Theoretical Boat Speed

[capn_billl]

Originally Posted by Dockhead

Well, actually, it is a law -- a rigid law of physics.

The thing is, however, that the law applies in different ways to boats with different hull forms, displacement, and power available. Some displacement hull boats can exceed hull speed fairly easily; for others it's like hitting a brick wall. Most are somewhere in between.

All the law says is that it takes a relatively speaking additional amount of force to exceed this speed in a pure dispacement hull with no hydrodynamic effects other than the speed of the bow and stern waves.

Few boats actually have their hull speed exactly at the hull speed due to other factors, I.E. changes in LWL as boat approaches hull speed, (designed or incidental), lift from hull curvature, or induced turbulence, ...

[BakerHansen]

I have sailed 8,5 knots on a 7,1 knot hullspeed... at 8,6 knots I broached.
Engine D1-30 will give me 7,2 knots wide open.

[LakeSuperior]

If your yacht is an older design having overhangs the theoretical hull speed will also depend on the amount of heel. Simply put, the length of the hull in the water increases with heel.

[BakerHansen]

Originally Posted by LakeSuperior

If your yacht is an older design having overhangs the theoretical hull speed will also depend on the amount of heel. Simply put, the length of the hull in the water increases with heel.

Sirius 310DS, so fairly new I`d say. Just massively overpowered

[capn_billl]

In short hull speed is more of a guideline than a rule.

[Patrigo]

I have read all the posts on hull speed, and scratch my head. Up to now I believed that at max hull speed the boat sits in a trough (created by the hull being forced through the water) with the wave crests at bow and stern. If it is under tow by a powerfull vessel, and exceeds hull speed, the trough deepens, and the bow and stern waves are now above bow and stern. There are several instances from history, of for example, sailing ships, loaded with grain from Australia, racing to be the first back to Europe and make more money than the next vessel. In a blow or storm, they were too late in shortening sail and went under. A total loss. Many years ago I was crew on a 40ft Alden Yawl and a friend with an 84ft steel ketch was towing us against a headwind. A few times we exceeded hull speed (when the ketch was on a down wave) and our hull shuddered, with water splashing over the cockpit coaming and foredeck. On the same trip I met a guy in a 28 footer, who had taken a tow from the US Coastguard. He cut the towline with an axe when they took of and were sinking him. The displacement hull makes the fore and aft waves, and cannot climb up the bow wave and leave the stern wave behind.It creates the waves. When below hull speed there can be several wave crests between bow and stern. This will create a bunch of arguments ! Lets go ! !

[capn_billl]

Tough to argue with observations.

And a deep keel is no way going to climb above the bow wave, and plane.

If we want to do a deep dive here, the bottom of the boat will create either lift, or suction, (negative lift), depending on shape.

If this suction exceeds the lift from buoyancy - total weight of the ship plus cargo, IE a heavily loaded grain ship probably with a flat bottom, then you will sink.

Some boats, especially modern production boats are designed like they expect to be powered by a big engine most of the time.

An older sailing ship, or one based on a an older design would not have this as a design consideration.

If I was a shipwright, designing the hulls of a new production motor/sailor, I would give it a wide flat stern with a dagger keel, and a gentle slope to the rear so when it squatted under full throttle it would have an angled surface to create lift, keep the bow high, and as speed approached hull speed the mass of water displaced would lift it on plane...

Pretty much like nearly every modern sailboat, I've looked at.

[sailingabe41ds]

I owned a 2004 H36 for 13 years and between
7.4 to 7.5 is about the right number.

Abe