19' Miller SJ, flat deck.

WJ pump, no set back.

383 Chevy, 375 pounds, dressed.

Hull is in the 575-600 pound range.

Would there be any benefit to moving the motor forward? If so, how much, and what dictates this? Center of gravity? Or?

Seating is not an issue, top speed and turning is.

Thanks
Dan'l

Aluminum block?  With a lighter engine, typically we have been going about 1.5" to 2" inches further forward than with a typical BBC.

First understand that our specialty is really drag racing.  This is how I would approach it if this were my build.

We are all about feeding the impeller, without putting the thrust down, the boat doesn't go.  It all starts with the keel and the intake.  Ultimately, the pump drives the location of the engine.

For a Circle boat, I would start out by moving the intake as far back as possible without doing a full setback.  I would remove the wood from the transom cut the glass so that there is only 1" for the intake to sit on.  This is much stronger than a full setback, less chance of cracking the Poxy Putty.

Next, I would shorten the mounting flange of the intake, immediately behind the shoe.  I try to avoid Jacuzzi, but on a Berkeley, you can cut half way through the back two holes.  Next trim the transom so it fits really nice around the back of the intake, between the shoe and suction housing flange.  This allows the intake to be installed as far back as possible and be as strong as possible.

When installing the intake, it should be as absolutely as low as possible.  It will probably be necessary to grind the hull in the rear most corners, where the mounting flange touches the glass.

I don't know what angle the Jacuzzi intake is, but a Berkeley, for example, is 4-degrees.  I wouldn't set the intake any less than 4-degrees (Berkeley).  If you push it to 5-degrees, that pivots the whole jet about the intake mounting flange behind the shoe.  This effectively straightens the water flow through the jet along the short turn radius and lowers the thrust point.

I would finish off the intake install by adding keel depth.  Keel depth is relative to horsepower and speed.  The more horsepower, the more water can be processed through the pump.  More water processed, more speed.  More speed loads the pump harder, more water needs to be processed by the pump.

Should you get to the point where you over charge the pump, you will develop some handling problems.  Water builds pressure before the impeller.  Instead of being converted into thrust, it pukes out the intake.  It can manifest as a porpoise or violently blowing the tail.

Adding keel depth gives you the ability to run less shoe and loader, for less drag.  A circle jet is going to load the pump a lot differently in the chop than a drag jet on a ripple free drag strip.  You are going to need to creep up on your final settings.  I think I would start around 3/16" keel depth and a back cut shoe.  The back cut of the shoe should be 3-degrees to keel (just in front of the intake).  The rearmost edge of the shoe, at the front edge of the ride plate,should be slightly above keel.  I would have numerous shoe shims (1/32", 1/16", 3/32", 1/8", 3/16", and 1/4").  I would also have an assortment of loaders with different length ramps.  It's going to be all about finding what the boat likes in the chop and that will probably change through the day.

Now, to address your actual question:

Several things influence the ultimate dimension, for example, presence of a Jet-a-Way.  I will assume that you don't mind making a custom driveshaft.  Drive shaft length can be an issue because the weld yokes can be only so short.  A drive shaft that is just slightly longer than the original is not possible.  There are a few different slip yoke lengths available, that can help you manipulate overall drive shaft length, and those can be shortened some if necessary.

I usually strive to get the oil pan completely in front of the intake.  This allows the engine to be mounted lower without interference with the four front intake bolts.

The idea of moving the engine forward is to get the back of the boat up out of the water.  It's not just the engine location, you need look at everything.  I think everything should be mounted as low as possible and made as light as possible (without failure).

Typically, we also move the driver back.  One reason is for safety, so the driver doesn't get caught under the dash in a wreck.  Also for Center Of Gravity (COG) manipulation.

My thinking is that the majority of the mass should be centrally located about the part of the hull that is actively riding in the water during the turn.  I would want to minimize the polar moment of inertia so that the boat turns faster and with better control.  Then accelerate hard out of the turn.

Finding the right COG is most likely going to take some testing.  For serious race boats, we typically make the rails with three mounting positions in 2" increments, especially where horsepower upgrades or an aluminum block are planned.  Unfortunately a new drive shaft is required for each position.

To be continued...

Of course, after you get everything mounted, the COG is going to be different.  Will you be installing a turn fin?

It sounds like you have the intake well under control.  Is your shoe a back cut shoe (ride plate is lower than the biting edge)?  If the biting edge is 5/32" below keel, plus another two 1/16" shims, THAT'S HUGE!  That might cause excessive drag, even though a more aggressive shoe and loader are beneficial to keeping it hooked in the chop.

Typically, to describe engine placement, I would measure, across the top of the stringers, from the transom to the block bellhousing surface.  Again, we do mostly drag boats and very few circle jet.  Measuring across the top of the stringer, for a Miller drag boat application, we would strive to install a BBC in the 25" to 26" range.  For a small block making big power, we would push it further a couple inches.  It really depends on how much power it makes.

Again, the objectives are 1) get the stern up out of the water and 2) leverage the bow down.  I believe that there is more to COG that is not being exploited.  For example, there is a rooster tail height limit in Comp Jet.  Limiting the thrust vectoring limits the attitude of the hull.  In a perfect world, if the boat were perfectly balanced to exactly the right attitude, the direction of thrust could be horizontal.  Any thrust direction, other than moving the boat forward, is wasted energy.  Of course glass doesn't exist in Comp Jet.

I wish I had more definitive answers for you.  I would make provisions for additional engine locations (2" increments) and go test.

Good luck,

Joe

Not saying you need to cut the shoe, just saying it's aggressive. I would try the shoe as-is first.  It may work good in the chop.  You just need to be ready while setting up for a turn or when the pump takes a gulp of air.

If you need to reduce the biting edge depth, remove material from the top surface that mates against the intake.  That way the back cut, counter sunk holes, ride plate step, and opening are unaltered.  If you don't blend the scoop back to the intake, you can add shims to gain the biting edge depth back, but with more adjustability.

I wouldn't put the fuel tank all the way back.  I like in front of the engine, on the starboard side (unless you are running a dry sump).  The fuel pump will work more efficiently and the weight bias would improve as the fuel is consumed.

You're always welcome