I have "borrowed" this from another site to help others including myself as I know I'll refer back to it many times.
RC Boat Hull Attitude
What we call attitude is the way a R/C boat behaves on water. This page explains the theory to help get the best performance out of your R/C Model Boat. Theoretically, when moving a perfect boat should:
This is the theory. In practice, things happen in a slightly different way. There is not such thing as an ideal boat and so, we must adjust it to make it behave as close to this ideal as possible. Let's exam each on of those items and discuss how to correct "bad habits".
Before any adjustment, it's important to have in mind that a boat on the water is in equilibrium over two lines. One of them cuts it from bow to transom and other transverses from port to starboard, at the boat's Center of Gravity - CG. This understanding is important to make it clear that all adjustments are complexes and seldom produce a single effect. For instance, lifting the transom makes the bow go down into water as a consequence. Lifting starboard makes the port side deep into water. To extend the prop far from the transom to reduce prop walk may result in porpoising. In other words, whatever you do in one side induces a counter-reaction on the opposite side; sometimes correcting a problem causes another.
Center of Balance - CB: line that cuts the boat in two halves from bow to transom. Movements from starboard to port or v.v. occur alongside this line.
Center of Gravity - CG: point of equilibrium of the boat, between bow and transom. Bow to transom movements occur over a shaft that runs through this point.
The higher over the CB the Center of Gravity, the higher the tendency for the boat to lean on turns. Higher/heavier engines tends to raise the CG - the farther from the keel the CG, more unstable the boat will be. Compare the race cars with their street cousins: lower=more stable.
Wet area: the smaller the portion of the boat in contact with the water, the higher the speed. This is pretty obvious because the drag of the hull is proportional to the area in contact with the water. The limit is attained when the wet area is so small that we lost the control of the boat.
How to correct the attitude of the boat
Center of Gravity: The chapter How to Assemble a R/C Boat, in the Technical session, shows how to calculate and adjust the CG. It's easy to understand that a CG towards the front tends to maintain the bow down into water, increasing the wet area and hampering the planning of the boat when at speed. On the other side, a CG too close to the transom tends to raise the bow too much, making the boat unstable at speed. As a rule of thumb, CG must be between 27% and 30% of the total length of the boat, measured from transom, for a completely assembled boat - ready to run except for the fuel. As very powerful engines tend to raise the boat's bow, under those circumstances a CG over the upper limit (30%) may be advisable. On the other hand, mild engines will be happy with the 27% rule.
Strut angle: A strut has a positive angle when the prop shaft points upwards, far from the water surface. In this situation, the transom is pressed down. A negative angle occurs when the prop shaft points down, towards the water surface, raising the transom. If you imagine the boat supported on a transverse shaft that goes exactly through the CG, it's easy understand that, when the transom is lowered the bow goes up, reducing the wet area. The contrary occurs with a negative angle for the strut. Although apparently we could use the strut angle to correct any incorrect attitude, the truth is that the neutral angle is the ideal, just a few degrees off are allowed to adjust the wet area. If more than 2 degrees are necessary, something else should be wrong.
(from a original design by Ron Frank)
Trim tabs: Normally 2 are used on each side of the boat. The outer ones take care of the behavior of the boat on turns, the inner ones of the attitude when traveling in a straight line. We will take care of those from now. Increasing the trim tabs angles - orienting down the adjustable portion - raises the transom, dropping the bow - the boat increases the wet area. Inversely, reducing the trim tabs angle - directing up the adjustable component - lowers the transom and raises the bow, reducing the wet area.
Outer trim tabs act on turns. Remember, adjusting them to rise the transom makes the nose dig on turns, making the turns eventually sharper than desired.
Once again, the degree of possible actuation is limited - if correcting the attitude demands more than few degrees of actuation, we must look in another place the solution of the problem.
Hooks and Rockers: Hook is a concave surface on the bottom of the hull, near the stern, that acts like a big trim tab and raises the transom - lowering the bow and increasing the wet area. Rocker is exactly the opposite - a convex area near the stern, that lowers the transom and raises the bow.
Some manufactures claim that those surfaces are made on purpose on their hulls, to determinate the attitude of the boat on water. I don't like the solution, I prefer to believe that both are defects that must be corrected. The attitude of the boat must be determined by devices that allow adjustments and not by this way.
The correction is very complicate. The rocker must sanded until be level with the bottom of the hull, the hook must be filled with and sanded.
Both corrections are complex and time consuming, reason why I suggest you to avoid a hull with any of those characteristics.
RC Boat Hull Attitude
What we call attitude is the way a R/C boat behaves on water. This page explains the theory to help get the best performance out of your R/C Model Boat. Theoretically, when moving a perfect boat should:
- have a reduced wet area: just a minimum part of the hull touching the water
- not have prop walk: tendency to turn right even with the rudder on a neutral position
- have no tendency to chine walk: side to side movements when traveling in a straight line
- have no tendency to porposing: bow oscillation, up and down when traveling in a straight line
- have no tendency to torque roll: rolling to the starboard as a reaction to the engine torque
- be stable: not slip too much and not roll on turns.
This is the theory. In practice, things happen in a slightly different way. There is not such thing as an ideal boat and so, we must adjust it to make it behave as close to this ideal as possible. Let's exam each on of those items and discuss how to correct "bad habits".
Before any adjustment, it's important to have in mind that a boat on the water is in equilibrium over two lines. One of them cuts it from bow to transom and other transverses from port to starboard, at the boat's Center of Gravity - CG. This understanding is important to make it clear that all adjustments are complexes and seldom produce a single effect. For instance, lifting the transom makes the bow go down into water as a consequence. Lifting starboard makes the port side deep into water. To extend the prop far from the transom to reduce prop walk may result in porpoising. In other words, whatever you do in one side induces a counter-reaction on the opposite side; sometimes correcting a problem causes another.
Center of Balance - CB: line that cuts the boat in two halves from bow to transom. Movements from starboard to port or v.v. occur alongside this line.
Center of Gravity - CG: point of equilibrium of the boat, between bow and transom. Bow to transom movements occur over a shaft that runs through this point.
The higher over the CB the Center of Gravity, the higher the tendency for the boat to lean on turns. Higher/heavier engines tends to raise the CG - the farther from the keel the CG, more unstable the boat will be. Compare the race cars with their street cousins: lower=more stable.
Wet area: the smaller the portion of the boat in contact with the water, the higher the speed. This is pretty obvious because the drag of the hull is proportional to the area in contact with the water. The limit is attained when the wet area is so small that we lost the control of the boat.
How to correct the attitude of the boat
Center of Gravity: The chapter How to Assemble a R/C Boat, in the Technical session, shows how to calculate and adjust the CG. It's easy to understand that a CG towards the front tends to maintain the bow down into water, increasing the wet area and hampering the planning of the boat when at speed. On the other side, a CG too close to the transom tends to raise the bow too much, making the boat unstable at speed. As a rule of thumb, CG must be between 27% and 30% of the total length of the boat, measured from transom, for a completely assembled boat - ready to run except for the fuel. As very powerful engines tend to raise the boat's bow, under those circumstances a CG over the upper limit (30%) may be advisable. On the other hand, mild engines will be happy with the 27% rule.
Strut angle: A strut has a positive angle when the prop shaft points upwards, far from the water surface. In this situation, the transom is pressed down. A negative angle occurs when the prop shaft points down, towards the water surface, raising the transom. If you imagine the boat supported on a transverse shaft that goes exactly through the CG, it's easy understand that, when the transom is lowered the bow goes up, reducing the wet area. The contrary occurs with a negative angle for the strut. Although apparently we could use the strut angle to correct any incorrect attitude, the truth is that the neutral angle is the ideal, just a few degrees off are allowed to adjust the wet area. If more than 2 degrees are necessary, something else should be wrong.
(from a original design by Ron Frank)
Trim tabs: Normally 2 are used on each side of the boat. The outer ones take care of the behavior of the boat on turns, the inner ones of the attitude when traveling in a straight line. We will take care of those from now. Increasing the trim tabs angles - orienting down the adjustable portion - raises the transom, dropping the bow - the boat increases the wet area. Inversely, reducing the trim tabs angle - directing up the adjustable component - lowers the transom and raises the bow, reducing the wet area.
Outer trim tabs act on turns. Remember, adjusting them to rise the transom makes the nose dig on turns, making the turns eventually sharper than desired.
Once again, the degree of possible actuation is limited - if correcting the attitude demands more than few degrees of actuation, we must look in another place the solution of the problem.
Hooks and Rockers: Hook is a concave surface on the bottom of the hull, near the stern, that acts like a big trim tab and raises the transom - lowering the bow and increasing the wet area. Rocker is exactly the opposite - a convex area near the stern, that lowers the transom and raises the bow.
Some manufactures claim that those surfaces are made on purpose on their hulls, to determinate the attitude of the boat on water. I don't like the solution, I prefer to believe that both are defects that must be corrected. The attitude of the boat must be determined by devices that allow adjustments and not by this way.
The correction is very complicate. The rocker must sanded until be level with the bottom of the hull, the hook must be filled with and sanded.
Both corrections are complex and time consuming, reason why I suggest you to avoid a hull with any of those characteristics.