Trailer cruiser revisited... as a trimaran

[marshmat]

Originally Posted by Rick Willoughby

Matt
I also consider the situation where you are at design cruise and also need to push into a stiff breeze. So at 10kts cruise and 20kts wind you have 30kts apparent. The wind is now costing you 10HP. Probably as much as the water drag.

Rick W

A good point. But I think if I were beating into a stiff headwind, I would not be doing the 10 m/s (20 kt) speed at which wind costs me ~4hp. More like half that, probably less. Doing 10 knots into a 30 knot headwind (insane skipper?) costs ~16 hp in windage, but the boat only needs about 10 hp to overcome water and wake at that speed... add a bit for bashing through the waves.... there's still plenty of thrust on tap to handle the headwind.


Ad Hoc:
I'm not aware of any regulation around here that a pleasure boat must have a power windlass. I don't think I've ever seen a boat under 8 m that does.
"lcg chase for your powering estimates" - not sure on your terminology here, "lcg chase" means....?
I think I've got a good grip on the load paths, but any advice on load conditions and paths will of course be welcomed

[Ad Hoc]

lcg chase...this is what you do when doing tank testing.

Basically you have your model at the DWL and/or predicted displacement., level trim...run the tests. You then perform the same by moving the lcg fwd (of current location) 0.5%, 1.0%, 1.5% and 2% and the same aft. This then produces a resistance curve showing how the EHP is affected by the lcg position. You then design you boat for the "optimal" location, which may not necessarily be where you want the lcg to be, since it means moving many eqpt around, again!

Once you ahve drawn up a basic scantling, I'd be happy to have a look over them for you?..or if you have some sketches of what you're proposing?

[marshmat]

Ahh, OK. Yes, model testing in various weight distribution configurations is definitely a good idea. I can sort-of simulate that with different trim settings in Michlet, but I would like to put together a test model before building full-scale. Right now I have no shop, and no access to a shop, so a model will have to wait for the time being. It's worth noting, though, that there really isn't much equipment on this boat that can be moved around- there's an outboard engine, its fuel tank and batteries, the anchor, bilge pumps... the rest is basically all loose gear. The crew weigh more than the sum of all the equipment; the LCG shifts by up to 5% of LWL just by the crew moving around the cockpit area. It's not nearly as big a boat as its LOA would suggest.

I'll post some more structural details when I get a chance. An extra pair of eyes to look things over is always good

[Ad Hoc]

matt

Having done the lcg chase you will then at least know when or why your speed has increased/decrease owing to minor changes. Most designs/designers say put a trim tab or fix a wedge at the back, that'll make her go faster!...the only reason why they say this is because of ignorance of their hulls resistance data. If no lcg chase is performed how else do you know trim tabs will work, you don't. But on sea trails when the boat is a bit heavier than predicted and she isn't quite doing her speed, lots of head scratching....then someone suggest, look the trim is a bit high lets fit a trim tab....low and behold she runs better. Magic, lets do all our boats this way!...nope, just basic poor design.

When you do your lcg chase, you will at least know how flat or curved your EHP curve is. A good design will have the EHP being flat or near flat (at your designed lcg) or as the lcg goes aft, well inside the 1~3% 'zone'. Since all boats from conception to finish, their lcg's move aft, never fwd! So if your hull has a relatively flat EHP with change in lcg, you know it is not onerous. BUT if yours varies widely, you may have to sit further fwd all the time!

We always build models and tank test them. I'm shortly going to build a 25m test tank in my back garden. A CFD prog is ok for just doing some basic parametrics, but i personally would never trust the results. I know others do as that is their only frame of reference; the numbers!

I'll be interested to see how she comes together..

[marshmat]

Without a few experienced experts and some solid real-world benchmarking, I generally consider CFD results to be about as reliable as my government's ten-year budget forecast.


Scantlings

There's no requirement, in my case, for the boat to be built to any particular standards body's code. The applicable standard (Transport Canada TP1332E) requires that "Structural strength shall be commensurate with the intended service of the small vessel, taking into account the maximum anticipated loads." Any applicable code- NBS, ISO, ABS, Lloyd's, BV, DNV, or common engineering practice- is allowable, based on the designer's engineering judgement. (There's also 150 pages of somewhat more detailed, performance-based requirements for all sorts of systems, but no prescriptive methods.)

I'm using the following design loads for the hulls, loosely based on ISO 12215 for class B service (even though this boat would actually be class C):
Vaka bottom skin: 14 kPa
Vaka topside/deck skin: 10 kPa
Ama skin: 10 kPa
Watertight bulkheads: 7.7 kPa
For a marine ply / epoxy / fibreglass skin with 20 MPa tensile strength, and using the largest unsupported span of each case, suitable dimensions would be:
Vaka bottom skin: 12 mm (9mm ply + 3mm glass)
Vaka topside/deck skin: 12 mm (9mm ply + 3mm glass)
Ama skin: 9 mm (6mm ply + 3mm glass)
Watertight bulkheads: 12mm ply
Non-watertight bulkheads / ring frames: 12 mm ply, minimum 100 mm in moulded dimension

The structure is essentially taped-seam plywood construction, plus a few longitudinals where appropriate (keel, chines- not shown in the sketches below).

Each of the four akas is designed to handle 28 kN in any direction, the force being applied where the aka joins the ama. (This figure is arrived at by simply finding the maximum buoyancy the ama can exert if fully submerged, splitting it between the two akas, and applying a safety factor of 3.0.) They are 240 x 240 mm box sections, laminated of wood (species TBA, but most likely fir). These loads are transferred to the vaka through a pair of closely spaced ring frames. The akas are pinned together at midships, and when extended, are also pinned to both ring frames at this point. There is also a link on each side, running from near the bend in the aka to near the step-like flare in the vaka (this link is not drawn in yet).

The total structural weight works out to roughly 800 kg. Many structural elements do double duty as part of the interior, so they're included in this as well.

Suggestions to make this structure more efficient would be very much appreciated. For our purposes, a bit of extra weight is OK if it makes the build faster and simpler. This is a lake/river cruiser, not a racer, so saving every possible kilogram is really not a priority. Being able to bash into the occasional submerged log, or to survive sideswiping a dock at three or four knots, is much more important.

I've attached a few screenshots and a very rough sketch of how the aka attachment structure will work. I'm not sure at this point whether to go with independent 4-bar linkages on each side (like a Corsair) or to permanently pin the akas in the centre, forcing the two sides to move together (the direction I'm leaning towards right now).

[Ad Hoc]

She's looking good.

"...I generally consider CFD results to be about as reliable as my government's ten-year budget forecast..." can't argue with that


So the structure has been based upon the ama's total submerged buoyancy times the lever from its CL to the deck edge of the main hull? Then a FoS of 3, is that correct?

Have you investigated the rotation/torsional moment of the ama relative to the main hull; depending upon the actual geometry, this can dominate a classic transverse bending moment as you have described. Easy way to do this is the basic assumption of the ama rotating about its midships. This leads to the classic pitch connecting moment.

The important part of the ama box beam to main hull connection is the shear/load path.
Is there adequate shear path capacity..both long.t and transverse?
Is there sufficient stiffness?...in other words, yes the stress may be calculated as ok, but what about the overall deflections?

Under the splitting loads, what deflections have you calculated using the 240x240 box? Since all your calculations shall be deflection driven. Very rarely will they be stress driven in composites.

If you wish to email me more details, i'd be happy to have a look at the details of the structural connections.

[Ad Hoc]

opps...forgot to add

Have you also considered berthing loads, such as coming along side, then a sudden wave passing (or accident/error in seamaship) and you hit the jetty/another boat/some other immovable object beam on. How will the box beams perform? A nice conservative 0.25g I've found is good in these scenarios, for small boats.

[marshmat]

Thanks for the tips, AH. The torsional moments you mention are being investigated. I will post more when I have time to work through those calculations. As for accidental impacts, etc.- the approximations I've been using so far result in any one aka being able to take roughly the entire loaded weight of the boat in any direction. They probably don't need to be nearly that strong, but it's a starting point until I become convinced that it's safe to get away with less. And I can see them being overstressed like this on occasion- hitting a dock or log, or propping them up for access to the underside of the hulls, or.....

[Ad Hoc]

mm
"...They probably don't need to be nearly that strong, but it's a starting point until I become convinced that it's safe to get away with less.."

Absolutely. Couldn't agree more. Until you have more supportive evidence, always err on the side of caution. Sounds like you have a 1g sideways load case then, ideal!

[marshmat]

Hi everyone,

My sincere thanks for the various pieces of advice that have been offered on this thread. Although I'm trained as an engineer, boat design (ie. real design, not just sketching) is a relatively new field to me. While I like to think I have a good grip on the overall process, there are numerous points where it's possible to slip up and waste a lot of time- it's nice to have more experienced folks around to point out these potential pitfalls.

So, some updates, now that I'm back in town and have had a few hours to sit down and work some things out:

Layout

We've worked out more details of how the interior will be laid out. (See attached images.) The aft dinette folds into a berth that is about 8 cm short of queen-size width. With the help of a tape measure, the largest and the smallest of the prospective crew and an adjustable office chair, we've found seat and windshield heights that work for both Katy (~160 cm) and me (~185 cm), both standing and sitting. The helm seat back folds forward to become a leaning post, and the second person can sit where they can converse with the captain. The captain can spread out charts to starboard- in the waters we cruise, you really want these right beside you (parts of the Rideau might have 10 bends, 15 possible wrong turns, and a dozen buoys, all within a mile or two).

This isn't a passagemaker; when we stay on the boat overnight, it will almost always be docked or beached. Think "boat camping". So the onboard galley isn't going to consist of much- a sink to port, the dinette table for food preparation, and some space to store dishes at the aft end of the chart table to starboard. Cooking will most likely be done on shore with a campstove or barbecue. We won't be fitting refrigeration- a couple of portable coolers will do nicely.

The addition of a small plank and a cushion makes the dinette into a nice nearly-queen-size bed, and a canvas enclosure will provide weather protection and privacy. The bow and stern compartments can be used as berths, albeit slightly cramped by powerboat standards (they're still larger than the "single" berths in many production sailboats). Still, with three or four people aboard, the odds are that a few of them will pitch a tent on shore- its 8.5 m length notwithstanding, this boat has less weight and similar volume to an average 23' cuddy.

Structure

There are a few more calculations to go before I can settle on the remaining structural details. Commentary on the validity of the following load cases that I've seen in various standards and references (and/or appropriate safety factors to use) would be appreciated:

1) Fully loaded weight supported on two points: stern of port ama and bow of starboard ama. (Roughly akin to being caught in a high quartering sea with a wavelength a bit less than the boat length.) I think this is about the most severe load case a trimaran can see?

2) One ama fully submerged in a wave, pushing upward on both its akas equally with its maximum submerged displacement.

3) Pushing longitudinally on the bow of one ama and the stern of the other (a "racking" motion that could be conceivable in the event of an impact with a dock or log)

4) Pushing laterally inward on both amas (similar to hitting a dock broadside, or having another boat raft against you)

A sketch of the aka folding linkage is attached below. The linkage consists of a simple pin joint connecting the two akas, through which a pin is placed when they are extended, locking them to the ring frames on either side of the aka assembly. A single-bar stabilizing linkage on either side connects each aka to a bracket on the side of the vaka. You'll notice that this system appears to have one too many degrees of freedom: left to its own devices, it is effectively rigid when down, but free to roll left and right when retracted. This is intentional- it allows a simple cable system (one cable per aka, running through turning blocks and joining into a single cable with a single winch) to retract the amas at the dockside, before trailering. The amas carry a greater portion of the boat's weight when retracted than when extended, and so will tend to extend themselves when the winch is let out.

I had considered a classic 4-bar linkage (a la Corsair), and that could certainly be done. However, I do like the idea of being able to retract or extend the amas with a single control (and maybe a bit of kicking) while in the water- many of our ramps are awfully narrow to launch a 4-metre-wide boat. And linkages are expensive (aluminum, with bearings, brackets, etc.) Whether or not the akas should also have a connection to the vaka "wing" structure is debatable. The akas themselves, by the way, will likely be of laminated fir or another easily available wood.

Weight & Balance

The on-trailer weight currently rings in at 1330 kg, including a substantial heap of movable gear that will probably never leave the boat. That comes up to 1645 kg by the time you top off her tanks (288 L of gasoline- probably too much, 83 L of fresh water, and 15 L of sewage). Six adult crew, with full tanks, bring the total to 2125 kg. So, pretty close to the design targets (1500 kg in normal trailering condition, 2500 kg fully loaded).

The crew are pretty much centred over the CB/CF location, and although they can change the trim a degree or two by moving around the cockpit, adding more crew does not change the overall balance noticeably. Depending on whether that heap of loose gear is kept in the bow or stern compartment, the CG can be moved between about +3% to -3% of LOA from the CB location. So we have the flexibility to adjust the trim as necessary, depending on how she actually behaves.

Outfitting

And a start on the equipment list:

Power:
50 hp outboard & associated systems

Electronics:
Dual batteries (capacity TBD)
Class D VHF
Engine instruments and fuel meter
Nav lights
LED reading/cabin/courtesy lights
Bilge pumps (2 x 1000 gph in each ama, 3 x 1200-1500 gph in vaka)
120+ dB horn
12 V distribution panel (note there is no 120/240 VAC onboard)

Ground tackle:
ROCNA 15 (I wish... they're rather expensive, might have to settle for a Bruce or CQR)
20-30 ft chain, ~100 ft nylon rode
Stabilizing bridle

Interior:
Galley sink w/ manual pump
Simple manual pump-out head
Foam or air mattresses for bow/stern berths
Dishes and other galley bits

Bosun's locker:
Fenders (lots of big ones)
Dock lines- six 20' half-inch or 5/8" nylon
Big lines- As much as can fit? Probably 80-100 feet each of 3/4" or 1" nylon or polyester, seems like a start.
Chafe protection gear and snubbers
Spare parts collection for outboard
Paddles (4) - yes, four people should be able to paddle this thing quite well if need be
Lots of lifejackets (duh!)
Ring buoy on 50' line
First aid kit



Commentary is welcome

[Willallison]

Don't get me wrong here... I have nothing against your tri project... but....
Is it worth suggesting that you go back and revisit the requirements for this vessel..? I wonder still whether the monohull would be a better solution. It would probably be lighter, have more useable interior volume, most certainly be less complex and expensive to build and I would venture to suggest that it may also be just as economical to operate.
Now, if you have your heart set on a tri (and let's face it, we must all be guilty of letting the heart rule the head, or none of us would have boats!!) then you can simply ignore this post and carry on!

[marshmat]

Hi Will, good to see you around here again

I wonder too whether the monohull would be a better solution. Hence the reason why both ideas are out here on this forum for more experienced friends to critique. (Recall - Playing around with a 10 m trailer cruiser ) The two boats are, after all, based on the same list of requirements.

Let's compare the two a bit more directly:

Size: Both are 8.5 m LOA, ~8 m LWL. Both weigh approx. 2500 kg fully loaded. The mono is about 300 kg heavier when empty (1700 vs 1400 kg with empty tanks but including all leave-aboard gear) due to its larger engine (140-190 hp sterndrive or jet, vs. 50-60 hp outboard) and a slightly beefier structure (the tri, having some wave-piercing ability and a more structurally efficient main hull shape, doesn't have to contend with the large slamming loads a low-deadrise planing mono can experience). I think the mono can stand to slim down a little, though... will work on that.

Speed: About 15 knots cruise / 20 max for the tri, 18-20 knots cruise / 25 max for the mono. Score one for the planing monohull.

Range: Depends on engine choice, but for the same fuel capacity, the tri should have substantially greater range. At 6 knots (many of our favourite canals have a 5 knot or 10 km/h limit), the drag on the tri is about 0.45 kN, versus about 0.8-0.9 kN for the mono. The tri's drag at 12 knots is 1.7 kN, versus 2.8 kN for the mono at the same speed. Of course, the mono gets slightly better once it's fully on plane, but even up to 20 knots, the tri still has a noticeable advantage. Lower resistance, of course, translates to lower power requirements and longer range. Score one for the tri (as long as we stay below 20 knots).

Life Aboard: The main seating/helm/galley area is about the same size in both boats. The aft cockpit area of the mono is about equal to the side decks of the tri. Either could be fitted with a simple windshield or a pilothouse.

Construction complexity and time: The amas of the trimaran are really quite simple stitch-and-glue hulls (a few plywood bulkheads, plywood skins, and a bit of fibreglass- maybe a week each to build). What's left is of comparable build complexity in both cases, except for the trimaran's folding linkage, which involves machining a bit of aluminum. The two boats are built from similar quantities of similar materials, I wouldn't expect much of a cost differential.

Trailering: Undoubtedly simpler with a monohull than with a folding multi.

In other words: the two are pretty much tied for "next boat" status, and there will be much debating over the relative merits of each before we move on to construction drawings. I'm still planning to start cutting plywood, one way or the other, sometime next summer. These being personal projects, they obviously take a back seat to "real work", so I'm only working on one at a time, and then only for a few hours here and there. An update on the mono, similar to what I posted on this thread yesterday, should be coming in a few days.

[yipster]

gee i keepmissing threads, read this one only today
tri is about half the power for the same speed rite?

[marshmat]

Hi Yipster,

At low speeds, yes, the tri requires about half (or less) the power of the mono.

By the time we get to 20 knots, the tri's 50-60 hp outboard would be maxed out, while the planing mono would be using about 70 of its available 140-190 hp. (In both cases, this is at 2.5 tonnes disp., and including gearbox/prop inefficiencies.) This is the tri's top end, and a comfortable cruise speed for the mono.

(Frankly, I don't see much point in going with a planing hull unless speeds of 20+ kt are going to be used regularly- by the time you get to 25 kt, that planing mono is using more like 110 hp, so its engine would be sized accordingly.)

[yipster]

20 knots is a fine speed Matt
maybe in your neighborhood it may not be the case
but i have that funny feeling in denser populated waterway's
police will point speed guns at the planing boat first