Just imagine you have $600 (which is an unfortunately small amount to "multiply"). Transaction fees and other costs guarantee an investment of $600 will likely have a lower risk-adjusted profit than if you had more. I can't imagine BTCjam or Havelock return anything close to 8% after defaults.
in BtcJam there are a lots of ponzis and doubtful projects, unfortunately it's scammer's heaven
Havelock on the other hand, have better projects but many of them pays revenues according sells of each company, but those sells are paid with dollars, so if the company sell 10.000usd in products, then your dividends in bitcoins are a part of this amount. If the price rises, the company income still are 10.000usd, but your dividends in btc will be lower
I would not invest in a company whose income are in usd if I expect multiply my bitcoins, but this could be a good opportunity if you have some USD