incline test

[skipperdude]

Have you ever tested the righting movement on your boat.

** I was talking to a friend who suggested using a weighted string hanging on the center line of the boat taped to the overhead with a stick or yard stick laying athwartship. a couple heavy people standing on the outside rail after marking the incline then have them hop off or moving back to the centerline.
** *then I got lost as he started talking about there accurate weight and using *triganometry tables to compute the righting movement.*I should have prefaced this by saying he is a blow boater.** I can't think of any real use for this information.

SD

 

[sunchaser]

CAD and related stability programs can do this sort of simulation, provided the boat design is well established on a hydrodynamic model. But, all sorts of gear added by the owner can confuse things. As can sea state etc. The best layman writeup on an actual vessel I've seen is in Dashew's website for construction of the Windhorse. Two years ago I saw some work done (by a well known naval architect) on a keel extension for a large yacht*with various software*used to predict the sizing. To get the desired effect they had to fill the aluminum extension with antifreeze to counter the added buoyancy.

 

[koliver]

If you want to carry passengers for hire, the incline test is one that has to be done to get your certification. On a friend's converted fishboat, they did it with barrels of water on the deck. So many Kg of H2O at the side of the deck = so many degrees of heel. If it heels too much, no CSI certification until modifications are done to correct it.

 

[Yorksafloat]

I did this on my boat with barrels of water in various combinations and locations.* Very good note taking is critical, as is careful measurements.* Once you have these measurements (raw data) you really need to take that data and the line drawing of the boat to a naval architect to have him do all of the stability calculations.* You can theoretically do it yourself but it's more than my poor little brain could handle.
I used John Simpson at Simpson Marine (www.simpsonmarinedesign.com).* Extremely competent, helpful and reasonably priced.
Good luck.
Jim
P.S.* When the dust all settled and the recommended ballast was added to my boat it made a HUGE difference in the comfort and safety of the ride in big seas.

 

[Forkliftt]

Jim,Did you add ballast to your current vessel? How did you add it?
I have wondered if adding some "displacement" would make for a better ride in heavy seas.

 

[FF]

On the banks Dories would flood the boat some to improve the ride , as well as the "carry" when rowing.

Unless its built for the weight in the origional displacement ,

I would doubt there would be any improvement with an overloaded boat.

FF

 

[Phil Fill]

On PMM we have discussed roll period and incline test.* However, the incline test or max point the boat will keel over before it turtles has never been answered.* I have even talk to Navel engineers and they seem to also get back to the roll period, not max incline test.*


*
I know the roll period will give some indication as it should be between 4 to 8 seconds.* If longer than the boat could roll over/turtle and a uncomfortable ride/feeling, and if to short a snap ride.* So my question is; If he roll period and incline test the same thing? **I mean both require a couple of Bubbas to stand on the side and then step off.* It seems this incline test is sort of a back box/mystical calculation that few know how to explain much rather than calculated it.

 

[Arctic Traveller]

An incline test is done to determine a vessels stability.The term stability refers to a vessels ability to return to an upright position after being heeled by an external force.Instability can lead to capsize.Stability can be looked at as the relationship between buoyancy and load.Load includes people, gear, supplies, and things like water on deck or ice. Loose water in the bilge can be a big factor (free surface effect)Stability problems can result from a combination of factors including a change in weather and sea conditions.Commercial vessels are required to have a stability book that determines the maximum number of passengers or freight allowed, and specifies theoperating parameters that must be followed to remain safe.Any changes to a boats equipment, conversions to structures, or any other large weight changes will require a new incline test, and will result in a new stability book.
*
As private vessel owners, a stability test in not required. A simpler roll period test (Phil /Fills bubba test) will give an idea of the comfort of the boat while under way, and indicate if there is a serious stability problem. I would give some details of how to test this, but there are many ways to interpret the results, and the last time I mentioned this, I ended up having to defend my self and have no interest in opening that can of worms again.For those interested, search for past discussions of stability.
*
Instability can be life threatening, and a basic knowledge of stability is well advised for all mariners.Anyone building a new boat, or converting an older one would do very well to have an initial stability test done, and again after making any major modifications.Most people will require the services of a Naval Architect to develop the stability book.
*
Arctic Traveller


-- Edited by Baker on Monday 11th of January 2010 04:34:48 PM

 

[FF]

The use of weight became mandatory after a ferry boat in NYC tipped over watching sail boat races in the late 1800's..

1/2 the free board can go under at the rated Pax count , with ALL on one side.

The rules have changed from the Double sized folks ,
so instead of 160 lbs per pax , the new rule is 185lbs.

As their certificates get renewed most Pax boats will be re -heeled with the new weight , and the number of Pax to be carried reduced.

Unless things like ports are fitted with storm covers which MUST be closed during operation , the free board is to the bottom of the opening.

The flipped pax boat in NY State was not a USCG inspected vessel, as it was only used in NY, so beware!

-- Edited by FF on Monday 11th of January 2010 12:38:40 PM

 

[Monterey10]

My 25 was rafted next to a 24'er, of the same brand make..** I walked down the gunnels between boats, one foot on each gunnel.* The 24 tipped significantly as I stepped down on it.* My 25 didn't move a bit.** The 25 was wider, heavier and had a different shape to the undersides. * It resisted tipping significantly, yet was only a foot longer.* So, I learned that there is quite a bit of differences between models and brands.

 

[Yorksafloat]

I added 4 tons of ballast to the boat.* 2.5 tons in one location and 1.5 tons in a second location - all on the advice of a naval architect.* The roll period is one of the data points that you give to the NA in addition to the incline test results.* You also will need to take the boat out in flat seas and record the heel of the boat at various speeds with various rudder inputs.* It will also be important to measure the trim and (if possible) do all of these tests with the boat empty (including all tanks) and full.* This last bit is not strictly necessary as long as the state of the tanks is accurately reported but it will help the NA get more accurate results.
When we added the ballast the difference was remarkable.* The roll period hasn't changed but the roll is more "ponderous."* Before it tended to snap to the maximum heel and hesitate before recovering.* Now it just gently rolls back and forth without any appreciable change in the acceleration of the roll.* To give you one data point - before ballast at 8 knots and a 20º rudder input the boat would snap heel 18º and "hang" there during the turn before deciding to recover after the turn.* After adding the ballast the boat will only heel about 6º and gradually recover to about 3º during the turn.* As you can imagine this has a significant affect on the mood of the Admiral!* I can also handle significant following seas anywhere between 0-30º off the stern with autopilot alone.* Before I had to hand steer and it was still nerve wracking.
While all of the data collection tests are easily done by an owner (it helps to have at least one assistant), the actual calculations and final determination on quantity and location of ballast should be done by a qualified and experienced NA.

 

[Forkliftt]

Thanks

 

[Arctic Traveller]

Yorksafloat wrote:

Before it tended to snap to the maximum heel and hesitate before recovering.* Now it just gently rolls back and forth without any appreciable change in the acceleration of the roll.

*

A roll that hangs before recovering is one sign of a very unstable boat.* It's not hard to visualize a boat in that condition having a high center of gravity, and being far more prone to capsize.* Doing an incline test, and having a naval architect design a ballasting plan was a very smart move from a safety standpoint in addition to making the boat more comfortable. * ......Arctic Traveller

*

 

[FF]

"at 8 knots and a 20º rudder input the boat would snap heel 18º and "hang" there during the turn before deciding to recover after the turn."

Who was the NA? , this is a basic design flaw and it would be good to avoid any boats from him?.

 

[Yorksafloat]

FF - This wasn't exactly a design flaw as much as it was an original owner flaw.* When the boat was built it was never inclined (as required and documented by the designer and builder).* Essentially, the PO ran out of time and money.* Going into the purchase we knew the boat was marginal (at best) and let's just say that factored strongly into the final price we paid.* We went into this knowing it was going to need to be inclined and ballasted.* I took the original designer's notes and calculations and then did my own inclining test and had an independent NA go through the calculations in order to satisfy myself that there were no gross disagreements.*
Now that the boat is properly ballasted it is a rock.* Very steady in all sorts of sea states and it will take a lot more than I can take!** I could go into great detail regarding the sea conditions she has seen in the last 10,000nm under the keel but suffice it to say I am completely satisfied with her stability and seakindliness and safety.
Jim

 

[Phil Fill]

Jim, Who*did you use to do the*incline test.?

 

[Yorksafloat]

Phil/Fill - I did the incline test myself (data collection) and I used John Simpson (www.simpsonmarinedesign.com) up in B.C to do the calculations and determination of how much ballast and where to locate it.* I wholeheartedly recommend him.
Jim