Trailer cruiser revisited... as a trimaran

[Willallison]

Originally Posted by marshmat

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.

Yes - I know... she sounds like quite a find...
But... let me tell you a little tale....
A friend of mine was have an argument with his wife. She was quite rude to him and when he asked what gave her the right to talk to him that way, she simply pointed to her wedding ring and said, "this".....

In all seriousness, I sincerely do congratulate you.... 16 years of wedded boating bliss myself and wouldn't have it any other way (no she's not looking over my shoulder!)

[Fanie]

And she's just as good with fibreglass as I am, if not better....)

Good choice there Mat Luck of the Irish in Ontario finding a gal like that !
If you are already looking for something for mure serious cruising then you ARE getting married Good luck, enjoy and come back dead tired or else we'll have to send you back to try again...

Er... if you make excuses to sneak away and come post on the forum, she's doing it right

[Ad Hoc]

Matt

I missed that one too...congratulations. Oddly enough my wife is quiet happy for me to take up half the back garden with my new 25m test tank...i think she sees it as me just being an odd Englishman in Japan!!!

If you have a load/geometry/force diagram of your linkage, be happy to cast my eye over for you...?

[baeckmo]

Remember lads, that among almost all mammals, including Homo Sapiens, it is the female who selects her partner on the basis of maximum genetic variation...........! The clever ladies just let us believe that we have any saying in the process.

[Willallison]

Originally Posted by Ad Hoc

Matt

...Oddly enough my wife is quiet happy for me to take up half the back garden with my new 25m test tank...i think she sees it as me just being an odd Englishman in Japan!!!

You think it's a test tank... she KNOWS it's a reflection pond....

[Ad Hoc]

hahaha...either that or she'll surreptitiously put some Koi in it, and say we now have a nice water feature!!!

[marshmat]

Koi shouldn't mind the occasional test model flying through their pond, as long as you can keep the raccoons out....

I will post some of my structure/force sketches as soon as I get a chance.... most certainly they're not up to the standards you 'pros' are used to, but should convey the general idea. (They don't bother teaching engineering physicists how to draw or do structural calculations anymore.... apparently quantum mechanics and mathematical modelling is the 'way of the future'?)

Thanks for the kind words, everyone; 8 days to go so I might be a bit hard to find for the next week or so.

[Ad Hoc]

Matt
"...They don't bother teaching engineering physicists how to draw or do structural calculations anymore.."
That's because you're a physicist, by training, not a naval architect. We did 2 years of structural theory and calculations. Mind you was a bit abstract at times! My first job after graduation, was upon reflection, a very very simple calculation, almost child's play. But, the abstract nature of the theory and designing structures was so complex when we finished the "basics" it was far removed from real structures, it took a while to "tune" my brain into the real practical world to bridge the link with theory. Once i did that, its a piece of cake...mind you, i can't remember how to do complex matrix calculations any more...i just do basic EId^2y/dx^2= bending moment calc's now, even then i get a bit lazy these days and break it down into the very simple bending moments, (taken from Roark's) saves doing lots of calculus.

Good luck on the nuptials...if you ever 'sail' your way around to Japan, we have a spare room..maybe some fresh Koi sashimi too..hahhaa

[marshmat]

I actually have time to work on boat stuff this weekend.... wtf?! (well, )

There are a few more structural load calculations to do before I'll be posting anything more on that front.... yes, they are coming, I just want to make sure I'm at least somewhat confident that I understand what's going on first.

I recently realized that I hadn't posted a decent linesplan yet.... since I generally work directly in 3D (and, in previous work, have often manufactured straight from 3D as well), it took a few minutes to brush up on my rarely-used AutoCAD. But here it is, lines and main particulars for the most recent revision to the hull.

The main hull is designed for taped-seam ply/epoxy and its lines should be accurate; the amas, however, are intended for compounded plywood- which, as we all know, can introduce some slight variations compared to the expected shape.

One of the power tri's big downsides- the added wetted surface of its amas- is evident here. In the condition shown (2500 kg disp.), 39% of the boat's weight is on the amas, yet they cause 47% of the drag at 20 knots (most of this being skin friction). In the end, a trimaran configuration is going to have slightly more drag (as well as being more complex) than a similarly slender displacement mono. And if I were doing, say, a commuter boat- or an ordinary cruiser- that's the route I would take. (Going to tiny amas, like Nigel Irens' power tris, would partially defeat the reasons I want amas to begin with, and the drag penalty for somewhat larger amas is really not all that large.)

BUT:
- At locks, docks and such, we are billed by length, while beam is rarely penalized.
- We need a stable platform with room for activities near the water; a tri's "wings" serve this function nicely and give it pontoon-boat-like stability (and ease of access) at the dock.
- This boat will likely be pressed into working service on occasion. We have to be able to carry lumber, etc. and work off one side of the boat; this tri can handle lopsided loads very well (up to a tonne and a half out on one wing before you're likely to get in trouble)

So despite the added complexity of construction, the trimaran form is looking like the preferred option for our next boat. She'll be built a fair bit sturdier than many trailerable tris (~830kg of structural mass for an 8 m waterline boat- the main hull skin is a hair over 8 kg per square metre, about twice what you'd find in a lightly built foam-core composite craft of similar size), as I'm expecting to run aground on Shield granite, beach her on gravel, load her down with concrete and 2x4s, and have newer helmsmen ram her into the dock at four knots (ie, the sorts of things you really try not to do to an F27 or Telstar on a regular basis.)

A test model, to verify that the hull performs as anticipated, is in the works for the coming summer. No launch date has been set, of course, but I expect to start building smaller bits and components next summer, with hull assembly likely to start as soon as I (1) can afford to, and (2) am reasonably sure of staying in one house for long enough to get something done.

[Ad Hoc]

Matt
Looking good. Your autocad skills would put many CAD draughtsman i know to shame! I would ahve expected the aft lines of the buttocks to run near straight and/or parallel; since you're running at higher Fn's. (forgot to add...rounding off the hard chines aft will reduce the drag too).

Your amas are 6.8m @ 20knots that a Fn of 1.26...that is residuary territory. These are creating a lot of waves, ie more residuary drag than frictional. The L/D ratio of the amas is 5.9, so this confirms a bit on the low side, hence, wave making is dominant.

However, as you have pointed out your SOR is not trying to produce the most ideal design/hull. The SOR is to suit what you want/need and works...and that is what design is about. Collating all the variables, often conflicting, and making it work. Well done

[marshmat]

Ad Hoc -
Thanks for the compliment

The amas are indeed operating at a Froude number where, for most forms, wavemaking would be dominant.... however, these are only about 40 cm wide on the waterline when she's fully loaded. (there's a mistake in the file I posted earlier today, where I copied from the wrong data table: L/B for the ama is actually 17 and each of them is carrying 275 kg of the total, and L/B for the main hull is 10 while it displaces 1950 kg. Brain fart , not the best day today.... ) My estimates indicate about 0.4-0.5 kN of skin friction and less than 0.1 kN of wave drag on the ama at 20 kt, loaded to 2500 kg displacement. A smaller ama would reduce the friction, but would sacrifice the extremely high at-rest stability we need.

Overall, we are looking at just a hair under 3 kN total drag at 20 knots.... or about 30 kW plus prop losses (say 50 kW, 65 hp rated power) to do this speed fully loaded, about 2/3 of these figures when lightly loaded. The likely choice will be a 50-60 hp outboard which would give 20+ knots running light while remaining small and cheap enough.

The reason for the slightly upswept run aft is to cut down on volume in the aft sections, necessary to balance the fine bow and reduce the tendency of deep, straight sterns to get caught by following waves.... a sacrifice in efficiency, yes, but not a huge one. Although we want to be able to sprint at 20 knots now and then, I'll take a slight bow-up attitude at top speed as the trade-off for a better motion in choppy seas at lower speeds. The bottom is nearly flat, transversely, from about station 6 aft.

[Willallison]

I was just looking at the lines as Ad Hoc made his post. I came to the same conclusion - I'm surprised at the amount of rocker you've designed into the hull. I too would have expected to see straighter buttocks to avoid excessive squatting

[marshmat]

The model testing may yet result in some changes.... but that has to wait until things thaw out here. Is a 4.5 degree quarter-beam buttock angle really that high? 3 to 6 degrees seems about typical, at least among the fleet around here, for boats that operate between the displacement and full-planing ranges....

[Ad Hoc]

In a word, yes.

[marshmat]

G'morning guys.... a bit more rested now and taking another look at a few things....

I should note that there aren't many power multis around here- we have a couple of F27s, a couple of condomarans, and a Hobie or three that are commonly seen on the lake here, that's about it. I have a heap of books and theoretical papers, but frankly, I don't put much faith in them without some real-world feedback. So I don't have as much data to go on as I would like- the feedback on this board is proving quite helpful.

I'm now looking at a few variations on the bottom shape that might alleviate the high-speed squatting issue that Will and AH have suggested could pose a difficulty for this hull- more to come tonight or tomorrow. This would involve reducing the midships draught slightly and reshaping the midships section to maintain a similar volume distribution- I don't want to add any more volume than necessary in the aft sections, but after thinking about it for a while, I think you guys are right, they can stand to be straightened out a bit.