I have used the double hose on my Wallas Diesel
heater for 15 years with no trouble. Exterior of exhaust hose is very warm where it goes into the heater unit, but would not melt anything. The center exhaust point might where it exits the hull, but I am not sure. I put it out the transom high. The unit came with a hanger that puts a high loop inside just as it exits the hull to prevent water
ingress. I understand some manufacturers of units with this double tube arrangement (inner tube for combustion exhaust and outer tube for combustion intake) have noted a shorter life for the combustion fan motor
because of the heat of the intake air--especially if you have a run longer than 36"--so I would keep the run short. I replaced my combustion motor
after about 1000 hours when it made some interesting noises and I have a 5' run. The benefits of forced air diesel heat are wonderful--even used it occasionally in the tropics to dry things out. These units are not maintenance
free, but worth the effort and not very complicated.
A friend put the exhaust hose on the side of his hull and has to put a cork in it every time he sails
in any weather
that heels the boat.