Re: What causes a Boat to Slow down in Turns
I thought everyone knew that the earlier you go boating, the slower your boat goes in the turns!
And knowing the physics of how things work makes it much easier, cheaper, and more enjoyable to be able to predict how changes to your hull, engine, prop, etc, will affect your boat!
Seriously, it's probably easiest for most people to understand if it?s explained as a thrust vector in relation to the centerline of the boat. The amount of thrust remains constant, but the percentage going into moving the boat forward is proportional to how hard you are turning.
When your boat is going straight, 100% of its? power is contributing to forward motion, like in the picture below.
The portion of the power going into moving the boat forward is the cosine of the turning angle (and you never thought that geometry crap in high school would be useful!).
For example, if our boat is running straight (turn angle 0 degrees) with the propellor delivering 200HP into the water, then the amount of power going into moving the boat forward is 200HP x cosine(0) = 200HP x 1 = 200HP. So all the power is going into pushing the boat forward.
When you turn, only a part of the power is directed towards the bow in the direction the boat is moving. The remaining part is used to push the stern to the side, as in the pic below. As a general rule, the boat will rotate around it's CG (center of gravity).
If you want to make a 30 degree turn, then the thrust is directed at a 30 degree angle in relation to the forward motion of the boat. The amount of power going into moving the boat forward in our 200HP example would be 200HP x cosine(30) = 200HP x .866 = 173HP.
If you wanted to maintain the same forward velocity while turning, you would have to increase the throttle setting until the ?forward component? of your thrust vector was 200HP. For a 30 degree turn, that would require increasing the throttle until the engine was delivering 200HP / cosine(30) = 200HP / .866 = 231HP. If you were already at WOT at 200HP, then you?re gonna slow down, nothing you can do about it!
As you turn sharper, more of the power goes into the turn and less goes into forward motion. For example, a 70 degree turn would mean that only 200HP x cosine(70) = 200HP x .34 = 68HP would be going into moving the boat forward. In other words, you?re gonna drop a LOT of speed!
Obviously, if you want to pivot in place with no forward motion (like one of the Mercruiser or Volvo POD drives), the drive would be turned at 90 degrees. That would mean that the forward component of your thrust would be 200HP x cosine(90) = 200HP x 0 = 0HP going into forward motion. All power would be going into turning your boat.
A straight shaft and rudder works similarly, however it converts less of the total thrust into a turning component. Some percentage will always remain as a forward component due to the fixed shaft.
