They feed air to prop. Since the prochargers don't make a ton of power in the planing rpm area, they allow the props to slip, unloading them, so the engines can get into the powerband. At least that's my take on it.

Kind of the opposite of what you and I been trying to do. We have too much slip, and want less slip. But we have roots blowers that make good power down at 2500-3000rpm .

Cool job on the fabrication Eric, cant wait to see the finished product. Never seen one made for a #4 drive

Forgot to check this thread. Like Joe said yes they allow the props to slip. I could get them to blow out if I wanted or keep the rpm at 3k. As the boat started to break over the props hook and off you go. I really feel that its about the perfect setup with the planing issues resolved. Just a recap it was only an issue planning when I put the 33's on that were new style merc 1500hp props with a ton of cup. WIth all the other props I had on it planning was not an issue.

I made one for my 28' profile, started with a radiator hose zip tied to the side of the drive, worked great. After talking to BBlades I ended up putting a vented bravo prop on it, I never thought they would work without exhaust going through the hub. Worked great, even had to downsize the vents.

Good to know. Thanks for the input!

soo then the problem with prochargers is not a problem any more- aerated drives and than you all done.Do we agree?

Made it a lot easier for sure. I still couldn’t believe the difference it made.

I missed this thread somehow but I found it interesting. I considered at one time selling SC's and going procharger however with having most all I need with current SC's I'm just staying with them. From what I gather is both have their purposes. Seems like prochargers would be a little easier on drives and trans. After boating for over 30 years I understand fuel economy is something that never even gets taken into consideration however say for instance if having two identical boats with one being procharged and the other with SC. Say both making same hp at 6,000-6,500 what would consume more fuel. Not a day running all out but just a normal port hopping etc day being on and off the sticks. Even if both engines dyno'd with very similar bsfc's. My assumption would be the roots would drain the wallet quicker however don't really know the facts. All I know is when I was running NA and my partner John Sr ran 525 SC's with identical boats and same top end on our 28's there was not even a question as to consumption then again the early on 525's ran pig rich.

But more curious about the other two builds I mentioned.

The load curve of a planing hull boat matches the torque curve of a naturally aspirated engine quite closely. Because positive displacement blowers have a relatively constant boost level, they just amplify this torque curve. An added benefit is they can extend the max rpm as well.

The centrifugal supercharger increases boost with engine speed one to one. 4lbs at 2000, 12lbs at 6000. Like mentioned above, when the boat is propped to use all of the top end power, it won’t have enough torque at planing speeds. Basically the torque curve doesn’t match the load curve of the boat as well anymore.

Ironically, I kind of think a straight bottom boat would be a good fit for a centrifugal supercharger. Because these boats never really free up and fly at high speeds, they need big power on the top end in order to keep going faster.

I think for tuning centrifugal superchargers you should probably keep the compression ratio up, spin the blower fast, and then deal with excess boost on the top end by pulling timing.

these are just my science backed theories, please experiment on your fancy motors :D

That's a good post but now I have a question probably addressed millions of times regarding static compression on say a 548 build with a tbs 871 blower. My head guy likes to spin the blower as much as you can get away with yet at the same time heat becomes a factor. I'm aiming for 8.2:1 +/- SC and see if I can get away with 6 lbs boost as a starting point with premium. (No innercooler). I am currently working on an alternative way to cool rather than a traditional innercooler/chiller with a closed loop system but that will be a different thread.

Btw I realize cam overlap, duration and timing events can play a big part in this but let's just say an efficient cam for a Blower build. I'd like to go 1071's it again my 871's are fresh from blower shop and would like to make good use of them and perhaps eventually whipples.

if he’s not a boat guy I’d take the advice with a grain of salt. Roots cars can make huge power on alcohol, but Suck relatively on gas.

my other theory is that for boats there’s no reason not to go as large as possible on the blower. Spin it slower and make the same boost. Probably less heat.

Also lower compression won’t really hurt you with a roots blower, as you’ll still be making boost at all engine speeds. Merc generally used very low compression on roots motors, use a little less advance on the bottom end.

ive thought that you could cool the fuel before the carb