The roll acceleration: What´s the best for crossing oceans?

[David Roberts]

David

I think we are refering to ocean crossing displacment boats here , not planing power boats.

Your suggested period is in what units ? If its in feet its not sensible for a ballasted sailboat but it is closer if it's in meters. 4 seconds is usually accepted as comfortable and a good target for a ballasted sailboat.
Roll period calculation requires the radius of gyration (k) without that the formula isclose to useless except for comparing boats with the same k .

Thank you for your useful reply.

One can get sick in all types of boats. My planing designs seem to have good motion on the open sea, reasonably comfortable. Perhaps the relatively short roll period helps with this. It is certainly a long way from the natural period of decent sized swells.

Yes, sorry. Period in seconds, WLB in meters.

From http://ittc.sname.org/2006_recomm_proc/7.5-02-07-04.1.pdf
and
http://www.naoe.eng.osaka-u.ac.jp/ittc-stability/umeda/7.5-02-07-02.5.doc
"In the absence of more accurate knowledge a value of 0.40B for the roll radius of gyration and 0.25L for both the pitch and yaw radii of gyration are recommended."

In any case, we have equations linking three variables, k, GM, and roll period. In any of our designs, we can calculate GM, measure roll period, and calculate k. Perhaps you could give us k's from some of your designs. Mr. Gerr seems to think it not an obstacle, given that we are really interested in values to one significant digit.

I haven't measured roll periods on any of my designs. Perhaps I shall start doing so. Using Mr. Gerr's formula, they all should have roll periods of between 1.5 and 2.5 seconds. As I said before, a little quicker than optimum, but so what. Interestingly, offshore fishermen report a greater chance of seasickness with deep-V hulls, which have a lower GM and thus slower roll than my designs.

FWIW, my small planing craft have Brewer CR values of from 13 to 25, mostly depending on size. The larger craft of course have a more pleasant motion in a wind chop, but I don't notice any great difference w/r to ordinary ocean swells, though of course larger vessels will be able to withstand heavier weather.

 

[TeddyDiver]

Wide hull forms, like most of the planning type (and especially all multihulls), tend to roll with the wave slopes, not with their own frequency. So in this regard it's a bit pointless to calculate the roll periods.

 

[divinesd]

Most of the time we use: Non-dimensional damping factor Roll: 0.062
But it is all depanding on so many things.

 

[robherc]

Gentlemen,

There are those whose opinions, up with which, I will not put!

(Never end a sentence with a preposition, Sir Winston Churchill).

In such situations I go to the recalcitrant offenders' profiles and add them to my ignore list. Thus, I am informed of a "ignored" contribution to a thread, but my sense of wellbeing remains intact, as my eyes are spared the task of informing my brain about the contents of the post.

Thank You SOOO much. As I'm still quite new to these forums, I did not know about/how tu sue that functionality.
Now this thread has (magically) become quite a bit more readable, and I don't have to worry about accidentally reading something Paul B said & believing it as fact. I'd REALLY hate to remember one of his comments later in a design, forget who said it, and end up getting free (involuntary) swimming lessons in the middle of the Atlantic for believing him!

 

[julleras]

David:

The formula given by Gerr is that if your boat falls between those to figures the boat will have 'maximum confort and safety'. WLB should be in mts. ('The nature of boats', pg.115)

 

[David Roberts]

David:

The formula given by Gerr is that if your boat falls between those to figures the boat will have 'maximum confort and safety'. WLB should be in mts. ('The nature of boats', pg.115)

Yes, thank you. I thought that this thread might be interested in this opinion by someone of Mr. Gerr's stature. Thank you for your further explanation. I don't have that text.

 

[Antonio Alcalá]

Up, the summer is close to us!!!!

 

[Guillermo]

Depends on the hemisphere....
Anyway: Ready for it, having been weekend-sailing the whole year round in our wonderful Rías

All the best.

 

[Antonio Alcalá]

jajajjajajajjajaj i´m back mate!!!

 

[Patrick BLOSSE]

Hi to all,

Thanks to all for that amount...
leading to safety and confort ratios to be experimented, and improved

Kindly.

Patrick BLOSSE

 

[Wavewacker]

I was attempting to apply this information in the selection of a boat. I'm seeing that some believe it's meaninless, applied to those that may be prone to sea sickness and as an expectation of a level of comfort that is not so well defined.

I did ask about it with respect to a specific boat but that fell without response, so, is a rating of 24.48 good? Is it even something to consider?

 

[Guillermo]

If you are talking about the Motion Comfort Ratio, MCR, see pages 28 to 30:

http://www.sponbergyachtdesign.com/THE DESIGN RATIOS.pdf

 

[Wavewacker]

If you are talking about the Motion Comfort Ratio, MCR, see pages 28 to 30:

http://www.sponbergyachtdesign.com/THE DESIGN RATIOS.pdf

Sorry, I can't down load pdf on a notebook, the screen doesn't go down far enough to accept the terms

Any brief comment on the MCR being appropriate in the selection of a boat?

 

[Guillermo]

As a rule of thumb, compare the MCR of the intended boat against the MCRs of several other boats of +/- the same LOA. The higher for the size, the better.

As an approximation and following the graph in Eric Sponberg's document, it is recommended that for average comfort the ratio MCR/LOA(ft) should be between 5/8 and 5/6, and for greater comfort it should be over 5/6

But be careful: If within the "greater comfort" zone, a boat under 35-40 ft would be what Sponberg calls a "lead sled", so very heavy and slow.

Cheers.

 

[goodwilltoall]

MCR is bunk just like the inventor of it claims, although faintly for notoriety it gives him. Giving tp52 a 16.0 ratio and catalina 27 a better 26.0 ratio is a double witness that its nonsense.

Any ratio should use optimum b-l ratio of 6-1, both for overall sizes as well as waterline. Why did he go with 70-30% ratio as the optimum for LAO vs LWL? Then claiming it gives more bouyancy when if you just add LWL you automatically get additional bouyancy. Why isnt the ratio 95-5% then or taking aesthethics aside, 100-0%.

Additional displacement helps comfort. If designing new and you desire more comfort, weigh the overall costs of heavier/slower boat, and as an example then figure: your boat hull scantlings call for 1" coldmolded hull- readjust the water lines for 1.5", this will spread the weight around and make it more seakindly, that much more stronger, and more comfortable for state of mind as well.