In the space of just over a week, SayHi Translate went from a tiny unknown to the #3 paid app in the iPhone App Store. In the last couple of days, the voice-translation app crossed 100,000 downloads, and today it’s at #6, sitting just below four games and the red-hot but totally useless UglyMeter.

That is remarkable for two reasons. First: SayHi’s voice translation software competes against Google Translate. Second: SayHi is a self-funded startup run by three friends in Connecticut.

SayHi’s 25-year-old CEO Lee Bossio came up with the idea for a voice-oriented translation app while spending a months-long stretch talking to interpreters at the hospital where his sister was recovering from surgery. The hospital employed a substantial staff of interpreters for Spanish-speaking patients, but there was still never enough. Wouldn’t it be great, Bossio thought, if an app could do part of the work for them.

Then followed what Bossio describes as “three years of utter misery.” The SayHi guys busted ass to develop the app, working after-hours while holding down steady jobs. They then tried to sell it to hospitals around the country, charging $15 a month per unit. To their immense frustration, they had few takers.

Eventually, they decided they couldn’t keep bankrolling a cash-bleeder forever. So, in February they decided to change course. They put a $2.99 price tag on the app (since discounted to 99 cents for a limited time), tweaked its architecture, and released it for the consumer market.

To their surprise, it shot up to the top of the app store, soon getting a helping hand from LifeHacker and Gizmodo. A similar app, the Robert Scoble-endorsed Vocre, provides a similar service for free, but it can’t match SayHi on user experience.

Users are drawn to SayHi’s simple interface and functionality. You need only select the language you want to translate to, tap a button, and speak into your phone. The software picks up on when you’ve finished, translates your voice into the foreign language text, and then speaks it back at you – most of the time with perfect execution.

Bossio is reluctant to reveal the app’s software secrets, but he claims it does a better job than Google Translate and Bing’s equivalent because of a “secret sauce” that augments those services through machine learning. Which leads one to ask why the guys took so long to make this available to consumers. Bossio confesses his initial naivety.

“When everyone tells you you should do something, and you kind of ignore them,” he says on reflection, “it should be a wake-up call for most people.” Somehow it took him a while to see the light.

Bossio’s protectiveness about SayHi’s IP is well justified. One opportunist has already put a call out for a developer to help build a direct clone of SayHi. (I’ve seen the job ad but, on principle, won’t link to it.)

Competing with Google and Vocre might seem tough enough, but it’s clear SayHi will have more than enough other challenges to keep them busy.