Re: 83 regal majestic 210
There's no way to say exactly what the top speed will be unless someone on here actually owns the exact same boat.
As a reasonable guesstimate, I'd say 40-45 with 2 people and fuel half full. Adjust from there depending on load and water conditions.
I'd estimate the fuel tank on that size of a boat will be around 30-40 gallons.
However. . .
Newbies tend to get very hung up on this question of top speed. In real use, you will very rarely wring it out at WOT for several reasons.
First, unless you have very smooth water with no wind, tide, or boat wakes, running around at WOT will knock the fillings right out of your teeth. And it can be dangerous to boot.
Second, a 21 foot boat ain't no Toyota Prius. At WOT, you'll actually be able to look down and see the fuel gage needle dropping toward "E" at an alarming pace that will cause your wallet to squeal in pain. Boats absolutely drink fuel at high speeds and that's why most of us are real fond of a setting called "best cruise".
Finally, WOT puts tremendous wear and strain on your engine. There's no coasting when it comes to boats. WOT will have you spinning your engine at 4500+ RPM and under huge load besides! When's the last time you drove your car around with the engine screaming for mercy like that? Probably never.
Bottom line is I wouldn't worry too much about top speeds and I certainly wouldn't make it a make/break decision point.
Grouse