Turbo 400 trans or other Multi speed transmission with Surface drive.

Yes, 12k is unreasonable outside of racing applications. Particularly when you can get a beefed up TH400 for a fifth that.

 

Why not manual trans. Decades ago, from late 40's, papa had a criss craft with a big blue gear lever. I supposed it clashed gears a little switching gears. That boat had 2 steering wheels on the side.

 

Big reason is that the 400 in already installed and working very well. I cant wrap my head around how a manual would work well in this application...

 

It wouldn't be as convenient as the automatic shifts from that old valve body, that's for sure.

What prop are you running to take advantage of the very low gear in first?

 

Right. 18x30 SD

 

I am running a 17x30 for now.

 

I'm curious about acceleration in 1st, top speed in 3rd.

 

acceleration in 1st is good, not great and comes on plane in about 20 seconds, Once it's on plane it seems to take off. I have only taken advantage of the 1st and 2nd gear thus far and I have not done a top speed run yet. I have far to much work to complete before I am ready for good shake down.

 

Interesting.

Top speed in 3rd is going to be very fast. Gerr's Propeller Handbook suggests about 10% slip. I don't see where you stated rpm for your 670hp mill, so I'm guessing around 5000rpm.

30"×5000×0.9=11250

That's 128mph

Actually, at that speed, slip is probably only 5%. 135mph. 217km/h

Assuming you have the power to get to that speed. If not, a lower pitch prop would really help acceleration.

 

I ran Crouch's formula, just to see. I don't know the all-up weight of your boat, so I guessed 7700#. Adjust accordingly.
Crouch's formula has been superceded by more accurate formulas in the almost a century since he cameup with it. But not knowing anything beyond 30', 670hp, SP 17x30, the inaccuracies in the formula are a rounding error compared to the other attributes. But using those values I got 56 knots, 65 mph.

I don't know what the expected slip for a SP prop is at that v. If it's comparable to an immersed prop, you'd do better with a 17x15.
@sandhammaren05 is the SP prop master on the forum. He will know.

 

Oh yes. SP prop. Faster than 65.