What is "Hull Speed" Anyway

[David M]

There is another way of thinking about this. Instead of a mathematical formula, it is when you have a bow wave, a stern wave and a trough in between. This makes it not an exact speed.

[Pete7]

Interesting picture of a bow wave that doesn't reach the stern so less than full displacement speed. The curved hull and counter stern won't help planning.

[Pete7]

The picture came from an interesting series of videos for anyone with some time. They feature the start of the 1968 OSTAR.

https://www.youtube.com/watch?v=RQbEMquxCRI

[Pete7]

Originally Posted by David M

There is another way of thinking about this. Instead of a mathematical formula, it is when you have a bow wave, a stern wave and a trough in between. This makes it not an exact speed.

This ^^^^^ is perhaps the easiest way to see what is going on by just looking over the side.

The is the PK45 featured earlier which shows the bow wave well aft of the stern so into semi planning mode. If it could go faster the stern wave will disappear as the hull planes and the hull is supported in a dynamic form.

Pete

[boatman61]

Face it guys.. hull speed is/was a calculation based on more traditional boats where each knot over pushed the bows downwards by maybe a foot..
The saucer shaped hulls of today largely put this calculation to bed.. my Beneteau's have come nowhere near the potential pitchpole point I have achieved in more traditionally designed boats I have owned.. bar bilge keelers which share this shape.
So.. its a pretty null discussion as 90% of folk own the shallow bilge fin keelers which kinda knock the hull speed calculations off the counter..

[IdoraKeeper]

Originally Posted by boatman61

Face it guys.. hull speed is/was a calculation based on more traditional boats where each knot over pushed the bows downwards by maybe a foot..
The saucer shaped hulls of today largely put this calculation to bed.. my Beneteau's have come nowhere near the potential pitchpole point I have achieved in more traditionally designed boats I have owned.. bar bilge keelers which share this shape.
So.. its a pretty null discussion as 90% of folk own the shallow bilge fin keelers which kinda knock the hull speed calculations off the counter..

Have had similar experience to your first post. Needing to de power the Yankee and mizzen with each gust and in time with each swell as the bow wave threatened the come on deck. Each surge forward showing 11kts on the log. Near as I would care to be to driving her under. Not a hint of surfing from an Ingrid.

[IdoraKeeper]

Merry Christmas to all you boatnics dreaming of big bow waves and shivering tillers.

[jtsailjt]

Originally Posted by boatman61

Face it guys.. hull speed is/was a calculation based on more traditional boats where each knot over pushed the bows downwards by maybe a foot..

:

Though it appears that way from aboard, if anything it’s the stern that’s descended into the trough of the bow wave as the bow rises slightly in its unsuccessful attempt to climb over the bow wave. A planing hull will be successful. The reason you see the bow apparently being “pushed down” is that the bow wave greatly increases in height as hull speed is approached in traditional displacement hulls.

[boatman61]

Optical illusion then.. 6" freeboard at the bow and 3ft at the stern.. could have sworn there was at least a 10degree inclination going forward..

[rgleason]

Good review, thanks. A good wind, A good reach, with new sails, set correctly, combined with rounded waves at a good angle to reduce bow and stern waves and promote surfing, can lead to some exhilerating sailing above hull speed. Our short Ted Hood designed CCA Bristol 32' (with lwl=22 feet) has hit 7.1 regularly in those perfect conditions and sometimes 7.5 knots and everything just feels right. There are times when the conditions remind us of these theoretical hull speed limits!

So, here is a question. Our boat has long ends and the faster she goes (up to HS) the longer her LWL gets, generally, until shes caught between her bow and stern waves. So with LWL changing from 22' up to say 24.5-26.5 feet (a guess), what values would be expected for HS and how would you calculate it, adjusting for actual waterline length or not?

[Polux]

Originally Posted by David M

There is another way of thinking about this. Instead of a mathematical formula, it is when you have a bow wave, a stern wave and a trough in between. This makes it not an exact speed.

Or when you have not wave at all on the stern. The JPK 38 is a great fast performance cruiser, very adapted to solo sailing that with the right conditions can plan. Look at the video at min 1.45

https://www.youtube.com/watch?v=0Sor7XrqySY

People tend to put on the same bag, Pogo's bag, all types of boats with the beam brought aft even if they have different hulls. For instance the JPK 38 has 3.99 beam, the same beam as the 2ft smaller Pogo 36 while the 2ft bigger Pogo 12.50 (that is a 40ft boat) has a 4.55m beam. The JPK 38 has a more narrow hull with finer entries, a very nice one, better adapted to upwind sailing.

Interesting to have a look at the hull, one that i like a lot and is a very good example of a planing hull on a performance cruising boat. This JPK is not a racer, they have other excellent cruiser racers (with many victories) on the JPK line.

https://www.youtube.com/watch?v=xwnNDJMVrk0

[rgleason]

Here is part of the answer .

he factor is called Speed Length ratio (S/L) and it is calculated like this:

S/L=8.26/(D/L ratio)^0.311

Then... Hull Speed = S/L*LWL^0.5

(LWL^0.5 is equal to the square root of LWL)

[B00B00]

Here is a view out the back of the beneteau at around 14kts. Clean(ish) wake and well and truly overtaken the quarterwave. This is surfing though, i think we would have to be pushing very hard to push through the hull speed in flat water.

Previously to the beneteau we had a jeanneau sunfast 3600 racer cruiser, very similar to the JPK 10.8m long and 5000kg. Would plane very easily in flat water and had emense hull form stability but you paid a hefty price for that in light airs with the extra wetted surface area....
Being wider allows for a generally flatter under water shape which helps with planing.
We got 20kts out of it going down auckland harbour once in flat water and 22kts surfing.
This is a video on the same day at about 17-18kts boatspeed. https://youtu.be/-ocw4TlNqU0
And photo of sufing at 21kts.
Huge amounts of fun in a boat that is also a very comfortable family cruiser.

[rgleason]

I think we all like to go fast, whatever that is, but your big boats are acting like a laser, 420, Viper 640, Vx One or Evo, whatever..., maybe not a sailboard or kiteboard, but it must be a thrill in a big boat like that. Do they get skittish and sensitive on the helm at that speed?

[estarzinger]

Originally Posted by El Pinguino

Moving along... a one metre sea and a 12 metre boat ? ... that is not a flat sea. the 'flat water rules' no longer apply....

This. Even smallish waves both impart significant energy and change the dynamics. It is probably an explanation for much of the 'disagreement'.

There are some decent calculations in the drogue literature about the energy imparted from waves, and at surprisingly small waves it will surpass the sail energy. Basically you have the boat's mass sliding/falling down an incline, plus any crest jet effect.

Originally Posted by rgleason

your big boats a . . . Do they get skittish and sensitive on the helm at that speed?

Actually, the helm is generally quite nice once you get over the hump. They tend to 'load up' as you are trying to push thru hull speed, but once you start surfing they become light and responsive.