Wakes...

Open rant time!
Wakeboarders tearing up your bay? Had your dinghy flipped by one of those not-quite-with-it yacht pilots cruising along halfway between plane and displacement speed? Jet-skis got ya down? Vent your wake-related woes here!
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(I'll start with my beefs. Wakeboarders- Stay away from the shore already (and while you're at it, get off my little cottage lake)! Sport-yacht newbies- Your boat either planes or it doesn't. If your bow is 20 degrees up in the air, either speed up or slow down. Jet-skiers- well, they won't listen anyway, but I recall something in an old thread about little strands of floating rope...)

 

Wakes

Are your wake worries as bad as this image?This wake is from fastferries in the sounds of Cook Strait New Zealand.Check out www.guardians.org.nz/fastferries

 

Jet-skiers- well, they won't listen anyway, but I recall something in an old thread about little strands of floating rope...)
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YES ,,,,pollypropeline line (floats) cut into 2 or 3 ft pieces , then unraveled into just the single strands will stop most jet boats.

Sometimes the internal seals get destroyed , sometimes the impeller just siezes in molten plastic. Fine either way.

Certainly makes anchoring more fun when not being buzzed/waked by Kamikaze children drivers with nothing to do & nowhere to go.

Install the line from the dink by rowing around and your good deeds won't be noticed , as the kids won't usually come near the dink , usually out of fear of an oar in the face.



FAST FRED

 

Tom- that's probably a similar fastferry to the Toronto-Rochester boat? When that one ("The Cat", they call it) comes into Toronto, crowds of surfers gather on a nearby point to ride its wake in to shore....

 

About the ferries, they should consider hovercraft ferries. A little bit noiser but no wake............lesser of evils.

 

Wakes

The plan is to build a new port where the ferries do not need to go through sheltered areas,They are a good way to travel.It may be a while before we use cushion craft style ferries.Does the image look familiar to you marshmat?

 

Yeah, Tom, that's a fairly similar boat. Toronto's is a bit larger and is closer to a true cat (no centre V) but with similar wavepierving hulls. And they make a hellva wave.

 

... groan - there are days when I wish Tassie wasn't the origin of the modern wave piercing catamaran..... sorry guys!

 

Make jet skis illegal!!!! But anyway, I have seen jet skis going up to the shore and the lifeguards not caring, but when a kayak comes...*WHISTLE!!!!! WHISTLE****

 

If you have two craft of the same displacement (fastferries,one a hovercraft or cushioncraft,other a jet propelled wave-piercing cat) both displace the same amount of water,so where does the large wave making force come from.They should create an equal size wake???

 

Interesting questions.............................

First, a hovercraft is NOT a displacement hull craft.

Secondly, a hovercraft still displaces it's weight, but over the entire surface underneath it equally.

3rd; Once the hovercraft is up over "hump speed" which is equal in concept to a displacment hull coming up on plane, there is no conventional wake.

4th; If you do look closely however at the hovercraft, you will see spray and some random skirt contact causing little more than a ripple.

5th; The hovercraft displaces it's weight in a way which does not cause a "bow wake". Any stern/tail wake is almost non-existant.

6th; Big disclaimer, getting a large hoverferry up over the hump will cause a large wake in the process until it gets up to speed. This is why hoverferries have gently sloped dry land launching pads. The craft comes up on cushion on land and maintains the cushion (captured air cavity) as it tranverses to water - no wake.

7th; On my small craft I come up on cushion instantly over land as I am not displacing anything. I have launched without wake in as little as one foot from the waters edge.

8th; Out of sequential order on this one - the depression in the water caused by the cushion of air captured underneath the entire body of the craft (flat bottom more or less) is about (1) one inch deep. This depression moves with the craft, think of it as a shadow. This depression may be deeper or less than one inch depending of the weight of the craft per surface area. I have read that this depression disappears after hump speed, but that might depend on the aerodynamics and speed of the craft as well as wave conditions. In practice the largest craft on a smooth river I have seen for myself was 12 feet wide and 20 feet long seating six people. It's wake if you can call it that was equal to a muskrat swimming in the river I saw earlier in the day.

 

A guy in a hovercraft forum wrote this for me:

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As for wakes on hovercrafts, my understanding is as follows:

A hovercraft at rest or below wake speed is essentially a displacement vehicle. It always, of course, must counter gravity. At rest the water is fully displaced in an area exactly the size and shape of the hull, just like any boat.

In motion, there is a force applied downward on a surface the size and shape of the "footprint" of the hull. Unlike boats, this force is effectively equal over the whole area, and the force is applied either by air or by a flexible skirt. The mass of the vehicle and its contents is applied to any unit of water for the duration of time the hull is over that unit of water. Since water is a fluid, the force applied accelerates the water downward for exactly the time the hull is over that unit of water. Since there is little or no surface contact, only the downward force applies, there is no adhesion between hull and water and there is no hard surface to cause a higher than average force on a small area.

Once over plane, the wake is determined by the momentum of any unit of water, and that in turn was determined by the force applied by the cushion over the time the cushion was over that bit of water. The force is pretty much constant on smooth water, so the only variable here is the impulse time. The faster you go, the lower the time and so the lower the speed of the water. Momentum will carry the water down further than it was when the hovercraft left that section of water, but at that point the water is trying to reach equillibrium, so all energy is either transferred to neigboring water or is absorbed by nonelastic forces. As the disturbed water is left alone, the wake spreads out like any other wake, and the effect on the shore or a canoe or a fisherman is diminished significantly.

In other words, you DO get a wake. It's just smaller at higher speeds. A good share of a hovercraft's water disturbance is, in my opinion, generated by the thrust system or by physical contact of the skirt. The actual lifting disturbance is very small.

In contrast, a common boat hull always displaces water by physical contact of the hull with the water. A V hull will have less contact with the water as speed increases, but it's at a higher pressure because of the lower area. If you have a 20 foot long boat which weighs 3000 lbs and you are going fast enough that only 1 square foot of hull contacts the water, the AVERAGE pressure on that square foot is 3000 lbs/sq ft. If this were a purely rectangular hovercraft and the craft was 10 feet wide, your pressure would be 15 lbs/ft^2, and it would be pretty much equal at all points in smooth water.

It seems to me that this means a boat displaces a small amount of water with a high force, where a hovercraft displaces a lot of water with a small force. There are no hard surfaces to create large disturbances unless you impact a large wave with the hull, and even small waves are disturbed less because the skirt is flexible.

 

wakes

I have had to print out so I can absorbe so much interesting info.We are getting away from jet skis..I was thinking in terms of 3000..4000 ton gross craft,However much of the principles must still work.The wake making forces generated by the jet propelled fast ferries was interesting me,whether they be from hull or propulsion systems.Maybe having a cushion craft propelled by water jet or surface propulsion,for more economy and perhaps less noise.

 

I think a lot of the trouble with fast-ferry wakes is that these boats are designed to run at speed-length ratios as high as 2.0 while still remaining in displacement mode- while open-water waves will naturally run about 1.34. So you're pushing a hull well past its natural hull speed, creating huge pressure differences along the hull and forcing water to go where it doesn't really want to go so quickly. Anyone who's been caught behind a newbie cruiser captain knows that pushing a hull faster than hull speed but not quite to planing or semidisp speed will give massive wakes.

 

Wakes

The designers of these fastferries would have all the latest research information to build these craft,the fact that such a wake would be created must surely have be known and considered not acceptable.Reducing speed to reduce wake damage from a craft designed to run 38 Knots defeats the purpose of the speed design.