Can you shorten the hose? It sounds like your pump is above the water line. Thus turning it sideways brings it below the waterline. I have a thru hull with a sentry pump and have never lost prime. Mine also has a tee from the pump feeding my washdown pump which is self priming. So if it ever lost prime I can run the washdown for a second to prime the inlet. I think yours just needs to be lower to the thru hull.
