The separation of single-channel underwater acoustic signals is a challenging problem with practical significance. In view of the signal separation problem with unknown numbers of signals, we propose a solution with a fixed number of output channels, enabling it to avoid the dimensional disaster caused by the permutation problem induced by the alignment of outputs to targets. Specifically, we modify two algorithms developed for known numbers of signals based on autoencoders, which are highly explainable. We also propose a new performance evaluation method for situations with mute channels. Experiments conducted on simulated mixtures of radiated ship noise show that the proposed solution can achieve similar separation performance to that attained with a known number of signals. The mute channel output is also good.