by propwobble » Sun Dec 13, 2009 11:31 am
I know I have seen a fan cooling system in one of the old model mags at some point but I can't recall where as it was years ago. I am not a boat guy by nature but the concept of a buried engine intrigued me and caused me to build a radiator for a water cooled two stroke. I wish I could remember the article,sorry. I used a thermometer, which I got from tower hobbies with a remote sensor to measure head temp . They no longer have it but I know there are others out there. A model engine tachometer will be helpful in your process as well.
Your application will be similar in some ways but not truly like a ducted fan jet airplane model where the fan provides thrust, with yours, the fan needs to consume much less power to just push air. For ease of finding an appropriate fan, one of the type for a ducted fan aircraft engine with a few ( perhaps half or more depending on if it pushes enough air) blades removed might do the trick. The less air pushed= more power to the drive shaft. For any fan system, the pitch, diameter and number of blades plus the size of the duct and the number of bends you have to put in i twill determine the power consumption and the air flow. The fewer bends the better. Of course the goal is to put enough air over the cooling fins of the engine, especially the head and definitely in the area of the exhaust valve. Heat increases with engine speed but so does shaft speed/fan speed half good, half not so good. You might even be able to use a fan from a computer fan if you can figure out how to attach it to you drive shaft; I think I'd prefer one from the model ducted fan industry. If you go another way, it would be wise to try to use a fan that is designed to run at speeds more than what you will run but still be efficient. Fans can come apart if run over-speed. Four strokes vibrate too, so a nylon type fan with a good sized hub might be best. Vibration is an enemy and causes metal fatigue, avoid a metal fan.
Clearly you get the idea and if you are willing to experiment, you will likely succeed. Be careful, use a protective shield some sort (clear plexiglas or better yet: lexan)_ when you are exposed to the running system until you confirm it is safe.
The boat you are building sounds like an ambitious project that will take some time to construct. The fact that you are thinking in terms of satisfying a specific desire and seem determined to create a realistic model indicates you have the tenacity to figure it out.
If you could find a water cooled four stroke you would save yourself a lot of time. In addition to the OS there is one from the UK called Gannet that occasionally turn up as well. They are out of production and spendy. They are spark ignition 15cc. There also is British Magnum out of production, not the Asian Magnum, made a glow marine four stroke of 15cc.
You might want to look at Ebay #150396257487
best of luck