Trailer cruiser revisited... as a trimaran

[Ad Hoc]

Matt
Looks fine. Do you know what your static waterline trim is, in light and fully loaded?

[marshmat]

AH,

Included in the "light ship" weight estimate (currently sitting at 1300 kg) is a 100 kg allowance for all that miscellaneous gear that builds up over time.... moving the "junk pile" between bow and stern compartments gives us +300 / -200 kg.m of fully controllable trimming moment.

The moment to trim 1 cm is estimated to be approx. 3.8 kg.m/cm (or 70 ft.lb/inch). (This is by an older formula for displacement mono shapes, corrected for a tri-hull configuration; it may not be quite right here as this boat is relatively full in the ends.) The cockpit is directly over the CF/CB, and depending where the crew place themselves, the trimming moment (normally ~0 at any displacement) can vary up to about +- 100 kg.m.

So to summarize, static waterline trim will typically vary by a maximum of about +- 10 cm at the stern and perhaps closer to +- 16 cm at the bow, and is highly dependent on where the passengers decide to sit.... I know of no accurate way to simulate trim changes with speed in a hull of this type, so we will have to wait until sea trials to find the trim conditions at which she runs best (the model, planned for next summer, will also yield some insights into this).

[Fanie]

Hello Matt,

Haven't chatted to you in a while. How's married life treating you so far


Wrt your boat. You still set on going for the tri... a trailable folding cat may have a lot of advantages. For one the hulls can be made much wider, more deck space and I can think of no advantage a tri will have over a cat...

Just a thought though.

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Matt

I assume from your post, you have estimated trims etc, via hand calculations? If so, if you have a dxf output, i can dump it into my stability program and run a set of hydro's if you like?

Assuming your figures to be "roughly" correct, the amount of trim over that length is not much. So your drive wont be affected really, so 'should' be good across the speed range. I don't have a lot of experience with out drives like that (other than my small old outdrive on my small fishing boat i had before i left the UK, and skiing boat when i was a teenager), so can't comment much more....since the boats i design are commercial, and not small add-ons installations like yours. Other than, are you selecting the prop size, or is the manufacturer suggesting one to you?

Unless you have run model tests, there is no other accurate of estimating or establishing trim, with speed.

[Willallison]

Hah.... great minds as they say..... I just sent Matt a PM suggesting the same thing!

[Ad Hoc]

"Im gonna have to get up pretty early in the morning to catch you out..!!"

[Willallison]

Well - maybe we could both do it and see if we get the same answer!!

Re the prop size and location, one of the great advantages in going for an outboard is that everything can be changed easily and generally at no cost. Trim and height are simple adjustments and most reputable dealers will allow you to test a number of props in order to make sure you get the best one. It'll be interesting to see how the performance from light to heavy ship varies with different props - if you are going to be travelling any great distances, you might find it worthwhile carrying a couple of different sizes - one for each condition. A spare is always a good idea anyway....

[Ad Hoc]

good idea.
Have one matched for the 'top end' speed...and then increased blade area for the lower end speed when 'you're pulling'.

[Willallison]

A good example - though perhaps a bit extreme for Mat - is the 'high-thrust' range from Mercury. I assume they still make them - we've had ours for quite a few years.
Very low pitch / high blade area, but the interesting thing is that the hub is shorter than the spline onto which it is attached. This allows the prop to slide back and fwd. In fwd the prop slips fwd and allows the exhaust to exit as normal. In reverse, however, the prop slides back against a 'cup' that covers the aft end of the hub. The exhaust gases that would otherwise flow aft then back across the blades, now exit between the prop and the gearcase, allowing the prop to swing in clean water. Simple - but very effective - we've used it a number of time to tow 20+ ton boats off the hard (with a 10hp outboard btw)

[marshmat]

Fanie - Good to hear from you We're getting along quite well (have survived 4 weeks so far....)
Re. cat vs. tri- I'm not sure if I mentioned this on here; if I did, it certainly wasn't in much detail. About a year ago I spent a fair bit of time doing preliminary sketches and calculations to help figure out what the "next boat" was going to be- and most of that work was based around a displacement cat design. The cost, complexity and size/weight were similar to the tri I'm working on now. The downside? Well, every time I tried to give it some sort of protected space on the bridgedeck, it ended up with the high windage and low clearance of a pontoon boat. If we were in a warmer area, I'd more likely go that way, with a more open bridgedeck. As it is, in the cool Canadian climate, we decided that the trimaran's lower, more protected central cockpit would be preferable. We didn't really look at proas, due to our length constraint, although I would seriously consider them in a case where excessive length was not penalized. (Some of you may recall a planing monohull proposal that I did discuss on here; I may yet finish the design of that one but will likely not build it myself.)

AH, Will - Yes, I am doing many of these calculations the "old fashioned" way, albeit with areas and volumes from the 3D model rather than from planimeter & Simpson's rule. This project is a learning exercise for me, the perpetual student. And, having grown up with computers and 3D CAD (and now programming the darn things as part of my job), I simply refuse to trust a computer's word on any calculation that I don't know how to derive and understand for myself.

That said, I have been using the hydrostatics features in Freeship (via an ACIS transfer from Rhino4), once I've done sufficient research to know how each parameter affects the boat's behaviour. W, AH - I'll PM you regarding your generous offers of assistance.

I hadn't thought about carrying multiple props, but now that you mention it, the idea makes a lot of sense. One higher-pitch, low-DAR wheel for lighter load cases, and a shallower, high-blade-area one for when we're called into "lumber barge" duty and need more thrust at lower speeds. 14" outboard props are not excessively costly and, for a trailer boat, are very easy to change.

[Fanie]

Hi Matt,

The first consideration is of course safety, I'm sure this won't be a problem with either a cat or a tri. Then secondly would be comfort. Imo this is important, especially if you and someone else is going to spend some time in it. When it gets to too little space it is something that can begin to irritate quite quickly. A mono hull as in a tri does not have space for one to move around on.

If you think in terms of a cat, how about a lobsided set of hulls (similar to a proa), the one the primary hull with liveabord stuff, berthing etc while the second hull is for storage ie batteries, water etc. The primary hull can extend over the secondary hull and obtain full trailable width. When you fold it open the extended part can rest on the beams.

I like the boat's size though, 8m is ok, 9m would be better of course. One should keep in mind while trailable it must be possible. Trailing is the secondary in line, I would rather trailer a bit more difficult but have more comfort on the water, unless you plan to spend more time trailing than sailing

Anyway, just thinking out loud. I'm not trying to convince you to go to a cat, a tri's center hull can be made very much the same in width, I've seen some very nice tri's insides. Personally I just favour the cat a bit more.

On your tri I would lift the ama's a bit, and make them a bit deeper. Both them ama's in so deep is going to slow you down quite a bit. Doesn't have to be the tri falls over from side to side, but on the little tri one hardly noticed if it slanted from one side to the other. Your's is larger and would be much more stable.

I'm not sure what your tri's BOA is, but an 8m tri can comfortably be 6m wide... Have you thought about a hard deck between the main hull and the ama's ? When weather allows it would be nice to loaf around on it.

[Fanie]

I've been thinking about your tri...

You can have a full trailable width center hull if you fold the beams and ama's like in the picture. I know it's rough, but you can work the details out yourself.

The sizes is roughly 2m400 wide center hull and 1m800 headroom. Folded out the tri is 6m wide in total, and folded in the total trailing height on the trailer is about 2m700.

To lower the ama's into the cavities you could use a gin pole with a winch. They would hinge up and when they reach the upper position, you can winch down and they will drop down into their sleeves. To fold out you would winch the ama's up, put a stopper pin in below the beam (red) and push them over so they hinge to the water as showed. They can then have another locking pin to keep them from hinging up or a single bold to bolt each beam down.

The beams can be located in line with bulkheads, so the bulkheads would serve a duel purpose, be bulkheads as well as be a housing for the beams.
It may look complicated but it should be fairly easy to make.

You would score the space in the center hull that the ama's would take up on the trailer.

[marshmat]

Hi Fanie,
You've raised some good points, I'll try to address a few of them:

Proas
I like the idea.... there are some proas that work very, very well indeed. I wouldn't rule one out as a future boat. For now, though, we aren't ready for something quite so radical- even a cat/tri is a rare sight around here.

Size
We're trying to keep this thing down to dimensions and weights that will allow it to be towed by a properly equipped minivan or similar. Length is penalized on the canals and can cause problems at smaller docks; 8 m on the waterline seems to be about the right compromise for how much we want to carry and do. (Even so, this is a metre or two longer than most boats of similar usable space.) Think boat camping and day-tripping, rather than cruising.

Layout
We are 4.6 m BOA with the amas out, and yes, there is a folding hard deck bridging the gap. (A good platform for fishing, diving, etc. especially with walk-through side gates, no gunwale to climb over.) At the passenger seating level, the main hull is a full 8' wide.

Amas
The folding scheme you describe is one that I looked at early on; once the main dimensions of the hulls were figured out, it became evident that the amas could tuck away in otherwise unused space below the vaka 'wings' in a similar manner to a Farrier sailing tri (although the linkage is somewhat different).
The amas barely skim the surface at light-ship displacement (1300 kg), yet they are quite large- almost sailing tri size- compared to the tiny "training wheel" outriggers of many power tris. Because we will often be working out on the wings, or carrying loads (sometimes asymmetric), the added buoyancy of relatively big amas outweighs the slight nuisance they pose when trailering. The added drag of the larger amas is more than offset by the corresponding reduction in L/B and D/L of the vaka. Obviously, this would play out differently if we could simply make the vaka longer, an option we have already exercised to the maximum extent feasible for our requirements.