Re: 14' aluminum v-hull transom
Charkbait,
Nice boat. From picture, it looks like your transom runs straight across full width of stern, so you probably won't have to adjust the transom height to accomodate spare motor. I assume that both motors have same shaft length (20" nominal?).
Width of my transom at top is 63" and at chines (~ water line where splash rail meets stern) is 54". It looks that your boat is a little narrower at transom, so you might(!) be a little tight for space with both mounted. Simplest way to resolve this is by experiment, ie. mounting both motors, with spare in operating position and also tipped up for running only with main (9.9).
I can't tell from your photo details of your transom construction. Is there a wooden transom core inside the stern (ie. where motor mount screws tighten onto stern)? & if so, how much of the stern aluminum does this cover? This info is important in two respects, one being whether you would have to extend mounting area (wood) inside as well as outside, and second wrt amount of support & rigidity that afford to your transom with two motors mounted when on water & esp. trailering.
There is a wooden mounting plate on outside of the stern for your main, and if you are to mount spare beside the main, one approach is to replace this with mounting plate sufficiently wide to accomodate both motors (for final install). For testing/experiment, I think that you can use a piece of plywood alongside the original outside mounting plate. A piece or two of two-sided tape can hold this in place whilst you're fussing with mounting the spare & clamps will secure for float/water test.
It may be useful if you're serious about this to scrounge or buy a 1/2 or full sheet of exterior, or better yet, marine plywood (1/2" thick is practical) and buy a qty of epoxy, hardner & 6" cloth, sufficient to seal/protect all your wood, once you've figured out the size & thickness of wood that you need, inside &/or outside existing Al on transom, but again it is a simple matter to experiment with ply or other material cut to size before committing. A hint is that measuring tape & paper or scrap cardboard, traced & cut as templates can save $ & aggravation. There are lots of links on the web wrt construction / repair of botas & wood preparation using epoxy as an adhesive & sealer. There are cheaper means to do this that can work well for a long time but will have to be replaced. It's your time & money.
A consideration with narrower boat & transom is that weight away from the centreline can make the boat a little more 'tippy'. Ideally when running with one motor & spare, the main is on centreline, but you might have to offset the main to accomodate your spare. This thrust offset from centreline (keel) can also make the boat less stable esp. in lighter craft, but it's not uncommon for people to have two motors mounted and run only on one much of the time. I suspect that one of the most important things in boating is how the operator behaves and prepares for problems. Others have said this much better. The stern of your boat looks relatively flat & may tolerate these these effects a little more than v-bottom, at least at rest.
A float test with both motors mounted as you'd like them & other stuff loaded as you would for travelling will give you a sense of how the boat rests in water with spare mounted, and some careful maneuvering (at first) under power likewise hint at how the boat will probably behave in new configuration, and what adjustment(s) are necessary to trim it out properly.
Two other things.
I can't tell for sure from your pic, but looks that your prop & anti-ventilation plate are well below bottom of the boat. You do have a 20" counter & long-shaft motor? As mentioned earlier in this thread & elsewhere, you can get some improvement in performance by raising motor a little, unless in fact you have a long shaft mounted on a 15" transom when significant changes are recommended in mounting or motor.
Extra weight on the transom can deform your hull when trailering if bunks do not reach all the way to the stern. I try to keep weight in boat balanced & least practical, esp. for long distance travel over rough country and launching if dry launch/retreive is necessary.
Good luck
