Are Island Packets as bad as some claim? - Page 8 - SailNet Community
 47Likes
Reply
 
LinkBack Thread Tools
post #71 of 98 Old 04-01-2017
Dirt Free
 
boatpoker's Avatar
 
Join Date: Jul 2008
Location: Toronto
Posts: 2,619
Thanks: 22
Thanked 147 Times in 142 Posts
Rep Power: 12
 
Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by Eder View Post
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
Now you're talking !
Don't see may of these in Ontario but on one grounding insurance claim I saw the front two feet of the keel opened up. I was shocked to see a boat in this price range with ballast of crumbled cement, various pieces of rebar, scrap metal and what I think was the damper plate from a 73' Nova.

After somewhere around 4,000 surveys I no longer believe builders and their marketing hype or boating magazines.

The hysterical laughter you hear as you drive a way in your"new" boat ..... is the seller.
boatpoker is online now  
Sponsored Links
Advertisement
 
post #72 of 98 Old 04-01-2017
Senior Member
 
fallard's Avatar
 
Join Date: Nov 2009
Location: Mystic
Posts: 2,354
Thanks: 20
Thanked 80 Times in 79 Posts
Rep Power: 11
 
Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by Eder View Post
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.
First we need to know your Froude (not Frouds) number!

Last edited by fallard; 04-01-2017 at 06:41 PM. Reason: typo
fallard is online now  
post #73 of 98 Old 04-01-2017
Mermaid Hunter
 
SVAuspicious's Avatar
 
Join Date: Oct 2006
Location: on the boat - Chesapeake
Posts: 5,687
Thanks: 0
Thanked 275 Times in 246 Posts
Rep Power: 14
 
Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by RichH View Post
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.
Quote:
Originally Posted by Arcb View Post
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.
Quote:
Originally Posted by fallard View Post
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.
Quote:
Originally Posted by Stumble View Post
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.
Well Rich, I'm pretty sure I do understand hydrodynamics. It says so on this piece of paper on the wall over here (Webb Institute '82) and is implied by this patent on towed bodies in the filing cabinet.

Back in the day when hull forms where all very similar we used empirically derived coefficients for a lot of things. Hull speed was one - 1.15xSQRT(waterline (ft)) = kts. As hull forms became more diverse there were a number of interesting attempts to take a lot of full-size and model data to correlate other characterizations (block coefficient, prismatic coefficient, Δ/L, and other more complex ratios and ratios of ratios) to the hull speed coefficient which is why you'll see multipliers between 1.1 and 1.4.

All of this neglects the fact that hull speed was never and is not a hard wall. It is indeed a fairly big region where the speed-power relationship curves up so that the power required for an incremental increase in speed increases dramatically. The region is a curve, so the rate at which the incremental power requirement increases itself increases. It's like the relationship between distance, speed, and acceleration. Naval architects used hull speed as a mechanism for comparing one platform to another not as a prediction of actual speed, although in the early days of steam it was a component (but only a component) in power plant sizing.

Fortunately as hull forms became more and more diverse our understanding of hydrodynamics continued to improve and other approaches pushed "hull speed" into the background. Model testing was part of that but far from all (which is the only application of Froude number, which relates to scaling between models and full scale). Over the last 50 years computer modeling has completely overwhelmed any other measure of the speed-power relationship.

Details:

Bulbous bows do decrease wave-making resistance, independent of "hull speed." Stern bulbs can do the same for low prismatic forms.

SWATH, like other catamaran hulls, take advantage of high L/B forms which are so far off the map of "hull speed" that they in fact demonstrate the limitations of simplistic parameters like hull speed.

Planing and semi-planing hulls work in non-linear regimes where "hull speed" just doesn't apply. Yes you need a lot of power to plane, but if you look at the speed-power curve you will see a real knee in the curve where planing begins (ish) that looks nothing like the region at "hull speed" for displacement hulls.

If you still aren't sure, consider all the 40' boats with "hull speed" around 6 - 6.5 kts that easily sail at 8 to 10 kts. Sails generate a tremendous amount of power.

"Hull speed" only has meaning when comparing boats of geo-similar hull forms.

The way that most people use it is meaningless.

sail fast and eat well, dave S/V Auspicious

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

beware "cut and paste" sailors


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
SVAuspicious is offline  
 
post #74 of 98 Old 04-01-2017
Moderator
 
Arcb's Avatar
 
Join Date: Aug 2016
Location: Eastern Ontario
Posts: 4,029
Thanks: 215
Thanked 202 Times in 198 Posts
Rep Power: 10
 
Re: Are Island Packets as bad as some claim?

Okay, this is how I understand theoretical hull speed as well.

My most intimate experiment with hull speed came when my crew at the time took possession of a 60ft high speed power cat with with a couple of big MANs turning a pair of surface piercing propellers.

Any way, she had reportedly been capable of cruising at over 30 knots in salt water, but when we tried to trim her out on freshwater, we were unable to climb the bow wave. My role was just helmsperson, but that did give me a pretty good perspective of what was going on because I was controlling the yoke, trim and throttle much of the time.

These sea trials went on for months as we tried to achieve plane from both a vessel handling and engineering perspective in soupy warm fresh water. The team was eventually able to put the puzzle together and got her running flat.

Long way of saying, I'm a big believer in the hydrodynamics principles that guide theoretical hull speed, but I do definitely appreciate, it's more of a rough guideline than any kind of hard rule, especially given how rare full displacement sailing yachts are in 2017.

Last edited by Arcb; 04-02-2017 at 09:06 AM.
Arcb is offline  
post #75 of 98 Old 04-01-2017
Senior Member
 
amwbox's Avatar
 
Join Date: Aug 2015
Location: Portland, OR
Posts: 632
Thanks: 23
Thanked 34 Times in 34 Posts
Rep Power: 5
 
Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by Eder View Post
lol @ this thread about a cruising boat but assuming only 1 metric being important...speed.
It's extremely annoying.

If I want to go fast...a sailboat...very nearly any sailboat, is just about the worst possible choice of vehicle. I've never understood this bizarre obsession so many have about making a class of conveyances that are, collectively, just about the slowest mode of transport available...."fast".

An IP is a big, heavy, beamy, cruising boat. If you're valuing cruising boats on their relative speeds, you've entirely missed the point.

Now, watch me go win an autocross event with a 1972 Chevy Suburban. Because that's what determines the usefulness and worth of Chevy Suburbans, doncha know?
fallard and seabeau like this.

"Freedom is the increased knowledge of what you can do without." —Thoreau
amwbox is offline  
post #76 of 98 Old 04-01-2017
Senior Member
 
fallard's Avatar
 
Join Date: Nov 2009
Location: Mystic
Posts: 2,354
Thanks: 20
Thanked 80 Times in 79 Posts
Rep Power: 11
 
Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by amwbox View Post
It's extremely annoying.

If I want to go fast...a sailboat...very nearly any sailboat, is just about the worst possible choice of vehicle. I've never understood this bizarre obsession so many have about making a class of conveyances that are, collectively, just about the slowest mode of transport available...."fast".

An IP is a big, heavy, beamy, cruising boat. If you're valuing cruising boats on their relative speeds, you've entirely missed the point.
If I want to go fast, I use my stinkpot (first thumbnail)--that is, when my son hasn't commandeered it to take the grandkids to Napatree Point.

Otherwise, I remain attached to my shallow draft sailboat that is parked at her home dock (2.5' MLW)--note the rudder is kicked up (second thumbnail)). This boat is very weatherly and fun to sail, especially when we are not towing a dinghy.

But when we charter in the Caribbean, we have become enamored of the Island Packets, like the IP460 (third thumbnail) with its 260 gallon water tank, which we topped off at Spanish Town before heading out to JVD. Two heads and daily showers are nice, especially with the kids and grandkids aboard for a week. Comfortable is fine on vacation!
Attached Thumbnails
IMG_1452.jpg   P1020176.jpg   P1030710.jpg  
chef2sail and seabeau like this.
fallard is online now  
post #77 of 98 Old 04-01-2017
Senior Member
 
Towguy's Avatar
 
Join Date: May 2016
Location: Northern Alberta
Posts: 282
Thanks: 31
Thanked 18 Times in 18 Posts
Rep Power: 4
 
Re: Are Island Packets as bad as some claim?

Fine looking sailboat Fallard.....Ralph. How shallow draft?

Northern Alberta. "85" C/L 16. ,"85"S2 7.9 under repair.
To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
Towguy is offline  
post #78 of 98 Old 04-02-2017
Senior Member
 
colemj's Avatar
 
Join Date: Jul 2003
Location: On the boat cruising
Posts: 3,403
Thanks: 5
Thanked 141 Times in 133 Posts
Rep Power: 17
 
Re: Are Island Packets as bad as some claim?

I know that boat and that dock! Lived in Mystic for 18yrs and walked by there countless times.

Mark

Dolphin 460 Catamaran "Reach"

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
- Current boat

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
- Previous boat
colemj is offline  
post #79 of 98 Old 04-02-2017
Senior Member
 
RichH's Avatar
 
Join Date: Jul 2000
Location: Pennsylvania
Posts: 5,066
Thanks: 54
Thanked 331 Times in 315 Posts
Rep Power: 21
   
Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by SVAuspicious View Post
Well Rich, I'm pretty sure I do understand hydrodynamics. It says so on this piece of paper on the wall over here (Webb Institute '82) and is implied by this patent on towed bodies in the filing cabinet.

Back in the day when hull forms where all very similar we used empirically derived coefficients for a lot of things. Hull speed was one - 1.15xSQRT(waterline (ft)) = kts. As hull forms became more diverse there were a number of interesting attempts to take a lot of full-size and model data to correlate other characterizations (block coefficient, prismatic coefficient, Δ/L, and other more complex ratios and ratios of ratios) to the hull speed coefficient which is why you'll see multipliers between 1.1 and 1.4.

All of this neglects the fact that hull speed was never and is not a hard wall. It is indeed a fairly big region where the speed-power relationship curves up so that the power required for an incremental increase in speed increases dramatically. The region is a curve, so the rate at which the incremental power requirement increases itself increases. It's like the relationship between distance, speed, and acceleration. Naval architects used hull speed as a mechanism for comparing one platform to another not as a prediction of actual speed, although in the early days of steam it was a component (but only a component) in power plant sizing.

Fortunately as hull forms became more and more diverse our understanding of hydrodynamics continued to improve and other approaches pushed "hull speed" into the background. Model testing was part of that but far from all (which is the only application of Froude number, which relates to scaling between models and full scale). Over the last 50 years computer modeling has completely overwhelmed any other measure of the speed-power relationship.

Details:

Bulbous bows do decrease wave-making resistance, independent of "hull speed." Stern bulbs can do the same for low prismatic forms.

SWATH, like other catamaran hulls, take advantage of high L/B forms which are so far off the map of "hull speed" that they in fact demonstrate the limitations of simplistic parameters like hull speed.

Planing and semi-planing hulls work in non-linear regimes where "hull speed" just doesn't apply. Yes you need a lot of power to plane, but if you look at the speed-power curve you will see a real knee in the curve where planing begins (ish) that looks nothing like the region at "hull speed" for displacement hulls.

If you still aren't sure, consider all the 40' boats with "hull speed" around 6 - 6.5 kts that easily sail at 8 to 10 kts. Sails generate a tremendous amount of power.

"Hull speed" only has meaning when comparing boats of geo-similar hull forms.

The way that most people use it is meaningless.
Wow thanks for the over-reactive introduction of marine hydrodynamics summary. My professional field of endeavor and with several degrees is applied and theoretical fluid and thermal sciences and most of the adjacent disciplines that support them.
The mention of hull speed was to highlight that the IP seemingly is a design targeted towards trade wind sailing of the (Caribbean) islands, etc. and other venues of where 20+kts. are common and where IPs will typically be at max. developed speed due to the typical wind encountered in such a venue. In case you missed it, Hull speed is a relativistic conversion/equivalence of the wave speed propagation of the moving hull (shape) wherein even the early observationists/empiricists defined that for the common ~3:1 ratio of length to width resulted in a 'typical' hull speed or wave propagation back-deriving to a 'cookbook' equational constant of 1.34. Yet, when the L-W ratio was at higher values (military destroyers, frigates, catamarans (even Hobie Cats) etc. whose L/W approach 5:1 or more tend towards a K approaching 1.15, torpedoes approaching 1.) again, all cookbook, but only approximate values well known since the 18th & 19th centuries.

Apparently you missed entirely the purpose of my comments .... an IP sailing in its targeted trade wind venue & crewed by a knowledgeable crew will usually be at or near a max. relative speed (vs. input applied forces/power). Its the same for any 'designed shape' operating in either open or conduit fluid flow - if that flow value is far from DESIGN the developed dependent variable parameters (output) will be less (OR MORE) than expected/estimated.
Again my 'point', the IP series seemingly was purpose/market designed for trade wind or near trade wind venues, in case you missed it. ;-)

Last edited by RichH; 04-02-2017 at 08:25 AM.
RichH is offline  
post #80 of 98 Old 04-02-2017
Mermaid Hunter
 
SVAuspicious's Avatar
 
Join Date: Oct 2006
Location: on the boat - Chesapeake
Posts: 5,687
Thanks: 0
Thanked 275 Times in 246 Posts
Rep Power: 14
 
Re: Are Island Packets as bad as some claim?

Quote:
Originally Posted by Arcb View Post
These sea trials went on for months as we tried to achieve plane from both a vessel handling and engineering perspective in soupy warm fresh water.
But hull speed has nothing to do with planing hulls.

sail fast and eat well, dave S/V Auspicious

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

beware "cut and paste" sailors


To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.
SVAuspicious is offline  
Reply

Quick Reply
Message:
Options

By choosing to post the reply above you agree to the rules you agreed to when joining Sailnet.
Click Here to view those rules.

Register Now



In order to be able to post messages on the SailNet Community forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.
Please note: After entering 3 characters a list of Usernames already in use will appear and the list will disappear once a valid Username is entered.


User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in











Currently Active Users Viewing This Thread: 4 (0 members and 4 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Similar Threads
Thread Thread Starter Forum Replies Last Post
Electrical - I did a bad bad thing (alternator / regulator) blowinstink Gear & Maintenance 21 05-05-2015 08:12 PM
Insurance Claim radovix General Discussion (sailing related) 16 06-24-2012 03:47 AM
Insurance claim issue..... Petar General Discussion (sailing related) 19 07-04-2011 05:24 PM
Free O Day - How bad is too bad ? ? ? nogin General Discussion (sailing related) 52 03-20-2011 09:40 AM
Rudder Problems-Island Packets clayton Boat Review and Purchase Forum 1 01-29-2002 10:57 AM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome