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 variable 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.

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 involves is the curved skeg found on the Yamaha SHO 200hp through 250hp and the Evinrude G2 series engines. 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

Good news is on the horizon with regard to engine porp torque and hard steering. The same torque correction found in the SHO engines is also incorporated again in the new Evinrude G2 engines from 200 hp through 300 hp, and it is standard in all shaft lengths. The SHO engines are currently only available with this technology in the 20” shafts.

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 instance 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