Monday, November 06, 2006

How we got here from there - Part I

The process of going from the idea of building a boat to purchasing the plans took about one year. As I mentioned moving from California to Maine was a big part of this process but everyone's road will be unique.

I remember googling 'catamaran' and 'boat building' and checked a few of the sites lsited. The site that got me thinking that building a boat was different today than 15 years ago when I was involved in several Star boat builds was seeing Derek Kelsalls site and the flat panel construction. This planted the seed that a one-off catamaran was something that could be accomplished by an amateur boat builder with some composite experience. My previous exposure to boatbuilding included lofting which was something I didn't want to deal with. However, modern day surface software seems to have all but eliminated the absolute need for lofting for the amateur boatbuilder.

Further re-enforcing the notion of "I can do this" was reading through several detailed photo journals recording different peoples builds. Going step-by-step through other peoples projects fortified my intent to build a boat.

The search for a set of plans for our coconut highway crusier (between lats 20N and 20S) was underway November 1, 2005.

With that we developed several criteria to help in our search process.. They were important in the following order..

  1. Performance
  2. Total build cost < $100,000
  3. Proven design - Robust
  4. On deck comfort - Cruising
  5. Ability to ride out a fresh gale
  6. Ease of build - Build Hours <>
  7. Accomodation comfort
  8. Resale value

With that we began the first pass search by looking into designs from Schionning, Gary Lidgard, Chirs White, Derek Kelsall, Kurt Hughes and Bob Oram. We narrowed it down to Bob Oram's 44C since it met our criteria better than the others. Bob is a bit less well known than the others in the states but has drawn up some nice lines and promotes a "keep it light and put the icemaker into the waterline" mentality. Somewhere along the line I was searching for build journals and stumbled into the Harryproa web site, saw the proa and dismissed the design as a horribly freakish experiment and continued on my way to the build journal for Blind Date which is a Visionarry built in Denmark as a boat to take 'vision challenged' people sailing. Pretty cool story.

On another visit to the photo journal on the harry site I took a quick read of the design concepts of the proa.


The things that presented the greatest strenghts in my eye were the following:

  1. All sailing loads concentrated in one section of leeward hull.. This makes the engineering problem quite simple as an excercise in cantilever physics.
  2. More LWL per dollar
  3. Less bridgedeck exposure to swells
  4. It was something unique and different
I'll explain the thought process here a bit more:

There's enough folks discussing the details of multihull structural design that are much more qualfied than I. However, the notion of trying to compress a mast through the center of a bridgedeck on a catamaran became quite silly.

These loads require a significant amount of resources in both time and cost.

As a rule of thumb we used a basis of $10USD per pound of boat and 1.5 build hours per pound of boat up to the sailing weight. This was derived from looking at several ACTUAL builds with construction methods using horizontal strip plank, vertical strip plank and flat panels using sandwich contruction without carbon in the hull lay-ups but speckled about for strength. It didn't seem to matter what build method was used. I'll hit this more in build methods.

This allowed us to literally stretch our dollars into more waterline. If there's one thing I've learned over 30 years of sailing is that nothing beats length at the water line for comfort, performance and safety. Below are a few illustrations highlighting what our budget would allow us to build. They represent comparisons between a 37' waterline cat and a 60' waterline proa.

Side profile of 60' proa versus 37' catamaran. Each with build costs less than $100k USD. A few things to note here would be waterline length and position of booms relative to water. Proa sailplan will start several feet closer to the water than catamaran reducing heeling moments.

Top-down profile of 60' proa versus 37' catamaran. Note larger tampoline space and reduced solid bridgedeck surface area. Less solid bridgedeck = less slamming. Bridgedeck slamming can be terribly unnerving in a poorly designed craft. Also notice larger beam.

Aft profile of 60' proa versus 37' catamaran.
Note righting moments and heeling moments. Heeling moment for catamaran approximately 15% higher due to increased length of moment arm between heeling point and center of effort assuming similar sail areas. Righting moment for proa approximately 20% greater due to increased length in moment arm between heeling point and center of mass.

An increase in righting moment combind with a decrease in heeling moment equates to more stability. Once again, this is a very simplified approach to viewing these concepts and there are numerous analysis covering the topic.

More to follow in Part II


robertd said...

The extra righting moment is a good deal more than the 20% seeing the ww hull is where the beer is kept.
Have you considered polypropylene honeycomb as a core? Rob seems pretty happy with it

Tony said...

Settling on a core materila feels alot like getting married where you think about it until the day it happens and then you think about it no more.

Although I have a lead on foam for 1/2 the boat I am stillplaying around with using balsa and honeycomb. I don't need to make this final decision for a while. Until then I'll make some test coupons and play around with the different materials.

Seems like there's four categories for determining core.

1. Cost (like honeycomb)
2. Weight (like foam)
3. Structural Characteristics (foam/balsa)
4. Preference to work with (balsa)

We'll see which shakes out at the end.

Doug H said...

I thought that surface area makes for more building time, ie. but suface area should be prportiopnal to weight. I mean a twice the length boat is taking 2 cubed which is 8 times longer?