Off Kilter – Part I (with so much to say about the Invincible hull design there is a Part II!)
by John Curnow, Global Editor, Powerboat.World 31Aug 2022
Just one little thing swung the whole deal. ‘It leans inward during a turn’ was a not a headline on the press release, just a little stanza, part of a more complete sentence, about the second or third paragraph in. Wow. To me this WAS the whole deal, right there. In flat water the Destroyer-esque list to the outboard side of the powercat at speed is simply a little unnerving, especially to the uninitiated. Out at sea, well you definitely want to plan the motion into your action!
So at the recent Sydney International Boat Show I got to see the Invincible powercats up close. Very close, for they were resting on their keels inside the Darling Harbour Exhibition Centre, and there was also one Ian Birdsall on hand to talk through the aspects below.
The other little tip, was another name. This one being Morrelli and Melvin, which – given our strong sailing associations – was easy to latch onto, for they do multihull design. So having the Invincible 37 exposed made it very easy to establish the ‘why’ and ‘how’ in relation to the turn in. Also, it is no wonder it is also patented. It’s smart. It’s ace (as in asymmetric). Boom. Boom.
Crack on – Crack it open
OK. First and foremost, there’s the asymmetric form. Then there’ll be the deadrise. A significant deadrise on the outboard edge, and shallower deadrise inside, which also works in with different rocker geometry. Then finally you’ve got an increase on, or if you like, a flair in the freeboard going down the gunwale, probably about a third of the way back down the boat, and then finally the two steps allow the boat to rest in the water lower at rest or trolling speed, and then obviously provide lift when under way.
Like everything, it’s always a compromise. So how have Invincible made their platform, as it were? Ian Birdsall explains, “There’s a massive difference between a sport fishing boat, and your regular catamaran design. The sport fishing boat’s got to be able to function in a variety of circumstances, including carrying a lot of weight in gear, ice, provisions, fuel, and humans.”
“That’s hard to design for, especially as to where it will be placed. Apart from the outboard list or heel, in a decent seaway, a cat can return the spray from inside the two bows straight back out and over the leading edge of the cat. Also, it comes at you with interest, due to the apparent wind as a result of your speed. Another is a certain predisposition to hobbyhorse.
“So you try to eliminate as many of those things as you possibly can, and to get those attributes in a 10-metre boat can be more difficult than it is in a 15, particularly when it comes to weight distribution. So the asymmetric hull design makes a huge difference to the performance capabilities of a boat, particularly of this size.
“Trainspotters will note the centre of the keel line is favoured towards the inboard edge, which is evidenced by the image with the height of the top chine. However, by having a steeper deadrise on the outside, as compared with the inside, you’ve effectively allowed the boat to fall, and this is what allows the boat to roll into turns.
“You’re reducing the lift on the very inner side of the turn. So when you go into a left-hand turn, let’s say, the outer hull is moving slightly faster of course than the inner hull. That creates more lift in itself, driven by the inboard edge of the outer hull.”
As mentioned, a look at the wide/deep chines also gives away a bit. The ones on the outside are significantly higher than they are on the inside. So this allows the boat to roll further before the chines start taking effect and creating lift. If those chines were equal, the boat would actually want to stay horizontal. If it did that, then the centre of gravity of course would roll it the wrong way automatically.
Now you also have to deal with rocker, as pertains to not going down the mine, and lift in the sense of CoG, and CoB. Ian continues: “There’s a fair rocker in this hull. If you look down the length of a boat, usually you’ll find that in most hulls, when you get to midships it will be fairly constant from then on to the transom. There won’t be much change in the actual surface running area over the boat in terms of rocker. On this hull there’s a significant rocker, basically from the start of the running surface to the stern, which essentially allows the bum to sink in.”
“With the Invincible there’s continual rocker, and what you’re creating there is further lift forward, particularly in the lower speed range where you really need it to keep the bow up, and if you’ve got a lot of weight in there, then of course it’s even more significant that you can do that.”
The thing to note here is that a cat offers huge storage potential up for’ard, and the Invincible 37 can swallow 250-300kg up there in a heartbeat.
“This particular hull shape worked out for us incredibly well. We put a huge amount of time into developing it. I should also point out that the 33 is the only Invincible with trim tabs, simply to provide a bit of bow down when running very lightship. Every other time, you simply don’t need them. It is a function of overall length, and the amount of inbuilt lift we have right up for’ard.”
Stairmaster (Yes, it is all about steps)
“I think steps are often misunderstood. A lot of people have steps on boats and I don’t think they really know why they’ve got them there; it’s sort of trendy. The steps on this hull serve two purposes. The first is to allow the stern to run lower at trolling speeds. Less lift, and CoG aft means bow runs high.”
“At higher speeds they come into their own and do what they originally were designed to do for high speed race boats. That is to reduce drag by removing some of the running surface area, which means less wetted area to push along. Now the steps on the Invincible are significant. These are massive voids in the hull designed to do a job, not look pretty,” said Birdsall enthusiastically.
So there remain the three key elements: a variable deadrise; significant change in the rocker throughout the length of the hull; and the size, positioning and the function of the steps.
Flat is good
Morrelli and Melvin, with massive experience in the yachting world, brought much to bear – even though a 55-60 knot offshore fishing boat, and a racing yacht/multihull might not seem comparable straight off the bat. “Yachts have to perform at different levels of weight distribution, different levels of heel, power, you name it. So there are a lot of similarities, but the one thing we really needed to get in this particular design – and this goes for all of our cats – is the maximum amount of interior space we could, for the size of boat.”
You may think that opting for long overhang with the bows would assist with that, but the trade off with weight for’ard of the waterline is abysmal at best. LWL and LOA need to be close. Very close, just as with a racing machine, where LWL may actually exceed that of the deck line (dreadnought bow).
You also don’t want any steps inside the hull, which is another issue with cats. As you go higher and higher moving for’ard, at some point you will need a step. Unlike the ones underwater, this is not good. Really bad in fact when you’re chasing a fish around. So Invincible have a flat deck on every model (and I have to say some of the most capacious under deck storage (including fish wells) I have ever come across!
Got a job to do
“Storage capacity was absolutely paramount. I mean we build Invincibles as workboats. They are built for a purpose, and that is to go out there and fish like hell. To be out there all day, and go 150 miles offshore if you want; no problem. To do 150 miles out and back in two and a half to three hours each way. Not surprisingly, we not only have private owners using these boats, but there are a lot of charter operators doing 2000 hours a year. You know these are workhorses,” commented Birdsall.
That’s busy, for you need to consider that the average private boat would be 250 hours a year. It’s not unusual for an Invincible to come back in for an overhaul, where they go back through our facility, with motors that have got 2500-3000 hours on them, and they’re only a season and a half old. So these get worked hard, and for the private owner it means the durability is incredible. A private owner can buy one of these boats and it will last them a lifetime if they want it to.”
“They’re built to such a standard; as a work boat; as a tool to do a job.”
Same ethos, different boat
Naturally this means there is retained value, as well, and that is revealed in build and structure. It has been an investment for Invincible too, for each is not a scaled version of the other, but rather its own beast, which is evidenced by the fact that the 33 is really the only one with a narrower beam, and the others are nearly the same as each other.
Technically I could get caught out there, for the soon-to-arrive 46 Pilothouse does have the same hulls as the 46CC, but different liner, so you get the point.
As Ian says, “Cats are really difficult to design well. It’s easy to build a cat; very easy to design and build a cat, and so very hard to build one that works well.”
Well, if that’s not a line to finish on… Stay tuned for Off Kilter Part II soon enough
Indeed, Off Kilter Part II is now ready to read right here
Finally: please look after yourselves,
Global Editor, Powerboat.World