Hull Speed? - Page 3 - SailNet Community
Old 07-24-2007
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I think what some of you are referring to are Semi-displacement hulls.
USCGRET1990 is offline

Old 07-24-2007

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Its only a Seawind 24'

Quote:
Planing is generally when the hull has generated enough lift and is mostly out of the water
Thats a fuzzy definition! So what is it when you are going faster than displacement hull speed but not mostly out of the water?

My Seawind 24 can pull upto 20 knots on a hard reach... there is no way that you can claim that is a "displacement speed" for a 24' boat of any sort. It simply must be generating lift at that speed and therefore riding up on the water to some degree (the amount of spray supports that! oih! she is wet!) That in my book is planing as is surfing, the only difference is the source of the power i.e. Sail vs Sail and Gravity (wave action).

In my book when the boat is displacing less water than its gross weight it is starting to ride up onto the surface of the water. The rest is semantics about degree, which relies on hull form, weight etc etc. Therefore surfing is planing, big heavy monohulls don't do it very well or for very long I grant that and they often need the force of gravity to make it happen (a v big wave) but happen it can. We have had our old Brolga 33 (heavy) planing at 14 knots (well 10 to 14 and back very quickly). You can see the mass of green water she shoves aside as she rides up on it....but alas it scares the poor old girl and she will only do it for 10 seconds at a time and then she wants to turn around and go right back home. You ended up with tiller up round the ears as she falls off her bow wave and the following wave starts to catch her. If you didn't anticipate it, it was ugly, wet, flappy, noisy and the water got very close to the helmsman.... fortunately she was built like a ...... , well nothing I have seen since.

I'm gunna go with Chris White on this one, multihulls plane, very efficiently with minimal fuss but they do it. Thats why I keep talking about sports boats, they plane so easily that it makes the fuss that goes on getting say a J24 up look like a circus. That however does not mean they are not doing it

Have you seen Mr Whites "Hammerhead 54'" .... now thats a cruising tri ! 20 knots in that !

Cheers
MBz

Last edited by MeanzBeanz; 07-25-2007 at 12:23 AM.
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Old 07-24-2007

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Its a Dragon... now its exceeding hull speed and riding up onto a wave. So is that planing?
MeanzBeanz is offline

Old 07-25-2007

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Quote:
Originally Posted by USCGRET1990 View Post
I think what some of you are referring to are Semi-displacement hulls.
Or low speed planing hulls...

Thats more a reference to form than the act of planing... as in what its designed to do best. That does not preclude the vessel from planing if the right conditions occur.
MeanzBeanz is offline
Old 07-25-2007
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Quote:
Originally Posted by MeanzBeanz View Post
Try asking at www.boatdesign.net there are plenty of designers there that will explain if you ask nicely.
I did just that last year and here is the result if you can wade through it. http://www.boatdesign.net/forum/show...hlight=yotphix

The short answer is that hull speed and wave making are not well understood and are argued about even by experts. From observation it is possible to make some generalizations which are adequate to design with but it is not surprising that every time the topic comes up there is a big discussion and no clear explanation.

This is from the boat design discussion sparked by my question.
"So, after millions of dollars and many thousands of man-hours spent
on the problem, we are still a long way from even a reasonable
understanding of the physics involved. CFD is, of course, an enormous
advance because we can now display our almost complete ignorance in full
colour!"
yotphix is offline
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Ok I think I have it, but have one other question. When looking at a boats specs is the measurement given for LWL taken when the boat is sitting upright or is it taken assuming X degrees of heel?

Using the formula given, a boat with a 26' 8" LWL would have a theoretical hull speed of about 7 knots. Would that figure take the longer water line resulting from heeling into consideration or is the theoretical speed actually slightly higher when the boat is heeled?

Thanks for all the great feed back and discussion.
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Old 07-25-2007
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A boat's LWL dimensional specification is always measured with the vessel at rest, not the extended variable length, which occurs during heeling.

True Blue . . .
sold the Nauticat
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Old 08-08-2007

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Quote:
Originally Posted by midlifesailor View Post
Ok I think I have it, but have one other question. When looking at a boats specs is the measurement given for LWL taken when the boat is sitting upright or is it taken assuming X degrees of heel?

Using the formula given, a boat with a 26' 8" LWL would have a theoretical hull speed of about 7 knots. Would that figure take the longer water line resulting from heeling into consideration or is the theoretical speed actually slightly higher when the boat is heeled?

Thanks for all the great feed back and discussion.
My understanding is at rest, so yes level, and fully loaded to the designs maximum capacity (Load Water Line). In a light load condition you will find that the LWL can be well above the actual waterline.

To get closer to real world you would have to calculate the waterline at the optimum angle of heel in a typical load state. Often designers will give the ideal angle of heel to sail the boat at for maximum efficiency, a couple of Farr designs I sailed it was 18 degree's, Farr reckoned above that you where slowing down and it was time to depower and flatten out the boat. "Flat is fast" was the motto! I guess at the end of the day it requires an intimate knowledge of your boat and then its a best guess calculation because sea state, freeboard windage etc etc (all the other unique features of the design and your boat) will impact the result. Not to mention the fact that is an approximation to begin with

Is the fog gone? Or just sitting around the ankles ..... LOL

So there you go, we don't really know for sure.... well ain't that just dandy

Cheers
MBz

Last edited by MeanzBeanz; 08-08-2007 at 06:47 PM.
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Old 08-08-2007
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As a hull heels the the waterline plans becomes assymetrical, the lifting from keel starts to diminish and the hulls slips and makes more and more leeway. It's all very complex and why heeling excessively is a bad idea.

And the wife complains more too.

jef
sv shiva
SanderO is offline
Old 08-08-2007
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I quit....I'm almost home now

Last edited by Giulietta; 08-08-2007 at 07:02 PM.
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