What is equivalent of “pottakkannente Maaveleru” means blindman throwing stones in a waterless well, but he thinks he is throw stones at mango tree

Question

What is equivalent of Indian saying "pottakkannente Maaveleru" means while actually the blind man is throwing stones in a waterless well (a job in vain), but he thinks he is throw stones at mango tree (where he will get mangos in return. mangoes are suppose to be valuable)

Example:

A expat community (from India) in a foreign country (say USA) is discussing about their home country rate (USD vs Indian Rupees) exchange rate). Everyone says the current exchange rate will remain the same (not exactly beneficial to the expat community). A person among them predicts that by end of the month the rate will increase (he does not have any proof or rationale) and so what he means is the expat community can get a better rate and end up earning better deal than today. One another skeptic amongst them say "pottakkannente Maaveleru" which in this context mean he is shooting in the air (same as stoning in a waterless well - no result) and that the rate will not improve (not benefit the expats).

Now according to the skeptic, but like a blind man the predictor's see himself stoning at the mango tree (which means predictor thinks his prediction will be true). The skeptic indirectly means that the predictor does not have any basis to say what he is predicting.

Importantly the general meaning is that

if his prediction comes true it is a fluke and if not than like any politician he will vanish into obscurity when the day comes.

Answer 1

I don't know of anything similar, but the general meaning that

if his prediction comes true it is a fluke

could be expressed as:

even a blind pig can find an acorn once in a while

or:

a stopped clock is right twice a day

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Although I should add that these are more often used in retrospect - i.e. after the prediction already came true - rather than to dismiss the prediction when it is made, as in your example.

Answer 2

Somewhat similar is the idiom "a shot in the dark".

TFD(idioms):

a shot in the dark

1. A guess or estimate with very little or no assurance as to its accuracy.
   Well, this is just a shot in the dark, but I'm going to say that the answer to the question is 52.

2. An attempt that is not expected to succeed or has very little chance of working.
   It was really just a shot in the dark when I tried to fix our washing machine, but I was actually able to get it working again on my first try!

Farlex Dictionary of Idioms. © 2015 Farlex, Inc, all rights reserved.