T&T: Stability testing, was Replace Bimini

[Gary Bell]

Ernest Nash responded to Terrence Neill's posting about choosing a bimini replacement:

I am always surprise when I look at the weight that is added high up on a 
boat.  Stability was always an issue in my recent past as a Marine 
Engineer in the CDN Navy and is stil today working with new vessel 
replacement projects.  I find stability informtion for most boats on the 
market sketchy at best,<snip a little>  
Personnaly, unless it was modelled and a set of stability curves were 
produced for the boat, I would think hard and fast about adding a 
significant amount of weight up high, hard tops are quite heavy.

Gary responds with a question:

If a hard top were made with door skins instead of much thicker plywood or with a really light foam core and with thin vacuum bagged FRP shell, couldn't we make a hard top with about the weight of a fabric and stainless bimini?  Not an academic notion, as I am hoping to come up with a large 'Europa' sort of cover for the back of my PDQ power cat.  Slender hulled catamarans don't have much roll to worry about, but we are correspondingly tender in pitching.  I do understand that unlike the symetric addition of weight in the rolling monohull example (where there would be no permanent list), my project would also shift the static fore and aft trim significantly.  

Ernest goes on:

I imagine that Terry's friend probably found that the role period 
significantly changed, it probably increased considerably, making one 
wonder if the boat was going to right itself.  Increasing the role period 
can be advantageous if the boat is too "stiff", but the change has to be 
well defined so as not to go too far.

Gary responds:

You are right of course, but it seems to me that it is the attendant roll excursion or amplitude more than its period that would be frightening.  I had a few experiences  in the US Navy on a destroyer which had a huge new (ll tons as I recall) air search radar mounted 128 feet up a tower which was added in a conversion from aft guns to missles (DDG 34).  Although in truth we spent relatively little time actually walking on the bulkheads, the amount of roll -- in degrees -- was a dependable heavy weather topic among the cast and crew.  

Ernest continues:

I would also submit that small boat manufacturing is a black art as the 
work required to get a set of stability calcs is time consuming and costly 
and should require significant modelling to ensure all is well.
<snip out Ernest's closing his quote of Neill's post>

Gary finally gets around to his point:

It seems to me that a huge amount of costly and time consuming  work are 
sometimes used to predict how a theoretical boat should behave.  That's 
fine if you are designing a brand new boat and you have the resources 
available to manage these calculations and modeling tests.  Most 
recreational boat builders, as you point out, lack the resources and 
motivation to produce all the arcane mumbo jumbo that only confuses or 
frightens their buyers.  After all, in the boat building biz, it is the 
money of the buyer rather than the acclaim of some obscure few ivory 
tower theoreticians that they need to make their payroll.   
Incidentally, the metastatic diagrams for my beloved PDQ are on their 
website. 

In this case we have the real boats right here at hand, and ample motive 
to invest a little in some peace of mind.  The bad news is that we 
operate on individual peoples' sized budgets, not those of shipping 
companies or governments.  The good news is that we really don't need to 
mess with scaling issues, wave tanks, expensive consultants and/or 
voodoo math.   I can imagine myself at my nice flatwater dock, first 
taking some static measurements of healing change per pound of weight 
added at the extreme beam (or static pitching and  right aft in my 
catamaran case) to determine if there is any significant risk in moving 
from this static test to a second dynamic one.  I can further imagine 
myself in a nice secluded flat water spot, with a buddy boat to make 
hopefully repeatable wakes, a few two-by-fours, some plastic buckets and 
the washdown hose to simulate the estimated weight of the proposed 
project placed at its estimated center of mass -- timing the roll (pitch 
in my case) period with my watch and using a cheap sailboat inclinometer 
for the amplitude.   If my test rig passes those experiments, I would 
then be willing to be seen on the waterways with the test rig in place, 
checking the real world boat motion.  If any other boater dares to ask 
what the @#$% is that, I will calmly reply that I could tell them but it 
will cost them a beer first.   BTW I have visually  estimated the center 
of motion for my boat in a video I took of the boat pitching at the 
dock, with the camera positioned approximately dead abeam of the center, 
and it agrees nicely with the metastatic calculations of the designer, 
with a little windage allowed for all our gear and variously full 
tanks.   The point is that we don't go to sea in theoretical boats, and 
our real boats are readilly available for some simple experiments. 

Isn't a similar static heeling test commonly done in some survey work?  
Does AYBC or some other standards outfit address stability testing?  
Should we always go trustingly along with the experienced eyeball of 
some 'expert' or 'experienced' stranger; or alternately use our own 
uninformed guestimations?  Do some of you already check boat stability 
and motion in an emperical and objective way?  If not, couldt we develop 
some methods? 

Hopefully,
Gary Bell (AKA that old drone himself, Mr. Science -- said with a 
flourish of his wizard cape)