TT720 float buoyancy

Colin Lindsay

   I have been reading the archives here and on the general F-boat list, in anticipation of taking ownership of a TT720 later this year. I am impatient to go sailing, but for now I am in total dreaming mode and trying to read everything about these boats.

I haven't seen the trailertri 720 float buoyancy quantified anywhere online, only the general description that it is something less than 100% of the total boat weight. I tried to guess by eyeballing dimensions from the plans and calculated that each float when fully immersed might displace 70-95% of the boat's weight (listed as 1,230 kg on But that is a big range (depends on how fast the width tapers towards the ends), and to get to the buoyancy force I'd have to subtract the weight of the float materials from the weight of the water displaced when it is immersed. Does anyone know the designed float buoyancy? 

Try Flying

Dont have a definitive number for you but can tell you that it is impossible to lift the main hull out with the float flotation. I've submerged mine by about 1 foot over the deck and it's slow. Ian purposely designed the TT to heel over and spill wind in a capsize type event. The narrower stance and limited float volume ensures this. You will heal more than a later design but not as much as  a mono... and generally go faster.

Desert Maid

No numbers, but the boat definitely pivots around the main hull. This is not an issue in general sailing as the float will immerse then rise as boat speed increases. Sailing with crew out to windward is a definite advantage. Where the design comes into it's own is being hit by an extreme gust. I have had this happen once as a big gust coming off a bluff. Sailing solo, the boat went over with the leeward float well immersed, the main spilt the wind and the float came up again. Exactly as it was designed to do! That was once in thirty years of sailing my 680 and another persons' 720 with big rig.
They are not the latest design by any means, but still great fun to sail! I humbly suggest that you keep the boat as standard as possible for an easy, comfortable sailing experience. Big rigs etc. are fine if you have a competent crew, but they can quickly get out of hand. I have skippered both and really enjoy my 680.


Hi Colin,
I bought TT120 #47 "Triple Time" in 2014, and have the digitized set of Ians plans.
I am a naval architect and have created a CAD drawing of the boat based on the plans.
Based on the CAD drawing I calculated the moulded (inside) volume of the float to be 0.853 m3 (that is exclusive the plywood cladding). 
Some years ago when crewing on another TT120 on Pittwater we once had a near capsize in a sudden hard southerly gust where the main sheet jammed and the boat heeled over until the mast was close to horizontal and the cabin side went under water, that is one float about 1.5 meter under water. The boat came up again very-very slowly as it rounded up into the wind. We were fairly shaken, but continued racing.
Based on my experiences with the TT120's I reckon Ian Farrier was a very good designer. As I understand it from Ian's responses on the multi-hull fora, he purposely designed the floats with a buoyancy less than 100% of the boat weight. When the boat heels over excessively and the float and beams go under water the boat will slow down and round up into the wind and the wind will then get in under the sails and help righting the boat up again. A very good safety feature, but of course a drawback when you are pushing the boat to it's limits for maximum speed.
Happy sailin

Try Flying

Yeah my experience was ;less dramatic but also on Pittwater being hit by the full force of the wind as we cleared Barrenjoey. The mono who was attempting to pass us faired worse and got severely rounded up...  he also had a spinnaker up. we dipped the float,  I dropped the traveller a bit and we accelerated away.

Try Flying

While we are on the subject of buoyancy the fore aft characteristics are also interesting. Given how fine the bows are and with flat decks I was initially concerned about getting tripped similar  to a Hobie cat. This has proven to not be a big issue in our experience and while we don't have massive fwd low down buoyancy and sharp wave piercing decks like a F85sr or more modern racing cat what I've found is that our sweeping fwd folding strut fairing acts a massive end plate that pushes the bow up if you get that far. I only have anecdotal evidence of this as I'm not that far fwd to see whats happening if we are that hard pressed but I did get the stern of our boat well clear of the water one fateful sail with a big arse square top and 20 knots up the bum while "surfing" 2m swell... yes I was way out of my comfort zone and I was a bloody idiot for not reefing earlier.

The one thing that  I can say definitively is that the TT's do take care of us idiots and is a very capable boat even while coastal sailing

Colin Lindsay

Thanks for the stories! I didn't ask in order to plan any major changes, just out of curiosity and to try to add what seemed like missing info in these archives. I plan to use the boat for family cruising with a toddler, so safety is good. As a monohull sailor I generally like what I have heard about how the TT720 gives a bit more feedback than more racy multihulls.

I also just like numbers. I realized that the weight of the float materials is included in the total boat weight that is trying to sink the float, so no need to separate it out. Starting with Hans' CAD number:

0.854 m^3 × 1024 kg m^-3 (seawater density) / 1230 kg = 0.71
The float probably displaces a bit more than 71% of the boat weight (in salt water...)

If the skin of one float is about 7.5 sq m of 4 mm ply, that volume displaces only another 2.5% of boat weight. Maybe you can get to 75% with the parts of the beams that go under. 

Interesting to think how sensitive that fraction is to weight added. And how there would be a couple percent less righting moment in fresh water!

Try Flying

Tris have the benefit over cats in that your righting moment is increasing and stays that way until you start raising the center hull out of the water whereas on a cat its all down hill as you push harder Probably saying this wrong but basically you know sooner with a tri.
With our narrower stance (compared to later models) you have zero chance of lifting the main hull significantly. but what you do have to worry about is the fact that once submerged they could take a bit to raise back up. This is where open weave nets are your biggest friend as the windward side pushes you over harder. The flat decks don't help either due to the water resistance coming back up but you really have to get smashed to put the boat over that hard.

As a family cruiser its an awesome boat and the wide deck space is a great place for the kids to play.

Its fine to mod but I tend to think along the lines of comfort mods compared to hull shape ones..... although I have contemplated increasing volume in floats if I did new ones... maybe. 

All Farriers tend to "squat" down at the rear so the biggest performance gains is keeping weight out of the arse and the easiest way to do that is have tiller extensions out to the nets. I've been toying with the idea of dynamic buoyancy by adding an adjustible T foil to the rudder cassette (ala Hullvane) but that's just because I like to tinker

Danny Mydlack

On Mon, May 10, 2021 at 08:59 PM, Try Flying wrote:
Mark, could you point me towards more information on hullvane. I google it but didn't come up with much. Thank you. Danny

Try Flying

Your google is broken :)
Not sure how much is hype but they had some interesting test cases and when your doing this on vessels that size its not vapourware. I was interested in their concept and explainations in their videos as a basis of trying something on the stern of our boat. I was thinking more a ~500mm wide T foil that is actually bolted to the cassette rather than the rudder and around 100mm to 200mm under the waterline. If I made it adjustable I could fine tune it and being bolt on easy to remove if it is a dud. Just back of napkin idea for now