Older 225 hp Yami on 80's vintage Sceptre
 

Estimated 30 mph top speed seems like either tired outboard, or heavy logged hull correct ?

80's 23's are heavier hulls and have a larger fuel tanks. You'd have to work hard to only get 30mph out of it . . . even with the bottom covered in barnacles dragging buckets and running on 5 cylinders you should still be able hit 30mph. There were different flavors of yamaha 225 (2.6L 225 1989 - 1992) yamaha excel 225 2.6L (86 - 89'ish) and then the large 3.1L displacement versions after 1992 (v76, ox66, etc.)

The old excels were strong engines. Had a friend with a Paramount that would bang 60 with one. Couldn't kill it.

What prop?

I have a 1977 Sceptre and run a 1988 Yamaha V-6 Excel (225hp). Last summer at WOT (5800rpms) I ran 43 mph with a light load.

That's what I figured, something likely wrong......

I agree. You either dropped a cylinder or are caring an external 3000#'s of aqua in the hull. That set up should run 40-42 wot with the the "wrong" prop. Something is amiss.

43mph = 37kts

"estimated"....

I have an '89 Sceptre with a 225 ox-66. With 1000# of people and gear, at 30 mph I'm around 4700 rpms.

hopefully That's not your top speed . . . or something is very very wrong.

I don't see anywhere on this thread of what wheel we're turning here. Estimated 30 mph (maybe 28-32?) could be more or less. Are we running in still water, or in current.
When I had my 225 ox66 with a 15x19 prop, I was turning about 4700 rpm at 38 mph. I switched to a 15x17 and turned it about 5300. The top end didn't change much, but the engine ran better in my opinion.
I think a tachometer would be helpful here. Just my 2 cents.
btw. I did try a much smaller diameter prop and the boat took off like it was a different boat, it would run in the 40's, but I did not like the ride in seas. The bigger diameter
kept the boat in the water better and gave me a smoother ride.

Prop calc
http://www.go-fast.com/Prop_Slip_Calculator.htm

OX66 ratio is 1.81


Let's assume it's a 15P prop the boat would have 18% prop slip to get 30mph at 4700rpm with a 1.81 ratio. 18% prop slip is awful.

17P would be 29% prop slip which is cement barge awful.

Mine's a "new to me" boat that I've only had out twice. I have a 15x17 prop. Most of the last trip was spent at 35-3800 rpms to keep from launching the boat. Where we could run faster, it was about 25 mph at 4200 rpms and 30 at 4700. I didn't run WOT, but there's plenty of throttle left over. Also we were probably closer to 1500# of people and gear with about 80 gallons of fuel.

I tried that calculator on the numbers I remember on the old motor (mind you that was over 12 years ago) and I get 19% slip on both props. I could not find a place to plug in the weight of the boat,load, type of hull, etc.

That same prop, a saltwater series 17 on an f225, same boat, gear ratio of 2 has 12% slip, per calculator.

I asked 👍

First things first.
Compression test cold and hot please. Cold first.
Max rpm under load?

Pic of prop?
Weigh station nearby?

My question came from info on a boat for sale, so I don't have any other data ref prop etc.

I was just wondering, and as usual, the good people that frequent this site have excellent experience to share. Thanks

good one