I don't know if this was Captain
Dana's rationale, but my inferred rationale for the two alternatives on offer (45 degree vs vertical)
is that in some installations there can be the risk, in big following seas, of the water backing up through the waterlift box to the elbow (at low revs or with the engine temporarily stopped but the seacock still open) - purely from momentum.
It may help if the hose loops between the lift
box and the transom, and between the lift
box and the elbow, are as vertical as possible, hence orthogonal to the direction of momentum flow. And/or if the waterlift box is close astern of the engine, a vertical elbow may be easier to pipe up.
Don't take too much notice of this: it's based on shallow supposition and ignorant inference. The following, however, might be worth reading, for some:
The stainless 45 deg elbows made by Yanmar
are quite affordable, but they do rot
out quickly, (in my experience, anyway) from pits which start NOT in the water gallery but in the inside tubing, at the point where the water is meeting the outside of that same tubing.
This is admittedly with salt-water cooled engines, where the water is cooler at this point in the process: my thinking is that the exhaust gases, particularly in the early stages of warmup, will condense at this point on the inside of the tube, and carbonic and/or sulphuric acid will be part of that condensate. Furthermore, the carbon buildup which is copious at this location, might end up being a problem for galvanic action (I say this because the resulting pitting looks quite similar to galvanic corrosion).
Carbon does not always feature on galvanic tables, but it's quite noble. Graphite is theoretically more noble, in fact, than gold.
So anyone putting in lots of hours on a salt water
cooled GM series Yanmar
might want to consider taking their 45 deg elbow to a stainless fab shop well versed in marine
questions, to get a copy made out of thicker-walled material. These generally last almost indefinitely, where the others can sometimes become problematic in only a year or two.
It's important to decarbon the OEM items regularly (with a small but stiff-bristled and aggressive rotary wire brush in a die-grinder or powerful drill motor), and do it vigorously, then check carefully to see if the carbon has been concealing pits which have already connected through, and actually allow a minute amount of seawater in to wet the carbon.
Naturally this will drastically accelerate any galvanic problem, by providing an electrolyte. Of course it will also lead to a salt-water atmosphere in the upper reaches of the exhaust port, and before you know it, your exhaust valves are pitted, and it's not long before you're into more expensive surgery from the downstream effects of that.