Question of the Month
GPS And Radar
Can you explain the difference between the radar and GPS for top speed on the boats in the Performance Trials? Sometimes they are right on and other times they can be off by a mph or two. Thanks for the great column.

Grant Feldstein
Salem, Ore.

An accurate GPS speedometer measures speed around the circumference of the earth. Usually the speed indicated by a fixed GPS is the most accurate. Most GPS units update the indicated speed reading about every second.

One thing that will obviously affect the GPS-indicated speed is current. For example, if you are traveling downriver and the current is 2 mph, the GPS will indicate the additional 2 mph even though you are not traveling that fast over the surface of the water. During Powerboat's Performance Trials, if we are testing where a significant current is present, we average the GPS speeds from two directions.

In order to verify GPS-indicated speeds, we also print the top indicated speed according to our Stalker Pro radar gun. Many times during the test, I also will verify GPS readings by validating the reading with the radar gun.

Today's Stalker radar gun readings are instant. Accuracy of the radar gun reading depends on what the signal is being "bounced back" from. The radar gun can "bounce" its signal off the surface of the water, a land mass or another boat, either standing or underway. Usually, we use the radar gun from inside the boat pointing it directly forward over the surface of the water. Radar speeds are only accurate if the signal is directly inline with the direction of travel.

How About 6.2S
I am planning to buy a new Sea Ray 290 Sun Sport. I am unsure which direction to go with for power. Should I select the twin 350s or the single MerCruiser 496 Mag? I want the best top-end and also nice midrange cruising efficiency.

I think I would eventually like to add a Whipple Supercharger kit to the power plant(s). Also, I am confused as to which Bravo outdrive is best in this application. Could you provide some of your expert advice on this?

Finally, are hydrofoils worth the cost and effort? We will mainly be using the boat in freshwater, but it will see some coastal use.

Ron Brockhaus Jr.
Rock Hill, S.C.

I would definitely select twin engines for the Sea Ray 290 Sun Sport over a single engine. The boat is on the large side for a single drive. The single MerCruiser 496 Mag will be working pretty hard all of the time especially to achieve a high-speed cruise.

Twin engines will result in a boat that cruises easily and gets on plane faster. The boat will stay on plane at lower speeds as well.

As a side benefit, it is much easier to dock a twin-engine boat. If they are available, I would opt for the MerCruiser 6.2-liter engine packages. They will fit in the same place that the 350s do, and the increased performance is worth it.

Whipple has pretty much discontinued building a kit for the Mercury 350-cubic-inch line. They are continuing to provide a kit for the 6.2-liter MerCruiser engines. With the 6.2s, you may not need to spend the extra money.

I would install Bravo One X-series drives at a minimum. If you can get the XRs, go for it. They are stronger yet. If you decide to go with a single 496 Mag, then you might consider a Bravo Three X-series drive, which will be more efficient with its dual counter-rotating propellers with the heavier boat.

By hydrofoils, I believe that you mean trim tabs. It is always better to have trim tabs than not. It is just another element of control for your boat. They are especially nice to have to correct the attitude of the boat in crosswind situations. And, tabs will provide for a more comfortable ride in rougher conditions.

Jet Boat Water Supply
I own a 1987 Eliminator with a 502-cubic-inch big-block Chevy with a jet drive. The engine is equipped with stock exhaust logs.

I just replaced the heads because of a blown head gasket and one head was cracked. The engine has never been overheated. Should I use a regulator or bypass system to adjust the water pressure for the engine cooling? Or am I worried about nothing?

Ian Williamson
Palmdale, Calif.

Most jet-drive boats like yours were set up so that the engine receives its cooling water from the bowl pressure in the jet pump. It is likely that you have a 3/8-inch pipe fitting on the inside of the jet pump on the port side.

Many jet boats are equipped with a gate valve on the pipe nipple that is screwed into the pump housing. The purpose of this valve is to control the amount of water going to the engine.

It is very important that an adequate flow of water is going to the engine at all times. Accordingly, the practice of restricting the flow of water too much by closing the gate valve too far can cause engine damage.

It is much better to allow your engine to run at a temperature of about 130 to 140 degrees rather than restricting the water in order to achieve a temperature closer to what an automobile engine runs.

Some engine builders are focused on running temperatures near 180 degrees because they believe the engine makes more horsepower at those temperatures. The additional horsepower that may be realized will not be noticeable in your boat, but may result in engine damage.

It is important to maintain some water pressure in the engine, especially when the engine is under load. Water flowing through the cylinder heads carries the heat away from hot spots, especially near the exhaust seats. If the water pressure is low (which lowers the boiling point of water), or the flow is low, the result is that hot spots develop, which can cause steam pockets in the cooling passages of the cylinder heads.

Once a steam pocket occurs, it continues to grow and insulates the castings from the cooling water. It is most likely that your head was cracked because a hot spot developed in this manner. It also is likely that the blown head gasket also was caused by excessive heat.

I don't think you need to add a system to your boat. It is sufficient to make sure that your engine is receiving adequate cooling water and that reasonable block pressure exists. You might consider installing a water pressure gauge in the boat that is connected to the intake manifold near the water outlets. Five to 10 psi of water pressure while under load would be a reasonable goal.

Where Has The Oil Gone?
I have a 2001 353 Formula FAS3Tech powered by twin Mercury Racing HP575s with Bravo XR drives. I grenaded the port drive when the lower driveshaft broke. We rebuilt the drive, installed new gears and replaced the lower case.

Later in the year, the clutch failed. This required another rebuild.

Now when I take it out, I lose a whole reservoir of gear lube. Twice, I have taken the drive off and pressurized it for days on end, with no apparent leak. There is no lube in the bilge and none in the bellows. And, there is no visible leak on the case or at the prop shaft.

As a precaution, I have replaced the upper O-rings. I'm afraid that I will suddenly lose all of the gear lube and blow the drive up again. Do you have any ideas?

Mark Kerber
Reno, Nev.

You have done a pretty good job of trying to find your leak. I can offer a couple of more checks and suggestions.

When you remove your drive from the transom assembly, there should be no oil at all in the shifting cavity. Many times, inexperienced technicians miss this because sometimes oil does leak from the drive lube line check valve momentarily during drive removal. The shift linkage cavity could hold up to a quart of oil or so.

There are a few ways that the oil could leak into the shifting cavity, including a leaking O-ring on the rear cover and leaky seals on the shift shaft.

I have seen cases where the Bravo shifting mechanism is relocated to a stringer below the level of the drive and oil from the shifting cavity has leaked through the inside of the intermediate cable sheath.

It is still possible that the oil is leaking out of the prop-shaft seals even though it does not leak during your pressure test. It is easy to inspect for this possibility. There are seals facing opposite ways on the prop shaft.

The one closest to the propeller is there to keep water out of the drive. The lip of the seal faces toward the water. The next seal forward is there to keep the oil in the drive. The lip of this seal faces toward the oil.

You can check for an oil leak by carefully pushing the lip of the outer seal away from the prop shaft with a small screwdriver.

There should be no oil in the space between the two seals. If there is, it is likely that you are losing oil past the prop shaft seal while under way. The rotating shaft and the influence of the propeller thrust may be enough to cause the leak that you are not seeing during your static test.

Finally, if your drives are the Mercury low water pickup design, it is possible that the oil is leaking into the water-pickup passage. The early version of the lower drive had an internal plate that divided the water passage from the area just in front of the prop shaft bearing. I have seen quite a few of these lower cases with oil leaking into the water passage.

Usually, it is detected when the boat is stored out of the water and eventually the oil starts running out of the water pickup and onto the floor. But many times the oil only leaks when the drive is hot and there is a positive pressure in the drive gear cases.

What Is Standard?
I am considering installing CMI headers on my 1997 Checkmate 253 Convincer powered with a big-block Chevy and a Bravo One drive. I have been asked by CMI if my boat has a "standard Bravo One setup."

How do I know if it is a standard or nonstandard setup? Are the CMI Elbow Tops a good choice for my boat?

Shaun Doyle Rock Falls, Ill.

I am not exactly sure how the criteria that I am about to provide you was originally formulated in our nonperfect world of boat manufacturing. With that said, at some point, someone, somewhere decided what the "standard" location would be for the centerline of the exhaust tips on a "standard" Mercury or MerCruiser engine installation using a Bravo drive with the engine mounted on the "standard" inner-transom plate.

I believe this applies to engines using the MerCruiser cast manifolds, and has evolved into the engines built by Mercury Racing that have used the CMI E-Tops, and now are provided with the CMI "Sweeper" headers.

CMI has "standard" tails available for their headers designed for "standard" Mercury Bravo installations that have the exhaust tips in the "standard" location. If your exhaust tips are not in, or close to the standard location, then custom tailpipes will be required to complete your installation unless you are able to relocate the tips on the transom.

Measurements are made relative to the engine crankshaft centerline. On the transom of your boat, the engine crankshaft centerline is in line with the center of your gimble ring. The center of the gimble ring can be identified by the location of the pivot pins on each side.

If you have a standard, non-high-performance transom assembly, the electric trim indicator and limit senders are located on top of the pivot pins. Once you have determined the center of the gimble ring, make a mark with a pencil on the transom directly forward of the pivot point on a horizontal plane. Then, make another mark on the transom with a pencil at the top of the center of the transom assembly to indicate the crankshaft centerline from side to side.

Next, measure from the centerline at the top of the transom out to the centerline of the exhaust tip to port, and then to starboard. On a single-engine boat, these are referred to as the "A" and "B" measurements. "A" is to port and "B" is to starboard. Then, draw a horizontal line on the transom at that point even with the gimble ring pivot pins.

Using a square, measure up the transom (flat on the transom angle) to the centerline of the exhaust tips. The port measurement is the "C" dimension and the starboard measurement is the "D" dimension.

So, what is "standard?" Standard is when measurements "A" and "B" are 13 5/8 inches, plus or minus a quarter-inch. Additionally, in order to be "standard," the "C" and "D" measurements must be 13 7/16 inches, plus or minus a quarter-inch.

If your exhaust-tip centerlines are compliant or close to these measurements, the "standard" tailpipes from CMI will work.

There are other variables. If your engine has a "tall-deck" block, the headers and tails will move out and up .400 inch. And if you are using aftermarket heads, most have the exhaust ports raised about a half-inch. If you have both, you can plan on the "standard" exhaust hole transom location moving out and up nearly an inch at about a 45-degree angle.

A version of the CMI E-Tops were standard on the Mercury Racing HP500EFI. They would be a good choice for your engines. The next step up would be the Sport Tubes from CMI. Both are a great choice.