This species of contract was unknown to the common law of this country, since by it no pledge of a chattel was valid, unless the article pledged was actually transferred to the possession of the pawnee. The right to hypothecate was, however, recognized by the civil law, and has been long adopted in the maritime law of England.
A Compendium of the Law of Merchant Shipping, 3rd Edition, 1864, F. Maude, C. E. Pollock, pp.433-434
Bottomry bonds as a legal instrument were considered to be outside of the Statue of Usury, as explained by Burnett J in The Right Honourable the Earl of Chesterfield and Others, Executors of the Honourable John Spencer, Esq versus Sir Abraham Jansen, Knt  EngR 492; (1799) 1 Wils KB 286, Hilary Term, 24 Geo II, 1750, at p.292:
This case is a contract upon a hazard that the defendant may never have any principal or interest at all, and is like the case of a bottomry bond; one of the first cases whereof is in Cro. Jac. 209, wherein the Court determinded it not to be within the Stature of Usury, because of the real hazard there was of losing the whole money.
But it was objected that bottomry bonds were held to be good in favour of trade. I answer that the true reason why the Court holds them good is, because they are not against the statute; for how can a man be said to take a greater sum for the forbearance of payment of a sum of money, when by a hazard he runs he may be entitled to neither principal nor interest…
Definition of Bottomry Bonds was given by Lord Stowell in The Atlas  EngR 328; (1827) 2 Hag Adm 48 at pp.52-53:
The hypothecation or bottomry bond known to the civil law, and acted upon with and undoubted authority of this Court, is a bond whereby the captain of a vessel, not having any credit in the port, is enabled to obtain money for the repair of the ship, and its equipment for the voyage, upon what is called maritime interest. In the Roman law, in which it was familiarly known, it was called usura maritime, or foenus nauticum. The extent or value of this security for repayment was not limited, for it was not certain, but only eventual, dependent upon the safe accomplishment of the intended voyage. If the ship arrived safe, the title to repayment became vested; but if the ship perished in itinere, the loss fell entirely upon the lender. Upon that account, the lender was entitled to demand a much higher interest than the current interest of money in ordinary transaction. It partook of the nature of a wager, and therefore was not limited to the ordinary interest; the danger lay not upon the borrower, as in ordinary cases, but upon the lender, who was therefore entitled to charge his pretium periculi, his valuation of the danger to which he was exposed. A contract similar to this, upon the cargo of the ship, is called a respondetia, but is of rarer occurrence.