SailNet Community banner
21 - 31 of 31 Posts

· Registered
Joined
·
43 Posts
Reaction score
13
Discussion Starter · #22 ·
LOL is right. The guy is Crazy Hardcore about doing The Science. He sailed within 100 Miles of The Big Island of Hawaii and blew right by without stopping.
Said it would waste a week and he'd a better use of his Time would be to be doing The Science.

He is now about 100 Miles from Wale Island
 

· Registered
Joined
·
626 Posts
Reaction score
150
Wondering what the motion comfort would be if he only plodded along at 5 Knts like most blue water cruisers ?
Not very much different, apart from the fact that you would feel any small wave immediately...
Bow goes into a wave, you feel it standing at the helm in the aft...
And you are able to react to it in time...

Probably sailing is about choices... Either being on a heavy freight train, which gives you the illussion of safety, or on a light craft and try to be reactive and challenge, or avoid "the sea"...
 

· Registered
Joined
·
3,113 Posts
Reaction score
1,462
I think that steel boat shown was a Colvin? Typically very flat bottom sections, relatively narrow and shoal.
 

· Mermaid Hunter
Joined
·
5,666 Posts
Reaction score
2,423
Lets reflect on the history of naval architecture. In the days of free-hand lines and planimeters various ratios were very useful ways to make apples to apples comparisions between boats, and between requirements and the designs that responded to them. The limitations of manufacturing technology meant a lot of complex characteristics were the same because they had to be.

Time marched on and splines and ducks were replaced by CAD. Parametric analysis gave way to complex calculations enabled by computers: FEA, SHCP, SDWE, and other tools changed the game. At the same time manufacturing technology provided incredible flexibility in form.

Let's look at Mr. Brewer's comfort ratio as an example.

"This is a ratio that I dreamed up, tongue-in-cheek, as a measure of motion comfort but it has been widely accepted and, indeed, does provide a reasonable comparison between yachts of similar type. It is based on the fact that the faster the motion the more upsetting it is to the average person. Given a wave of X height, the speed of the upward motion depends on the displacement of the yacht and the amount of waterline area that is acted upon. Greater displacement, or lesser WL area, gives a slower motion and more comfort for any given sea state.

Beam does enter into it as wider beam increases stability, increases WL area, and generates a faster reaction. The formula takes into account the displacement, the WL area, and adds a beam factor. The intention is to provide a means to compare motion comfort of vessels of similar type and size, not to compare that of a Lightning class sloop with that of a husky 50 foot ketch."

The comfort ratio formula is as follows: Displacement in pounds / (.65 x (0.7 LWL + 0.3 LOA) x B^1.333). Brewer says ratios vary from 5.0 for a light displacement daysailer to the high 60.0's for a super heavy ocean cruiser.
As Mr. Brewer will tell you, length and beam affect waterline area. On the other hand a full sectioned boat like a Passport 40 and one with finer sections but greater draft can have the same comfort ratio even though their performance is quite different.

The comfort ratio doesn't even consider weight distribution. There is no consideration of moment of inertia. A boat with dinghy and engine on the back, a full enclosure, packed lazerette, and 400 ft of chain for two big anchors can have the same comfort ratio as one with the dinghy on the foredeck, a dodger, a light load, and 200 ft of chain for one big anchor.

Length to displacement ratio has the same limitations. SA/D - the same, limited by VCE and VCG being ignored. There is some residual value to ratios if you understand their limitations but they cannot be applied across the board in a meaningful way.

Weight provides inertia. The result on seakeeping is dependent on hull form and sail plan and other things. The complex function of so very many factors can lead to a boat that slams in a seaway or one that slices through the sea.

There are certainly typical performance characteristics such as those that Jeff described, but there are enough exceptions that undue dependence on out-of-date ratios may lead a boat shopper to pass up a great boat because the limitations of the ratios aren't considered.

There is still a good deal of art to hull design and sail arrangements. Environmental externals (seas and wind) are nondeterministic and evaluation of real world response is difficult. Model testing is helpful but scaling is complicated.
 

· Registered
Joined
·
5,111 Posts
Reaction score
2,973
The comfort ratio doesn't even consider weight distribution. There is no consideration of moment of inertia. A boat with dinghy and engine on the back, a full enclosure, packed lazerette, and 400 ft of chain for two big anchors can have the same comfort ratio as one with the dinghy on the foredeck, a dodger, a light load, and 200 ft of chain for one big anchor.

Length to displacement ratio has the same limitations. SA/D - the same, limited by VCE and VCG being ignored. There is some residual value to ratios if you understand their limitations but they cannot be applied across the board in a meaningful way.
I've always liked Jeff H's dismissal of the utility of the 'Capsize Screening Ratio", by his pointing out that by repositioning a boat's ballast at the top of the mast, will make no difference whatsoever in its resistance to capsize according to the formula... :)
 

· Over Hill Sailing Club
Joined
·
3,689 Posts
Reaction score
934
It seems that two different issues are being discussed. The comparative weight per length and how it affects a boat's motion is one issue. Seaworthiness relative to weight is another much more complicated equation. Simple displacement has little to do with the sea-kindliness of the boat. It's more a function of design. "Heavier" has some connotations that are misleading, leading one to equate heavier with a more comfortable ride. Boats with the same displacement can have radically different shapes and characteristics.
 

· Super Moderator
Farr 11.6 (Farr 38)
Joined
·
10,918 Posts
Reaction score
5,297
What you are saying is very interesting, can you please explain to me the difference underwater profiles makes with regards to motion? I see a lot of posts criticising the 'soap dish' shapes of modern cruisers.

I would like to see more boats with more 'V' shaped underwater profiles but that isn't from understanding of physics, more intuition.

I read about a boat recently called a riptide 55 which I thought was very interesting.
While a case can be made that vee shaped sections near the bow can reduce pounding when pitching, vee'd sections do nothing good for motion beyond that. The current thinking is that an elliptic shaped section progressively build buoyancy with heel angle so that there is less of a jerky motion as the boat rolls and the progressive dampening reduces roll speed and heel angle as well. Vee'd sections have no impact on the five other types of motion.
 

· Registered
Joined
·
3,113 Posts
Reaction score
1,462
Colin Archer 42

speaking of 42 - Matt and Nicole just posted their weekly report 42 days at sea

oceanresearchproject.org
If you are talking the yacht he rode in the Atlantic in 2013, pictured in post #15, that is the RV Ault, a 42' Colvin Gazell.

12' beam
4' 6" draft
25,000 lbs displacement.
 
21 - 31 of 31 Posts
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top