Posted by ken on October 19, 2003 at 18:02:49:
In Reply to: Choosing inboard transmissions, How? posted by Jamie on October 19, 2003 at 15:37:51:
You get to the ratio by the expected speed of the boat. Approximately 100rpm for every 10mph boat speed (prop rpm). this is very approximate in nature. A small boat like Cracker box with a V8 would usually go 1:1 or maybe a step-up ratio to get the prop faster than crank speed. You get into deeper ratios like you mentioned by putting larger boats around a given engine, to turn larger props. In your case, you are putting a very small hull around a V8 and reduction is not needed because of the high attainable speed, hence a high prop speed. remember that the faster the boat can go the faster you turn the prop, and likewise the slower the boat can go, the slower you turn the prop (with reduction).