Are Island Packets as bad as some claim? - Page 7 - SailNet Community
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post #61 of 98 Old 04-01-2017
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Re: Are Island Packets as bad as some claim?

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Originally Posted by SVAuspicious View Post
There is no such thing as hull speed. Search for my previous rants and I'll save you from repeating myself.
Well maybe its that you dont understand the hydrodynamics of what entails 'hull speed' .... the underling tech definition is the speed of WAVES generated by the moving hull as a functional result of the sqrt of the length of the hull, not the speed of the apparent max. speed of the hull which is the dependent variable - a resultant. The wave speed propagation is the independent variable at the max. condition. Beyond 'hull speed' a displacement hull functionally loses WLL due to the bow overriding the bow wave ... and the stern 'squats'. Look at all this from the perspective of (hull supporting) waves, not the hull, and this will become 'more clear'.
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post #62 of 98 Old 04-01-2017
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Re: Are Island Packets as bad as some claim?

I'm also curious about the hull speed comment. Theoretical hull speed was covered in first year hydrodynamics and ship construction, along with the various methods of cheating it; bulbous bows, SWATH hull, semi planing hulls, planing hulls, excessive amounts of power, but I've always found theoretical hull speed to be a reasonably accurate limiting factor to speed with regards to conventional full displacement hulls.

However, looking at both the IP and Beneteau hulls, I'm not convinced either one would behave like a full displacement hull, it looks to me like either one would be capable of some degree of planing.
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post #63 of 98 Old 04-01-2017
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Re: Are Island Packets as bad as some claim?

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Originally Posted by SVAuspicious View Post
Charter boats are rarely fully loaded. No spares. No extra sails. No deep storage of provisions and personal "stuff." Charter boats are often worn but rarely loaded up for cruising.

Agreed. When people ask me what they should buy to make their boat move faster I say "lessons.".
On the first quote above, I was relating to my experience with a chartered IP460 that had a serious AC installation and a serious solar charging system. We had provisioning for 6 adults and had just topped off the 260g water tank and were carrying close to 150g of fuel and were towing a dingy. The spec displacement of the IP 460 is about 32000 lbs, vs about 20000 lbs for a Beneteau 44.7 that has a 160g water tank and a 53g fuel tank. The difference in the weight of water and fuel for the 2 boats is about a ton. So, in our case we were not loaded to the gills, but were still fairly "heavy" compared to the lighter weight Beneteau. That was my point: we were close to hull speed (a recognized metric) on a run in about 20 kts of wind. That said, I will grant that the Island Yachts IPs are not typical for the charter business in the Virgins.

To your second quote, I couldn't agree more. And that applies even more so to folks who don't normally race--like IP owners who may not be tweaking as they go from point A to B. I learned my lessons years ago when I sailed--and raced--a catboat for 15 or so years. The conventional wisdom was: " It's easy to sail a catboat, but is isn't easy to sail it well." That perspective is not limited to catboats!
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Re: Are Island Packets as bad as some claim?

Hull speed is a theoretical rule of thumb. I know several boats that defy the formula including my own displacement hull. My fantail gains 4' waterline length when she squats and will hit 8.1knts. at full power. A previous displacement boat with 36' at the water line and grossly overpowered would do 11knts. it was ugly, noisy and the transom was below the water line but she would do it.
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The hysterical laughter you hear as you drive a way in your"new" boat ..... is the seller.
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post #65 of 98 Old 04-01-2017
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Re: Are Island Packets as bad as some claim?

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Originally Posted by boatpoker View Post
Hull speed is a theoretical rule of thumb. I know several boats that defy the formula including my own displacement hull. My fantail gains 4' waterline length when she squats and will hit 8.1knts. at full power. A previous displacement boat with 36' at the water line and grossly overpowered would do 11knts. it was ugly, noisy and the transom was below the water line but she would do it.
Hull speed is not an absolute limit, but indicates the point of diminishing returns for speed gained vs. applied power for boats in displacement mode.

There are curves routinely available for many planing power boats that show fuel consumption vs speed. Typically the fuel consumption rate climbs as speed is increased beyond hull speed and then falls off as the boat gets on plane.

A cruising sailboat may have flat areas of the hull, typically toward the stern that can put the boat in a semi-displacement category, but the benefits to speed/fuel economy may be minimal. A racing sailboat designed to plane is a different animal.
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Re: Are Island Packets as bad as some claim?

Good lord, not another hull speed thread please....

First it is a real number that is actually derived from real measurable physics. It is a function of the S/L ratio of free running waves in the open ocean, and Frouds numbers. It is not meaningless. The problem is that AT BEST all it can tell you is the point at which wave making resistance goes from a linear increase to an exponential one. It in no way says you can't go faster than X, it just says that to go faster than X the input force climbs really fast from this point forward.

Secondly there are a lot of tricks that can get around it like using very skinny hulls. Which because of the nature of long skinny shapes haveing a reduced tendency to make waves. Or bulbous bows which act to create a second wave that interferes with the standing bow wave.


Hull speed is real, it matters, and for ships operating with a Frouds number around .4 it is a pretty solid wall, for other ships it is more like a suggestion. But since about 95% of all vessels built prior to 1900 operated with a Frouds number in the .39-.41 range it really does describe a large portion of all vessels.
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Re: Are Island Packets as bad as some claim?

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Originally Posted by Stumble View Post
Good lord, not another hull speed thread please....

First it is a real number that is actually derived from real measurable physics. It is a function of the S/L ratio of free running waves in the open ocean, and Frouds numbers. It is not meaningless. The problem is that AT BEST all it can tell you is the point at which wave making resistance goes from a linear increase to an exponential one. It in no way says you can't go faster than X, it just says that to go faster than X the input force climbs really fast from this point forward.

Secondly there are a lot of tricks that can get around it like using very skinny hulls. Which because of the nature of long skinny shapes haveing a reduced tendency to make waves. Or bulbous bows which act to create a second wave that interferes with the standing bow wave.


Hull speed is real, it matters, and for ships operating with a Frouds number around .4 it is a pretty solid wall, for other ships it is more like a suggestion. But since about 95% of all vessels built prior to 1900 operated with a Frouds number in the .39-.41 range it really does describe a large portion of all vessels.
So, maybe we should compare boats on the basis of Froude number in addition to PHRF? BTW, most of us would not consider a boat made before 1900.
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Re: Are Island Packets as bad as some claim?

I drove a yacht club tender for a couple years that was built in 1894, iron hull, very much a displacement hull. Even she could exceed her hull speed, because she had the power, but you knew it as soon as you did because the passengers feet would get wet on the stern, and that speed was pretty close to the theoretical figure.

I doubt a deep very boxy vessel like a Laker or crude carrier with limited propulsion power could.

This is why I'm curious to see SVauspicious theory on this.
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Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by Stumble View Post
Good lord, not another hull speed thread please....

First it is a real number that is actually derived from real measurable physics. It is a function of the S/L ratio of free running waves in the open ocean, and Frouds numbers. It is not meaningless.
Don't think anyone said it was meaningless although pre-1900 ocean going ships have little to do with my interests.

The hysterical laughter you hear as you drive a way in your"new" boat ..... is the seller.

Last edited by boatpoker; 04-01-2017 at 02:35 PM.
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post #70 of 98 Old 04-01-2017
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Re: Are Island Packets as bad as some claim?

lol @ this thread about a cruising boat but assuming only 1 metric being important...speed.

What about tankage,build quality,real bilge,storage,kindly motion,protected prop,encapsulated keel,proper deck to hull construction,real chain lockers,and...err...nm.

Lets get back to how fast we can sail around the beer cans in 6 knots of wind.
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