Streching plans

If I have found a plan I like except its to small can I multiply all dimensions by 25% to get the size I want? I realize it would increase hull and bridgedeck height and there would be some customizing to make it all work.
I am a Cabinet Maker and Corian Fabricator, so working with wood and finishing and sanding does not scare me.

 

From what I have read 25% might be pushing it, but before anyone can give you solid advice, which boat design are we dealing with ?

 

Welcome to the forum Maurice.

Simply put, no, you just can't scale up the plans by 25% and have a reasonable expectation of good balance, underway dynamics, weight, etc. Do a search for "mechanical similitude" and you'll get the picture. Things don't scale proportionately or at least as they would seem at first glance.

This thread might be helpful: > http://www.boatdesign.net/forums/bo...n-talks-up-scaling-up-down-designs-45644.html <

What design are you interested in and why can't you find one 25% bigger that serves the same role?

 

Thank you for replying. I am new to this and trying rap my head around it.
I understand the theory behind mechanical similitude. It's the same reason you can't have an Ant the size of a dog. as you scale it up its physical needs change to keep up with it's scaled up mass. What I don't understand is why then do scale models work for testing ideas? May be I'm not wording my question correctly. I know I would not be able to scale up all dimensions, such as over height of the hulls(it would make the standing head room over 8') I guess what I want to do is stretch the hulls by 25%. I've looked at cross sections of the boat I'm looking at(28') and a 36' and the measurements are almost exactly 25% larger on all points. By the way the boat I want to grow is the Bernd Kohler KD860. In larger boats he doesn't have any thing with the same layout. until you get to over 45'. I don't like the "Pelican". His "Voyager is nice but it has Asymmetric hulls which are too narrow in my humble opinion. I like his vortex panels, and in all the tests I've done, they work well. I've seen mentions of a scaled up KD860 they were calling a KD920 but I cant find any info on it. Also if you look on his sight there is a picture of a boat called "Atalante" and if you look at the people on deck and scale the people to the boat, the boat is about 36'. I am also not sure of the legality of changing some ones plans that much. I am aware of the different calculations for Catamarans. I thought I could make it work but I won't know for sure until I buy the plans. I also don't want to get Bernd Kohler mad at me for changing His design.

 

Have you asked Bernd Kohler about drawing what you want ?

 

Pete Hill (of "Voyaging on a Small Income" fame) lengthened a KD860 to 10 meters, raising the bridgedeck a bit, kept the same breadth, and installed a self-designed bi-plane junk rig.

You can see photos here (free Yahoo membership req'd). There are also a few posts from/about Pete and 'Oryx' on the k-designs forum.

Also a few threads and photos on the Junk Rig Association forum.
"Oryx"
Oryx
Pete Hill's side-by-side/split junk catamaran 'Oryx'/Tabernacles
Boats of the Month

And a few photos on the Wharram site

Many of the KD860 builders have modified Kohler's design, to a greater or lesser extent. Sometimes he approves, sometimes not. Kohler did approve of Pete's modification -- after the fact.

 

Model testing works because the data is manipulated through formulas that scale up the data. That is, they don't behave in direct proportion to the scale, but it can be calculated. Also, the larger the scaling is, the less accurate the data becomes.

 

Thanks for the input.

 

Hello Maurice,
There is now reason why you cannot scale that design up by 25%. There are only two major things you need to worry about.
1. Making sure it stays in one piece since the loads increase by more than the 25% (Righting moment could be 1.25*1.25^3)
2. Making sure that she floats on her lines. It is likely that the boat would be build lighter in comparison. That would mean she would float differently.

For a cruising boat, as long as you keep those 2 points in mind, the boat will sail fine (and have similar characteristics to the normal one)

Give me a msg if you want to check my credentials etc.

 

Contact K-Designs (ikarus342000.com) and see if they can pen up a 36' version. As you might have concluded by now, you're looking at a stretch, not a proportional scaling (wholly different things). Scantlings will obviously need to change, but you'll get the length you desire.