Thursday, February 21, 2013

What could possibly go wrong?

I really like the idea of stabilizing a small solar/electric boat converted from sail using two paravanes attached to the top of the mast.   I think I can make it work.   But it seems prudent to look at what could possibly go wrong.  So here is a list.

If the forces are too great we might snap the rigging on the mast.
I remember in the MacGregor26 sales video that 130 lbs of force was needed at the top of the mast to hold the boat on its side.  So I think that 100 lbs is not going to be too much for the rigging.   Also, I will probably use a string or fishing line that breaks at 100 lbs so that it would snap before the mast was damaged.

The paravane might hit the boat and damage it.
I will probably use a PVC pipe as the float and aluminum as the keel.  If things went bad and somehow the paravane hit my boat I would expect the PVC not do damage it.  The strings will be like 50 feet, so if it is next to the boat is is not being pulled, but just sitting in the water.   A 5 MPH collision between my boat and a PVC pipe with maybe 20 lbs of keel should not be bad.

Getting the paravanes in and out of the boat could be trouble.
I will make a loop on the top that I can grab with the mooring pole.  They should not be to heavy to lift up.  As we drive the big boat toward one it will just be sitting in the water, not pulling.  A regular trawler paravane resists if the boat is moving, but with out long line we will be drive close to it and snag it withe the pole without having to stop. 

If the ropes are too stretchy it might not help too much, but if they are not stretchy they might snap too easy.
I will have to try to see if it really can be made to work.

The drag from the mast, rigging, and paravanes could be too much for a small electric motor.
Will have to see what speed we can get with mast down, mast up, and mast up and paravanes.

Tangled lines or lines in propeller.
Need to be able to wind in lines to pravanes.

If mast tips away from paravane it will increase the speed on that paravane.
So paravane must be design for more than just the 4 or 5 MPH of the boat.   Also needs to be designed so that it does not pull out of the water in a way that messes it up.

Waves making paravane go up/down/left/right could make for peculiar motion on the mast and undesirable ride.
If the paravane is 80 feet to the side the up/down motion should not matter too much.    The left right might.  If you were going straight toward or away from the waves then the paravanes would be experiencing the same wave motion as the main boat.  In this case there would not be a tipping due to waves.   A wave right at the edge of the boat can move the side up some amount.  A wave at the paravane would move the mast a similar amount.   But the long lever of the mast means that the boat is moved much less.   So it seems like it will still be a big improvement.  However, I think we will need experimental results to really answer this one.

If the mast tips toward a paravane the string might go slack and it could be like a kite in the air that stops flying.   
Some stretch in the string could reduce this issue (though making others).  Experimental results needed.

Both paravanes have a downward component to their pull.  When the mast tips this full downward component is on one side and the more it tips the more lever this downward force has.  So there is a instability issue.
If the forces are small compared to the righting force of the boat, and the mast never tips very much, this could be ok.  But it could be a problem.  The longer the lines to the paravanes the less downward component to the force vector.  So longer lines could reduce this problem.

The paravanes pull back a bit and when the mast is on one side they will tend to turn the boat toward that side.
This could make stearing more trouble but my boat has two rudders and the engine so it should be possible to compensate for this.

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