmental consultant (Marine) who happens to live in Aus - on a canal system with low wake zones etc - I will find the answers provided here most interesting, I would hope.

Perhaps something that also might be of interest to you (in regards to your question and how you've phrased it) - would be wake speed - or the speed at which the wake wave intercepts the banks it operates against - as well as the angle of interception (incidence or reflection)!

I would wager - that in any serious study of such phenomena (and I believe this could be modelled well either in tank tests or computer simulation), that in the end it will be a function of the total wake wave energy that determines the amount of damage (however significant wave height will also factor in there as well), that occurs to banks, combined with angle of interception.

These are very tough problems and the subject of quite a lot of research. Here are some images of wakes in finite width canals that illustrate why generalisations, or simple formulae, are unlikely to give reliable estimates:

http://www.cyberiad.net/wakefw.htm

Cheers,
Leo.

It would appear (to me at least) that the photo's don't really depict a great deal of difference in the wave form except for the distance they travel.

As explained elsewhere...in a thread recently - in a travelling wave - the water really doesnt move - only the molecules move up and then down in a verticle fashion as the wave form energy passes.

As we've seen with recent tsunami's in the last 12 months - that energy doesn't seem to dissipate greatly sometimes even after the waves travel substantial distances...

So - are we in effect saying that canal width has almost no discernable effect on diminishing the energy of the waves and hence their destructive (erosive) power?

I might have imagined - that in effect canal depth (or lack thereof) may be a better buffer to reduce the erosive effect of wake waves?

In effect - canals with shallow well vegetated benthic zones along the edges are well known to better resist erosion...while depth in iteself is an aid to safe navigation of those same canals...

Quite a conundrum - no?

My interest at the moment is in artificial reef structures for canals to limit just the effects we are discussig here, not to mention encouraging more marine lifeto take up residence in the canals systems..

Reef balls are currently being trialled hereabouts...but it's a complex issue, and taking discussion away from the original question / purpose of this thread that Kate has so thoughtfully commenced.

Cheers!

[marshmat]

Data and codes regarding the production of wakes by ski and wakeboard craft are plentiful. This field has been studied in great deal for many years. Unfortunately, virtually all of this data is considered trade secrets by the very few companies that have gathered it at great expense.
There are a few software codes that can give reasonable estimates of wake shape. Leo's Michlet code, for instance, works really well for a lot of slim displacement hulls, although I'm not sure if it would appreciate being asked to analyze a planing wakeboard boat with length/beam ratio of 1.5.
As far as minimizing wake damage goes- in general, based on what I've read and what we get on Lake Ontario: Vertical breakwalls, very bad. Vertical breakwalls that have jogs and kinks, slightly better but still bad. Gabions, they fall apart. Best is large boulders at about a 30-degree angle, continuing down underwater to the bottom, with smaller rocks and plants throughout. Wave energy is then dissipated over larger areas, not concentrated at one point- and it's absorbed, not reflected onto the neighbour's property. Since they put something like this in at my local marina a decade ago (including some 'fish habitat' structures mixed in with it), erosion's almost nil and the harbour's almost dead calm all the time.

[yipster]

high wake waves dont come from ski and wakeboards they are made by heavy displacement, usually big slow boats. its a common misconception.

[FAST FRED]

In most cases the slope of the bottom is as important as the shape of the sea wall.

Just as a nice slow upward slope gives surfers a great ride on huge waves , the presence or absence of bottom slop will detirmine the wave hights.

Which makes each bit of shoreline different.

FAST FRED

[Leo Lazauskas]

Originally Posted by trouty

It would appear (to me at least) that the photo's don't really depict a great deal of difference in the wave form except for the distance they travel.

As explained elsewhere...in a thread recently - in a travelling wave - the water really doesnt move - only the molecules move up and then down in a verticle fashion as the wave form energy passes.

As we've seen with recent tsunami's in the last 12 months - that energy doesn't seem to dissipate greatly sometimes even after the waves travel substantial distances...

So - are we in effect saying that canal width has almost no discernable effect on diminishing the energy of the waves and hence their destructive (erosive) power?

Yes, for the situations I have shown.

Originally Posted by trouty

I might have imagined - that in effect canal depth (or lack thereof) may be a better buffer to reduce the erosive effect of wake waves?

That depends on the (depth-based) Froude number. Near the critical value, a large wave that looks like a shock-wave is produced. There some images of ship waves in shallow water at the site I gave you before.

Regards,
Leo.

[Leo Lazauskas]

Originally Posted by marshmat

Data and codes regarding the production of wakes by ski and wakeboard craft are plentiful. This field has been studied in great deal for many years. Unfortunately, virtually all of this data is considered trade secrets by the very few companies that have gathered it at great expense.
There are a few software codes that can give reasonable estimates of wake shape. Leo's Michlet code, for instance, works really well for a lot of slim displacement hulls, although I'm not sure if it would appreciate being asked to analyze a planing wakeboard boat with length/beam ratio of 1.5.

Quite right, Matt. For planing vessels it might be better way to use other codes, for example "flat ship" methods as described in:
"Wave patterns and minimum wave resistance for high-speed vessels"
at:
http://www.cyberiad.net/wakepredict.htm

On the other hand, I suspect that far from the vessel, the actual hull shape is not that important. However, it probably is important to get the displacement right, i.e. after accounting for dynamic heave and trim.

Leo.

[Leo Lazauskas]

Originally Posted by wdnboatbuilder

so your looking for a craft to create a wake to jump on? If so, are trying to make the wake long also?

My program Michlet allows you to define a desired wave wake shape and then finds the hull shape, length, draft etc, that makes that shape. When I write the code I wondered if it would be useful for designing moving bodies (not necessarily ship-shaped) that could make "interesting" waves for skiers, indoor surfers, etc.

Cheers,
Leo.

[D'ARTOIS]

As Leo already stated, there are no reliable figures availablke, if at all. Firstly, who cares?
That might appear a bit blunt, but sometimes bluntness evokes unnecessary discussions on the workfloor (a reason that boatbyuilders are not diplomats!)

Many factors are influencing wake and the more, there are a few kind of wakes.

A wake of a bulkcarrier is different from the wake of a semi-glider. Commercial craft above a certain tonnage may not travel/sail under own power through the North-Sea channel, but must ne towed by tugboats. Other craft may go under own power but need to travel at dead slow speed. This to avoid erosian of the wallsides.

A deep-V craft, travelling at high speed can leave a very nasty and destructive wake, spoiling the wallsides of riverbanks and chanals.

In this particular case you may say: the flatter the aft (bottom) the less wake the craft will arouse.

Then, speed is of lesser importance as the displaced volume is minimal.

Fishing boats in this respect, being mostly half-gliders (I am not talking about US fishing craft - let that be clear) travelling at max. 15-17 knots are the real problem - they are always in a hurtry and have very little respect for environmental issues - to put it nicely.

A second issue is the draft of the river/channel one travels in. The less deep, the more of a wake is to be seen and felt.

A wake in the Congo river is no issue at all, but in the Canal du Midi it is a total different matter!