ITA 14.99

[Catmandu]

Multihull Dynamics is a great resource, I never said otherwise. I only suggest one be careful with the numbers that MD gets from the builders.

[catmancan2]

Having just returned last night from spending 2 weeks at the ITA boatyard let me add some comments to an already good exchange of comments from others.

The comps of other similar sized cats noted, as well as their MD ratios, are not factoring in the hull shape below the waterline. The comps being used are of U-shaped semi-displacement hulls whereas the ITA has a flat planing hull. So the 1.50m LWL/Bh is the same as the hull bottom of the ITA minus a few mm loss from hull side to hull bottom transition.

The ITA design brief was for a performance cruising cat loaded with the creature comforts many buyers require and cruising gear extended voyagers need. While the ITA transom immersion of 10.4 cm at full displacement may cause a slight drag in light winds, the Polars show very little loss for both lightship and full displacement. Of course the location of engines (diesel or electric) also play a role in transom immersion. This is one reason why the propulsion systems of ITA are approx 2+ m forward of the rudders.

Attachments below show waterline at full 14.5 t displacement plus how shallow the hull rocker is which is a hull depth of 0.57 m.

Below are replies to a few other questions recently posted.

• Bridgedeck clearance is 0.92 m at full displacement
• Optional split air condition system (chiller/fan coils) and heating system have been mechanically engineered (plumbing/electrical) plus discharge and return vents built into the furniture. Same is true for watermaker and (God forbid) washing machine
• All salon windows, 2x sliding cockpit doors, and hull windows are tempered
safety glass
• There are 3 locations to install a genset, engine room, foredeck locker, or
my preference under the port cockpit daybed for easiest service
• Hull/deck/coachroof insulation is typically the core of the laminate, yet the
engine compartments will totally insulated for sound
• JEFA is supplying all steering components with the rudder stocks being
stainless AISI 630 but I do not know diameter. The composite rudder
tubes were installed and a lot of JEFA components arrived while I was at
the yard. All I can add is I saw one of two aluminium quadrants and it was
huge

Let me end by saying while technical specifications and polars are great for the sake of comparision, there is only one true test, and you know what that is . . . sailing various boats that are of interest. I'm not a naval architect or an engineer . . . unless "seat of the pants" qualifies!

[iliohale]

Just to highlight the light ship vs. heavy ship waterline, the photo attached here of Hull No. 1 clearly shows the estimated heavy ship waterline of 14.5T metric or 16T US (32,000 lbs). This is 4.4T US (8,800 lbs) above light ship per ITA.

Gary

[SVNeko]

Thanks for the additional info. Can you explain what you mean with the following:

Originally Posted by catmancan2

The comps of other similar sized cats noted, as well as their MD ratios, are not factoring in the hull shape below the waterline. The comps being used are of U-shaped semi-displacement hulls whereas the ITA has a flat planing hull. So the 1.50m LWL/Bh is the same as the hull bottom of the ITA minus a few mm loss from hull side to hull bottom transition.

[catmancan2]

Originally Posted by SVNeko

Thanks for the additional info. Can you explain what you mean with the following:

There are 3 basic hull shapes. A flat hull bottom displaces more weight than a U hull bottom and a U hull bottom displaces more weight than a V hull bottom. What I posted yesterday was only in response to hull immersion (sinkage rate). There are other factors involved in hull design. And each hull shape has its pros/cons.

The photos I referenced in my post yesterday didn't appear, so trying again, see below (maybe!)

[SVNeko]

Yes, that is what I thought. But I don't believe hull bottom shape matters for hull immersion calculations. Its already immersed. I believe all that should matter is the buoyancy of the horizontal cross sectional area at the water line and the volume it describes as it is immersed. Or am I wrong?

[catmancan2]

Originally Posted by SVNeko

Yes, that is what I thought. But I don't believe hull bottom shape matters for hull immersion calculations. Its already immersed. I believe all that should matter is the buoyancy of the horizontal cross sectional area at the water line and the volume it describes as it is immersed. Or am I wrong?

Good question but Neko I'm afraid I've reached the end of my rope of knowledge. So before I hang myself let me reach out to the architect for clarification. It may take a few days since the Paris Salon Nautique is in progress. In the meantime maybe someone smarter than me (it wouldn't much) can answer.

[iliohale]

Originally Posted by catmancan2

Good question but Neko I'm afraid I've reached the end of my rope of knowledge. So before I hang myself let me reach out to the architect for clarification. It may take a few days since the Paris Salon Nautique is in progress. In the meantime maybe someone smarter than me (it wouldn't much) can answer.

Assume an average hull width of 4’ bow to stern. Likely conservative assumption based on eyeballing the hull shape and the topside flare

49ft long x 4ft wide = 196 sq ft
1cm = 0.03281 ft
196 sq ft x 0.03281 = 6.43 cu ft per cm of hull depth
6.43 x 2 hulls = 12.86 cu ft per cm of hull depth
12.86 x 62.43 lbs per cu ft = 803 lbs or 364 kg per cm of hull depth

Back of envelop math = 364 kg per cm of hull depth which is in the ball park of the 480 kg cited by ITA. Based on the actual hull shape it is reasonable that the 480 kg per cm is correct. Let’s see what the designer comes back with.

Gary

[Multihull]

Originally Posted by iliohale

Back of envelop math = 364 kg per cm of hull depth which is in the ball park of the 480 kg cited by ITA.


Gary

Am I missing something?

A deviation of approx. 33% is in the ball park

Rob

[Catmandu]

Hey, maybe its a Big ballpark ...

[Catmandu]

In all seriousness, considering the tools that the designers of the ITA have their disposal it should be very straightforward to predict with great accuracy the volume of water displaced at various immersion levels. Once you have the volume it is simple math to determine buoyancy ... of course with some slight variations depending on the salinity (e.g. weight of the water displaced) etc ...

I will be interesting to see what the architect comes back with.

[BigBeakie]

I would be much more interested to hear the designers thoughts on the immersed and flat sterns re parasitic drag. To me that is the most noteworthy hull design feature.


Sent from my iPad using Cruisers Sailing Forum

[fxykty]

I’m curious about the flat bottoms, especially in the forward half of each hull. Going to windward or reaching in any sort of seas, the front third of the windward hull and first couple of metres of the leeward hull often come out of the water. The u shape of our hulls make it much gentler than flat hulls would. From my old IOR racing days, if we weren’t heeled over at least 25 degrees the flat fore foot was a boat breaker. Obviously not an option for a cruising cat.

Regarding planing, the reality is that most cruising is done at sub 10 knots average speeds, so most of the time the boat will not be planing AFAIK. If the boat is designed for coastal sailing and racing then maybe more planing, but at what speed do you consider to be planing? Is it anything after nominal hull speed, or something higher?

Regarding the transom immersion, very not good at lower speeds but probably becomes a non issue as the boat accelerates past 8 knots. But it will require more sail power - not a feature of a performance cruising catamaran.

All that said, an interesting new entrant and possibly will signal a viable new design direction. Thanks for all the information.

[catmancan2]

Originally Posted by BigBeakie

I would be much more interested to hear the designers thoughts on the immersed and flat sterns re parasitic drag. To me that is the most noteworthy hull design feature.


Sent from my iPad using Cruisers Sailing Forum

BigB raises a good point. A flat stern section will in fact induce parasitic drag. To counter that drag requires a hull shape to alter the water flow forward of the transom and of course not exceeding designed maximum displacement. ITA feels it is a better solution than the unintended drag of an overloaded raised transom.

Let's face it, boats are a series of compromises, the question each of us must ask is where do you want to compromise.

[catmancan2]

Originally Posted by fxykty

I’m curious about the flat bottoms, especially in the forward half of each hull. Going to windward or reaching in any sort of seas, the front third of the windward hull and first couple of metres of the leeward hull often come out of the water. The u shape of our hulls make it much gentler than flat hulls would. From my old IOR racing days, if we weren’t heeled over at least 25 degrees the flat fore foot was a boat breaker. Obviously not an option for a cruising cat.

Regarding planing, the reality is that most cruising is done at sub 10 knots average speeds, so most of the time the boat will not be planing AFAIK. If the boat is designed for coastal sailing and racing then maybe more planing, but at what speed do you consider to be planing? Is it anything after nominal hull speed, or something higher?

Regarding the transom immersion, very not good at lower speeds but probably becomes a non issue as the boat accelerates past 8 knots. But it will require more sail power - not a feature of a performance cruising catamaran.

All that said, an interesting new entrant and possibly will signal a viable new design direction. Thanks for all the information.

Yes, when beating to weather in certain sea states a U forefoot will pound less than a flat forefoot and a V forefoot will pound less the a U forefoot. I don't know if using an IOR design is a good comp, but I get your point. But there are ways to mitigate such pounding due to hull design and sail management.

The current hull designs with wave-piercing bows are intended to power thru waves as opposed to a U forefoot riding on top of waves. And for sure the less weight forward of the mast the better (for any hull shape). The ITA transitions from a V bow stem to a U forefoot before broadening out to a flatter planning hull shape forward of the hull rocker (which is also reduced in depth).

Of course there are times when any sailboat hull shape will pound giving the right (or is it wrong) conditions and as you know that's when a prudent sailor simply takes his foot off of the gas pedal and reef down which is a lot less stress on crew and boat.

The ITA sailplan is fairly comparable with other similar sized performance cruising catamaran designs with a 91m² main and 49m² self-tacking Solent. And the sail management system was designed for single-handing (really). This isn't our first attempt with this type of design since the design team, the build team, and myself were involved in a previous 14.30m project of which 3 were built. So the ITA will benefit from what has been learned and looking forward to nr.1 hitting the water next March. Only time will tell.

Did you follow the ARC Rally? See how well a sistership to yours did?