My experience is only with the MD17D and one thing I haven't had to deal with is the gearbox
( fingers crossed etc).
I do know however that after leaving the water
pump">raw water pump on the back of the engine
the water goes in one side of the gearbox
and out the other... then onwards and upwards into the exhaust
This as I understand it is what is going on.... different engine
... found that info here Foley Engines | 100 Years of Expert Advice
Coolers. Despite your best intentions for a variety of reasons some transmissions will run hot. In addition to the problems caused by severe use and getting tangled in a line, some gears are simply underspeced. The Perkins
4108 is rated at 50 HP at 4000 RPM
yet many boat builders equip them with the HBW 100 which is not rated for this. One could install a more appropriate transmission
such as the HBW 150 with 37% more fluid (0.63 quarts versus 0.44 quarts), and shorten the shaft to accomodate the longer HBW 150. Or, recognizing that heat is the enemy of gears you can increase the torque capacity of the gear that you now have. The first step is to upgrade to a better transmission fluid which we discussed above. The next step is to install a transmission cooler. Hurth
mechanical gears don't have a provision for the long oil
coolers composed of a "tube with a bundle inside" as seen on Borg Warner and Hurth
hydraulic gears. To mount a cooler on a Hurth mechanical gear, Foley Engines can supply a small aluminium box that installs on four pre- drilled holes on the outside of the transmission's case. Raw water is run to and through the small box (about the size of three stacked decks of playing cards) and it cools the fluid inside the transmission.
This Foley Cooler Kit works so well that it effectively changes the torque capacity of the Hurth HBW 100 to that of the HBW 150. It is a very cost effective way to upgrade your drive train."
gearboxes have a built in cooler...