Multihull Capsize Prevention <split>

Norman Cross had an aeronautical background (like Lockie).
He made the underside of his wing decks like an upside down aerofoil. The Idea was that the windward deck would have a downward "Lifting" force and so help reduce any upward lifting force when heeling to a strong wind or gust.

Also, when a wingdeck multi, and in particular a Cat, is anchored from the stern, --not only is the windage of the superstructure greater, but the drag of the superstructure and rig has a moment about the anchor rhode which will tend to push the bow down and create an angle of incidence of the flat deck or tramp, which may then provide detrimental lift.

 

Phil
That comes into the SOR. Distances to shelter are pretty good on the east coast of Oz even with a vessel that draws 9' and I'm not sure you'd be in that much danger if you didn't get into a nice quiet spot. But you do get a perfect nights rest while the deep keeled monohulls are rolling around and dragging anchor. But don't forget there are also monohulls with retractable ballasted keels.

Coastal cruising and offshore passages are quite different scenarios. Offshore regulations >50Nm are a lot more stringent than coastal in the commercial world for good reason, and it's not just being unable to run for shelter but also the delay in rescue services reaching the stricken vessel. Also a factor is the accuracy and availability of detailed forecasts. For example you can easily have 24 hours notice of an impending event, and just watch the fishing boats come in the evening before the day the gale blows.

Yes ballasted monohulls are more forgiving, they cannot be capsized from excess sail, the reason is because (in NA parlance) the wind heeling moment eventually intersects the GZ curve and even if this occurs at 90 degrees the vessel is at it's maximum righting moment. Then a very chastised crew get some sail off and clean up the mess in the galley.

Multihulls have wind heeling moments at some wind speed that will not intersect the GZ curve and it might be helpful if operators had this data from designers.

Multihulls are very dependent on operator skill and they are not forgiving of mistakes. An issue I was raising before that effectively your skill goes down with heavy weather considerably everything takes longer mentally not just physically. That's a double whammy in heavy weather and I said earlier it's also going to be contributory to the multihulls that inverted in storm conditions.

 

Off topic but Lachlan Crowther (Lock) was a grad engineer. He studied the two years of common core engineering and materials, and then he specialised in electrical and control systems of all things.

 

My thought is when you anchor or pull a drogue from the stern where is the lowest bridgedeck clearance you will have a venturi effect which creates a lower air pressure under the bridge deck and thus downforce. When you do it the other way around you will have a presure build up and lift under the bridgedeck.

This can easily be tested by anchoring both ways and mounting a barometer sensor under the bridgedeck, preferably a number of them distributed from entry to exit, and read the build up or decrease of air pressure under the bridgedeck and also compare it to the normal barometer, which is preferably of the same brand and calibration.

Perhaps someone should test this . . . .

Good luck !
Angel

P.S.

Not only the height but also the open width is much greater at the bow, which increases the above mentioned effects.


click pic for source

 

The Nicol Vagabond 1 literally had a airfoil wing to help lift the boat at speed. After the flight and barrel roll at 28 knots he inverted the foil like Cross. The decks were kept cambered as like Newick he felt they would provide downforce in a side wind. In large seas the strong wind is usually much reduced by the wave height at underwing level. Going downwind with a drogue the big advantage is the boat speed reducing the apparent wind and wave impacts. When anchored or using a parachute having the streamlined pointy end into the wind will minimize the force on the rodes and damage from wave impacts.

 

And increases your chances of being flipped by pressure build up under the boat for boats that are sensitive to that.

I've added a PS to my above post about this.

Cheers,
Angel

 

Just made a sketch to visualise what you say here.

Basis drawing is a Chris White Atlantic Cat 57 with Single Reef but think of it without the sails.

Red is anchoring from the bow. Green is anchoring from the stern.

- - click to enlarge​

Forces on the rig are the same for both ways of anchoring and are forcing down the side of the boat opposite to the anchor and lifting the anchor side of the boat. Which is worse for the anchoring from the bow as the the bridgedeck clearence in front is already higher and also the negative effect will be greater for the anchoring from the bow as the bow is lighter and thus easier to lift which increases the problem.

Wind catching surface of the superstructure is the same for anchoring from bow and stern but the aerodynamic profile from aft is worse ie more force from wind on that side. But that force has only a short arm to create a moment to push the bow down.

So after visualising I'll stick to my theory in post #244.

Cheers,
Angel

 

I suppose now the bridge deck could become a inflammatory subject, as to hight and angle. The problem with bow on the front of the bridge deck is higher so the air flow has trouble exiting past the aft part of the bridge deck, which gives you the air packing up
underneath creating lift. Then you have the curvature of the hull as well to consider.

May be a pod catamaran could reduces most if not all lift problems aspect for less accommodation

 

Sorry Angel, It looks like a bad idea to me......70 knot driven rain coming through the companion hatches etc....The wind on the mast would force the bows down easier giving you the underwing angle you are worried about.

 

I think Angelique was meaning it would be better to be anchoring from the bow. As she said she will stick to her theory in post 244.

 

And it's all about the design, what might be good for one might be a killer for another. From that we come back to the dilemma;
To have model specific tests and risk analysis and recommended to do when tshtf".. or not..

 

From his post I read that Cav has understand what I meant but disagrees about the effects which is OK.

But your assumption post #244 is about anchoring from the bow is wrong, it's about anchoring and pulling a drogue from the side with the narrowest opening and the lowest bridgedeck clearance which is the stern. I should have been explicit about the side I meant there. Will edit the post to do so.

Cheers,
Angel

 

Angelique I totally Agree with your Last post, It was just a interpretation of the post #244.

Another idea that could be to add something between the rear cross beam and transoms to reduce air getting underneath. The only problem could be cabin doors from the cockpit with wind and rain.

 

You may be able to optimised the venturi effect with a curtain between the rear beam and transom, by adjust the hight of the slot combined with the wedge pushing down.

It may be necessary to limit the amount of the wind going underneath when you are getting to much suction.

 

I watched the Nicol from the beach at night when a less than well forecast storm squall caught us by surprise. It was unloaded, very light,on its mooring, the wind was well past 50 knots and on the beach wet sand was airborne so it was like being in a sand blaster. There was flat water to start so no waves to hide behind and the wind in the multitude of rigging wires sounded like a jet engine on takeoff as it howled. The boat's bow did rise a few inches but I think it was from the windage on the mast,if air pressure builds in the rear the bow is forced back down then downforce pushes the boat down a bit. All the power on shore went out within a minute as trees snapped and branches fell. A good mooring test but I'm glad we didn't have more windage. Not many boats have achieved lift off when anchored, when winds are forecast for 50-60 knots plus from the wrong direction I try to put my boat in the "hurricane" hole, a enclosed cove so I don't worry as I am then kept busy by other storm chores. Putting models in a wind tunnel is an idea and a safe way to put theories to test. When the **** hits the fans in real life I tend to be conservative and stick with what has worked before. If you have a stern anchor set too it could help prevent kiteing. In the hole I anchor off all 4 corners but if a real hurricane was coming I'd use shore lines too.