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Tips & Techniques

High Performance Lower Unit Modifications

Having a CLE or a Sportmaster lower unit would be great but it may not be in the budget. Installing a nosecone and modifying the skeg can be accomplished by most prop shops, but if it is done incorrectly, it may break or fail at high speeds. This article will describe how we do it at R&R Prop Shop and why we believe it is a safe and viable option to high performance boaters.

The first thing that should be considered is the dynamics of the lower unit. Installing a nosecone has some benefits and some negative considerations. First the benefits: Increasing the length of the lower unit increases its aspect ratio (ratio of length to width) which allows the curve from the point to the back to be shallower or smoother. At high speeds, this evens out the impact that the bullet makes with the water and makes it less prone to gearcase blowout. Most nosecones incorporate a low water pickup which allows the gear case to be raised to the surface of the water without starving your engine for water. This in turn decreases the drag that the lower unit creates when it is dragged through the water, and less drag means higher speeds.

Here is the downside: making the lower unit longer increases its drag on the water, if it is held underwater, since now there is more surface area for the water to drag on. Thus, if the engine is not raised and a nose cone is applied, the speed could be slower. What we have done in this case is make the thing we are dragging through the water bigger, and that’s bad. The rule of thumb here is, if the lower unit cannot be raised such that the propshaft is within 2 inches height from the bottom of the boat, it may not be a good idea, at least from a speed standpoint. The other downside is the trim angle, if your setup requires lots of positive trim, dragging this lengthened bullet through the water at an angle not parallel with the direction of travel has increased your drag, not reduced it. The prop should be used to generate lift if the hull does not generate enough natural lift from hydrodynamics or aerodynamics. Setback may be used to re-balance the boat and this will change the lift as well. Over-trimming is generally a bad practice and usually points to problems with the setup or the prop selection. So before you consider a nose cone you should ask yourself these questions. 1) Is my boat and engine capable of speeds greater than 80 mph? If not, gearcase blowout shouldn’t be a factor and you might invest good money on something that will only slow the boat down. 2) Can your rig utilize a low water pickup and a surfacing propeller to good effect? That is, if the boat is too heavy or the prop cannot generate enough thrust or lift when raised out the water, this might be a bad idea. Imagine a big cabin cruiser with a small prop piercing the water on only half of its rotation --the prop slips on the water too much to generate thrust.

So we passed the test; we have a fast boat, a lightweight hull, a sturdy jackplate and a big motor. Now let’s make it go fast, safely! If Santa didn’t put a Sportmaster under your Christmas tree perhaps a call to Bob’s Machine Shop or your "gofast" outboard shop is in order. Our testing has proved that Bob’s makes one of the fastest nosecones, so that’s what we like to install. There are some considerations that make putting on a nosecone less than desirable. One of these is freezing water trapped in the recesses of your lower unit can damage the lower unit beyond repair. We fabricate "C" channels and apply them to the lower unit to create a tunnel from the nosecone to the normal water pickups (see photo) with the screens removed. This eliminates the rubber hoses, the drilled and tapped fittings, and the difficulty of finding clearance around the hoses for the bolts that attach the lower to the mid-section. In addition, no water can be trapped in a bypassed section of your lower unit to freeze. Once the pump stops pumping, water runs out of the (now covered) normal water pickups, through the c channel tunnel and out the low water pickup. The "C" channel is welded to the gearcase and fared in with filler to reduce its drag at non-planing speeds. Once moving, it comes out of the water and has little effect on the hydrodynamics of the gearcase.

The Torque Tab

Another downside of raising the lower unit is when the prop is raised with respect to the water surface it tends to "paddlewheel." That is, along with pushing or pulling your boat forward, the prop wants to walk on the water like a paddlewheel. This generates a rightward motion to the transom, which causes the boat to turn left. To compensate the driver must turn the wheel right in order to maintain a straight path. Now we have the undesirable effect of "crabbing" the gearcase when the boat travels in a straight line. This creates drag, just the thing we are trying to avoid. At high speeds it also tends to form a bubble on the "shadow" or port side of the gearcase. As the speed increases this bubble trails back further and further until it reaches the propeller. This causes the propeller to blow out or ventilate. Now the prop suddenly stops generating thrust and lift and the bow drops and the boat slows. This is known as gearcase blowout and is usually very dangerous because if one side of the bow catches the water before the other side (or if the steering wheel is not straight) the hull will "hook" and change ends violently. So how does one avoid this "ugliness"? The simple answer is to apply a torque tab to the skeg. This wedge, applied to the right side of the skeg, tends to apply a leftward force to the gearcase which should compensate for the propeller forcing the gearcase right. Now the gearcase doesn’t crab through the water, no bubble, no blowout, no hook and hopefully no accident. Remember when a hull flies the only thing other than the prop in the water is the skeg so it is the single thing left that gives the operator control of the boat. Like flying an airplane with only one small control surface, to say it is crucial is a vast understatement.

The rock and the hard spot

Remember that this whole exercise is about reducing drag and improving speeds. In order to lift the lower unit we have to mitigate the effects of propeller torque and gearcase blowout by applying a wedge and thickening our nice, smooth and thin skeg. Wait… we’re going the wrong way again? Wouldn’t it be better to thin the skeg down and decrease its depth and area? Technically, this might be the right approach but the sacrifice is great, the one crucial thing left to control the boat is now in danger of breaking off (bad things will follow) or not be large enough to control the boats’ path, more bad things…

Ideally, the skeg should be thin enough to create the minimum drag and thick enough to prevent breaking. On high performance skegs like the Mercury Sportmaster we leave the thickness in the middle and thin the trailing edge and sharpen the leading edge. This is accomplished by removing material from the port side, creating a "wing" shaped cross section. The right side is nearly flat until the rearward section, which flares at the torque tab. Like a wing with the aileron down, it creates "lift" in a horizontal leftward direction. The fluid dynamics of air going over an airplane wing is remarkably similar to water going over a skeg. The goal in each case is to create "lift" while generating the minimum amount of drag. On some skegs which have less profile area and no tab we add a pre-cast skeg with torque tab to the existing skeg. (See Photos) The old skeg is cut such that "teeth" marks are left in the remaining "nub." Now the new skeg is held behind the old and a tracing of the old skeg is made on the new one. The part of the new skeg that was shadowed is cut away on the band saw. The new piece is ground and fitted until it overlays the existing skeg. Both pieces are Vee-d or sharpened where the weld joint will be accomplished. This may sound like a lot of extra trouble but since the joint is not made in a straight line it is extremely resistant to fracturing. The pieces are vee-d to provide a strong weld after the new skeg is smoothed. We use a 5000 series aluminum-welding rod instead of the more common 4043 aluminum rod. Although more costly, it provides a stronger weld. The welding is done with a TIG machine and we take the time to assure that no porosity or voids remain in the finished weld. The skeg is filled with a minimum of filler material and block sanded to achieve a "factory like" finish. The finished product is purposely made larger and deeper than necessary. When the boat is tested with the desired prop and engine height the flare can easily be cut down until a minimum of wheel torque is generated at high speeds.

Is it faster or slower? A better question to ask: is it faster and safer? Taken as a whole, these modifications will almost always improve speed. The engine is jacked out of the water reducing gearcase drag. The skeg is modified to compensate for propeller torque and the reduction of blowout. The low-water pickup provides water at extremely high engine heights; in fact on most applications the water pressure is over 30 lbs. at high speeds on the Bob’s bigfoot nosecone. On some applications the outer two holes are plugged to decrease the drag even further. Is it safer? This is a hard question to answer. In most cases it would be extremely difficult to achieve speeds over 85 mph without such modifications and if one did, it might be dangerous at best. The prospect of traveling at 100 mph is inherently dangerous and could only be more so if one could not control the craft at these speeds. We feel if you are going fast, these modifications are not only important, but also necessary. At R&R Prop Shop we take your speed and safety seriously, just like you do.

Don’t forget your lifevest and helmet.
Kevin Taylor, R&R Prop Shop


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