There is no other place you can learn about 5/8 runner bottom v-drives except here !!! (OK.. maybe I'm stretching that a bit) Anyway, without showing a picture of one upside down, here it goes. There about 2' wide 'runners' running from the back of the boat forward. They're designed to manage air passing under the boat to minumize the amount of wetted surface to reduce drag. 5/8" is the depth of the runner. From the back it might look like this.
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Generally they will have 3 separate cavitation plates. They are usually used for Drag racing, but I have seen circle runners, but they are rare.
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LeE ss13