Boat nearly threw us out - why?

[workingonit]

Hi all,

I'm new to owning a boat. Purchased a second hand 6 metre alloy plate, narrowish width, single hull, with 70hp Johnson cable steer. Hope I got the terminology right

Situation, moving with outgoing fairly glassy water, at speed, in Elizabeth River Darwin. However, the tide is coming in, so we start to encounter standing waves(?) at 45 degrees to direction of travel coming up river at us. We chop through that quite happily for a bit when suddenly the boat turns violently right, but tilts left, nearly throwing us out.

Anyone can advise what might have caused this? Not aware of hitting any shoals or objects.

As an aside, the bottom of the stern is finished off in some box like shape rather than a 'v'. Any reason for this? Other boating friends don't know.

Cheers.

[Tombie]

Your boat sounds like it tracked the wave which then via inertia threw it out wards...

[shining]

Just a guess but.... A large obstruction on the river bed with incoming tide rushing over it could cause turbulence not necessarily visible on the surface. Once you are slewed across the current by the turbulence the pressure on the side of the hull could pull the boat down. Were you at speed (at least relative to the current flow?). As I said just a guess.... Or a very big saltie? :-)

[workingonit]

I would say we were moving down stream faster than the out flowing water.

My impression of the standing waves, 200mm?, is that they were probably the result on incoming water (tide change) running over the top of outgoing water, and we happened upon the interface of the two forces. The incoming water created the waves at 45 degrees on our right.

I could understand if the 45 degree angle of the wave tilted us left on the long axis, but I would also have expected the boat to turn left as well. Much like a grader blade would both roll and move soil in the same direction. But in this case we were rolled left, but steered hard right. The difference is like water in a bucket. Swirl the bucket around your head and the water stays in. Make a throwing action and the water comes out - that was us. Don't tell me I have the aquatic version of the land rover steering death wobble

Crocodile would be a nice answer to the problem.

[shining]

I could understand if the 45 degree angle of the wave tilted us left on the long axis, but I would also have expected the boat to turn left as well. Much like a grader blade would both roll and move soil in the same direction. But in this case we were rolled left, but steered hard right.
.

The river was running out into an incoming tide. Correct?

It sounds almost like what happens when one tries to turn too hard, screw cavitates and the wake catches up and rolls the craft around.

Being pushed from the side often leads to cork-screwing especially in a boat with a shallow draught. There isn't much in the bows for the wave to catch whereas the extra mass (and less freeboard) and motor in the water at the stern give the wave more to push against.

[workingonit]

Correct.

We were travelling in a straight line, cutting the standing waves at an angle rather than straight on.

Using some of your suggestions I wonder if the outflowing water was also being directed left by the incoming tide, catching our tail and turning us right, while the standing wave rolled us left.

Next time I see this I'll just cut my speed

[Ean Austral]

The actoin of the boat is similar to say a car with bad suspension, turn the car hard right the body rolls to the left, the boat is the same by turning hard right your force more pressure on the left side and momentum and weight transfer will cause the body roll left.

As for why, I dont understand how you have outgoing water on an incoming tide, unless you have hit a wider section or arm where water hasnt quite turned and you have met the tide that has already turned coming around the point of land.

My guess would be we are on spring tides at the moment and that is the time of greatest water movement, the waves were likely caused by the incoming tide moving over a shallow bank and thus causing the waves, think of surf over a reef, same thing.

Knowing the river you were in its natorious for bad tide eddy's and am guessing you hit one which caused the boat to turn as it did. When we had our hire boat business we had 1 sink in that river because the guy threw the anchor over with all the spare rope just floating on the surface, the current caused the rope to wrap around the leg of the outboard and held them side on in the spring tide eddy and flipped the boat over. That was a 5mtr tinny.

Cheers Ean

[workingonit]

Thanks for dropping in Ean with your local knowledge of the Elizabeth. Can't remember if we were north or south of the Eliz Bridge outside Palmerston, but that general area.

Need to clarify - I'm assuming the tidal eddy is basically a different phenomenon to the wave form over sand bank.

I can understand your description of the boat turning hard right and tilting left. Something turned us right, from our dead ahead course.

From this discussion I certainly have a better understanding.

[Ean Austral]

Thanks for dropping in Ean with your local knowledge of the Elizabeth. Can't remember if we were north or south of the Eliz Bridge outside Palmerston, but that general area.

Need to clarify - I'm assuming the tidal eddy is basically a different phenomenon to the wave form over sand bank.

I can understand your description of the boat turning hard right and tilting left. Something turned us right, from our dead ahead course.

From this discussion I certainly have a better understanding.

Easiest wayto explain the eddy;s im talking about, think whirlpool, but without a big hole in the centre. Usully caused buy the water flowing past say a large rock, it goes around each side and causes the water flow to speed up , if the water can also flow over the top of the rock, then it causes the water to swirl and in some cases churn.

You need to remember that on spring tides we have high tides of 7 mtrs low of 1mtr, its not that much today think its 6.8 to 2.0 so 4.8 mtrs of movement in 6 hrs, first hour after or before the high or low tide is not alot of movement, so really you are looking at 4mtrs of water in 4hrs, thats alot of force. Thats why I couldnt work out outgoing water with incoming tide.

I would doubt you hit something, you would have heard it, and if it was the bottom, you would likely have gone over the bow not the side.

A guess is a tide eddy of sorts, but hard to say.

Anyway, be careful on that part of the harbour, its a very tricky place .

Hope your next trip is less eventful

Cheers Ean

[Bearman]

G'day workingonit. That is a fairly big tinny for a first boat. Most boaties get a bit of experience in smaller dinghys before progressing up to ones that size. As others said there are lots of scenarios that can cause what you have described. It is hard to say what was the cause without being there. Things like tide movement, wave height, angle, speed, trim (bow up/down) wind direction, and boat characteristics all can have an effect on how it handles. Get out there often, take it steady and get the feel of how the boat handles in different situations before giving it too much.
The rear of the hull is probably shaped that way to give stability at rest. Quite a few brands have rear hulls shaped rather square so they ride good at anchor. As Ean said, big tides and lots of water movement up that way so as it's all a learning curve take it carefully until you get the hang of the handling characteristics of your boat. I am sure you are going to have a lot of fun with it. One of my favourite pastimes also