Prop Selection Help

I have a Lund Impact 1675 and that recently threw a rod in the lower jug on my Merc 90 Elpto (2 stroke).

The boat is in great condition so I opted to repower to a 2023 90 Merc Elpt CT. Command thrust for greater durability.
Most of the info that I am finding for propping a 90 CT, is for larger heavy boats and pontoons.

I am purchasing from a local reputable Merc dealer. They were uncertain and are doing research on the CT for a 17′ boat. A standard Merc 90 is a 17 pitch 4 blade. The CT opens the door to the full line of V6 prop selection.

I was running a small diameter 12.5″ x 21P prop on my 2 stroke. At 5,400 rpm it would harbor around 40 SOG.

Hole shot will likely not be of any concern, but I would like to get good top end: 40+.

Anyone with experienced advice out there willing to give a 2nd opinion?

Thanks!

Should have got the Mercury 90 XS, I thought the Command thrust for Pontoons boats. What ever prop you go with you want to sit at just below the Red line RPM at WOT trimmed up. The 4 Blade give you better hole shot and are more stable then the 3 blade but supposably the 3 blade gives you 2-3mph more on top end. I went from a 3 blade to a 4 blade on my new boat and like the 4 blade performance better.

bigcrappie
The command thrust has the same lower as the 150. Its sturdier and will last longer. It also provides and incredible hole shot due to gearing.
I am not concerned with the prop for hole shot, looking more for top end.
The CT will provide plenty of hole shot on its own.
Good point about the 3 blade… that might be a better option in this case.

I assume you’re looking to stay with aluminum props?

I would start with 2″ more pitch for a CT 90 on that boat, but you may need to go up 4″ of pitch depending on where you land for engine height. To take full advantage of the engine you may need to play with engine height. The old 2-strokes were different lengths than the new fourstrokes, and the CT’s are slightly longer than a standard 90 due to the larger gearcase even if they’re all advertised as the same length. Very little info will be out there for a starting point because very few people put CT’s on Lunds. When comparing 3 and 4 blade aluminum merc props it depends on the boat. The 3 blade will settle the stern and give better bow lift if the boat needs it. Four blade will almost guaranteed give better hole shot, but normally they don’t give better top end(not a rule just a guideline).

It seems like four blades have taken over now days. Some of that I believe is marketing, at least when it comes to aluminum props, but 3 blades still have their place. When I hung the new 40 on my 16′ tiller it was faster with a 3 blade since I needed the extra bow lift. However with less stern lift running the 3 blade I had to raise the engine one hole higher than with the 4 blade which let me go up an extra size in pitch. I ended up 2 holes higher than would have been stock.

Aluminum may be a safer bet in the river. Having said; the majority of owners that I know, run SS props on the river. I asked that question to the marina, and they said the difference in performance between the modern Aluminum matrix props and SS is negligible.
If that’s true, I will probably stick with Aluminum for cost and its ability to inflict less lower unit damage if I strike anything underwater.
Merc recommends a 17″ pitch for the standard Merc on my boat. Interesting, because I ran either a 19P or 21P on my two stroke. I preferred the performance of the 21P.
I don’t think hole shot will suffer with the command thrust… so maybe best to go 3 blade for a bit more top end?

Yeah, your old engine was whole different animal(power and gear ratio). Don’t get overly hung up on what you ran for a prop on that. Some boats will see larger gains out of stainless than others. My little tiller boat is 2 MPH faster with stainless because it stays hooked up with more trim angle, but the whole shot sucks. The 40hp just doesn’t have enough power to so spin it from a dead stop. I’ll eventually vent that prop to see if a little slip helps.