Start by removing the corn kernels from half the corn cobs. Why only half? The other half will be grated to produce a pulp that has tons of flavor and helps produce a smooth texture.
With the remaining ears of corn, use the large holes of a box grater to take off the majority of the corn. Do this over a bowl so you don't loose any of the pulp and juices. If there is any remaining corn on the cob, scrape it off with the back of your knife.
As long as you are grating the corn, why not grate the onion? This will also help give the chowder a smoother, creamier texture. If you don't want take the time, just chop it finely.
Heat the butter and oil in a large heavy bottomed soup pot and sauté the onion until translucent.
Add the chicken stock and then the potatoes. Bring this to a boil and then reduce heat and simmer for a couple of minutes.
Add milk, the grated corn pulp and juice only, and thyme and pepper. Let this simmer for about 8 minutes until the potatoes are tender.
Add the whole corn kernels that you removed from the corn cobs and the heavy cream. Let this simmer for 5 or 6 minutes until the corn kernels are cooked. Be careful not to overcook them.
Add the shrimp. They will only take a few minutes to cook in the hot liquid but watch them so they don't overcook.
Taste and adjust seasonings with salt and pepper.