Re: 90 hp mariner vs 70 hp evinrude
The '89 Merc should be a 3 cyl and if so, is loop charged as is your rude. Good for that. Problem is that you have to equalize the thrust from both engines. If you had a dual setup whereas you are running the same engines side by side, same gearboxes, same rpm's, same pitch prop (counter rotating or not) you will have two power sources working in tandem to power your rig.
If you have one that has more "thrust" the lesser one will become a drag and fight you rather than help you. Years ago I was at a launch and a guy pulled up with a 25 hp (50's era engines) and 15 hp rude. I thought that interesting so I finished getting my rig roadworthy while watching him. He got launched and motored out from the dock, punched both of them for his hole shot and the 15 kicked up in the air because the driving force against his lower unit was less than the water pressure against it caused by the more available thrust from the other engine. So, in that senario I would say that if he added pitch to the 15 hp he could have done a better job of helping to push the boat with the lower unit remaining in the water rather than being a load.
My 2c,
Mark
Edit: If I totally missed the point here, i.e. you were only going to run one engine and you asked about what to expect from the 90 hp then "scuse" me for the error. In that instance, the Merc is a very fuel efficient engine and with the added power and all, you should get better mileage and better performance from the Merc. I have owned a 70 rude looper and currently own a 90 hp triple Merc looper. Course your prop selection and setup will influence just how much you gain, but I sincerely doubt you will loose anything.
