Trailer cruiser revisited... as a trimaran

[Ad Hoc]

matt

you seem to be plugging away rather nicely

As for mono-tri...it is your project and you must live with it. Forget which is "most" efficient" or "better at lower speeds" etc. All nonsense.

Focus on what YOU want. If your boat in either configuration can deliver the SOR of your choosing, then fine. So it comes down to costs. If both ahve similar costs, not just capitol costs but running costs and maintenance costs, fine, if not go with the cheaper long term one. Finally safety. Which would you feel most safe in following an accident/collision.

That is the bottom line now you've come this far.

[Willallison]

That's what I was getting at when I suggested that Matt re-visit the mono, which I think would be a much easier boat to live with. Having said that, efficiency was one of his earliest stated design objectives.

Matt,
Pitty you can't build them both... it'd be interesting to see just how close to your predictions are to the reality.
Must say I'm surprised that the mono works out heavier and that the (theoretical) efficiency of the two is so far apart.
For 'typical' production boats, I agree with your comments regarding the use of planing hulls for lower speeds. A solid case can be made for the planing hull, however, as lonmg as bottom loadings are kept low

[Mat-C]

What's SOR short for?
I gather it's an acronym for the design objectives....

[Ad Hoc]

Statement Of Requirements = SOR

..in other words, what you want the design to achieve. This is then condensed into a specification and General Arrangement.

[marshmat]

Thanks for the insights, Will / AdHoc / yipster

Just in the interest of putting all this in perspective (and, of course, just for the hell of getting out on the water), Katy and I have been taking Sunset Chaser (a 5-metre Bolger runabout) out into some of the cruising grounds we'd be frequenting in the new boat. Some observations:

- At our cruise speed of 16-17 knots, we literally fly past the car ferry and virtually all the other powerboat traffic.

- 22 knots, in these waters (Lake Ontario, 1000 Islands and the Rideau), is just screaming along, fast enough that it becomes hard to keep track of the 40 other boats around you, and puts about fifty interesting destinations within less than an hour's trip.

- There are a lot of heavy, deep-V boats in this area that like to plow around at 15 knots, bowsprit pointing at the clouds. The resulting confused mess of large, steep wakes makes it hard to sustain high speeds for long.

- Canadians pay A LOT for gas, currently about a buck a litre (= $3.25USD / USgal), it was up to $1.40 during the spike last summer ($4.55USD / USgal, still cheaper than English petrol, though). Although there's no "official" evidence of collusion, it's well known that Canadian fuel prices will, at all fuel outlets, simultaneously skyrocket in response to any glitch in the market, and take weeks to settle down again. So a low fuel demand is a bigger priority than speed.

[Willallison]

There's no doubt that speeds on the high side of 20 knots are the exception rather than the rule for most cruising folk. That's why I was determined to produce a craft that was comfortable throughout the lower end. It's nice to have it on tap, of course.

They don't exactly give fuel away here either. Petrol runs at about $1.20 / litre. Diesel uo to $1.50 / litre
To be honest, I don't think it makes much difference. If it costs you $100 to go away for a weekend, as opposed $80, does it really matter? Of far more importance is how easy the boat is to operate.
Additional range is an obvious advantage for lower fuel consumption - and indeed that's why I went for the diesel in Graphite. The additional cost of the unit (about 20K) will never be recovered.

[marshmat]

Hi Will,

Being able to do the whole Rideau on one tank, with a fair bit to spare, is the absolute minimum range we need. (The Rideau is about 100 naut.miles if you go straight through, but with a bit of exploring it's easy to do 150 in a week.)

I'm currently looking at carrying about 250 L (66 gal) of fuel. Now, as we all know, outboard manufacturers are not keen on publishing detailed fuel consumption charts. So taking the old assumption of 10 hp/gal/h for a small gas engine, and using the boat's estimated drag (2.5 t loaded displacement) of 1.5 kN at 5 m/s (10 kt), propeller efficiency 60%, we find ~17 hp being used at 10 knots. So about 6.5 L/h (1.7 gph) to go 10 knots, ie. a mile and a half per litre (3 km/l, 32 L/100km, or about six nautical miles to the gallon).

That would suggest a range at 10 knots of about 375 nautical miles. Gunning the throttle to 18-20 knots drops that to about 200 miles; dropping to 7 knots gives about 11 nautical miles to the gallon for a 750 mile range.


Also, some structural updates, now that I've had a chance to sit down and run some numbers:

I've been looking at combinations of broaching, torquing and racking loads suggested in various standards (oftentimes they refer to catamarans only, so there's some engineering discretion involved in translating the load cases to a tri). So, for the engineering types here, a couple of the loads we're looking at are:

Wave torque (crest under port stern, crest under starboard bow, trough under vaka): Approx. 41 kNm

Broaching / bow burying: Approx. 11 kN up or down, approx. 2 kN lateral, acting on the bow of the ama cantilevered out ahead of the crossbeam

Now, this boat is likely going to get beat up on occasion- hitting rocks and logs, running up on beaches, bumping docks rather hard, landing incorrectly on the trailer (or hitting potholes while on the road), etc. I don't want to hit this stuff, but I know it will happen now and then. So I'm designing the outriggers, crossbeams and their attachments such that it will take a 28 kN load up, down or inward on the end of any one crossbeam to break anything.

Overkill? Definitely. It means, among other things, that the lower folding strut will have a breaking strength of 73 kN, about seven tonnes. And, importantly, it means that if everything does go to hell, things will fail in the order I want them to fail: damage to the outrigger will be contained to the outrigger, without affecting the integrity of the crossbeams or their mounting points on the main hull. But, lo and behold, the level of strength I'm aiming for in the crossbeams is achievable with a box-beam of oak or other decent-quality wood, 2.5 cm thick walls, 24 x 24 cm. And, under that 28 kN load, such a beam only deflects about seven millimetres.

The ring frames in the vaka at these points will of course have to be strong enough to safely pass these forces into the hull. But the crossbeams still end up being a substantially lighter structure than I had been planning on.

[Ad Hoc]

Matt

One Q...have you included any FoS in your load cases, or just used the derived values as absolutes?

Good to see you're doing a deflection check. The low modulus materials you're using means that deflection is the 'driver', not stress.

One load case, perhaps?
Have you considered you chugging along full chat, when the port or stbd ama hits a floating abject head on?...this collision slows down and stops the boat...using your values, this gives around 40kN load applied on the ama. So the cross beams will be in shear and tension (depending upon the stiffness possibly direct in-plane bending too), as it tries to pull away from the main hull.

In your fuel consumption figures, allow 10% unusable at the bottom of the tank. And and 10% for genny use too.

BTW, always wondered, what is vaka?

[marshmat]

Vaka = largest hull of a multihull, ie. middle hull of a tri.

Safety factor = yes, the 28 kN up/down/inward load is after the FoS is applied. Torquing is the worst-case situation on the water (ie, no hard objects involved) and I want an FoS of about 3 in this case.

10% unusable fuel capacity has already been deducted when I talk about 250 L capacity. No generator is fitted as the electrical system will be minimal, only as much as can be charged by the outboard's 25 amp or so alternator.

The case of hitting something is one that I'm not entirely sure how to handle. It could be treated as a simple racking motion (ama stays parallel to main hull, but is forced directly aft). Or one could consider that the boat will yaw sharply on impact, thus the ama would tend to pull out and up.

(Each ama has at least two, possibly as many as four watertight compartments, and their skin is simple and easily repaired. The vaka has three separate watertight compartments in the first three metres. Thus, a hull breach- undesirable as it would be- is not a major threat to the safety of the vessel. As a result, in a major impact, it would be preferable for the ama or the forepeak to be damaged- much like the engine bay of a modern car- rather than breaking something more critical.)

[Ad Hoc]

Vaka, i suspected, but not used to the nomenclature.

I would take the momentary impact. The ama is forced aft, relative to the main hull. So you have a racking moment & shear (depending upon the stiffness) and possibly bending too, since it cannot perform as a truss. (ie classic truss theory m=2j, j=no. of joints).

Once yawing occurs, the load transfer is slightly more complex and somewhat difficult to fully ascertain without spending a lot of detailed analysis, since how much yaw will occur and will the yaw of the 2 hulls be the same relative to each other?..which ever way, the load would be less in a yaw case, owing to the damping.

[Willallison]

No - it's frustrating the way diesel engine manufacturers happily provide power / consumption curves, but outboard manufacturers do not. I guess all you can do is look at some of the published test data from reoutable magazines... though not definitive, I'd think they'd give a more accurate indication than the old 10 hp/gal/hr....
I installed diesel in my current build for similar reasons - there are few places that provide petrol refuelling and the increased range was one of my objectives.

[marshmat]

OK, I'm back. And no, the boat hasn't died. I'm getting married in a couple of weeks, though, and have just started a new job- so, obviously, the boat has had to wait.

HULL

Two shots of the underwater surface are attached- one at 1.3 tonnes displacement (ie, empty) and one at 2.5 tonnes. This may help to clarify what I'm really after with this hull shape. The aim is for something that's quite efficient at low to moderate speeds, and will cut cleanly through the short, heavy chop we get on Lake Ontario. (Pounding into this stuff on plane just doesn't work- but we have a few Farrier tris and various midsize cats around here that can knife right through it without much pitching or spray, and that's the behaviour I want here.)

At the same time, it needs to be a hull that is stable, can carry a fair bit of cargo, and offers enough deck area to be a suitable "party boat" for day cruises. This way, the central cockpit can be kept sheltered and separate from all the busy and/or wet activity out on the amas.

As I've mentioned earlier, in 2.5 tonne loaded condition, the total resistance at 20 knots should be roughly 3.0 kN, ie. 30 kW indicated power before propeller inefficiencies are considered. We would likely be considering a "big foot" style 50 or 60 hp outboard as a suitable match; such an engine would be quite comfortable loafing along at 1/2 to 2/3 throttle which would give 12-16 knots or so in typical loading conditions.

The boat could also be built with an inboard diesel, if desired. However, it is unlikely we will go this route, as neither the range advantage nor the fuel price advantage is substantial enough to warrant the extra cost in this particular case.

COCKPIT

I don't want a pontoon boat- they aren't pretty, IMHO, and they are no fun once the waves are higher than deck level. But they do have those wonderful, ranch-style gates that let you walk on, level with the dock- no gunwales to clamber over while carrying a 60 lb cooler. Those same gates will be found on this boat.

Whether it's swimming, boarding, fishing, carrying lumber or just lounging around, we've decided that fold-down rigid decks to bridge between centre hull and outriggers would be preferable to the trampoline nets used on sailing tris.

There's no need for much of a galley, since we'll usually be docked or beached when spending the night on board. But a small sink will likely prove useful.

A comfortable helm station is a must, along with a convenient place for charts within reach of the helmsman. On the Rideau, one will typically have several sheets at 1:20,000 that are being referenced every three minutes or so; I want them easily visible at a glance.

Whether it's for dinner, for cards, or for prepping fishing tackle, there's something to be said for a decent size table- so a table for 4 is included; it'll drop down to form a double berth or, with the addition of a filler cushion at the boarding gates, a queen-size.

STRUCTURE
I'm plugging away on structural calculations as time permits. The vaka 'wings' are non-structural, supporting only their own weight plus whatever people sit on them. A beefed-up stringer at the wing-to-vaka joint will handle the longitudinal hull girder loads in this area. Outrigger strut (aka) loads will be resolved via a pair of ring frames at each aka, distributing these stresses into the main hull.

The folding linkage- I don't have a good drawing to post yet, but I'll put it up when I do- bears a slight resemblance to that of a Farrier tri, but with somewhat different geometry. The lower (diagonal) strut, running from the bend in the aka down to the main hull side, is the most highly stressed component of the assembly; it will have to have a breaking strength of about 70 kN in pure tension or compression. I'm thinking stainless steel or aluminum box section with a simple bolted pin joint at each end, suitably sized.... but would proper bearings be preferable?

[Ad Hoc]

Matt

Nice to see you're still plugging away.
So, with your 1.3 tonne, i assume you have now done a detailed weight estimate, with margins?

I'm confused when you say a breaking strength of 70kN. If you haven't selected the material, nor the section, how can you say a breaking strength of 70kN? Or do you mean, with the current geometry that you have drawn and thicknesses etc, the load imparted to this structural member, you ahve calculated as 70kN?

[Willallison]

I'm getting married in a couple of weeks

Haven't read the rest of it, but congrats (or comiserations...)
This being a boat design site, I feel it encumbent upon me to point out that this move will SIGNIFICANTLY alter ALL design decisions you will make in regards to your own boat.
Shower is the 1st word that come to mind
Closely followed by sunpad.
Galley, sadly, will probably change from one of 'her' priorities to one of yours.

The list is endless... but fear not... we are here to support you....

[marshmat]

Clarification: I mean that, given the geometry of the linkage and the loads applied to the outrigger, with suitable safety factors, the load on that strut when it fails in pure tension or compression should be 70 kN. There is also a small bending load on that strut under some conditions, that I have yet to fully analyze.

Yes, ~1300 kg empty is the result of a full weights-and-moments calculation, including a healthy complement of loose equipment. The bare hull is closer to 830 kg; the rest is driveline, tankage, interior fittings, equipment, etc.

And, Will, this may come as a surprise to many.... but the lovely lady has been involved in this one pretty much since the beginning, and has been guiding many of these decisions. (And she's just as good with fibreglass as I am, if not better....) This is going to be our "general purpose local" boat- camping, weekend cruising, carting lumber around, fishing, partying with friends. We're fully expecting to build something bigger and more elaborate once we're experienced enough to be comfortable with some more serious cruising.