Hull speed.

[Allan]

I have a Westerly 33 bilge keel with a waterline length of 28' 5" Can someone tell be the theoretical hull speed?
Allan

 

[Salty John]

7.14 knots.

 

[Allan]

Many thanks, can you remind me how you work that out?
Allan

 

[sailorman]

in your dreams 6.5kts would be nearer the mark.

 

[FullCircle]

There is a simple calculator here.....


http://www.sailingusa.info/cal__hull_speed.htm

 

[Skyva_2]

1.4 x sq root of waterline length.

But it is theoretical, boats can go faster if the hull lifts, even without planing.

 

[sailorman]

And Slower /forums/images/graemlins/wink.gif
a westerly 33 will never attain 1.4 sq root /forums/images/graemlins/cool.gif

 

[Allan]

Many thanks, that certainly works as the theoretical figure.
Allan

 

[Allan]

Can you direct me to where you got that figure please?
Allan

 

[William_H]

You must remember that theoretical hull speed is just a point on a graph of power versus speed where the power need to increase speed becomes increasingly large. (steep curve)
A heavier boat will have a steeper curve at around this point while a narrow boat (catamaran) a flat bottomed or light will have a less pronounced steepness to the curve. It will always be possible to exceed the hull speed just very unlikely in some boats.

A friend claimed (and I believe him) to have achieved 11.3 knots on GPS on Sat afternoon in a 24ft Farr. It was near running with small spin up reef in the main and a following sea. (no tidal assistance)

Theoretical hull speed is a good number to keep in mind in that around that speed you are wasting effort trying to go faster so think about comfort or if on the wind trying to point higher. olewill

 

[Refueler]

My 25ft Motor-Sailer Tub hits 9kts with engine ... flat out she's just over.

Cruising economy setting she hits 5kts average.

All witnessed.

 

[sailorman]

i have 31ft w.l 37ft o/all (one ton rated boat ) with 37.5 hp 7ton displacement max under engine with a clean bottom 7.5Kts.
no Westerly 33 will get near that /forums/images/graemlins/wink.gif. and with bilge keels??

 

[Noddy]

I have an Iroquois Catamaran. With two relatively long thin hulls (27 ft WL) it easily exceeds the theoretical hull speed. (6.96 kts)

I havn't done any carefully measured analysis but it seems to me that there are steps in the water resistance. There is the expected one at 7 kts, another at about 9 or 10 kts and another at 12 kts.

They tell me that it all gets very wild above 12 kts and I have not done that for sustained periods.

Any thoughts on this?

 

[Salty John]

The theoretical hull speed formula applies to displacement boats. The theoretical limit to speed is the length of the bow wave; once it runs from bow to stern the only way the boat can go faster is to ride over its own bow wave which takes a huge increase in energy. Planing boats such as most catamarans don't have this problem because they, in a way, sail on the water not in it. If you heavily load a catamaran it becomes in effect a displacement hull and the rule starts to apply - hence you shouldn't overload a catamaran.
Monohulls will appear to break the rule when they heel because in many hull shapes this leads to an increase their efffective waterline length.

 

[AngusMcDoon]

[ QUOTE ]
I have an Iroquois Catamaran. With two relatively long thin hulls (27 ft WL) it easily exceeds the theoretical hull speed. (6.96 kts)

[/ QUOTE ]

The theotetical hull speed is just a point on the speed/resistance curve where the curve starts getting steep, as described by olewill above.

That's one limitation to the formula. Another limitation to the formula is it applies to hulls that have a waterline length (lwl) to waterline beam (bwl) ratio of approximately 3:1. This fits approximately for most monohull yots.

The formula doesn't apply to multihulls, because the lwl to bwl ratio is vastly different. Therefore that's why you easily exceed the 6.96 speed you calculated - the formula doesn't apply at all to your hull shape.

My Dragonfly has a lwl:bwl ratio of 10:1 for the main hull, and 18:1 for the floats. It regularly exceeds 15 knots, I have taken it to nearly 19, and the nutters can get to 22.

 

[Tranona]

You seem to be confusing a rule of thumb theory with practice. The "theoretical" max on your boat is 7.79. You are probably not reaching that because you do not have enough power. As others have pointed out it is the power required to make the hull get over the bow wave that is the constraint. The bow wave is a function of waterline length, which in itself is a variable - ie it is not necessarily the same as the static waterline length. So the theoretical maximum hull speed of a Westerly is the same as any other similar hull with the same waterline length. Whether it achieves it or not is a function of propulsion power, either sail or engine..

 

[sailorman]

i have pwr aplenty, enough to bury the transom 12" /forums/images/graemlins/wink.gif

 

[davidrainsbury]

Theoretical hull speed only applies in flat water. If he had a following sea he was probably surfing.

 

[lw395]

Hull speed is a pretty vague term. It's usually either taken to mean square root of twice lwl, or some point at which a particular hull's resistance goes through the roof, the speed at which a hull will sit running in 20kts of breeze, without surfing.
By this definition, the hull speed of the same hull can be improved by removing weight. It's not a very useful concept, except for simple purposes like estimating the speed around a course for different size boats.

 

[Tranona]

Therefore increasing the waterline length and therefore the theoretical hull speed! All you are doing is illustrating that practice is different - the rule of thumb is there, but as many have pointed out the power required to increase speed at that point is substantial.

What your figures do say is that, assuming you are running the engine at maximum power and revs to achieve that speed it is propped correctly.