How can I make my boat a little faster?

I have a 1961 Nylox Ski Master. Vee hull front, but pretty much flat in the back. It's 13' long with a 60" beam. Bare hull weight of 325lbs. With a 1968 50hp Mercury short shaft with a 12x14 prop, I got 33mph(@5200). So I built up the solid transom and put a 1978 70hp Evinrude on it and got 37 mph(@5300rpm) with a slightly damaged 13x19 prop. That was with the middle hole with zero porpoising. The motor swap added 80lbs. So, apart from trying different props and trim positions, what can I do to the boat to make it faster? Would adding flotation foam at the transom get it out of the water a little bit more? Would moving more weight to the back help? I do have a spare 115hp Johnson I might try for fun. But I would like to avoid that urge. It's a cool little boat and I'd like to get 40+ out of if I could. Thanks.

Stephen

You can't go fast with a damaged prop... or an undamaged aluminum prop, for that matter.

Normally you would add a cheap water pressure gauge (to avoid overheating) and start raising the motor up higher for less drag.

Dittos on the water pressure gauge. How far below the bottom of your boat is the centerline of your prop shaft?

I do have a pressure gauge. The cav plate is even with the bottom of the boat so the prop shaft is about 8" below.

Jack your motor up 4” and see what she does. Ultimately you probably want to be 2” below or so. Any setback on your motor? Raise it up, watch your water psi gauge, and get a bigger pitch ss prop.

I’m referring to your prop shaft centerline on the 2” comment. Watch your psi gauge!!

You'll probably want a good solid mounting plate through bolted to the transom and raised so you can mount the clamps securely to that to get the height you want.
Are you lucky and have a 15" 70HP?

I'm pretty much maxxed out for height right now. It's a 16" transom with a motor board of 2" thick laminated plywood to accommodate the LS Rude. I'm bolting supports connecting the motor board to the rear seat bench/air tank using 1x2" steel. I have a Panther electric trim that I could use on there that should raise it at least 3". But I would be stressing that transom pretty good at that point. I might need to just rebuild the transom completely to the max height and use a jack plate. I appreciate your suggestions.

Yeah, or hunt a 15" leg. Don't think you'll want much for setback if you can help it with a jack plate.
Just get it a couple more inches up , enjoy it.

I could just build a new motor board a few inches higher, without adding any more set back. At what point would ventilation become a concern? I have a 17' Larson with a Mariner 115 that has the cav plate even with the bottom of the transom. I can't trim it that far before it starts to ventilate. But it will do 45.

Check this out , Must be a lot of those exact boats around in OR
https://portland.craigslist.org/wsc/...579155776.html

Yes, that is the exact patient in this case. LOL:) You can barely see the bow tip of my Larson in the garage.

What if I just put the 50hp SS back on there and use my motor board to have that sit 3 or 4 " higher? How much of a potential mph gain would I be looking assuming the prop stays in a good zone?

Try the merc 15" again. It will get you closer to proper engine height. Its not how much power you have its a matter of having the engine properly set up, weight, and balance.

Proper height and correct prop is most important.