New to me 89 Scarab Excel 28 + Prop help
 

Hey guys, first post here and it's about the first boat I've ever bought! A 28 foot scarab excel with 454/Bravo. Unknown mods to engine, seems like aluminium heads, cam, intake, demon carb, looks like stock chevy mechanical hei ignition, 4 inch through hull exhaust. Been fixing things here and there, but considering the age and what I paid, I'm feeling pretty happy! That being said, I want to change from the mirage 23p 3 blade to a 4 blade to improve holeshot, cruise and handing, but I'm running into an issue on what pitch to get and if I need more bow or stern lift. 4800 rpm puts me at 60 on GPS. Here's the weird part: it's consistent. Me with half tanks in the calm? 4800/60. 4 people, full tanks and chop? 4800/60. It's like there's a rev limiter, but I don't hear one and I don't get the effects of one kicking in, killing a bit of RPM and then coming back up. So I'm unsure if going up or down in pitch would be a better option. Also if the boat starts to porpoise around 50 partially trimmed out but seems to become less with speed and more trim, is that an indicator of needing more bow lift or more sternlift? This is my first boat, so take it easy on the new guy! Thanks!

How heavy is that boat? 60mph might not be too bad (people tried to tell me that too) Do you know what your gear ratio is? How many hp is the motor supposed to have?
Prop Slip Calculator | Mercury Racing

Supposedly it's 4600lbs dry. I have no idea what the motor HP is supposed to be, but with the speed and the 23p prop with the 1.5 bravo ratio I'm guessing around 500hp with a 14% slip. I'm not upset about doing 60 mph with that boat on our little lake, I just find it weird how consistently it hits that. I would expect to hit a limiter or go faster when I'm light and slower when I'm heavy, but it's the exact same no matter what the conditions are. That's what is making my prop pitch choice so difficult though is I don't know where the limiter is or if I'm lugging the 23 around. (Plus the whole bow vs stern lift issue)

3 blade mirage/bow 4 blade bravo/stern. I hate how my straight bottom 30 runs with bravos. The bow floats and handles rough water better with 3 blade mirages. (much better angle of attack)
You mention flat water and just you and half fuel. That will slow down a boat considerably. Also, it might seem like a weird phenomenon, but if you have more weight in the tanks in the center of the boat and your props can carry the bow. The leverage point might pick up the stern so you boat is less wet when running. If I recall my Scarab is just as fast with 1/2-3/4 fuel and weight in the cabin than it is empty. This was the only rational way I could think of. (hope that makes somewhat sense)

Basically implying that the forward and down change in the center of gravity is enough to substantially change the effect of the x dimension to a more efficient value? I'll have to do more research and see if that makes sense or not. You would think moving the CoG forward would hurt the efficiency due to a longer effective fulcrum causing less bow lift out of the water at a given trim. But again, I'm hitting the same GPS speeds and RPM independent of the loading... Are boat rev limiters not the same as automotive limiters where you can hear the engine bouncing off them?

Yes, that is the idea. Changing the wetted surface (X does not change, it's fixed)
Also, don't assume Wellcraft had the optimum placement of CG of the boat. These were production boats, I'm assuming they are close... I hope.

That's where a prop that can continue to carry the bow with trim comes into play. I have some 3 blades that won't and some that will. My best prop is a Mirage non-plus 27.

I understand the dimension doesn't change, just specifying the amount of the effect should change based on CoG.

So why does my 23 mirage non plus feel like the boat is flat when trimmed up?

Get a quick video and see how wet it rides. (full/low fuel) A good excuse for a couple good fly-by passes.
My boat is a transmission TRS boat, so 2 different animals. Just throwing out ideas.