Can I modify a pattern for a tight knit?

I’ve come to this forum for help after a long fruitless search of the great internet cloud. I am an older guy who loves wool on cold days and I’ve always wanted a “fisherman’s sweater” full of lanolin so that I could wear it in the rain. To my surprise, I’ve learned that it is the tight knit and 5 ply worsted wool that makes it water resistant more than the lanolin. So here is my problem; I have found the wool, 5 ply with lots of lanolin too, and I have a knitter, my very elderly mother, but she only knits with two needles and isn’t “about to learn anything new at her age”. The only patterns I’ve found with a tight knit are the Ganseys that use 5 needles and knit the sweater in one piece, tube like. My question is, can I take any pattern that uses 2 bigger needles, substitute smaller needles and just multiply all the pattern numbers by the same ratio to get a tighter knit? For example, a pattern with 4 sts per inch changed to 8 would just mean that I multiply all numbers by 2.

Welcome to the forum!
This is so kind of your mother to knit one of these Aran/Gansey’s for you. One thing to do is to make a gauge swatch with the smaller needles and see how many sts/inch you get. The pattern will have a recommended gauge in order to get a certain size. You (or your Mom) will be working at a tighter gauge so you’ll have to follow the instructions for a larger size. So if the measurements for the large size at the pattern gauge are good for you, your mother may have to go up to the extra large in order to get the same measurements.

I would add only one thing to salmonmac’s most excellent explanation. As your loving mother isn’t “about to learn anything new at her age” you might want to find a pattern that is written for knitting flat so that nobody needs to convert it from in-the-round to flat knitting. Have you looked at at all? If you haven’t joined yet, you should. It’s free and you won’t get spammed. I just did a search there and if you look you’ll see all the things I specified across the top of the page.|aran&pa=worked-flat&pc=sweater&craft=knitting&view=captioned_thumbs&sort=popularity&availability=free

A question for salmonmac: Could it work the other way? Would using a pattern written for a thinner yarn and using a different size work? I suppose if the gauge with the thicker yarn matched the gauge of the pattern, no adjustment in size would be needed. My brain is complaining. :eyes: It does not like thoughts like these. :sad:

Maybe too many things to consider changing at once. That’ll do it to anyone.
You could knit a pattern with the given yarn on larger needles. That would make the sweater fabric lacier and the sweater larger. Knitting a smaller size would bring the measurements back to the original size while still leaving the lacey fabric.
If you take a pattern written for a thinner yarn and work it in a heavier yarn, the measurements will be larger. To keep the original measurements (or close to them), knit a smaller size.

Thank you Salmonmac and GrumpyGramma for the excellent advice. Now that you brought it to my attention it’s easier than I thought. I’m still stuck on the math part though, so if the stitches are twice as tight than I should look for a size twice as big across the chest. I’d hate to have mom knit me a sweater that is too small. I’ll check out the and I noticed some other related questions at the bottom of this page. Thanks again you’ve both been very helpful. By the way, I would have answered yesterday but I had to help mother.