I have a stock merc prop (kinda thick) that`s a pitch or two too large . 18x31 R15
The cruise is great 70mph at 3900 but I would like less engine load at cruise , Thats about 3-4# of boost versus 1-2# with a 18x29 R18
If labbed how much cruise speed should I expect to lose?

usually labbing will increase cruise/efficiency. Why not use the 29's and get them labbed? What's the difference in speed/rpm between the 2 sets of props?

The 31s are mine .
The 29`s are borrowed , owner deciding if he will sell them to me.
The speed at 4000k rpm is 10mph , it`s a big difference.

After your disastrous interaction about the story on Stu Jones, I can't believe you are at it again so soon.

I don`t even know how to aswer that and honestly I don`t whats wrong with you .. you certainly have a different way of thinking and that`s being nice.
There was nothing "disasterous" about it. I voiced my opionion end of story.
You constatly post the dumbest questions and in the wrong forum and you think you`re going to talk $hit to me?
Like Elon told Disney CEO ... GO .. you know the rest .. weirdo

What HP rating are you props? I would think your boat likes the 18 rake more than the 15 as well, no?

100% right, it loves more rake. I`d love to try a 20* rake.
I spoke to Cutting edge, he could get me some more rpm in the midrange, I would lose some speed but also run less boost and the props would be thinner .. hmm . I got some decisions to make.

The cruise is with 31 pitch is really nice .... 4000k rpm and 3.5# of boost.

Have you talked to Brett at BBlades? Yes, hard to get a hold of some times but he knows his stuff.

And they have their winter deal coming up, which is nice

Have you talked to FJ Midwest? LOTO prop maker/shop. I use Cool Breeze, but I am running B1's. George at Cool Breeze can do anything, I am just not knowledgeable of his cleaver style magic.