And unless you have access to a laser you will not be able to concentrate the light sufficiently to reach ignition temps on a wooden ship at distance. The Mythbusters fixed array consisted of >100 mirrors all aimed to a single point at a set distance. This configuaration was decided upon after having the staff try, and fail, to accurately aim the mirrors by hand. The tar sealed target was tethered at the requisite distance and no ignition was achieved. To have 100 troops, even well trained troops, hold aim on a single point on a moving target long enough to achieve ignition is not believable. Remember, this is not tinder being lit by the concentrated beam of a Fresnel lense at 6 inches. You're talking about large dimension lumber at hundreds of yards. If you choose to believe in this fantasy then I will leave you to it.
Ed