Thread: scaling down

The laws of mechanical similitude prevent proportional scaling attempts, except in exceedingly small increments. This is because area changes by a cube function, weight to the 8th power and stability to the 16th power, making any up or down scale attempts all but imposable.

The 12 meter look-a-likes aren't actually scale representations, but just similar looking, with hugely different ratios to compensate for the stability loses, at such a reduction. All one has to do is look at a successful pond yacht and compare it to a full size boat. The most obvious difference is the scale of the appendages and most importantly the ballast ratio, which in a pond model may be 80%+, but in a real boat may be 35%.

Trailerable live aborads are very rare and difficult to design. Live aboards by their nature require standing headroom, yet trailer born yachts generally can't offer this. I have a 33' design that is technically trailerable, with standing headroom, but this isn't what you'd what to do very often with a 33' long boat, trust me. This particular design is just intended to save winter storage costs and is lifted out by crane or travel lift, dropped onto it's trailer then motored home for winter or repairs.

For this discussion, I think you need a much more refined SOR, so the list of possible designs can be narrowed down to a manageable number.

Hi Nemo.

PAR is 100% correct.
If you really want to try scale modelling and want to do it properly, try this link for advice.

Australian Maritime College

Paul.

*sob*
you are getting me all nostalgic.
Was a student there in the early '80s.
A most enjoyable time in my life. (wasn't a particularly studious student...)
Was where I got started in kayaks & alcohol.
Neither were relevant to my course.
AJ

Hi B.O.A.T.

During that period, I might have enjoyed, 1 or 2 "cleansing" ales with your instructors. (LOL).

Regarding, nemo43, original idea.

The college is the only place in Aus, where I think his idea might be able to be experimented with. ?????????

I don't know how much access is granted to private individuals, for experimentation.
I also don't know if there is a fee involved for non boat builder types.. ????

The facility is great, if nemo43 can access it.

Paul.

A classic example of what I mean. A scaled down ship and the quite obvious modifications just to let it sit upright while afloat. This one sails quite well and probably has a 90% ballast ratio. Look at the huge increase in lateral plane and rudder area necessary as well. If this was real life, the boat would be 100' long (30.4 m) with a 60' (18.3 m) draft!

Thanks guys

The sound you hear is me running screaming into the night.

'Tis as i feared, far too involved for a mug like me to be doing calcs

But, if i build a boat that is motor powered, ie has a minimum of top hamper and sits 'mostly on' the water rather than in it, have i not eliminated the principle of moments stuff and also the odd relativity of ballast amounts?

(yeah yeah Boatmik i know, "finish the dory nemo!!!")

Hi Nemo.
Now that you have specified, "power only", you have changed your original question.

I do believe there are trail able launch designs, which could suit your needs.

Search the net, for what you have in mind.

I have seen designs to 30ft, which have a narrow beam and small engine requirements.
You must ask yourself, if you have the towing capacity.ability to handle this type of rig.

Good luck.

Paul..

hi hooked up

can i tow it

some few years ago I purposely bought a lightly used Navara twin cab diesel ute but bought a couple of years older and spent the $ difference on an LPG gas injector.

Its original rated tow was 3000 kg and with the gassie it handles it much better than fine. My story for the first mate is that the gas injector saves money, it doesn't really, but Holey Moley the truck goes like a cut cat when empty and, while slowing down a bit on hills, will tow two tonnes at highway speed at a penalty of 1-2 extra litres per hundred over the 9-10 it gets usually. My petrol Patrol mates would take that increase any time (grin).

And if it all becomes too much I'll look at a fifth wheel set up which increases trailer load considerably.

As for boat design I think i am leaning towards an Atkins tunnel hull as a base hull. Most of the work will be on the Gippsland lakes which, due to their proximity to Bass Strait get fierce winds which, over the shallow lakes, builds up a 1-1.5 metre short interval chop. There are NO houseboats on the lakes, the craft has to handle this junk.

Nemo

The real problem with down sizing anything is the lose of stability, which is a factor of the 16th power, so you lose huge amounts of stability very quickly, meaning any scale attempts will flop over on their side come launch day, unless you've addressed this issue.

The Atkins tunnel hull, wouldn't be my first choice for a boat to suit your needs, particularly in a heavy chop.

Originally Posted by nemo43

Ahoy folks.
I have in mind a retirement project of a trailerable live-aboard but would like to wet-test a ride-on scale model for seakeeping.
Nemo

Ross Lillistone's Three Brothers? Trailerable (820kg/1800lb), big enough to live aboard(26 ft), designed to handle a heavy chop.

I don't think you could gain anything useful in the way of information regarding seakeeping by wet testing a scale model given the scalling issues mentioned above, I guess the main thing is to decide what your (and the other half's) needs really are so you can narrow down your choice of designs, have a talk to the designers and if possible some who have built/used them before. Designers seem very open and honest about their designs, are quite prepared to recommend someone elses design if they don't think their's will fit your needs.

Ian

Ian

Originally Posted by nemo43

Thanks guys

The sound you hear is me running screaming into the night.

'Tis as i feared, far too involved for a mug like me to be doing calcs

But, if i build a boat that is motor powered, ie has a minimum of top hamper and sits 'mostly on' the water rather than in it, have i not eliminated the principle of moments stuff and also the odd relativity of ballast amounts?

(yeah yeah Boatmik i know, "finish the dory nemo!!!")

I didn't say a word.

MIK

Howdy Nemo,

On a more serious note - the others are quite correct and the picture of the full rigged ship model gives a very good idea.

For something simple like a liveaboard there is not a lot you can get wrong if you aspire to reasonable performance requirements and modesty in all other things.

One useful thing to know is that exceeding the sizes of the plywood for the bottom tends to be expensive in labour and materials If you stay within a bottom width of 8ft it really helps keep the build very simple and the materials cost down.

This means stretching the boat out lengthwise. However if you have to pay by the length to store the boat then that becomes a criterion too that might make it worthwhile building a wider but shorter boat with more complex construction.

A flat bottomed boat is not the only alternative of course Nemo, but it might match the time you have available better than other alternatives.

Best wishes
MIK

Nemo43.

Check out this link.

You might find something of interest.

Duck Flat Wooden Boats - Mundoo Riverboats

Paul.

This is trailerable, though once you get into these lengths, it becomes exceedingly difficult to consider them truly trailer boats.

The left hand image is the narrow version of Floom (RYD-29.6) and she's 33' 2" (8.42 m) LOD, 29' 6" (7.49 m) LWL, 7' (1.77 m) beam and not much draft with the engine lifted clear. She's trailerable any where, but finding a suitable ramp for a 35' - 40' (8.9 to 10.1 m) rig isn't easy, let alone dragging it around.

The other version of this hull is 9' (2.28 m) in beam. This usually means you have to pay a special over size fee, but normally not much more then this. The 9' version has a lot more accommodations with the additional elbow room. Of course the 9' version isn't nearly as efficient underway as the 7' version either. Everything is a trade off in yacht design.