Join Date: Apr 2006
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"Regarding air flow around the engine. Pop the hood of a Chrysler 300M and tell me how does the air flow in there?" Well...I seem all out of Chrysler's tonight but I can assure you, every car maker studies airflow. There's a fan in the front, in every car and truck, designed to suck air in past the radiator. And, often, intakes under the front bumper, plus a road pan under many engines now, to help ensure linear air flow past the engine block before the air is routed--and it is often routed with plastic covers and baffles--out the rear someplace. Sometimes up the base of the hood, sometimes out the bottom of the engine compartment in the rear (where the road pan doesn't extend) but it is all intentionally routed. Partly for better gas mileage and better grip at highway speeds, but very much for cooling. Remember also that radiator with the forced air flow on it. Granted, marine diesels will be water cooled in some way and water grabs heat 14x faster/better than air. But as you will notice if you try to touch an engine block after you shut it down, there are some damn hot spots on it--even though there is cooling water in it. Airflow over the block is all those spots get for extra cooling, in a car. It's all they are tested and designed for, and there's a long history of engines that are known for little things, like the old Ford 289 engines, great engines in the Mustangs but they'd burn the #2 valve if they overheated even once. Or the Vega 2300cc engines, which shared the technology of Porsche--but overheated if you let the oil OR water get low. And that killed their reputation real fast. They were designed to run at 210F instead of the more typical 170-180F, in order to get higher efficiency. Just a 30F difference--but enough of a change to make things go critical and melt.
In a raw water boat engine, you might be running at 140F so maybe that's an extra tolerance, but go with a closed loop heat exchanger (to me "fresh water" is too confusingly similar to raw water) and you're back to 170 again. then you've got to ask, can anyone relly save money by kitting together bits and pieces, and doing it on a small assembly line, as opposed to a real marine engine with everything designed right (we hope) from the start?
You're certainly right about almost anything being able to do a lot of 50-hour seasons and I suspect that lets a lot of folks get away with hell. There are folks who take the Volkswagon "Pathfinder" air-cooled diesel and use it in boats. It's a workhorse industrial engine, found all over the world. That's certainly one way to go. But of course, air cooled engines are less efficient, even the original Beetle was really lousy at economy once you figured out how little weight it was pushing around. Still...if the engine is cheap enough, you can afford plenty more fuel. You just have to hope EVERYTHING is done right, for the saltwater location with no airflow.
Which is also why many diesels have engine room blowers--to cool down the engine room after the engine is shut down. That's when oil is no longer flowing, coolant no longer flowing, and the oil sitting still on hot parts cokes up and turns to tar in the engine.
Then again there's resale value. Assuming you might sell the boat one day...having a known brand engine in it will increase the value. Having a DIY special in it...the next buyer may just take off $10,000 figuring he has to replace it.
Lotsa stuff works, and works "well enough". Sometimes saving $5000 is what counts, well enough.