EDIT: Even with a zero-risk collateralized situation, you would be hard pressed to find someone who would be willing to lend their BTC at an interest rate of 8% for 6 months.
17% per year is a very high interest rate!
Are savings banks give as much?
It's not a lot for crypto lending. Exchanges that offer margin funding (Bitfinex, Poloniex) occasionally get to this APR without you needing to make a 6 month commitment - and the only risk you are taking there is with the platform itself. If the lender is taking on an extreme risk of default, they would want even more interest.
you can check the interest rate offered margin funding (Bitfinex, Poloniex) and compare. Why not?