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rmm1
16th April 2006, 11:55 AM
Hi everyone, I need a bit of advice. Someone has given me the plans for a 14 ft canoe designed by Tom Hill. It includes full-sized plans for molds, stem,etc. For various reasons,mainly space in my shed,I would prefer something between 10 and 12 ft,and I was considering reducing plans by about 25% on a photocopier. I was wondering if this would be too detrimental to the finished product.I just wanted something to muck around in the local creeks for a bit of fishing. Thanks ,rmm1

Bob Willson
16th April 2006, 12:04 PM
Try this (http://www.woodworkforums.ubeaut.com.au/showthread.php?t=10486&highlight=eureka)

bitingmidge
16th April 2006, 01:15 PM
Well done Bob! :D

Scaling down plans the way you describe rarely works satisfactorily for many reasons, mostly because the volume decreases by the square of the dimension. ie if you half the scale of the boat, you'll decrease the volume by four.

Sometimes you can get away with (note the words "get away") keeping hte same moulds and building them closer together, but in my view if you are going to go to the trouble of building a boat, it may as well be one that rewards you in every sense.

I've seen people build horrible boats "because the plans were free" too often (not knocking Tom Hill's mind, just the logic behind the decision making process.

A friend of mine has come over this weekend and wept tears of blood after seeing the Eureka for the first time. He wanted to build a canoe, but was too tight to part with the $70 or so for the plans, so downloaded a freebie from the net. He spent about 30% more on materials than I did, and has this awful thing that no one can sit in. He was just amazed at the difference.

You will find that the cost of purchasing plans from a reputable designer will pay for itself in wasted materials and redundant work as well.

Michael Storer does have plans for a 12 foot version of the Eureka as well, but talk to him first as it was designed for children and may not carry the load you intend (his contact details are on his website : http://members.ozemail.com.au/~storerm/

I'm not sure if it's room to build or room to store the canoe which has you looking at the smaller size, but either way, the Eureka is light enough to suspend from the ceiling very easily, and a normal person (not me) could build one in a few weekends (see Deepdug's posts on the same thread).

With the Eureka or it's ilk, you won't need moulds, stems or any of the other time (and dollar) consuming stuff.

Or build the boat full size and be done with it.
:D
Cheers,

P

Boatmik
17th April 2006, 01:19 PM
Well done Bob! :D

Sometimes you can get away with (note the words "get away") keeping hte same moulds and building them closer together, but in my view if you are going to go to the trouble of building a boat, it may as well be one that rewards you in every sense.

Or build the boat full size and be done with it.
:D
Cheers,

P
Not only do you have to reduce the spaces between the frames but you have to reduce everything that is measured longitudinally which means reducing the stem moulds in length but not in height.

A general caveat with reducing boats is that it is sometimes ok to do it to a rowboat, canoe or motorboat but you run really big risks doing it to a sailing boat unless you really know what you are doing.

There is a general rule of thumb that you can shorten or lengthen a boat by 10 or 15 percent, but there may be repercussions even to such small changes - so there are still risks doing it to a non-sailboat.

For example if the Tom Hill boat is designed to carry just one adult comfortably at its original length - if you shorten it by almost 30% it won't be able to carry that original weight - you will have to reduce the weight of the person by 30% - nothing a good sabre saw can't manage :-)

You could call the boat "Procrustes"
http://en.wikipedia.org/wiki/Procrustes.

Shortening boats will also have repercussions for the stability even if you don't change the width of the boat. The shortened boat will have 30% less stability even with the lighter person aboard - let alone the original crew weight.

It might be OK - but it depends how Tom has designed it. I think he has plans for shorter boats anyway- including a copy of the Wee Lassie at about 11ft long - so get one that is right for the job.

You wouldn't cut 30% out of a Holden Commodore and expect it to do the same job!

Best Regards

Michael Storer

jackbat
18th August 2006, 03:52 PM
I probably loft 60 or so boats a year and get frequent requests from customers to change the size of a hull one way or another and I can tell you that it definately is not a matter of shrinking it on a copying machine.

As a general rule of thumb, you can increase or decrease a boat by about 15% without messing with the beam, displacement, chines etcetera.

There really is a science to the flow of water from the cut of the bow throught the hull. Now I don't want to suggest that the boat would not float but you may find yourself swiming instead of paddling.

Remember when you shrink something down on a copier you shrink it in all directions. That includes the Depth at mid ship. Think about it. You just took a boat with a depth of 10 inches or so and made it 7.5". Depending on your weight you will be lucky to have 4 or so inches of freeboard. That is the same freeboard as a typical kayak.

A canoe with a beam of 30" will shrink to about 23". This is way to small for a canoe. Again, this is more typical for a kayak.

However, before you get the idea of turning this into a kayak, a kayak that is 23" wide needs considerably more length than 11 feet to keep you dry.

If you do a little research you will find that canoes from 20' to 12' have very similar depth because displacement is handled with varyations in beam, hull shape and length.

Bottom line, Bad Idea.

Jackbat

WWW.sandypointboatworks.com

Boatmik
18th August 2006, 10:09 PM
=As a general rule of thumb, you can increase or decrease a boat by about 15% without messing with the beam, displacement, chines etcetera.

Bottom line, Bad Idea.

Jackbat


Howdy Jackbat.

I agree with you on your conclusion - you stated the above about shrinking boats being "OK" and then have gone on to show that it is wrong.

I concur - it is so completely wrong that anyone stating it and holding it to be true should wash their mouth out with cabosil.

So am glad that your writing and conclusion doesn't support it.

What it should say is that an old time boatbuilder, apprenticed in the trade as a young man and now with 30 years of building boats can make the decision that such a shrinkage might be OK for a PARTICULAR hull.

Can I show you all what it does if you decrease the dimensions to 85% of the original.

(just realised I had answered this before above - the figures are a bit different because I worked it out for a 10% decrease back then)

Carrying capacity will go down 0.85 cubed = 0.61 - you lose 40% of the carrying capacity. - so your 14ft boat which carried one person now only carries 0.6 of a person.

Stability will go down 0.85 to the 4th power = 0.52 - so you lose half your stability.

And if it is a sailing boat the sail area is now 0.85 squared = 0.72.

So stability has reduced by 50% but sail area has only been reduced by 28%

Still with me!!!!

THE ACTUAL ORIGINAL QUOTE IS

THAT THE LENGTH OF A BOAT MAY BE INCREASED BY 10 or 15% without changing the depth and beam without damaging the performance and handling of the boat and has some capacity to improve both.
Philip C. Bolger

If any of you have read my article in the most recent Amateur Boatbuilder magazine - you will know exactly why I am over-emphasising this point!

I suspect a boatbuilder mentioned the same line as Jackbat and a client took it to heart with one of my Goat Island Skiff designs. It was unsailable and the client was asking me why (without telling me he had shrunk it initially)
_________________________

So if anyone quotes that one at the top to you ... ask them how they know it is true!

MIK :-)