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

[DogCavalry]

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

 

[sdowney717]

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.

 

[MRover]

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...

 

[DogCavalry]

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?

 

[DogCavalry]

Right. 18x30 SD

 

[MRover]

I am running a 17x30 for now.

 

[DogCavalry]

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

 

[MRover]

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.

 

[DogCavalry]

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.

 

[DogCavalry]

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Crouch's formula has been superceded by more accurate formulas in the almost a century since he cameup with it.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.

 

[DogCavalry]

Oh yes. SP prop. Faster than 65.