TOE-IN OR OUT?

QUESTION: Can you explain the correct procedure for setting the drive alignment on a twin-engine boat? I would think that if they are not in correct alignment, you could be scrubbing speed.

On a car, there is a certain amount of toe-in, and when the car starts moving, that position will change. Is a boat the same? Does the procedure change with regard to the direction the props are turning?

ANSWER: Traditionally, the front suspension on an automobile is set up so the wheels have a slight amount of toe-in. Being that there is (probably more in the old days) a certain amount of slop or play in the suspension or steering components such as ball joints, drag links, bushings and so on, if the front wheels were not set slightly toed-in to keep a constant load in one direction, the result would be what is called a shimmy. A shimmy can lead to a violent shaking of the front wheels and the steering wheel.

The amount the drives should be toed-in or toed-out on a boat depends on a number of factors. The most common considerations are hull type, drive type, propeller shaft depth and propeller rotation.

Generally speaking, twin-engine V-bottom boats with the propellers spinning out are usually set up with the drives toed-in slightly. It is common to set the nose of the drives about 1/8-inch closer together than the aft end of the prop shafts. The theory is that water exiting from the back of the hull is being parted slightly by the keel, so setting the drives at a slightly toed-in angle would result in them running more aligned with the water flow. That is only a theory. What we do know is that certain boats handle better with varying degrees of drive toe-in or toe-out.

If the propeller shafts are high enough so the propellers are surfacing, the side load created by the propeller blades seeing cleaner water in the lower sweep of their rotation will tend to push the drive in one direction. Similar to an automobile suspension, there is a slight amount of play in the gimbal ring and transom assembly pins and attachments.

The side force created by a surfacing propeller will load the drive and transom assembly in the opposite direction of the propeller rotation. In other words, if the propeller is a right-hand rotation (clockwise from the rear), the result is that the force is the same as turning (or flexing) the drive to the right, and vice versa.

If your propellers are surfacing and turning out, the toe-in (nose cones closer together) will increase under load. And, if the propellers are rotating inward (port clockwise and starboard counter-clockwise), the force will tend to pull the prop shafts closer together resulting in toeing the drives outward. The deviation is related to the type and condition of the gimbals and transom assemblies, and how high the propeller shafts are when the boat is underway. The result of the movement under load should be considered when deciding what the ultimate setup is relative to drive toe-in or toe-out.

Catamaran hulls that are running a high X dimension with the drive placement very close to the tunnel present additional criteria to consider. If the drives are close enough to the tunnel wall so that the sweep of the propeller blade is seeing clean water exiting from inside the tunnel, there is a tendency to neutralize the affect on the propeller created by water exiting from the angle of the running surface of the sponson.

The bottom line is to get some advice from the manufacturer or someone who knows a lot about your particular type of boat and setup. Then start from a setting and make minor adjustments testing the boat each time to gauge the affect on the handling and efficiency.

Always measure the toe-in or toe-out with the drives trimmed to a neutral position and the steering straight. With an assistant, measure from the front of the center of the nose cones or bullets compared to the center of the aft end of the prop shafts. If you run the boat and discover that the toe-in measurement has changed by itself, the indication is that there is significant slop in your transom assemblies, tiebar or steering. A change in the measurement also could be because the transom assemblies themselves are moving on the boat’s transom and need to be re-torqued.

BLACKHAWK REPLACEMENT

QUESTION: I own a 1995 Donzi Marine 22-foot Blackhawk Limited Edition boat with a Mercury 502 Magnum engine package. This boat was originally equipped with a Blackhawk drive that no longer exists and has been replaced with a Bravo One performance drive. As I understand it, Donzi built more rocker into the bottom of the boat, which now does not perform as well without the surface-style drive.

I also understand Arneson has a drive conversion kit, but it is kind of pricey for me at about $30,000 for a turnkey kit. I’m looking for other suggestions as to what I could do to regain performance and handling for about half the price.

ANSWER: The Blackhawk drive was a unique performance drive that had limited applications and did require special hull designs in some cases. It was very efficient, and with opposite rotating propellers, it worked well even on single-engine V-bottoms. Even though the propeller shafts were high compared to the running surface of the boat, the counter-rotating propellers balanced the torque.

At that time, most of the performance propellers for conventional drives had only three blades. A Donzi similar to yours would have been equipped with a propeller such as a Mercury Mirage on a Bravo One drive. As propeller designs and types improved for application on Bravo-style drives, boat manufacturers were able to start raising the X dimensions resulting in standard drives becoming closer to surfacing drives.

Today, many manufacturers are installing Bravo-style drives on twin-engine V-bottoms with the prop shafts above the running surface of the boat. They are able to do this primarily because of the four-, five- and six-blade propellers that are now available.

Mercury realized the need for a single-propeller, surfacing Bravo-style drive and developed the Sport Master lower gear case, which has a racing-style nose cone and a longer skeg to maintain steering control. This lower is available in stock length and one that is 2 inches shorter. The Sport Master lower will bolt directly onto your current upper. An add-on cavitation plate also is provided. The Sport Master was primarily designed for use on catamaran hulls and is not quite as fast as the aftermarket IMCO Marine lower gear case on V-bottoms.

IMCO also has lower gear cases that incorporate a nose cone and are available 1, 2, and 3 inches shorter than stock. If you use the 3-inch shorter case, you also must use the IMCO upper case that is designed to provide for more prop clearance under the cavitation plate. IMCO also provides an add-on cavitation plate for use with its shorter lower cases.

I think you would gain a tremendous amount of performance by installing a lower unit that incorporates the 2-inch shorter IMCO gear case on your drive. This would result in a less wetted gear case and that would reduce drag. Then install one of the five- or six-blade propellers available on the market today. You will have to do some experimenting. I would keep the overall diameter at a maximum of 15 1/4 inches. Too much diameter will result in a torque reaction during acceleration and deceleration.

If you want a lot of bite, try a lab-finished Maximus in a 15-inch diameter. On the Maximus propeller, if the boat comes on plane well with the shorter lower unit, I would also trim about 5/8-inch off the diffuser. The Hydromotive P-5X would be a good choice if you are looking for a propeller that creates a little bow lift. The Hydromotive P-5 Race (pointed ear) would be a good choice if you are looking for a little transom lift. And Hering offers a good selection of six-blade propellers that also would be worth a try.

If you install the shorter lower unit on your drive, and do a little prop testing, the boat may end up performing better than it did with the Blackhawk drive and be more manageable around the docks.

LAZY SHIFT

QUESTION: I have a 2005 32-foot Advantage Victory with twin MerCruiser 496 Mag HO engines and Bravo One X drives. The starboard drive is very slow to fully respond to the shifter at the helm. When shifting into gear in reverse and forward, the prop begins to spin immediately, however, it spins with very little power.

It takes 5 to 10 seconds and then the clutch locks up and the boat responds appropriately. The port drive responds immediately as did the starboard drive prior to having it serviced. I recently had the drives serviced, which included removing them, checking the alignment and replacing the drive oil.

The problem with the starboard drive started shortly after servicing with a brief delay that has grown to 5 to 10 seconds. I had this same problem with the port drive a year ago, but was unable, at the time, to link it to drive service due to some other factors, although the drives were serviced within the time frame of the problem. The shop ultimately replaced the cone clutch assembly in the port drive unit.

Is it possible that there is actually air in the starboard drive unit that has accumulated creating an air pocket that is preventing full engagement rather than a problem with the cone clutch assembly?

Do you have any other suggestions as to the cause of the problem, diagnosis and the fix for it before I replace another cone clutch assembly? I always enjoy your section in Powerboat.

ANSWER: There is no way that it’s related to air in the drive. A Bravo drive is a wet-sump drive and is filled to the midpoint of the upper input shaft. When the engine is running, the upper pinion gear and the driven gears are always in motion. When the cone clutch is engaged, one of the driven gears is locked to the upper shaft, which is coupled to the lower vertical shaft. When the lower driven gear is engaged, the propeller spins clockwise, and when the upper driven gear is engaged, the propeller spins counterclockwise.

There are brass rings attached to the two driven gears. When the drive is assembled, the driven (forward and reverse) gears must be timed in order for the cone clutch to shift properly. The brass rings are high on one side and low on the other so there is a “wobble” when they rotate. The shifting fork that rides in a slot in the cone clutch goes between the two brass rings on the gears. The shifting fork also has some cam ramps on it that contact the brass rings. When the drive is shifted, the uneven motion of the brass rings helps to kick the cone clutch into an engaged position.

I don’t believe your problem is from having the drives serviced unless they put the wrong oil in the drives. I believe that the brass rings on the gears may be worn or there is a problem with the shifting fork or cams. There’s a possibility that the problem is only being caused by the shifting fork or the linkage that connects to the intermediate shift cable. There is a setscrew on the shift shaft that also might have come loose. Remove the back cover from the drive to inspect the linkage, shift fork and the setscrew.

It’s possible that the intermediate shift cable wasn’t properly engaged in the shift linkage when the drive was reinstalled on the boat. It is sometimes tricky to get the linkage to latch on the cable end. To inspect that connection, the drive will have to be removed. The linkage is connected to the drive’s starboard side. It must be unlatched to remove the drive fully.

If all of the shifting mechanism is in good order, it is likely that you will need to replace the cone clutch and shifting fork. If the brass rings on the driven gears are excessively worn, replacement of the upper gear set also will be necessary. If there is any metal on your upper plug magnet, there is a possibility that the thrust bearing is failing. The debris that is generated can cause the cone clutch to act weird.

MECHANICAL TRIM INDICATORS

QUESTION: I have a 1990 26-foot Scarab CV. I have had an ongoing problem with the stock Mercury trim-limit switch. After a couple of trips out I need to adjust the switch. I want to go to a mechanical indicator setup, but wasn't sure of one thing: When using the mechanical indicator setup without a trim-limit switch, what limits the drive from being trimmed too far out while the engine is running? Is it just the driver and the gauge?

ANSWER: Many of the Mercury Racing transom assemblies do not have a trim-limit feature. Heavy-duty Bravo transom assemblies have a heavy-duty, stainless-steel pin held in place by screws tapped into the gimbal ring.

Transom assemblies that are equipped with the heavy-duty gimbal ring are not equipped with electric trim senders or trim-limit switches. It is up to the operator to make sure that the drive is not trimmed out excessively while the engine is running.

Usually, it is safe to run the engine with the drive trimmed to any reading where movement is still detected on the indicator panel. It is up to the operator to familiarize themselves with the maximum running trim position.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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