Deep Vee Setup
December 1997 Roostertail ( USA model race boat newsletter )
BY JOHN FINCH
As Mono Director, it is my job to foster the growth of the class. To accomplish this goal, I will be clarifying design criteria and even letting out some of the secrets that put the top drivers in the winner's circle. I can't do it all in one article, so stay tuned throughout the year in this section of the newsletter.
As you know, if you have been reading your newsletters, the Rule Book section on Mono hull classifications has been cleaned up a bit in an attempt to better define a Mono hull. By definition, a Mono hull is a boat that has a continuous wetted surface while operating at racing speed. This means that it cannot have steps, sponsons, or any other appendage that keeps the model from having one continuous wetted surface. This includes the bottom profile when the boat banks to negotiate a turn. I know that the real boats have steps and wings, but IMPBA does not recognize these as legal features in the Mono class. There are modelers that feel threatened by these advancements, so these changes have yet to take place in IMPBA.
To be successful in the Mono class it helps to have an understanding of Mono hull design in it's purest state. By that I mean the understanding of weight, balance, aerodynamic and hydrodynamic drag, lifting area, planing area, and power to weight ratios. You see, a pure Mono is far more difficult to tune for all out potential than a three point Hydro. The key word is potential! When it comes to a beginner or intermediate level model the Mono is by far the easier to handle and trim than the Hydro. But again, that is because we have accepted the Mono as a heavy sluggish vehicle with no hope of going extremely fast.
On the other hand, as a modeler who has two Mono hulls that consistently run over 70 MPH, I know that we can do better. The problem with a 70 MPH Mono is that it wont finish a single heat in a race with five other models cranking up the wakes. So, it is a compromice of stability and speed that we must make as we put together a Mono hull that goes fast but still finishes races.
In todays heat racing scene, a 50 MPH Mono is capable of winning races in all four engine classes. The fact is that most Mono hulls clocked at big events are only running 45 to 48 MPH. They look like they are going faster, but that is only because they are not trimmed properly. The boats that look slower, because they are trimmed properly, are the ones that are going faster and winning the races. So, the first subject to be covered in this series will be that of running hardware on the boat and how it affects the trimming of the model.
Deep Vee models are the best type of Mono for heat racing. They handle traffic and rough water conditions very well. Vee angles ranging from 19 degrees to 23 degrees seem to work best for heat racing. Shallow vee boats are faster on calm water and deeper vbee boats work best in rough water.
If a model is designed well, it will work with almost any hardware setup. If a model is lacking in some areas, you can sometimes help the handling of the model by changing the hardware setup. Today I will cover trim tabs.
Trim tabs are those plates at the transom that are used to adjust the ride angle of the model. Usually, they are mounted on the transom about an inch up from the keel to as far out as to the chine line of the model. The chine line is the corner where the bottom of the hull meets the side of the hull. The chine line extends from the transom (back of boat) to the bow (front of boat). The tabs are usually split into two adjustable plates on each side of the hull which makes two separate adjustable tabs on each side of the model.
The innermost tabs are adjusted up and down to direct the bow of the boat either up or down. Adjusting the tabs downward will push the bow of the boat down as water leaving the trailing edge of the tabs causes lift at the transom, thus pushing down the bow. Raising the trailing edge of the tabs will reduce the lift at the transom and allow the bow to run lighter on the water. If the boat leans to the right because of engine torque, which it most probably will, the right or starboard tab is adjusted downward at the inside trailing edge, (edge closest to the prop), to shoot water downward off the right hand tab, which in turn will lifts the right side of the model. Keep in mind that lowering the right tab will not only lift the right side of the model, but also cause the bow to be pushed down as well. If the boat is leaning to the right and also running very wet, it would be better to raise the left inside tab (tab closest to the prop on the left side of the boat).
So, you are adjusting two things when you adjust the inside tabs: the hull attitude bow to stern and the ride left to right. Most hulls that are set up properly for heat racing are adjusted with the left tab slightly down and the right tab about 1/16 of an inch deeper than the left tab.
The outer tabs, close to the chine line, are used primarily for adjusting the ride thru the comers. The right outermost tab adjusts the ride for right hand turns and the left outermost tab adjusts left hand turns. If they are flat with the hull, the boat will turn a predetermined radius. If they are pushed downward, the model will turn a tighter corner 'because the water shooting off the tab lifts that corner of the hull at the transom, which in turn causes the keel at the bow to be forced into the water with more authority. You see, a Mono slides as it turns, and if the bow digs in, the boat turns sharper.
If you raise the outside tab, the boat will become less sensitive in the corners as the bow rides lighter on the water and slides through the turn.
Most models I have worked with are too sensitive in the corners, so the outside tabs have to be raised considerably to get good linear turns out of the model. Some boats work better without an outside tab, and some models, like the Seaducer, don't use tabs at all. The Seaducer type hull does not bank hard to negotiate a turn, so the tab scenario described does not apply to it or any other Mono that stands up in the corners. Now you have the knowledge it takes to trim tabs to your advantage. There are other things that can be done to trim your Mono too, so stay tuned for the next issue of MONO MANIA.
Because there's no such thing as a boat that's TOO fast!
March 1998 Roostertail
BY JOHN FINCH
We talked about trim tabs in the last issue of the newsletter and how they affected the ride of a monohull. In this issue of MONO MANIA, I will elaborate on the strut and how it affects the ride of the boat. We already learned that we could compensate for torque roll by bending the right inside trim tab downward to shoot water off the bottom of the right side of the model. The downward stream of high speed water lifts the right side of the boat and fights the natural need for the model to roll to the right as viewed from the transom. Torque roll can also be adjusted out of a model by positioning the strut off center or by raising the strut.
Monohulls tend to roll over to their right in a clockwise direction as viewed from the rear of the model. This is because the prop grabs water in a counter clockwise direction and the resulting action is that the engine, which is attached to the hull, is forced to rotate clockwise as the prop encounters the resistive force of the water.
To help fight torque, you can position the strut to the right of the keel. With the strut just a bit to the right of the keel, one eighth of an inch or so for example, the lifting action of the prop is distributed more to the right side of the model. The added lifting force on the right side of the model helps to deter the engines rotational force and reduces the amount of down trim tab adjustment that is needed on the fight inside tab for keeping the boat from leaning on it's side. That was a secret, so don't tell anyone!
The angle of the strut up and down will determine if the bow rides light or wet. Negative angle, which angles the prop thrust cone downward, will cause the transom to be lifted and the bow to be forced downward. Kicking the strut back, so that prop blast is pointed up will cause the bow to ride light on the water. I recommend that the strut angle always be perfectly parallel with the keel or within two degrees of parallel. The reason is this: When the boat leaves the water during a race, the tabs will have no affect on keeping the boat from blowing off the water. If the angle of the strut is positive, the prop may push the boat into blowing off the water. If strut angle is negative, the bow will be pushed into the water and the boat may submarine. If the strut is parallel with the keel, the prop will push the boat straight and level until it settles back into the water. Of course this is assuming that the model is balanced aerodynamically with the proper center of gravity.
Another variable is strut depth. The deeper the strut is in the water, the more the boat is lifted out of the water. As the strut is made more shallow, the boat is allowed to settle into the water. Surface drive struts are popular because they allow the model to settle into the water. This helps to get the trim tabs into the water to stop chine walk.
Now you know how to fight the chine walk problem. We will talk about prop walk in the next issue, so stay tuned and read the Roostertail from front to back. An informed model boater is a better model boater.
June 1998 Roostertail
BY JOHN FINCH
We talked about trim tabs in the first issue of the newsletter and then struts in the second issue. Now lets get into prop walk and add all three issues together for a smooth running Mono.
Prop walk is the action that causes the model to veer to the right when at full speed. This is caused by the propeller dragging the transom to the left. The transom is dragged to the left because the prop, rotating in counter clockwise direction, tends to walk like a paddle wheel in that direction.
As a visual aid to understanding propwalk, visualize the propeller spinning counter clockwise as viewed from the rear of the boat. Draw a line from the left to right through the middle of the prop. The line divides the propeller into two halves; an upper half and a lower half. Blade movement on the upper half goes right to left. Prop blades on the lower half travel left to right. On a surface drive model the lower half of propeller rotation has a strong bite in solid water. The upper half, however, is partially above the water and has less bite in the water, so the prop paddle wheels to the left. With a subsurface setup, the strut blade can cause a stream of air bubbles to interfere with good prop bite in the upper half of rotation, which allows propwalk to run wild.
COMPENSATING FOR PROP WALK
One way to compensate for prop walk is to bend the strut to the left, much like an outboard engine that is turned to the left to make the boat turn left. This method can cause other problems, though, so pay attention to detail. If your strut exits the boat dead center out of the transom, twisting the strut to the left will put the prop left of the centerline of the boat. The prop being to the left of the center of the boat may cause the boat to chine walks badly after the adjustment. Reason being, the prop lifts the left side of the boat more than the right side. With torque lifting the left side of the boat along with the prop being off center, the left side of the boat is lifted easily and can cause the boat to rock violently.
A BETTER SOLUTION
The biggest cause of propwalk in a Mono is the strut blade. When water hits the strut blade, it is separated and runs on both sides of the strut blade. The problem is that it doesn't immediately blend back together after it passes the trailing edge of the strut blade. A trail of broken water leaves the strut blade but does not become a solid water mass for some distance. As I said earlier, the upper half of the prop rotation is located directly behind the strut blade and rides in the disturbed water flow.
The solution is to extend the strut hub far enough behind the strut blade so that water has time to join back together before entering the prop blades. I find that an inch is usually enough. This works with surface drive models too, because the new prop location far behind the transom of the boat puts the prop deeper into clean water. While the extended prop shaft length does allow more water to hit the top half of prop rotation, and the prop becomes more submerged, propwalk is reduced and increased speed is the result.
An advantage is also seen in the extended prop shaft as the prop, when further from the transom, tends to keep the bow positioned without the need for radical trim tab adjustment. As the bow rises, the prop gets deeper in the water and pushes the bow back down into the water. That is top secret info!!!!! If you get the distance just right, it takes less trim tab to keep a fast boat on the water. I have found that three to four inches work best for me, depending on the model. If you get the prop too far behind the transom you will know it, because the boat will hop, especially in the corners.
In an earlier issue I said that I prefer a strut angle that is parallel to the keel, and that is a good benchmark for most boats, If however, you have a surface drive model that is really hauling the freight, some negative angle and a long strut hub will give you the control you need to drive in the 55 MPH range with full control.
A combination of slight strut twist and an elongated strut hub used together will sometimes give the best mix in compensation for prop walk. If you plan on twisting the strut, build the stuffing tube so that it exits just to the right of the centerline at the transom, so that when you bend the strut to the left, the prop ends up dead center on the centerline of the boat. Good luck in taming that wild beast we call a Mono!