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John Robertson

Bit bumpy!
« on: May 16, 2019, 8:50 pm »

Experienced hovercraft pilots recognise the advantages of sideways across waves (not for boaty types) rather than straight on!

Warby

Re: Bit bumpy!
« Reply #1 on: May 16, 2019, 10:31 pm »
Not for boaty types??? Driving boats at an angle INTO the waves rather than straight at them softens the ride somewhat. If you are in a "following sea" then depending on size of waves staying on the back of the wave is safest rather than trying to overtake it and broaching with the possibility of turning over!!
On that video it didn't look particularly rough, no white horses!

John Robertson

Re: Bit bumpy!
« Reply #2 on: May 17, 2019, 8:31 am »
You can actually safely operate a hovercraft at 90 degrees to the waves with little risk of broaching - unlike a boat.  Not particularly comfortable as it rolls up an over each peak - but better than crashing into every  face and trough!    Same applies with following seas - the craft will be faster than the waves so it's usually better to angle over each peak rather dropping into every trough (sometimes requiring some tacking back and forth to minimise discomfort!).
Whitecaps are just wind:

Philip

Re: Bit bumpy!
« Reply #3 on: May 17, 2019, 8:38 am »
90 degrees to the wave is not a problem (within reason), we sometimes go hover surfing off some of our local beaches. Need lots of control to hold it on the wave 😀.

Nick Flint

Re: Bit bumpy!
« Reply #4 on: May 17, 2019, 10:24 am »
Personally I feel a slight following sea (wave) at perhaps 45 degrees needs caution as the tail can weather cock due to tail wind hitting rudders (duct) coinciding with less rear friction, (as rear lifts) just as the nose has an increase in friction.
Resulting broach (spin round) can cause an unwarranted manoeuvre!
Solution.
Pretend you wanted to do it. ;)
Stay calm, (walk about a bit) then put on shades----- 8)
Memories are BETTER than Dreams---"Capn" FLINT

Warby

Re: Bit bumpy!
« Reply #5 on: May 17, 2019, 10:41 am »
"White caps are just wind"
As wind increases they then start to "break" and can become dangerous!☹️☹️

John Robertson

Re: Bit bumpy!
« Reply #6 on: May 17, 2019, 10:58 am »
I'll re-phrase - white caps are caused by wind (pushing the narrow wave peak over).  You can get lumpy water (caused by tidal movement and/or turbulence) without whitecaps and also whitecaps on relatively smooth water.if the wind is high enough.  In themselves, whitecaps aren't dangerous for hovercraft and are just an indicator of wind speed (i.e you probably shouldn't be out there if a lot of them appear!).

Weather cocking is mainly caused by too little thrust in windy conditions (usually because you are trying to maintain sensible speeds down/cross wind).  Judicious use of the brake to limit speed (instead of reducing power) usually prevents it.


Hovercraft corkscrew much less than a boat does when crossing waves as they aren't pushed around by the water itself (just by the slope changes).  Their downside is they always try to prevent water contact so climb (or try to!) every little wave!

Ian Brooks

Re: Bit bumpy!
« Reply #7 on: May 19, 2019, 12:56 am »
Following wind is the worst condition in my view, for the reasons John mentioned. Loss of rudder authority can lead to loss of directional control. The Sev / Otter has the brake which helps enormously, the additional drag keeps speed under control whilst allowing for increased thrust and so maintaining rudder authority. Having said that, even this will be overcome eventually and I've experienced the result - not something that I care to repeat.

I have experimented with a small drogue, just a trailing rope with a large knot in the end,  to provide additional drag in following winds, it works well and can really stabilise things when getting to the ragged edge.
Ian Brooks
Gloucester, UK

Ian Brooks

Re: Bit bumpy!
« Reply #8 on: May 19, 2019, 9:26 am »
Its interesting to compare broaching in a boat and shuttlecocking in a hovercraft. They are related but different phenomena, related because in both cases they happen because the yaw (turning) forces on the hull exceed the maximum rudder force. That is, something is trying to turn the craft and the rudder is not strong enough to prevent it, so the craft yaws (turns) uncontrollably.

The difference is that the boat experiences mainly hydrodynamic (water) forces whilst the hovercraft experiences mainly aerodynamic forces. I say 'mainly' but recognise that both classes of vessel experience both forces.

To understand boat broaching you need to appreciate that waves are a bit wierd, in that the water moves both forward and back ward. At the crest, the water moves forward in the direction that the waves 'seem' to be going in, whilst in the trough the water actually goes backwards! If a boat sits with the stern on the crest and the bow in the trough, and it is not perfectly at 90 degree to the wave, the bow is pushed backwards whilst the stern is pushed forwards, turning the boat until it aligns with the wave. The end result is that the boat is left sitting parrelel to the wave right in the trough, and the next wave can roll it over. Very dangerous.

In the hovercraft, things are a bit different  because it's the wind not the water that causes the issue. Often, the wind and waves are aligned (but not always). Hovercraft tend to have more structure (duct rudders etc) at the stern and in a side wind this results in more side force at the back than front. This makes the craft turn towards the wind, and if the rudders cannot create enough counter turning force, the process continues until the craft points directly into the wind.

So we see two main differences with hovercraft, which respond to wind forces and turn through 180 deg, and boats which respond to water forces and turn through 90 deg. The hovercraft is much safer, because it leaves you pointing into the weather and able regain control, whilst the boat leaves you sitting in the trough waiting for the next wave to turn you over.

I did say that both classes of craft do experience both forces , and the hovercraft pilot should be aware of this. When you surf down a wave, if your bows bury themselves in the next wave, they will experience a strong back ward force, and if you are not perfectly straight on to the wave, this will turn you almost instantly. I have experienced this, it is unbelievable how fast the craft spins in its axis and in my case rolled over, literally catapulting me into the water. This is a very dangerous  combination of forces. Fortunately, it is easily avoided.

In a hovercraft it is imperative that you maintain enough lift to keep the craft out of the water. If you can see that it is inevitable that the craft will bury it's nose in a very steep breaking wave, it is vital that you hit it straight on.

Practically, the best advice is to maintain enough thrust so that you have decent air over the rudders to keep control, so use the brake (if equipped with one) or throw out a trailing rope with a few knots tied to the stern. Then turn out of the wind so that you are not running directly downwind, tacking if necessary to maintain your course. If you are still struggling to  maintain control, shift some ballast (ie yourself!) to help turn the craft out of the wind. But above all, maintain your lift!

Ian
Ian Brooks
Gloucester, UK

Warby

Re: Bit bumpy!
« Reply #9 on: May 19, 2019, 4:08 pm »
How NOT to cross an Estuary Bar

Warby

Re: Bit bumpy!
« Reply #10 on: May 19, 2019, 4:13 pm »
Another type of Broaching, when overpowered by the wind