Hi Mark,

Good points … you do need somewhere to start from. Whether it is to look at a boat (or drag racer) that is similar or search for some formula, equation or methodology.

Sometimes rules of thumb can work. And that’s why context is important.

The difficulty is that the “rules of thumb” are often used as a one shot method as a package solution.

But if there is a tuner/tinkerer’s culture like drag racing it can be a very successful method.

However, most boats are built just to use and not to worry about.

And I’m sure in the drag racing world there are lots of people who have read a bit … and then those who actually have been committed for a long time and actually gone up the practical learning curve of actually competing and seeing how the ideas pan out.

The first group are not so useful. But the guys up the real learning curve who have tested the rules of thumb and found them worthy .. or unworthy .. their words are like gold.

Hi Dewey,

I would make enquiries of the Moth Association in Australia to see if they can direct you to someone who can give you some information.

They also have a good website and some of the state websites and their old website has considerable information … maybe you can join their chat forum (ask them) and see if anyone has info they can share.

I am pretty sure most of the trifoiler development was manufactured stuff. I don’t know if there might even be some parts for sale.

Main thing is that whatever it is attached to has to be pretty light – I think Moths were around 80lbs all up in that era, and also a fairly fast intrinsic hullshape so that when you crash off the foils the hull can kinda keep on sailing.

http://www.moth.asn.au/

HOpe this helps
Michael.

Michael,
Could you provide or tell me where to find some more information on the trifoiler Moth? I am very interested in constructing one as it seems much simpler to construct and sail than a “normal” foiling Moth.

Thank You.

The only rule of thumb I cling to religiously is that when someone starts flinging around ‘rules of thumb’ too casually, he gets beaten with a stick no bigger than my thumb.

Seriously, though, this is a great discussion! As a designer of things wooden, I find some of my most exciting failures and successes come from challenging my own developed-over-time patterns and assumptions. “Hmmmmm…. it’s probably crazy, but I wonder what would happen if I ignored reason and…”

Hi Bill,

Thanks for the comment.

I can only agree. Like PAR says above, that going straight in and grabbing the pocket calculator and thinking it is going to work is a mistake.

I use computers for my design work, but really, it is only for the laying out of the lines and doing the calculations. You still need to know what the results of the calculations mean! :)

Similar with rules of thumb. The middle one is probably correct for a certain type of boat the builder was used to building, but will fall apart for different types of boats. For example a light dinghy 5 ft wide and 13 ft long compared to a classic square metre style yacht which might be about 30ft or longer on the same beam. But kept within its original intention of size and type it probably performs admirably.

I rather tend to trust constructional rules like your other two, but usually do check them against the result from more modern standards. Maybe not all that modern if we are talking about rivets – maybe I mean I will check a couple of other sources just to make sure I’m not overdoing or underdoing it.

The first couple of boats I designed were laid out full size on the floor. Because that’s the way I had been building them until that point. As I got used to computer design tools I understood very clearly that it really is the same process as drawing on the floor, except the calculation results are available more quickly and more accurately.

The computer model has to come back to wood at some point – so if you try to make the lines on the computer do something that wood won’t do when the boat is actually built, it will turn around and bite you quite badly.

This is probably a good thing.

MIK

The penny (size) of a nail used to fasten a plank on a wooden boat is one penny for every eight of an inch of thickness of the plank. For softwoods, add two pennies.

The general rule for lower mast diameter was 7/8 inch for every foot of beam.

Beveling for caulking: the outer edge should be beveled 1/16 for every inch of plank thickness; beveling from the center of the plank.

https://web.archive.org/web/20151025144750/http://www.tsca.net:80/CRBB/thumb.htm

For the most part we’re in agreement in that these rules of thumb are worthless, for the novice sailor or budding designer and can cause much more harm then good, if considered more then what they are, which is a very rough and general guide, typically for comparative uses.

I have a secret rule of thumb for finding a wife, but it’s use hasn’t worked well for others, for their lack of understanding of it’s implementation.

Completely agree Paul,

The first para in my article talks about the “context” of the rule of thumb. We all have shortcuts as designers.

The risk is when the “rule of thumb” loses its original context.

As designers we have some experience to back up our use of rules of thumb. Often because we have the chance to make mistakes – usually of the smaller kind – like the centreboard on my original BETH sailing canoe being too small. Handling could only be describes as lazy and performance disappointing upwind. I made another board with 9″ more depth and she became pretty good upwind – not quite good enough to stay with a well sailed Laser, but not embarrassingly far off the pace either.

Or sometimes further experience can show us that the rule of thumb is just wrong. Like the excessive glass recommended for kayaks one or the “you can shrink or expand a boat 10 or 15% with few problems” one.

Though it is hard to know if these are wrong because of a change of context or loss of context.

But I would generally think, that if a simply stated rule of thumb appears to be applied universally then it is highly suspect.

Also a good rule of thumb is if something is made heavier and stronger over the years … then it is very probably being overbuilt or overdesigned.

PROBABLY!

In other words there needs to be a realistic justification for it, rather than some sort of nebulous “creep”

Thanks again!

MIK

Another example would be a fast daysailor I did not too long ago. She was a narrow V bottom boat, with a fairly flat chine, so she’d be good up wind, if you can hold her down and have less leeward bow wave resistance, then a hull with a dramatic chine sweep at the bow. This naturally meant the sail area would be somewhat less then the ideals for a fast dayboat. So a moderate WS/SA and SA/D are the result. Just looking at these to figures you’d think she wasn’t particularly impressive, but having sailed the prototype at over 16 knots, this isn’t the case. She does all the things I expected, including pound hard going to windward, because of the flat chine choice (see, I didn’t say compromise, even though you know it was).

Most of the “rules” are as Mik suggests and just a general guide to compare against other, similarly sized craft. Once you generate some experience and more importantly success with design, you harbor favorites, a keenness for certain things and an understanding of the “relationships” with the “numbers”. These become second nature after a while, as you build on successes and learn from not so successful attempts. This understanding then becomes a set of personal rules of thumb. You may wish to test or stretch the rules from time to time, but most of the designs you work on, will usually fall right inside the comfort zone your personal rules create.