Trailer cruiser revisited... as a trimaran

[Rick Willoughby]

Mat
Can you post the light load linesplan for comparison.

Rick W

[marshmat]

Rick,

For the purposes of direct comparison, here's the tri and the cat both at 2.5 tonnes. The topsides have changed on both since I switched the models to Rhino, but from the waterline down there have been no major changes since these drawings. The undersides are pretty much straight out of Godzilla- faired smooth for the cat and made developable for the tri. (A little bit of strip-planking on the bottom of the cat is fine by me, but I want to be able to use sheet ply whenever possible.)

I don't have a light-ship condition weight for either yet, but the aim is for no more than 1.5 t on the trailer.

At 2.5 t, the cat's resistance at 10 m/s is 2.58 kN and the tri's is 3.07 kN, at least to the extent I trust Michlet calculations at such high a speed without accounting for sinkage and trim. 26 vs 31 kW indicated power to reach 20 knots, the fastest we would ever want to go in these.... I know that's a big gap by your uber-efficient standards, Rick (I've seen you fret on here about tens of watts), but for my purposes the values for the two are close enough to not be a factor in deciding which one gets built.

And yes, Fanie, my fiancee and I are planning to build one of these, in some form, hopefully soon. Sunset Chaser is starting to feel a little small, especially when we have to camp at lockstations in the rain.

[Fanie]

Question please... I work in Delfship. How accurate are the resistance compared to ie that of Rhino or Michlet ?

My cat shows a peak of 1400N at 8kn at 1990kg per hull. How bad or good is this ?

especially when we have to camp at lockstations in the rain

Sounds like perfect outdoor weather Back in the office the weather is usually perfect...

[marshmat]

Fanie,

At my maximum load of 1750 kg per hull, the "Godzilla's best" of the cat I've been mentioning here would have 1.16 kN resistance at 8 knots. So 1.4 kN at 8 kt / 2 tonnes would be very close to my calculations, and about the best one could probably hope for in a boat like this.

I am trying to figure out a way to get Michlet offsets out of Rhino, that would make life much easier.... I know it should be possible....

[Fanie]

Rick, what is the light load linesplan ?

[Rick Willoughby]

Mat
I am interested to see the light load linesplan for the tri to understand why you have them submerged so deep at full load. They seem much deeper than necessay. At light load I expect that it would be designed essentially as a monhull with the outriggers as surface skimmers.

Rick W

[Rick Willoughby]

Originally Posted by Fanie

Question please... I work in Delfship. How accurate are the resistance compared to ie that of Rhino or Michlet ?

My cat shows a peak of 1400N at 8kn at 1990kg per hull. How bad or good is this ?


...

Michlet will be within 2 to 3% of actual drag. If the others agree with Michlet then they are also accurate. If they do not agree with Michlet then they will be inaccurate.

Generally I have found using Delftship drag data for slender hulls is next to useless for design purposes.

Rick W

[marshmat]

Originally Posted by Rick Willoughby

Mat
I am interested to see the light load linesplan for the tri to understand why you have them submerged so deep at full load. They seem much deeper than necessay. At light load I expect that it would be designed essentially as a monhull with the outriggers as surface skimmers.

Rick,

The attached is at 1.25 tonnes total, which is in the ballpark of what I'd be aiming for in trailering condition (so including engine, fuel, etc. but not crew or cargo). I have not yet done a detailed weight estimate.

The relatively large amas are the result of a bunch of experiments with Godzilla. In my earlier drawings, they were fairly small and just skimmed the surface, each carrying less than 1/6 of the total displacement. Optimizing at various speeds in the 5 to 8 m/s range (10-16 knots) with various combinations of my own hulls and spacings versus Godzilla's unconstrained series-8 result, Godzilla would almost invariably make the amas wider and deeper than I had originally planned; at 2.5 tonnes displacement, each ama is carrying about 1/5 of the total weight.

I am not entirely finished with the Godzilla tweaking, and still need to do some runs on the tradeoffs between optimizing for different speeds and different goals. A 32-parameter unconstrained problem is a bit much for it to solve in a single shot. I may yet modify a few things, including the vaka/ama ratio.

The resulting wetted surface area is about 19 m^2 at 2.5 tonnes- quite high. Surprising to me, since viscous drag is usually so dominant in a fast multi. But constraining the amas to be small and the vaka to be large, given my length limit, always seemed to result in something with sufficiently higher wave drag to offset the reduced wetted area.

As drawn, the tri's wake with 2.5 t displacement is pretty tiny- see below. Despite being almost 10x heavier than my current boat for only a 60% increase in length, it can't make as big a wake as I can now...

[marshmat]

Ok, so a lot of tweaking and a heap of Michlet/Godzilla output files later....

See attached. This is at 2.5 tonnes displacement (1.9 t for the vaka and 0.28 t in each ama) with the ama centrelines offset 2.0 m to each side, vaka is up to 81 cm beam thus L/B = 10. These proportions are a bit closer to what I was originally thinking of.... Godzilla needed a bit of coaxing with tweaks to various constraints to get where I wanted to go. But all is well, and about 700 N of drag have been shaved since I first posted the design on here. We're down to a hair under 3 kN at 10 m/s. I've been smoothing and fairing things a bit in Rhino to make things close to developable, and can't yet get shapes from Rhino to Michlet, so there's probably about a 6 to 10 percent uncertainty on the resistance values mentioned above.

[Rick Willoughby]

Mat
The light load makes sense. Not quite what I expected but Godzilla has the habit of making sense once you reason it out.

My faux-tri is really a monohull with trainers. I have not looked carefully at a range of loading states.

Rick

[Fanie]

Hi Matt,

You may gain some space if you fit the shape of the ama's and the centre hull into one another, so folded there's no waisted spaces.

[marshmat]

Thanks for the advice, everyone.

Here she is at present (13th iteration): folded and open as viewed from the bow.

Displacement figures, to give an idea of how big she is and how that volume is distributed (corresponding to the two waterlines in the images):
35 cm draught, ama 44 kg, vaka 1180 kg, total 1268 kg.
50 cm draught, ama 272 kg, vaka 1939 kg, total 2483 kg.

Next up is to confirm loading and stresses on those crossbeams, and get their linkage figured out.

[Fanie]

Like your terminator look alike on the boat

Have you got a wire frame drawing you can post of the rh drawing ?

[Willallison]

Matt, have you considered using what is often referred to as a displaning hullform for the vaka? By this I refer to the type of shape originally conceived (I believe) by the late Malcom Tennant and widely adopted throughout the powercat world these days.... Basically a canoe stern below a wide flat transom / chine, like below...
There are a number of advantages, not least being their ability to resist squatting as speed increases. Also, I think there may be some benefit in terms of load carrying capability.

[marshmat]

Fanie,
More drawings will be coming, soon. Hold yer horses

Will,
I'm somewhat familiar with Tennant's 'displaning' stern, but had not considered it for this case. I'm under the impression that it's more of a pure-powerboat shape: tracking like it's on rails and performing at its best when pushed to relatively high speeds. I'm not sure how such a hull would perform under sail (I would like to be able to sail this boat, although it is more on the motor side of motorsailer)- but any insight would be appreciated