Trailer cruiser revisited... as a trimaran

Same size, more space

OK, I'm back. Yes, I know it's been a while. I have good excuses

After a fair bit of time spent moving bits and pieces around, my wife and I have come up with a somewhat more spacious, more versatile layout for this boat. The structural weight (approx. 875 kg) remains similar, the hulls haven't changed, the power and equipment selection hasn't changed.

What I've done is to move the crossbeams closer to the ends of the boat- leaving 4.3 m of cockpit space between them. The 'wings' have also been reshaped, opening up more usable space inside.

Up in the bow, as before, is the head and its holding tank, along with some storage space.

The captain's and admiral's chairs are at the forward end of the cockpit, with enough room to lay out charts, pilot books, etc. and a suitable complement of instruments.

The midships area can be configured as two inward-facing bench seats, a dinette for six, or a queen-size berth.

Further aft, on each side, there is a walk-through gate (as seen on pontoon barges). The galley spans the aft end of the 8-foot-wide cockpit. There is enough room here for a sink and a small stove or barbecue.

Tankage plan: 2 x 140 L fuel near midships, 2 x 60 L water side-by-side under helm, 1 x 96 L waste in bow, a maximum of 3 x Gp31 batteries (~120Ah ea) between fuel and water tank bays. Fuel, battery, water tank bays are isolated and watertight. Including the anchor locker, there are five separate watertight bays in the main hull. Those access panels in the cockpit sole are large enough to install, remove, and reach around all sides of a tank in any of those bays.

The weights and moments, with this new configuration balance out very nicely for the load cases we are considering (maximum static trim is approx. 8 cm down by the bow, typical is < +-4 cm).

Additional drawings and numbers will come as time permits......

 

Congrats are in order, I believe Matt.
You've come up with what looks like a very workable layout - enough stuff to be comfortable, but not so much that there's no space to enjoy it.
Not sure if you've covered this before, but I assume you'll have a canopy type of arrangement over the cockpit for shelter from inclement weather?

 

Thanks Will

Yes, some form of canvas top will be necessary- this is likely to wait until we have the boat partially built and can play around at full scale to figure out exactly where to put the poles, zippers, etc. A hardtop would be a possibility but adds windage problems when trailering; I think canvas is probably more versatile for the conditions we travel in.

 

Yep - lighter too
And you could always have a tonneau type cover for trailering

Only one concern re your drawing.... your bum appears to be wider than the helmseat. I note that the same can be said for navigator...though I hesitate to cast such aspersions on your newly betrothed..... ;-)

 

I was thinking of putting a swivel chair on each of those seats.... otherwise, the cushion can be larger than the storage bin....

 

Some tantalizing numbers for my engineer friends

Just in case anyone is still following this thread.... (yes, I know it's taking a while- hey, I have "real work" to do as well.)

Have a look at the attached graphs and tell me I'm nuts. Going into this calculation, we have:
- Wave drag (and inter-hull interactions) from Michell's method
- Skin friction from Grigson's method
(the above two c/o Michlet and checked against some old rule-of-thumb formulae)
- Wind drag, assuming a Cd of 1.1 on frontal area of 4.0 square metres in 1.2 kg/m^3 air
- A 70 hp outboard engine consuming 400 g/kWh of gasoline (I know this improves in some load conditions, but I've assumed 400 g/kWh throughout)
- A propeller with power factor Bp ~25 (it actually varies from 13 to 25, depending on engine/gearbox choice and assumptions about the wake fraction), thus a propeller efficiency of roughly 0.62 (could be as high as 0.70 in ideal conditions)
- Assuming the outboard cannot idle below 5% of its WOT power (hence there is a lower limit to the fuel consumption of about 0.8 L/h)

These figures suggest that in typical cruising trim (about 2.1 tonnes displacement), the boat can do better than four miles per litre at six knots (canal speed) and stays above one mile per litre up to 15 knots.

I think I'm sufficiently satisfied with the preliminary design to move on to the detailed design phase.... time for some detailed structural analysis, construction drawings, etc.

 

It looks flashy ! Hey, I thought these posts were made from there already

I'm sure you calculations would be ok. Ok, almost sure.

Why a 70hp, that's fairly large. Do you plan to do any planing, because if you don't imo you should look at a smaller motor. The hull length would only allow a certain speed for displacement which can be achieved with a relative small motor.

Alternative propulsion ? You never know.

 

My friend has a 50hp 4 stroke honda (I think) on his 9m house boat. It would have to be one hell of a wind for him not to make it with that motor.

On your boat, nicely streamlined I would thumb suck and say a 40hp (with a little 10hp as a backup) should get you not far off planing.

 

I cannot help being a bit cynical here. Why did you bother determining wind drag when you were told that it is negligible and unimportant.

As a matter of interest what is it as a percentage of total drag through the speed range in calm conditions and into 8kts true wind?

I do consider your overall Cd air is somewhat conservative even allowing for the bluff beams.

As an aside if you look at relative ratings of paddled boats, OC1s are usually placed behind touring kayaks. The poor performance is not solely related to the water drag on the ama. They get hammered in any sort of headwind. Their high seating position and the wind drag of ama and supports is significant in overall performance.

What I found is that, as you improve the efficiency of the hull below the waterline, you need to pay more attention to the windage of the superstructure. When pedalling I am regularly putting more energy into moving air than moving water.

Greg Ks boat Within is hardly affected by wind. He has been surprised how windy it is when he opens the hatch and pops his head out. From the original concept I aimed to have a low windage shape. The ocean rowing boats cannot make way above about 30kts. Greg can still go forward in 60kts - something I regard as primary safety.

Rick W

 

A 40 hp outboard would certainly suffice for speeds of less than 15 knots. 70 is what's required to reach a 20 knot sprint speed (might be nice to have, but I'm not yet sure how often we'd actually use it). It won't plane, but at high Froude numbers its wave system is so small that it can stay (mostly) in displacement mode without being held back by the "planing hump". I expect a couple of degrees of bow rise above 10-12 knots but no significant planing lift.

Because, like most engineers, I never believe what I'm told Besides, it's a fairly straightforward calculation and something that's useful to know.
Wind drag, using the above assumptions, is roughly 0.26 kN at 20 knots. Small, but not negligible (about 10% of the total at top speed). The Cd of 1.1 is a very rough guess and it could well be lower (but probably not higher). I prefer to err on the conservative side at this stage.

Yup.... I've experienced this first-hand in canoes more often than I'd care to remember.... you get halfway across the lake, then the wind comes up, and you fight to make two knots into a 20-knot headwind the whole way home.

Exactly. Make one part of the boat slipperier, and another part- that could be safely neglected previously- is now, proportionally, a major factor.

If I'm thinking of the right boat, that thing's very low in profile and quite streamlined- it's not surprising that it wouldn't be too sensitive to headwinds.

 

Ah, so it's a sprint boat

All in all I think your setup is quite ok. I like what I see. I would still consider the limp home option.

The cleats you have on the windscreen corners are going to be a problem to get to the foredeck when it's rough, and you'll have to have something there to hold on, going foreward and back. Otherwise you have to send the wife can't lose the skipper eh. Maybe a single pipe in the center you can hold on or lash your butt to if you have to work with both hands.

I don't know what all you have planned, but shade could be an issue and I would suggest something from the windscreen to cover you in case you run into a storm. If you have to head into it you don't want to get battered.

What have you in mind for the interior ? No one ever mention this.

 

Long time no see, guys....
No, the project hasn't died. I know it's been a while since I've posted an update. What can I say, things have been busy around here (conferences, moving, etc.)......

The design has a (temporary) name, "Starwind 800"- the name coming from a successor to the Starship Enterprise that I sketched out when I was a kid and Cap'n Picard was still commanding 1701-D. We'll come up with the boat's real name when she's ready to choose one.

There are a few more details to work out regarding the crossbeam linkage. It turns out my CAD skills far outstrip my metal shop skills, and the latter will be the limiting factor in building the linkage. One can easily draw almost anything on the computer; drawing a part so that it can be fabricated quickly and inexpensively is rather more complicated. I'm now satisfied with the vaka and ama structures and the laminated wood/epoxy crossbeams.

Exactly how the canvas top will be set up is another matter that has not yet been resolved. Frankly, I don't think it will be completely resolved until I have the boat partly built and can mock up the enclosure full-size. The boat can handle a hardtop, but it looks like canvas will be more suitable for our needs. Sudden storms are a very real concern around here, and while the boat is really not meant to run in conditions above Force 5, she'll be built and rigged with the intent of keeping her crew safe (although probably not comfortable) in winds gusting to Force 7. But most of the time, we'll be in Force 3 or less and wanting the boat relatively open.

As for interior: It's been refined a bit further, but no significant changes from before. The galley spans the aft end of the cockpit; as you move forward, there are port and starboard boarding gates, then a dinette that folds down to a queen-size bed, then the helm and navigator chairs. The head, some dry storage, and a pipe berth are in the bow section. I'll be mounting a central grab rail about 50-60 cm high to the foredeck, so that I have something to hang on to while messing around with the anchor.

 

Good grief Matt. What a long honeymoon

Getting explicit already and you're hardly back

Put a drawing up if you want. Maybe someone could make some suggestions...

How about using fiberglass tent poles, just use heavy duty ones. You simply bend them and stick them in holes where they should go, similar to setting up a tent. Two advantages - it will shape the canvas and keep it stiff.

I would reconsider the canvas and for the following reasons -

In wind it is very noisy. The wind will flap and move it constantly, very irritating and becomes very ! frustrating if over a prolonged period of time.

It becomes VERY cold VERY quickly (ok I know you're still very romantic but trust me on this one !)

IF it tears, or something goes wrong it is difficult to fix, especially when it's raining, and bloody hell, it ALWAYS happens at night and in pitch dark.

There is nothing like a nice hard cover to seek a bit of shelter and warmth in. You can always add ports that can open or even make the hard to hinge if you want a bit of open air, but I will always vote for the hard.

 

Better make that for clipping the safety harness to - for those winds you talk about, YIKES !

If you can make it so that the anchor can be handled from inside rather than having to climb over the deck it will make a big difference in comfort. Someone I know recently changed his setup and I have to admit it's a LOT easier, especially in rain - you only get half wet

 

Hey Matt....
Not sure I would agree with Fanie's thoughts on canvas enclosure's. My experience is that if they are well thought out, well set up and taught then they can make for a very flexible arrangement. The tent pole idea would be good if all you are after is a sun-shade on still days, though.
It's true that canvas doesn't hold heat well. In a space as small as yours, however, a small metho heater works wonders (I use a heatpal and have been very pleased with it - simple, safe and convenient). Assuming you will have as much as possible of it made of 'clears', it will warm up in no time with just the meerest hint of sunshine....
The downside to covers is that if they aren't well thought out they can be a real PITA... And it's pretty rare to find ones that don't leak - at least a little...