What is "Hull Speed" Anyway

[Panope]

Originally Posted by roland stockham

Absolutely but I would add a couple of points. Theoretical top speed is based on what the boat could do given enough power and without considering either sea state or heal. The idea of overhangs (ignoring when it has been used to 'cheat' rating rules) is that when upright at lower speed whetted surface is less so the boat can be quicker in light airs. when the wind picks up , so sails develop more power, the water line extends increasing top speed. This works well on traditional heavy displacement boats but is limited to purely displacement hulls with no planing ability.
Block (or prismatic) coefficient is complex, I don't think that high BC necessarily equals higher hull speed. If that where the case the fastest hull shape would be a lighter!!! At the stern it can create lift and allow a hull to plane but at the stem it creates high wave resistance requiring more power for the same speed in any sea. At 1/3 distance from the bow a high PC makes hulls prone to slamming which, apart from being uncomfortable, also slows the boat down. So it is more a case of the PC must 'fit' the overall design.

Yep, all this is very complex. So much so that making generalizations (including the ones I made, up thread) very difficult.

I managed to dig up a graph that I created during Panope's 2012 sea trials. Conditions were: zero wind with zero waves. Engine power only.

To negate the effect of current (less than 1 knot) I ran each RPM at four headings (N.S.E.W) and averaged the resulting GPS speeds. I then repeated the entire sequence on a different day and averaged both results.

Panope's design WL is 27' but she was heavy and therefore had an at-rest WL closer to 28'.

Using the 1.34 multiplier, we get a hull speed of 7 knots. However, the graph shows a significant "kink" at 6.73 knots.



Here is a sailing video I made in flat water. At the 0:50 second mark, a gust of wind gives a nice jump in speed to about 7.5 knots. Note how the secondary wave moves well aft during the gusts.

I can say with confidence that the boat makes a smaller wake when the rail is buried (sailing) than when upright (motoring) - at the same speeds.

Steve

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

[Kenomac]

Originally Posted by Kenomac

Basically, the difference is the weight of a tank of fuel or water. Not much. So... is it safe to assume that since you don't have a water maker which necessitates carrying along a rather full tank of water on passages, and we do have two water makers which means we usually only carry a partially filled water tank, that our boats generally weigh within one thousand pounds of each other? Maybe at times, our Oyster might even weigh less than your Moody, if running on less than half a tank of fuel.

Oyster 53 vs Moody 54 specifications

Oyster 53
LOA 53.83ft
LWL 45.58ft
displacement 48000 pounds
sail/disp 15.82

Moody 54
LOA 53.92ft
LWL 46.08
Displacement 44,132 pounds
Sail/disp 16.59

OYSTER 53 (HUMPHREYS) sailboat specifications and details on sailboatdata.com

Moody 54 MOODY 54 sailboat specifications and details on sailboatdata.com

Originally Posted by Dockhead

LWL of the M54 is 47 point something feet, not 46, and as far as I know your boat is 50,000 pounds light ship -- at least that's what the Oyster factory told me when I was looking at it. Sailboatdata.com is notoriously unreliable. Longer waterline makes a big difference in D/L because the denominator goes up with the cube of LWL. That's why the M54 has D/L in "racer-cruiser" territory, and the Oyster is "medium displacement".

To the previous poster -- no one is arguing about "who's boat is better". Everyone agrees that the Oyster is a magnificent boat. I love them and almost bought one myself -- had a deposit but the boat failed survey.

Originally Posted by Dockhead

Does your waterline change with empty tanks? D/L is a cubic function of LWL.

Here’s a 2003 review of your Moody 54 from Sailing Magazine done by Robert Perry using the factory published boat specifications. D/L listed at 201 with a 46ft waterline.

Moody 54: Bob Perry Review - boats.com

[Steve1944]

There are many factors to come I to play to add to or detract from the hull speed of a boat. Some are controlled by the mariner, and some factors control the mariner.

[StuM]

Originally Posted by Panope

Using the 1.34 multiplier, we get a hull speed of 7 knots. However, the graph shows a significant "kink" at 6.73 knots.

Convert those RPM to HP or fuel consumption and you will get a very different shaped graph.

[Panope]

Originally Posted by StuM

Convert those RPM to HP or fuel consumption and you will get a very different shaped graph.

Yep. Power required goes to the moon at the higher speeds. I figured this rather "flat" graphic would show the "drag rise" better than a exponential curve. Perhaps not.

Steve

[Polux]

Originally Posted by Kenomac

Polux,

Can you please send me the link to your personal blog.

Thanks

Just click on the link that is below on my posts (interesting sailboats).
I cannot believe that you never been there. I have about 2500 hits a day with a top of over 7000, from all over the world.

[Steve1944]

Hull speed is the calculation of maximum speed of the hull of a boat through water. A mariner can improve or have that speed diminished by effects like tide and winds. It’s often confused with the maximum speed of a boat. It’s a professional term.

[daletournier]

Originally Posted by Kenomac

Polux,

Can you please send me the link to your personal blog.

Thanks

http://interestingsailboats.blogspot.com/?m=1

[Steve1944]

Polux: i have no personal blog

[Kenomac]

Originally Posted by Polux

Just click on the link that is below on my posts (interesting sailboats).
I cannot believe that you never been there. I have about 2500 hits a day with a top of over 7000, from all over the world.

I really like your site, and I also found another I like very much; made up of doers minus the dreamers.

www.morganscloud.com

I’m checking out eastern Greece and placing it more on the immediate to do list per your recommendations. Maybe late this summer or most definitely 2019.

[Dockhead]

Originally Posted by Kenomac

Here’s a 2003 review of your Moody 54 from Sailing Magazine done by Robert Perry using the factory published boat specifications. D/L listed at 201 with a 46ft waterline.

Moody 54: Bob Perry Review - boats.com

I'll be sure to correct my design blueprints and as-built survey, with stuff you found on the Internet! Thanks!


Hope everyone had a good Christmas!

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[Joli]

At the end of the day it's mostly about waterline length although it's possible to press a boat and sail above theoretical hull speed. Most cruisers seem to be happy to sail to 70% of theoretical hull speed and very very few are willing to press the boat for additional miles even if it's a planing hull (which makes a crappy cruising boat for all sorts of reasons).

[malbert73]

Originally Posted by Joli

At the end of the day it's mostly about waterline length although it's possible to press a boat and sail above theoretical hull speed. Most cruisers seem to be happy to sail to 70% of theoretical hull speed and very very few are willing to press the boat for additional miles even if it's a planing hull (which makes a crappy cruising boat for all sorts of reasons).

I mostly agree with this as a summary but it overly generalizes which I think this thread demonstrates. There is probably no one size formula which determines every sailboat’s “maximal displacement speed”

The formula quoted numerous times above proves repeatedly unreliable for boats with narrow beams, or light displacement, or full aft sections, or powerful sailplans, or some combination of above.

But for non-planing sailing, the statement that I and others have said- which is that we regularly exceed hull speed- really just means our hull speed doesn’t calculate well by the 1.34*sq root LWL. That formula is too simplistic for more modern designs, or older ones with overhangs that immerse, or any other number of variables. Our “hull speed” is some different calculus.

And regardless, all hull speeds can be exceeded in the right conditions.

[Kenomac]

Originally Posted by Joli

At the end of the day it's mostly about waterline length although it's possible to press a boat and sail above theoretical hull speed. Most cruisers seem to be happy to sail to 70% of theoretical hull speed and very very few are willing to press the boat for additional miles even if it's a planing hull (which makes a crappy cruising boat for all sorts of reasons).

What about a boat which seems to have a variable or adjustable waterline length? :-)

[TJ D]

Originally Posted by Joli

At the end of the day it's mostly about waterline length although it's possible to press a boat and sail above theoretical hull speed. Most cruisers seem to be happy to sail to 70% of theoretical hull speed and very very few are willing to press the boat for additional miles even if it's a planing hull (which makes a crappy cruising boat for all sorts of reasons).

Right on all, except for the last part. A planing hull can make for a FANTASTIC cruising boat! I happen to think the world of mine...