Sry late reply , been in the process of moving house/ workshop etc
Big end bearing.. on a couple.. that indicates classic slow death of an engine by incorect loading of the power head due to incorect propping for the end use of the engine
Ok will deal with each of these bits of info independently
16 1/2 foot ply 'Rifleman'
I would guesstimate an open ply boat this size about 800kg (??)
tops out at 23knts on a flat sea
(assuming the motor is correct height and at best trim for WoT at that speed,) calculates to approx 750 kg
I think would be safe to assume trim and maybe height not exact, so estimate around 800/850 kg
we usually fish 2 up with a big livebait tank & a big fish bin. Loaded like this it does 17knts comfortably but is running towards WOT,
This calc out to (assuming trim etec is correct) to around 1250kg, take a bit off for incorect trim etc say 1100kg
once you get into a biggish sea (particularly from behind) it can struggle to stay on the plane & its WOT the whole way.
Classic under powered.. min power will be you have to work the throttle a little to stay on the plane when hit the bigger set of waves in a set
We played around with a bunch of props & ended up with a fairly expensive cupped stainless steel one that gave us a very good compromise between power & speed.
"play around" espec to get best cruise speed etc generally ends up with an engine over propped, no or little reserve power right thru rpm range and not reach manufactures recommened rpm WOT rpm range.. or just scarpe into the very bottom.
it give good economy and faster cruise speeds
But it has the powerhead (bearings crank ) loaded up all the time.
Like a little under powered tractor ploughing a feld as fast as it can go all the time.
The bit of critical info missing is what are the rpms at WoT normal load?
Your ball park target on a tohatsu/ nissan 30 and 50 hp would be around 5350 rpms with a pitch around 7 to 8" of the corect diameter/ slip.
This would give a cruise around 10/ 11 knt assuming it could even be up on the plane...!!!!!
Personally I think your expectations as to cruise speed and planing is way to high for such a boat..
Most planing hulls around this size and weight drop off the plain around the 18/19knt mark.
To get a planing hull boat, say a old sea nymph gull wing ( far better designed plaining hull) around this size and weight well powered would have around a 115hp motor and a cruise speed @ 4000 rpms around 30 mph.
I dont know much about this hull or boat, very little.. Going on the numbers I suspect the hull is more a displacement or semi displacement hull.. Very different to a plaining hull and that your expectations as to the use and the powering are very incorect.
Trying to meet these expectations WILL RESULT IN SERIOUS OF LOADING of the engine power head and failure.
Will try to isustrate like this.
Take the example of the sea nymp gull wing speed/ cruise above.
Now we can also put that 115 hp motor on a say 28' displacement (non planing or semi plain) launch to nicely putt around the harbour.
And it will putt all day at a very reliable and comfortable 28' launch cruise speed @ 3800/4000 rpms...
Same engine just different applications...and propped accordingly one a 19" pitch prop, the other maybe a 5 or 9" pitch, both with very different diameters and cupping.