I was told by my marine mechanic that, with the same pitch prop, a labbed version will provide less
mid-range cruising efficiency than that a stock version of the same prop. I also had noticed the difference when testing both a labbed 24p B/1 4-blade as well as a stock one on my 23' hull.

I assume this is caused by slippage due to the thinner blades.
Is this true?

The recipe used when Labbing determines the end result.

A "standard" Lab recipe developed 15 years ago at Merc Racing for a Bravo 1 will allow for a 1-2 mph cruise loss on a sport boat. It may gain 4-5 on the top though.

Hull design , power, and Lab recipe determine the outcome.

Remember, Labbing was designed for speed. There are other advantages to Labbing also.

Thanks for the input ...
Other than WOT top speed, what are the other advantages?
Also, is it fair to assume that thinner blades allow more prop slip?

Thin blades do not induce slip.

Advantages often overlooked that I really like to tell folks about is Acceleration and buttery smooth operation.

Brett

What makes the "labbing" cause a loss in cruising efficiency?
Wouldn't you have the same amount of pitch and blade surface?

I just changed to a 2 inch shortie and my slip number went from 8% to 13% with the same prop. Why is that?

Drive height

Any way to get the slip numbers down?

The acutal pitch of a lab finished prop, in most cases, is lower than that of a non-lab finished prop. Your slip really does not go up with a lab finished prop compared to a stock prop when you take into account the true pitch of the lab finished prop.

sounds like your X demension is to high. I would try a 1" spacer. What kind of drive?