My new prop selection

[LORDY611]

Background: 16' Lund (aluminum), simple wooden bench seats and plywood factory floor, Owner-installed mahogany side-console, 1996 40hp Merc 4cyl 2cycle OB.
The motor came off another rig from previous owner. I think it was a bigger heavier bass-style fiberglass boat, not positive. I also think he installed a 90hp on that other boat.
I installed a TinyTach which reads approx 4500 RPM WOT and 35mph on GPS. I believe the motor should run 5000-5500 at WOT according to a website.
The motor starts quickly, idles fine, accelerates rapidly and smoothly. I get some porpoising at full speed and trimmed (power T&T) and I feel like everything is going as it should (other than porpoising). Then I look at the tach and feel like I should change the 10-3/8 x 14 aluminum 3 blade (GUESSING original equipment, but no visable ID) to a 10-3/8 x 12?? Seems like a drastic drop so that is why I look to those of you who may have gone through something like this before to either say "dropping 2" pitch is not drastic" or "something is wrong and you need to recheck your..." Any educated guesses would be appreciated. Trying to get it right within the next week while on vacation.
Thanks to everyone.

 

[steelespike]

Re: My new prop selection

I would call 35 mph respectable.I'm no gear ratio pro but it seems a little odd to get 35 at only 4,500 rpm.Before messing with the prop You could first fine tune your setup.Starting with the anti vent plate even with the bottom of the boat raise it a little at a time to a point just before
venting becomes a problem.We used to add 1/4 inch sticks to support it at the top of the transom.perhaps yours has holes for vertical adjustment.
Be sure it pumps water.If you can gain an inch you could get a couple of mph some rpm and perhaps less porpoising.If you have no venting now you should have room to raise it.When trimming out go easy see if you can avoid the porpoising starting and still get max speed.

 

[LORDY611]

Re: My new prop selection

Dont get me wrong, the 35 mph is fine with me, I have no reason to go faster, especially on a small lake. I just worry about what everyone here talks about with not reaching the manufacturer's spec of 5K to 5500 RPM at WOT. It doesnt sound like it's lugging at all. Should I worry at all?
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