drynoc - does your Yanmar have a raw water strainer at the top of the exhaust elbow? If so, it is possible and very easy to have the engine load itself with antifreeze by filling the strainer bowl a time or two with the engine running between fills. This technique avoids the trouble of the plumbing modification altogether. If you have a helper, fill the strainer bowl with antifreeze - have the helper start the engine and run it only as long as there remains antifreeze in the strainer bowl, then have the helper stop the engine and you refill the bowl and repeat. Generally my Volvo D2-55 will expel the first of the solution after only 3 fills of the bowl. If you shut it down at that time (being sure the bowl has not been emptied) the entire raw water side of the engine will be filled with the antifreeze. The number of fills will vary of course depending on the strainer bowl size and raw water capacity of your engine.