Hypothetical Question - can the point where wakes cross between hulls be moved by hull shape?

Discussion in 'Multihulls' started by ElGringo, Feb 12, 2019.

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DCockeySenior Member

Note that the paper linked to above concerns a vessel of 34.13 m length and 1.1 m draft operating in water with a depth of 2.5 m, not deep water. The water is shallow compared to the length of the vessel. Water wave behavior and resistance due to wave generation is fundamentally different in shallow water.

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Richard WoodsWoods Designs

I agree with you David. And yes my Skoota 20,24,28 powercats all have asymmetric hulls. Seems to work OK. I write this on board our Skoota 28. Even loaded for cruising we can average 11 knots with twin 20hp outboards. Over 16 knots when empty. The inner sides are not flat though, the have curvature. I have seen the monohull cut in half to make a catamaran concept in a couple of powercats. Doesn't seem to work well

But if the water is shallow, ie under 9ft we cannot do more than 9 knots, however wide we open the throttles. Get to 11 ft or more and we get up to normal speed.

Currently we are in the Bahamas and the water is rarely over 10ft so we keep speeds to 7 knots

Richard Woods of Woods Designs

www.sailingcatamarans.com

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alan craigSenior Member

This paper contains two images which show that the wave between the hulls doesn't disappear with flat inner sides. Maybe water moves laterally under the hull:
https://www.sciencedirect.com/science/article/pii/S1026309812000855

R.Woods, this is why I like BD.net, I would never have guessed that water 9ft deep would have affected a 28ft. narrowish hull so much.

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Angéliqueaka Angel (only by name)

And now I'm wondering, does it affect a cat and a shallow draft monohull alike . . ? ?

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DCockeySenior Member

Short answer is yes, the change in shallow water wave mechanics is independent of hull type. In deep water (depth long compared to wave length) wave speed is a function of wave length, which leads to the "hull speed" concept. In shallow water (wave length long compared to wave length) wave speed is a function of the water depth. At intermediate depths wave speed is a function of both wave length and water depth.

With sufficient power and suitable hull it is possible to go faster then the wave speed in shallow water. Such speeds are referred to as "supercritical". In shallow water resistance increases at lower speeds than in deep water. At supercritical speeds resistance may be less than in deep water at the same speed. A good overview of the effects of shallow water on resistance is "On High Speed Monohulls in Shallow Water" by Dejan Radojcic and Jeffrey Bowles which was presented at The Second Chesapeake Power Boat Symposium (SNAME) in 2010.

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Angéliqueaka Angel (only by name)

Thanks David !

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fallguySenior Member

Do you expect the same performance with the Skoota 32 in shallow water?

I think it is an excellent problem to have ftmp because it means any grounding event is at slower speeds.

But do you expect the Skoota 32dm to behave similar-like the wave interference in shallow water is higher? Seems best to ask here.

My boat is 12' 6" cl to cl.

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Mr EfficiencySenior Member

Depending on the slope of the bottom, presumably.

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I wrote a brief summary here...if this helps too.

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That shows you have reached your critical Froude depth for your hull and as such there is insufficient power onboard to overcome the increase in resistance at that depth for your hull/speed.

Here are some measurements we did years ago, with one of our cats, running in deep then shallow water:

The increase in wash and hence the resistance too (not shown on the axis)..can be significant under the right conditions.
But once past this "hump" from subritcial to super criticla region...the boat will go faster than when in deep water.

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fallguySenior Member

It sure would be nice to understand a z axis of say cat l/w on this chart.

I'd say you answered for RW as well. Deep =?
Shallow =? Or did I miss it?

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What do you mean by z axis and cat l/w...in this context?

Not sure I understand the Q...or did I miss it?

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fallguySenior Member

So, is the width of the cat hulls a factor?

And what is the definition of deep bs shallow? Like Richard says 10 feet is shallow? 11 feet+ is deep?

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Richard WoodsWoods Designs

I normally sail in shallow tidal water in Plymouth UK. There is a big mud bank at the entrance to our creek. It doesn't matter what boat you are sailing, you can see the stern wave rising and the boat slowing as you come over the bank. Boats tend to "sink" in shallow water. Even ships like the QE2, which hit rocks when going too fast in shallow water, even though the rocks were theoretically below keel depth.

It is also why test tanks are so big relative to the models. The walls and bottom have a significant effect

That's a good graph Ad Hoc, thanks, what size boat was that? My Skoota28 has a 450mm draft and is about 2T right now. The difference in speed is huge. In 8ft water and WOT we do 9 knots. In deep water WOT we do over 16

More here Sailing Catamarans - Skoota 28 transportable minimum live aboard cruiser http://sailingcatamarans.com/index.php/designs-2/6-powercats/264-skoota-28

Richard Woods of Woods Designs

www.sailingcatamarans.com

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Since the square of the beam of the hull is proportional to the residuary resistance, theoretically.
But if the WSA comes down, then not really...since WSA effects the frictional resistance which is also a component in the drag.
Thus is is all about the "total" drag residuary and frictional....and like everything in design, it is a compromise and as such, you gain in one area loss in another.
Hence everything must be looked at holistically.

As noted here. It is related to the water depth.

So with:-
Fd = V/Sqr.root(gh)

h = water depth
V= speed of craft

If your vessel is running in water that is 2m deep.... and given Fd = v/Sqr.root (gh)

The critical Froude depth number is 1.0, so the speed which is critical, major increase in resistance is simply rearranging the formula:
V = 1.0 x (9.81 x 2.0)^1/2 = 4.43m/s or 8.6 knots.

So when you are running in water that is 2.0m deep if you are running around 7.5 -9.5 knots, you will experience a massive increase in resistance, with the greatest at 8.6 knots. The amount of power required to get over the "hump" can often be greater than the power onboard. Thus one is 'trapped' by the water depth, or limited in speed as such. But if you are able to go beyond 9.5 knots, you'll find that you can go faster than compared to deep water. You will also notice a chnage in the wash pattern - as noted above by DC too.

This is one of our 45m passenger cats.
We ran the tests in 2.0m of water...and then going from shallow to deeper (over 10.0m depth).

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