Adding RAVI to a core strengthening pose like boat pose is a unique opportunity to target the strengthening effects. We are able to manipulate the center of mass (balance point) of the body by strategically adding weight. RAVI allows you to do this in a safe and effective way.

Setting up the pose:

Boat pose strengthens the core.  It is important to go at your own pace and maintain alignment.  Strength is increased over time by putting pressure on the muscles in the core.  Be mindful of when you feel the pressure is enough.  Repetition over time is desired.  Be mindful of too much, too soon to avoid straining the muscles or destabilizing the low back with incorrect alignment.  Once you find the correct level for you – maintain the pose for 5-10 breaths.
Begin seated on the ground with legs in front of you.  Bend the knees with feet flat on the floor and lean back holding on behind the knees with the hands to help support you.  Bring your awareness to your seat and engage your core.  Work towards a long spine and neutral pelvis as you move the work of holding the pose to the core .  Engage the core by pulling in the low back and low belly and up from the perineum towards the heart.  Watch out for rounding the back.  Keeping a long spine, transfer the work of holding the pose from your arms to your core muscles mindfully. 
 Beginner Level:
Hands are behind the knees.  Lift the feet off of the floor (knee is bent so that the shin is parallel with the floor + straight spine + core engaged.  Draw shoulders down, hugging the shoulder blades in to flat to the back of the lungs and press heart forward to widen across the chest.
As you feel stronger:  Hold less with the arms and more with the core.  Eventually gain the strength to let go of the knees and reach the hands forward.
Balance adjustment:  Keep the feet on the floor and lean back with arms forward maintain the straight spine / neutral pelvis.
 Adding the RAVI
The RAVI placement will change how you experience the pose, by adjusting your Center of Mass. 
Center of Mass is the balance point of the body,  It is determined by how weight is distributed. When you add weight – your are influencing the center of mass.

Navasana with RAVI Heavy Shins

Adding weight to the shins in this yoga pose lowers the center of mass.  You will feel your lower abdominal muscles work more to hold the shape, because your center of mass has shifted down towards the extra weight held on the shins

Navasana with RAVI Heavy Chest

 Adding weight to the upper torso in this yoga pose moves the center of mass higher in the body. You will feel your upper abdominal muscles engage more and your sense of alignment more keenly, because your center of mass has shifted higher up the body towards the extra weight held across the chest.
You will also need to optimize balance when you add weight to one of the levers.

Straight Leg Boat – Variation

Slide the Ravi down the shin and over the top of the foot and toes.  Holding on to the RAVI from the sides or the edge closest to you, extend the legs straight and balance. 
The closer the weight is to the center of mass the lighter it will feel.  The further the weight is held away from the center of mass the heavier it will feel.

Strategic manipulation

Using the weight of the RAVI to strategically manipulate how you experience Navasana – boat pose. Increase the strengthening effects by targeting upper and lower abs separately and increase awareness of foundation, balance and alignment.