Expansion tank for M25?

[barely adequate]

I've noticed over the past couple months of cruising that the m25 in my E27 goes through a lot of coolant. Recently I noticed some in the bilge, and almost none in the reservoir, so I realized I had to deal with it right away. I added coolant, started the engine with the covers off, and stood by with a flashlight watching for leaks. I'd expected a faulty cheap hose clamp or old hose (have been replacing these as fast as I can get them since I got the boat a few months ago), but instead, when the engine reached around 150°, coolant started dripping out of the unoccupied small hose fitting just under the reservoir cap. I assume this is why the old alternator kicked the bucket (it was right under the drip, severely corroded, and non-functional), and I'm hoping to prevent this from happening again. The engine otherwise has run well until now and holds temp at just under 180° once it's warmed up. So, a couple questions:

-Is the coolant reservoir cap (or something else) likely defective, or could the loss be due to normal cyclical expansion/cooling of the fluid?

-can I add an overflow/expansion tank to solve this, if so, can anyone recommend an affordable aftermarket one? I'm guessing this isn't something you'd need to go for OEM parts on? Could I maybe even find one at an auto-parts store?

Here's a photo of the current situation:
https://drive.google.com/file/d/0B8-8UI9ISKB_eDQwX2p0aW1oY0U/view?usp=sharing

 

[bgary]

-Is the coolant reservoir cap (or something else) likely defective, or could the loss be due to normal cyclical expansion/cooling of the fluid?

-can I add an overflow/expansion tank to solve this, if so, can anyone recommend an affordable aftermarket one? I'm guessing this isn't something you'd need to go for OEM parts on? Could I maybe even find one at an auto-parts store?

Yes, the coolant expands at temp and needs some place to go. Just like in your car, which has an "overflow" tank... when warm, the excess goes into the tank, when cool, it can flow back into the block.

My setup was the same as yours - a length of hose hanging off the barb just under the coolant reservoir cap. In my case, the hose was long enough to hang down into the sump below the motor, so alternator wasn't affected but excess coolant sloshing around down there.

I went to O'reilly auto parts and bought a generic overflow tank. Cost about 10 bucks. Mounted it in a convenient spot (slightly higher than the cap), attached the end of the hose to the barb on the bottom of the tank, works great. Now I can see at a glance if my coolant level is where it is supposed to be, and no more coolant overflow into the sump.