Sailboat Speed Question: Can a Motor Increase Speed Beyond 10 Knots with Sails?

[DaveC426913]

As the boat speed increase a bow and stern wave are created and increase in height with a concomitant reduction in the water level between them. A sailboat has a greater percentage of buoyancy at mid ship where the beam is greatest with much less at the bow and stern. Thus relative to amount of the boat above the water level at the beam the bow and stern are more underwater. If the boat goes fast enough and has too little buoyancy at the bow you could submerge the bow. This usually doesn't happen under the boats own power but if it is being towed by a more powerful boat it has happened apparently.

Ah. I see.

 

[desertbike]

This isn't true at all, or boat racing would be a very boring sport. It's true that power required goes up pretty dramatically as you approach and exceed hull speed, but when you do, the boat definitely doesn't sit lower and deeper in the water. It starts to raise the nose instead, and depending on hull design, might even get up on full plane (at which point it can go almost arbitrarily fast if you have the power to do so).

You are only talking about specially designed racing sailboats. That can only be true on certain designs of sailboats, ULDB's (Ultra Light Displacement Boats) which are primarily off the wind sailboats with little in the way of keels and hulls designed specially to plane, they don't go to weather well at all, or multi-hulls, these days usually catamarans. Multi-hulls have come up with designs and systems which allow the boat to sail faster than "apparent wind" speed, and have incorporated foils to raise up out of water which is what you suggest. But the typical sailboat is a displacement hull and doesn't follow your premise. I've sailed well over hull speed in hurricanes unintentionally, and pushed a 60 ft boat deeper in the water in the process, increasing drag and load on the rest of the rigging and structure.

 

[desertbike]

As the boat speed increase a bow and stern wave are created and increase in height with a concomitant reduction in the water level between them. A sailboat has a greater percentage of buoyancy at mid ship where the beam is greatest with much less at the bow and stern. Thus relative to amount of the boat above the water level at the beam the bow and stern are more underwater. If the boat goes fast enough and has too little buoyancy at the bow you could submerge the bow. This usually doesn't happen under the boats own power but if it is being towed by a more powerful boat it has happened apparently.

As I stated in another post on the subject I've nearly sailed a boat under before in a hurricane down south, but we reduced sail to keep from plowing it under. I know of boats under tow being pulled too far past hull speed and being submerged, and have read historical accounts of it occurring during battles and hurricanes. It's not an issue with multi-hulls or special hull designs made to plane or surf. They have their own bad things which can happen when going too fast, but the engine (remember, this all started about the engine!) can help mitigate the pulsations of sail power and provide a better smoother sailing experience, but it's not going to let you get too far past hull speed.