A Research Program for Quantitative Wake Analysis

The historical reputation of the Hickman Sea Sled proclaims that it generates much less wake than a comparable deep V hull, and visually that certainly looks to be true. But the Mk 1.0 Eyeball doesn't generate numbers. In science and engineering a claim must falsified or verified.

I know the wake of a boat is a complex series of waves, each with their own energy. In the case shoreline damage is most relevant.

What component of a wake is most concerning, and how might that be measured?

 

Depends on your values but fuel economy would certainly be a popular one.
And that can be measured visually by the magnitude of the wake generated.
The bigger the wake wave, the bigger the fuel bill.

What is it you're trying to quantify here and why?

 

Shoreline damage. Where I live, that's a huge issue.

Also the difference between that aspect of an inverted V and a deep V are intrinsically valuable.

 

Are there folk in your region who have classic deep vee hull forms or semi-displacement hull forms (a pal once described these as 'blood & thunder' machines), who like to go places with the hammers down?
I can see that shoreline damage could be caused by these beasts - but is it worse than a typical winter gale that occurs naturally?
Either way, I would have thought that a sea sled wake would be much less significant in comparison.
In a simplified world, vessels basically have skin friction resistance and wavemaking resistance (altho' there is a bit more to it than that if you want detail), along with a bit of air resistance - but even for a slab sided vessel like Serenity the air resistance is not going to be significant unless you are motoring into a full blown gale.

I like how everything is explained in the link below - it is for a big ship, but the same basic principles apply to small craft as well.
https://www.usna.edu/NAOE/_files/documents/Courses/EN400/02.07_Chapter_7.pdf

 

I need to do some sort of numerical demonstration. My brother-in-law has a boat of pretty much the same length and beam and power. We can do runs side by side and see. I hope to make a business case for a sea sled water taxi in a sensitive area.

 

Actually, while it is height^2 that determines how much energy is in a wave of a given period, it is the period that determines how much energy is available for scour. And you also need to look at the foreshore slope for shoaling and the foreshore's composition WRT the Keulegan-Carpenter (KC) number.

See this old thread Simple wash rule? https://www.boatdesign.net/threads/simple-wash-rule.47981/

And this wiki Keulegan–Carpenter number - Wikipedia https://en.wikipedia.org/wiki/Keulegan%E2%80%93Carpenter_number

 

Crumbs! Those are non-trivial areas of endeavor good sirs. I am but a humble wood-butcher.

Well, direct comparison is useful. And as LL pointed out, energy in the wake is work done on the boat/water system. That at least I can address without a few years of remedial studies.

 

DC

There is an encyclopaedia of information on this subject, much of it, rather complex and…it depends..too.

The basics, tells you that for far-field waves, the length-displacement ratio is dominant, and that the wake period (wave)varies according to SQRT(L). Secondly, solitary waves. These are very long period waves and in shallow water, can travel at several ship lengths ahead of the vessel, and they are generated at sub-critical and near critical Froude depth numbers.

Then getting into shallow water effects and restricted waters, such as around an archipelago and/or a ‘confused’ estuary like a Fjord. These alter the characteristics of the waves, somewhat too.

But for those on the shore it is all about the energy, as JEH notes.

The energy per metre length is proportional to: wave height(^2) and wave length.

Thus a tsunami of “just” 1 m, but a wave length of hundreds of km’s….has massive amount of energy, compare to a boat with a wash/wake height of 1.0 m but around 2-3 boat lengths for it wave length.

So depends, upon your speed, your water depth, whether close to or influenced by islands, or other large blockage effects etc etc…

 

So I guess the only realistic approach given that we have relevant boats, and the beaches in question, is to drive the two sample boats padt the beaches, and observe the action on the shore.

 

Re the action on the shore (ref the wakes of your two boats) - how would they compare to the waves generated by a Force 7 - 8 winter gale blowing up the sound?
I am thinking that in some directions there would be a lot of fetch, and hence the wave height would be much greater than anything generated by the wash of a passing boat?

 

What the shore looks like is a function of the natural ocean environment, near-shore currents, and the surrounding geography. Basically, you can say silty-muddy foreshores are the lowest energy environments, transitioning through sand, gravel, cobble, bolders, and finaly bare rock/coral in the highest energy environments. There is a lot of transport of material to/from a foreshore over the seasons (i.e. the difference between a "winter beach" and a "summer beach"...or the gradual movement of barrier islands). The issue for wash comes when there is sufficient continious energy to keep the small components of the foreshore moving or suspended. In this case the foreshore can be rapidly depleted. For a full discussion of this see Oceanographical Engineering by Wiegel, or the USACE Costal Engineering Manual Knovel https://app.knovel.com/kn/resources/kpCEM0000P/toc

 

Is that why there's a 5 knot speed limit in your North Arm?

 

Get a wave probe ...get real quantitative results, rather than qualitative...data is always your friend

 

Define relevant boats.
A deep V 24 degree hull compared to Serenity?

I think that it would be too hard to quantify the parameters to produce " two comparable" boats.

I have skied behind two 21 foot boats, a Ski Nautique, hardly any wake, and a Mastercraft X14V, crossover (wake surf/ski) boat. The Nautique, hardly any wake at 32 mph. The X14V, knee destroying wake at 32 mph.

What speed are you thinking about for a ferry?

As a BC resident and old enough to perhaps remember the (google) BC Fast Ferry Fiasco, the fast cats generated unacceptable wakes. (perhaps politically unacceptable wakes )

 

Need to be careful not to conflate visual wake, with the actual energy in the wave system; which more often than not is not really perceptible by the eye.
Whilst 'some' may look bad, owing to a large(er) amplitude, we can't really see the wave period, and whether it is a soliton or a series of waves.

Yup, poor route planning and analysis.
Given there is and was at that time, a plethora of data to suggest what would actually occur, and did. But those boats were a political decision, and just "given" away in the end.
Shocking.