My boat ('82 35' Kevlar Cigarette Mistress) runs very well SSM #3's with Herring 3 blade 18 x 26 props....
looking to buy a backup set....Where to search for -and- what price should I expect?...new or even used!

So no opinion on price or where to buy a 3 blade 18x26 prop??...surly someone has thoughts?

Nobody make 3 blade props anymore.
We can do some great things with 4 and 5 blades. Give me a call and I can explain some options you might find good for you.

Brett

What is the largest diameter prop you can put on a speedmaster III drive?

Pm sent

Please give me the reason why they don't? ..........Also, If I'm currently using a 18 x 26 3 blade on my ssm3....would a 4 blade have to have more or less diameter?

3 Blade props work great, but if all the prop manufacturers didn't reinvent the wheel with the 4 and then 5 blades. everyone would be happy with what they had and prop companys would be out of business. Raising drive heights led to more blades. stepped hulls as well. still the fastest is the good old 2 blade, just doesn't work on a lot of hull types. I run old school hulls, no steps,no notches and the 3 blades carry the bow better, IMO..
4.5.6 blades work also but there might be a lot of testing to get the handling you are comfortable with changing over to more blades.
Brett will be able to help you, I see 2 sets of 5 blades in your future and your 3s on the shelf.
See if Hering has something.

Raising drive heights, stepped bottom boats and cat boats tend to run better with additional blades as stated by class6.
Because 4 and 5 blades work ok, good or better on all applications, Mercury Racing and Hering dropped 3 blades. It was actually a good business decision since less and less 3 blades are being sold and manufacturers wouldn't need to maintain that inventory as well.
I wish 3 blades were still available for specific set ups. Even more importantly, I really wish 4 blades were still made. If I was still at Mercury Racing, I would lobby hard for a small inventory of 4 blades.

Brett

Gentlemen, Thank you!......If I were to step up in blade (4 or 5) where would I start if currently have 18 x 26 3 blade?......

17 X the lowest pith you can find.

Brett