1979 Mercury I6 90HP WOT?

[Napps11]

I have a 1979 90hp merc on my Swiftsure Power Squadron (which is the exact same boat at Glastron cvx-16). I am running a 17p aluminum prop. I got 42 MPH at 5000RPM. I believe the diameter on it is 13". I get out of the hole extremely fast, even with 4 adults in it. Any recomendations as for pitch props for me to go faster? What is the WOT on a 1979 90HP Merc? I have read throughout the site and have not found a 100% sure answer. Recomendations for stainless and aluminum props would be greatly appreciated!

P.S. What would the differences for a 3 vs. 4 blade props?

Thanks!

 

[jimmbo]

Re: 1979 Mercury I6 90HP WOT?

Great little engine. Rpm range for that engine is stated as 4500 - 5000 rpm. So you are already at the peak rpm as per spec. That said there are several people on this board who like to prop I6s for 5800 - 6000 regardles of the spec. Being that the !6 90hp has very mild porting, its horsepower will drop rapidly above 5000 rpm.

Does your motor have power trim? If so, are you fully trimmed out when you are getting 5000? A More Performance type Stainless prop(Laser II) might give you a couple Mph more. Raising the engine higher on the transom is not an easy option with your engine, unless you use a jack plate, even then they don't help much a speeds below 50 mph.

You said you were doing 42mph. Was that from a gps, or the pressure guage speedo? Doing the math, that engine running @ 5000 rpm with a 17 in prop should be running a round 37 -38 mph. a 19in would be closer to 42 - 44 mph

As for the 3 vs. 4 blade? I can't say since I have never tried a 4 blade on any of my boats. Lower top speed I assume due to higher drag from the extra blade, less vibration.

 

[Napps11]

Re: 1979 Mercury I6 90HP WOT?

Yes, my motor has power trim. I am fully trimmed out when i am getting 5000RPM. 42 MPH was from a pressure guage speedo. What is the purpose of a jack plate? I understand it is to get the motor out of the water more, but would it not have the problem of getting water into the cooling system?