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As long as the deep cycle battery can safely supply the "impulse current" or surge current needed by your starter motor (which can be 1.5-2.5 kilowatts easily) without overheating and warping the plates or internals, it is perfectly safe.
I had a discussion about this with JCI (possibly the largest US battery maker) some time ago, for a starting battery that had to sit for long periods and would be better served by using a deep cycle. Their answer was that if the deep cycle had enough capacity--it was actually the better way to go.
So, depending on how much power your start draws, it can be perfectly good to do it this way. On a 41? Dunno, you'd need to find out how much power that is.