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Old 01-01-2017, 05:02 PM
Abby cat Abby cat is offline
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When does a monohull become a multihull??

I have noticed the trend in wide monohulls that are meant to gain hull form stability while decreasing the reliance on lead ballast thus lighter and faster, but they are slowly evolving into multihulls are they not.

So where does the line cross between a monohull and a multihull, ie how much like a multihull can a monohull be before it is classified as a multihull.

Example an imoca 60 has a length/beam ratio of about 3/1 and the 100 footer Comanche has a ratio of about 4/1. and most offshore cats have a ratio of about 2/1 so the beam is getting there.

But also in hull shape can a monohull have "hulls" that are only below the water but the boat only has one piercing of the surface of the water, but when sailing the windward "hull" comes out of the water as a catamaran does.
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Old 01-02-2017, 02:15 PM
TANSL TANSL is offline
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I think I understand what you mean but the first answer I can think of is that a monohull can never become a multihull. And forgive you for the simplicity.
To answer properly, if there is an answer, we should first clarify what each one understands by a hull and how it differs from a float / ama. Clarifying that concept maybe the answer is easier.
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Old 01-02-2017, 02:52 PM
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Angélique Angélique is offline
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For power boats the Cathedral Hull goes a bit towards a Tri, here's a subject thread . . .
Cathedral Hulls ? ? - - - - Wikipedia: Cathedral Hull
Quote:
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Last edited by Angélique : 01-03-2017 at 08:19 PM. Reason: the definition in post #10 made me interchange the pictures in the quote for this one
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Old 01-02-2017, 03:07 PM
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Doug Lord Doug Lord is offline
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Some said the boat in the first picture was a trimaran but it was a mono with molded in leeboards. In 1898 the boat in the third & 4th picture-Dominion- won a series of races that got her type banned. And the fifth picture shows a classic relatively"modern" tunnel hulled scow shape similar to the M20.
The last picture shows the worlds first foiling keelboat that has some features of a catamaran in its forward sections but is definitely a monohull. http://www.quant-boats.com/documents..._July_2016.pdf
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I think if the boat has more than one distinct hull in the water when it is level it is not a monohull. Lots of beam, in and of itself , doesn't make it a multihull.
Attached Thumbnails
When does a monohull become a multihull??-tantra-molded_in_leeboards.jpg  When does a monohull become a multihull??-tantra_with_us_1_rig_002.jpg  When does a monohull become a multihull??-dominion.jpg  

When does a monohull become a multihull??-tunnel-hull-scow.gif  When does a monohull become a multihull??-inland-lake-scow-001.jpg  When does a monohull become a multihull??-quant-23-no-credit3.jpg  

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Old 01-02-2017, 03:14 PM
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Angélique Angélique is offline
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Just saw there's a sister thread: When is a mono a tri ?
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Old 01-02-2017, 03:27 PM
Abby cat Abby cat is offline
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Thanks for the replys
Angélique thanks, but I am more speaking in sailing terms but is still interesting.
So TANSL what do you define as a hull, I have attached a grey area hull whether it is a mono or a multi, I would be interested at what your opinion is.

thanks

Nick
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Old 01-02-2017, 04:15 PM
TANSL TANSL is offline
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I rather tried to get you to say what a monohull is to you because, interpreting literally the words, if a thing you define as monohull it is impossible for it to become multihull.
For me a hull could be defined as the part (or tarts)s of the boat that generates buoyancy and helps to support the weight of the boat. From this point of view a float is only a buoyancy reserve for certain cases, damage, large heelings, ... (This is my way of trying to answer Abby cat. Others can think what they want and it will be interesting to know other opinions)
The boat in the picture of your last post, for me is a monohull with sponsons, or with a tunnel ???. I do not see it well. In any case, hydrodynamically speaking, in my opinion, it is very deficient, so whatever it is, I would not call it a boat.
Your welcome, Nick.
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Old 01-02-2017, 05:05 PM
Ad Hoc Ad Hoc is offline
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Abby cat

Your question is an interesting one but a bit misleading; especially since it covers several different disciplines (often at variance with each other) at once. However you may find the following HERE an interesting read that provides some insight to why
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Old 01-02-2017, 06:36 PM
Abby cat Abby cat is offline
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Hi

yea the image was of a concept and would require much more to make it efficient but what I believe the understanding is that a hull is the bottom, sides, and deck but not the masts, superstructure, rigging, engines, and other fittings.
But the difficult part is when does the bottom of the hull become the sides as to whether a boat is a monohull or a multihull.
The reason that i had these thoughts is because the sydney to hobart yacht race has not allowed multihulls to race, so I have been thinking about how you could get a multihull to be, by definition a monohull so it could do the race, this is because an equivalent length racing multihull is faster than a racing monohull.

thanks
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Old 01-02-2017, 08:26 PM
Ad Hoc Ad Hoc is offline
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It is very simple.

At the design waterline, if you cut the hull(s)...what do you see...one, two or three hulls?
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Old 01-02-2017, 10:51 PM
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Ike Ike is offline
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Both the USCG and ABYC use a simple definition: If the waterline forms a continuous line all around the hull it is a monohull. This is with the boat sitting level on the boats waterline with normal load. If it makes two or more footprints under the same conditions it is a multihull.

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Old 01-03-2017, 04:39 AM
Squidly-Diddly Squidly-Diddly is offline
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Quote:
Originally Posted by TANSL View Post
I think I understand what you mean but the first answer I can think of is that a monohull can never become a multihull. And forgive you for the simplicity.
To answer properly, if there is an answer, we should first clarify what each one understands by a hull and how it differs from a float / ama. Clarifying that concept maybe the answer is easier.

http://www.wildernesssystems.com/us/products/ride-135

Kayak has tunnel hull for stability. Still a mono IMO but blurring the line.
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Old 01-03-2017, 04:54 AM
Mr Efficiency Mr Efficiency is offline
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There'd be a few planing cats and tris that would qualify as monos based on that continuous waterline rule.
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Old 01-03-2017, 05:00 AM
TANSL TANSL is offline
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The reasoning that I want to expose, although with little success, is that if a floating object is a "monohull", it can never be "transformed" into a multihull. It is what it is. I mean, the question is badly posed. You can ask if a floating object is mono or multihull but one thing we have already described as mono, can not be multi.
On the other hand, I think explanations of Ad Hoc and Ike are very clear although, in occasions, in case of hulls of very different sizes, it would be necessary to qualify them.
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Old 01-03-2017, 05:36 AM
Mr Efficiency Mr Efficiency is offline
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Quote:
Originally Posted by TANSL View Post
The reasoning that I want to expose, although with little success, is that if a floating object is a "monohull", it can never be "transformed" into a multihull.
Dynamic lift can do it, in some cases.
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