Multihull in Fast Flowing Water

[Fanie]

Hello Tom,
Thanks for taking the time, although you're a bit late I'm back already.

I wasn't allowed to launch the tri on the sea. I'll follow this up with the so called 'authorities'. It seems the sea in SA now belongs to some and no one else.
I was charged R200 to launch the tri on the river which is rediculous There are no facilities so they just want.

So as for my original enquiry -

In the case of a prop, if the boat is capable of doing 10 kn and the water flows at 8 kn then of course the boat will do 2 kn. I'm fine with that.

However, a sailboat's speed is determined by the wind. Granted, there would be more drag on the hull, and upstream sailing would be slowing the boat some.

The only time the current would take the sailboat downstream would be when the hull drag becomes more than the sail force, but there should be an advantage.

The faster a sail boat sail the greater the aparent wind angle and you lose sail force (you can only sail so fast in a given wind). If you are slowed down mechanically then the force from the sails should be more than when you are at speed.

Does anyone know how the water drag on a hull increase if you keep on increasing the water speed ? I know it will eventually end up in planing or flying but Freeship does not indicate much speed.

[Rick Willoughby]

Fanie
Here is something to ponder.

Wind over land is zero.

Downstream water flow is 10kts.

Are you able to sail upstream?

Rick W

[tspeer]

Originally Posted by Fanie

...However, a sailboat's speed is determined by the wind. Granted, there would be more drag on the hull, and upstream sailing would be slowing the boat some.

The only time the current would take the sailboat downstream would be when the hull drag becomes more than the sail force, but there should be an advantage.

The faster a sail boat sail the greater the aparent wind angle and you lose sail force (you can only sail so fast in a given wind). If you are slowed down mechanically then the force from the sails should be more than when you are at speed.

Does anyone know how the water drag on a hull increase if you keep on increasing the water speed ? I know it will eventually end up in planing or flying but Freeship does not indicate much speed.

You need to look a the problem from a couple of different frames of reference. First consider the frame of reference of a person floating in an inner tube as the sailboat passes. The boat speed through the water, as seen by the person in the inner tube (or the sailor) depends on the wind speed relative to the water. This is no different than when sailing without current.

Next consider the speed of the boat as seen by an observer on shore. The speed relative to the shore is the vector sum of the speed through the water and the current. This is the same for the power boat and the sailboat.

What's different is the speed of the wind as observed by the person in the inner tube vs the person on shore. The speed of the wind experienced by the person in the inner tube is the speed of the wind seen by the person on shore, minus the current. If the wind is with the current, and at the same speed, there will be zero wind over the water.

You can see this at work in the example of short-tacking up the shore that I gave earlier. The current and wind were in the same direction, so the wind relative to the water dropped over the part of the channel where the current was greater. This is why the monohull had difficulty making any progress through the water. When I went close to shore, the current was less, so the wind over the water increased. This allowed me to sail faster through the water. It also meant that the set to leeward from the current itself wasn't as much, so there was a double benefit.

Another way I've found useful to deal with current is to imagine the water is stationary and fixed points, like buoys, are moving through the water. So if I need to round a buoy in an adverse tide, I imagine where I'd have to sail to intercept the moving buoy as though it were a boat steaming at the speed of the current.

It's all relative.

[DaveJ]

I might throw my 2 cents in as well. Some thing that i believe you guys are overlooking in the inertia of the boat. Lets see if i got this correct.

The river speed is 10kts and all things aside the boat is pointing upstream at 10kts hull speed in a straight line against the current, you could say the boat is at a state of rest, it has no energy. Now you want to change direction and turn, it will feel more responsive and turn better than lets say a boat doing 10kts on still water, because the boat on the river has no inertia, and doesn't have to deal with the forward inertia.

Now lets turn that around, the river is 10kts and the boat hull speed is 10kts down stream, so its forward inertia is at twice at 20 knts than a boat doing 10kts on still water. Now if it wants to change direction it has 20kts worth of inertia for only 10kts worth of steerage, so its going to be quite sluggish and can get very dangerous on very bendy rivers.

This is where twin engine over single engine will be at advantage, as you still have steerage even with no hull speed. Now as for sailing, my head hurts too much trying to figure that out, and its home time.

Dave.

[Fanie]

I think you guys are ab so friggin lutely right.

The wind is going to blow downstream at 10 kn and the current is going to be 10 kn. Marvelous sailing at 10 kn in 100% wind still conditions. Just think, I can sail at 10 kn side ways and having a braai aboard and the smoke would go streight up.

I never thought the wind would go with the stream, and while we were there it actually did at one stage.

I got my answers, thanks everyone.

[Richard Woods]

For a multihull in fast flowing water you may want to look here

http://player.video.news.com.au/couriermail/#1180586374

Richard Woods of Woods Designs

www.sailingcatamarans.com

[DaveJ]

yer, the south port passage can get very tricky sometimes. Just a question, in that instance, would it be better to do that with dagger boards up or down, i would think up, as would want the pivot point of the boat as far aft as possible and allow abit of side slip if the stern started to overtake the bow. Having the dagger boards down would give the pivot point around them, and if the stern started to overtake the bow, there would be no side slip, the wave would then have the leverage to swing the bow around until it is side on with the wave, and who knows what will happen then.

Dave.

[Fanie]

Hi Richard,
That was well done with that cat, one would expect nothing less from a multihull. Daggerboards up and you can surf. Why not. Just because it's not a hobie doesn't mean it has to be sluggish.

[Fanie]

That actually is a good example of how a cat would would run down a wave in high seas and where one would use a drogue to prevent it.