Jack Plates and Steering Torque

 

Steering effort and jack plates:

 

Now and then we have a case where a new owner of a bay boat has a problem with the steering. This is almost always caused by the torque created by the propeller which is neither a boat nor a steering system problem, but because it is so often misunderstood I thought it a good idea to explain how and why this is happening and what can be done to help cure the problem.

First I’d like to comment that steering problems where the steering effort exceeds expectations have been growing steadily in the last few years. In part this is attributable to the consumers themselves. If you think of it, most of them have never driven a vehicle of any kind without power steering. The truck or car they drove to the dealership had power steering and usually every other vehicle in their lives. Then they drive a boat which has of course an outboard motor that can operate at different speeds, trim angles, engine heights, and can use an all but infinite number of varying props. All of these variables can have an effect on steering torque, and the only method of correcting that steering torque is usually the torque tab on the engine’s anti ventilation plate.

I’ve told people for many years there is only one speed, trim angle, engine height, etc where the torque tab can neutralize the steering effort, if any, and all other engine speeds and trim angles will result in some steering torque. The degree of that torque varies tremendously depending on the application going from barely perceptible to impossible to overcome. It is important to remember that all of this torque is created by the propeller rotating through the water. Not the engine, the steering system itself, or least of all the boat. The boat is nothing more than an inert piece of material to which the other parts are attached in this situation.

Notice I said only one set of variables can be neutralized by the torque tab, if any. There are cases, and that number is growing, where the tab simply cannot do that job. Why? Look at the typical tab on let’s say a 50 hp engine. Then compare the size of that tab to the one on a 250 hp engine. Even though there is five times as much power spinning a much larger propeller the tabs are nearly identical in size. If it’s just right on the 50 hp, it’s way too small on the 250 hp.

Another cause of increased steering problems caused by prop torque is the props themselves. In the last ten years I’ve noticed a trend to larger diameter props. Often this results in more efficiency especially at mid range speeds, but it also contributes to the steering torque transmitted from spinning it through the water. This increased prop torque has not been met with increased torque correction in most cases, and I fault the engine builders for that oversight. The technology is there and has been for many years to do so. Going way back to the mid to late 70’s when bass boats were very popular and faster speeds were needed to get to the good fishing spot first the owners and dealers started experimenting with elevated engine heights via jack plates. This resulted in the same increased prop torque we have now as a result of taking the torque tab out of the flow of water, as illustrated in these rough drawings.

 

 

 

 

 

 

 

 

So why does this elevated position result in more prop torque? It’s really simple physics. One of Newton’s laws regards the natural equal and opposite reaction with regard to a moving object. The prop spins to the right, twisting the engine to the left, resulting in a pull to the right at the steering wheel. When the prop is completely immersed in the water the blade on top partially counters the blade on the bottom, but not quite because it runs behind the engine’s gear case where the water flow is disturbed and the blade doesn’t get quite the bite as the blade on the bottom in undisturbed water.  As the prop is raised the blade on top begins to surface dramatically reducing the drag on that blade and its ability to counter the twisting force of the blade on the bottom. At the same time the torque tab is clearing the flow of water coming from under the boat and it eventually has no effect at all in countering steering torque.

 

The cure for this is illustrated in the bottom right corner of the second drawing, and it’s not very clear, but what it involves is the curved skeg found on the Yamaha SHO 200hp through 250hp. I first saw this used on the Evinrude Rude Ram in 2000 on what they called their Lightning Gearcase. These high speed gear cases not only include this very effective torque correction in the skeg, but also utilize a low water pickup for the elevated running positions. This is to keep a constant water flow going to the water pump to cool the engine.

After market companies such as TH Marine and Bob’s Machine also offer an add on wedge to be attached to the lower skeg of engines not built this way. These simple and inexpensive devices go back to the early bass boat days and are still inexpensive and effective today. If you have an engine mounted on a jack plate allowing the engine to be raised on the transom, whether it be fixed or hydraulic, I highly recommend these devices if you do not have a SHO engine on the boat.  Info at this link. http://www.thmarine.com/products/Outboard-Performance/Outboard-Torque-Tabs  and pictures/ pricing at this link http://www.amazon.com/T-H-Marine-Torque-Tab/dp/B0000AXV99/ref=sr_1_fkmr0_1?ie=UTF8&qid=1410549736&sr=8-1-fkmr0&keywords=the+marine+outboard+torque+tab

 

Another problem with steering torque is the confusion floating around hydraulic steering. People often confuse hydraulic for power steering. While hydraulic steering can offer less resistance to steering effort than mechanical it is done through the number of steering wheel turns required to move the engine the same distance as compared to the mechanical system. For example the typical mechanical set up requires around 2.7 turns to go from hard right to hard left. The average hydraulic system requires around 5 turns to do the same thing, and some of them as many as 7 turns. Like changing gears on a winch,  more turns equals less effort to turn the wheel. But even though hydraulic may be easier than mechanical it does nothing to remove the torque from the prop, it only serves to help the driver overcome that torque. As the torque increases from one or more of the aforementioned variables it comes to a point where the operator of the boat is stressed to steer and often times they now assume something is wrong with the steering, or the boat.

Power assist steering can help overcome this complaint. Remember it is an option on most boats equipped with hydraulic steering but it should be offered before the complaint whenever possible. It is up to the dealer to anticipate this need depending on the set up being sold. I personally don’t like this solution since it does not relieve the pressure created by prop torque, it only overcomes it. In extreme cases such as an elevated running height where the top of the prop as well as the torque tab are out of the water flow the torque can be tremendous along with very high hydraulic pressures inside the steering cylinder, hoses, and the helm. If something breaks in the steering it can result in catastrophic results including loss of property and life. Because of this I always recommend that steps be taken to reduce the torque before steps are taken to overcome it.

I hope this dissertation is helpful and can help us all to keep the new owners happy with their Key West Boats. Remember this is not an issue isolated to Key West Boats in any way, but part of high speed recreational boating for many years.  While this article was written primarily for bay boats equipped with hydraulic jack plates many of the facts covered here with regard to prop torque and exaggerated steering effort apply to many other applications as well.

 

Thanks

Tom Marlowe

Key West Boats Inc.

                                            What causes a boat to list?

 

What causes a boat to list? There was a time years ago when hardly anyone paid attention to their boat listing. Today it’s a common question, and often perceived as a problem with the boat. Keep in mind a boat can experience a list at rest, while running or both, but the list of possible causes is really short:

 

Weight is distributed off center.

Prop torque.

A hull anomaly.

A trim tab issue.

 

Taking these in order, weight distribution is the most common cause and easiest to cure. Move something; whether it be a battery or your big buddy, if it can be moved to level the boat out, why not?

 

Prop torque is the next most common cause and possibly the least understood, but it is only possible in a running list and most prominent in a single engine application. Simply put it’s one of Newton’s laws of motion. For every action there is an opposite and equal reaction. Most props spin to the right, or when viewed from the rear they spin clockwise. The “reaction” is the twisting motion the engine places on the boat’s hull to the left, or in nautical terms it creates a port list. Many years ago most small boats were a side console set up with the operator’s position on the starboard side. This was to allow the captain’s weight to offset the prop torque. They also would typically locate the battery on the starboard side, since the captain will usually have some passengers that offset his weight.

 

There is a trend in the last few years towards more centered console designs, and that orientation eliminates the ability to offset prop torque created list with the operator’s weight. The battery can be located to the starboard side, but with the console centered, and a great location for the battery, that weight is often centered as well. This leaves us with nothing much to counter the prop torque other than stored gear. If you can locate the heavy gear off centered to the starboard side, do so. Or, if the boat is equipped with trim tabs the tabs can be used to level the running attitude if necessary.

 

On the subject of a running list it’s worth noting that engine trim angle and speed can have a surprising effect on the tendency for the boat to list while underway. Hydrodynamics can be confusing at best, but simply put the engine’s thrust angle can force the bow lower or higher in the water. Since the bow is usually the sharper angle of entry to cut through the water, that part of the hull is less stable, and less able to resist the twisting force of prop torque to the port side. Trimming the engine out results in the bow running higher which shifts the lifting force of the water to the stern which is the wider and usually flatter part of the hull which offers more stability, and the running list can be reduced.

 

Speed affects the running list as well. Again it’s hydrodynamics at play, but at higher speeds the water becomes more like a solid in its ability to support the weight of the boat. Therefore as speed increases the effect of the prop torque will often diminish as the lift or support of the water increases. Once all of the forces at work on a hull are better understood it becomes easier to accept that a perfectly balanced boat running level at all times is very rare in real life, and often it comes from an operator who understands what is happening and how to balance the boat with the controls at his disposal. For those acutely bothered by a running list trim tabs and proper use of the tabs are a great asset.

 

A hull anomaly? This could be something as simple as a transducer, or as unusual as a misshaped runnin surface. I’d like to cover the transducer possibility first since it is so common. Transducers for recreational use have historically been relatively small and flat on the bottom, but that has changed dramatically in recent years and some of them are huge, while practically none are flat. A flat transducer can be mounted flush to the bottom of the hull where they have very little effect on the running attitude of the boat, but those with rounded bottoms are often mounted below the running surface to some degree. If they are angled down this creates lift, and the bigger the transducer the more lift it can create. Uneven lift from one side or the other of the boat creates a running list. Unfortunately for these transducers to deliver the side scan images, they must be underwater, and incorrect transducer installation is one of the problems I most often see.  

 

Moving on to a misshaped running surface. This can be distortion caused by the pressure of sitting on a poorly adjusted trailer or lift bunk, or is less common cases, flotation foam that over expands or shrinks. Whatever the cause, if there is distortion in the hull it changes the pressure the water running over it creates. More pressure on one side or the other can create a running list. Some hulls will have a hook or wedge designed into them, so not all irregularities are necessarily a problem.

 

And finally a trim tab issue. Tabs are installed by real people and sometimes human error will result in a tab installation issue causing a running list. A slight difference in angle on the tabs when fully retracted will result in more lift on one side than the other. With everything else balanced this results in a running list. I check them with the tabs fully retracted and look for a difference from one side to the other. That being stated, it is not uncommon to install the port side slightly lower than the starboard to offset the prop torque. It is a legitimate and useful tool to combat the list.

 

I’ll wrap this up with a short commentary. Recreational boats have developed over the years into some very impressive pieces of machinery, and are much more complex than they were years ago. Of course they are much more expensive as a result, and with the increased cost the consumers are more demanding than ever and deservedly so. Water sports are a great way for an active family to enjoy being outdoors, but while water is great and absolutely necessary in our lives, it is still an imperfect means of support. As a result, almost every boat will list to some degree. Sometimes we just need to embrace it.

 

 

 

 

Fuel Supply Diagnostics:

A common problem as boats and engines age is fuel delivery from a built in tank to the engine. A popular test to determine if it’s the fuel system in the boat or the engine is to run the engine on a portable tank. The logic is that if it runs good on the portable tank, the problem is in the boat fuel system.

The 6 gallon tank test is helpful but it’s not conclusive. It’s easier to pull fuel from a tank sitting in the boat than it is from a tank under the floor because of less lift, and the tank in the boat likely has an anti siphon valve in the fuel barb on the tank itself. The anti siphon puts a bit of vacuum in the line to prevent a leak from emptying the tank into the bilge, but that resistance can overcome a borderline fuel pump on the engine. A portable tank doesn’t have this valve so it can occasionally mask a fuel pump problem.

The most reliable test of fuel delivery from a built in tank is with a fuel vacuum gauge and a piece of clear plastic fuel line leading to the gauge. With this installed between the tank and the engine you can easily determine exactly how much vacuum is needed to pull the fuel from the tank to the engine. Most engines tolerate as much as 3.5 inches of mercury vacuum without a problem. Once the vacuum gets over 4 we can expect problems. I’ve seen fuel actually vaporize at that much vacuum, and that’s where the clear plastic fuel line comes into play. Placing that line on the supply side of the gauge allows bubbles in the fuel to be easily seen. Bubbles can represent an air leak, and in extreme cases you can see as the fuel vaporizes inside the line from extreme vacuum. Just as water will boil at lower temperatures with higher altitudes where pressure is decreased, so does fuel. High ambient temperatures and alcohol content in the fuel increases the chance of this vaporization inside the fuel lines.

If you see a vacuum reading of over 3.5 it’s time to find out why the fuel supply is restricting the fuel flow to the engine so much. A clogged filter is obviously a concern and easy enough to bypass in most cases. Filters used in boats are usually high capacity and not often the problem, but be sure. Beyond that a restriction can be a pinched or collapsed line, but the more likely cause is either the anti siphon valve or the fuel pick up tube in the tank.

As mentioned earlier the anti siphon intentionally ads a bit of resistance to the fuel flow but occasionally they work too good or get sticky and need to be replaced. The amount of resistance depends on the depth of the tank with deeper tanks requiring more resistance. A good average would be something in the 2” mercury range.

 Most DIY folks do not have the fuel vacuum gauge, and many techs do not have one either but checking the vacuum needed to pull fuel from the tank is the only dependable method of determining the source of the problem. Short of that, I’d start with that anti siphon valve and replace it temporarily with a straight fuel barb to take that resistance out of the system. If it works like that many are tempted to just leave it out, but it is an integral part of the safety features in the fuel system and should be there.

The fuel pick up line in the tank is most often a stiff pipe, nylon and aluminum are the most common materials, and many of them have a wire mess on the bottom end to prevent particles from interfering with the anti siphon valve. Particulate that reaches the valve could hold it open negating the anti siphon function. Obviously trash shouldn’t be in the tank but things happen and it doesn’t take much to clog this screen. Most pick up tubes screw in a fitting on the top of the tank. If corrosion isn’t an issue it’s easy enough to remove the tube and inspect it. Check that screen to be sure it’s clean, and check the tube for cracks that can leak air. If you find trash on the screen it’s a good idea to clean that tank out completely before wrapping up this project.