I don't think they had any real concept of how shock waves travel through water. I believe, but am not certain, that no actual tests using the attached, full-size, spar torpedo on Hunley were carried out prior to the sortie that sank Housatonic. Hunley's second, fatal sinking had roughly coincided with David's October attack on U.S.S. New Ironsides, which didn't sink that ship but was nonetheless considered by General Beauregard a genuine success, and a model to follow in the future. Beauregard was very skeptical of the Hunley project by this point, but reluctantly agreed to allow Dixon to take over command of the boat, and for him and Alexander to recruit a new crew. One of Beauregard's conditions for continuing with Hunley was that it be refitted with a spar torpedo, and abandon the towed-torpedo model previously employed, which had already resulted in a number of close-calls, and was deemed to be of great danger of all concerned. (Beauregard also claimed, after the war, that he had insisted that Hunley's operations be confined to running on the surface, but Chaffin's book discounts this claim, as there's no contemporary evidence of such an order or policy, and Dixon and his crew continued to routinely and openly practice diving right to the end.)
David itself was very nearly sunk in attacking New Ironsides with a spar torpedo, but another of the "lessons" they learned in that incident was that the torpedo itself had to be much larger -- twice as large, as it turned out.