Ratio...

Can someone comment on the above water to below water ratio. That is the height of the boat from the waterline divided by the draft. According to the article a maximum ratio of 2.6 allowable for venturing into rough waters. Any credence to this or is it useful as one variable in buying a boat. Still prepping for a purchase 2 years down the road.

NewbieFromNJ said:

Can someone comment on the above water to below water ratio. That is the height of the boat from the waterline divided by the draft. According to the article a maximum ratio of 2.6 allowable for venturing into rough waters. Any credence to this or is it useful as one variable in buying a boat. Still prepping for a purchase 2 years down the road.

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So for a typical 4.0 ft draft cruiser the max would be 10.4 ft. Most sailboats would qualify. I guess it depends on how you define "height of the boat from the waterline" and "rough waters".

That is just a rough rule of thumb...

I have never heard of it and I have been an active boater and professional captain for 16 years and in the USCG for 23 years before that and acting e in boating.

Stability is designed then altered by real life use of a vessel.

I think it is a silly rule of thumb because there are so many variables...just the difference between a cat hull and a mono would alter that rule greatly....as would beam in general.

psneeld said:

That is just a rough rule of thumb...

I have never heard of it and I have been an active boater and professional captain for 16 years and in the USCG for 23 years before that and acting e in boating.

Stability is designed then altered by real life use of a vessel.

I think it is a silly rule of thumb because there are so many variables...just the difference between a cat hull and a mono would alter that rule greatly....as would beam in general.

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Strong second. If you contemplate big open waters get stability information about the boat along with other important issues for open water that are not as important for protected areas. Little things like how big are the windows and how strong? How much flat surface is there where green water can do its damage. how strong is the rudder? When I was involved with a CG hurricane rescue in the North Atlantic a very stout Motor Sailor needed crew evacuation small round ports caved in rudder smashed. Most recreational motor boats are not meant for open water long haul use and if that is the OP intention a lot of education and reading is suggested. Most ocean going sail boats particularly older designs are far better suited to open ocean use.

Never heard of this horse pucky in a career designing of designing boats. Rules of thumb like this can be dangerous in uniformed hands.

I will throw this in:

Much to much is made of length to beam ratios. Take two identical boats and bolt them together at the transom. The new, long skinny boat, will then have the same roll period and resistance to being heeled by wind forces as each boat did separately. Beam to underwater depth ratios together with freeboard are much more significant.

There is a basic primer on stability here:

Stability of Boats and Ships

It's primarily oriented towards sailboats but the basic principles apply.

I think that the a/b ratio is another one of those beebeism's that needs to be taken with a grain of salt. Unfortunately some boat manufacturers aren't helping. As Roger has pointed out no naval architects bother with computing it as they know better.

The author of that ratio was Robert Beeebe who "designed the original passagemaker and fathered the modern day trawler yacht era".

http://www.passagemaker.org/Documents/Friends of PM/Robert Beebe & PM.htm

To the op's question..is it useful...imo no

Here is a good summary, said better than I could concisely articulate......

http://oceantrawleryachts.com/comparing-trawler-displacement-hulls/

"A/B ratios are controversial. Some naval architects take issue with the entire concept. Critics correctly observe that the factors considered in stability studies don’t include an A/B ratio. The factors do, however, consider a vessel’s center of gravity. The lower the center of gravity in the hull, the less likely a boat will rock excessively in a beam sea or capsize. Voluminous superstructures perched on a shoal draft hull are more likely to have a higher center of gravity.

A high A/B ratio will have some ramifications that are completely separate from stability. The greater the amount of surface above the waterline, the more “sail area” is subjected to wind to confound close quarter maneuvering. On the positive side, more mass above the waterline permits cabins with more headroom and larger windows, and may even raise the main deck enough to increase the vertical clearance in the engine room.

Conclusions:

The sea is no respecter of brand names. Storms do not defer to ad campaigns or marketing hype. Informed and intelligent selection of an ocean going trawler intended for long range cruising can be somewhat simplified by comparing and contrasting the afore-mentioned ratios and understanding their effects on predictive performance of a hull."

Your comments have been helpful. Thanks to all.

I would be more interested in the location of the center of gravity of the boat. I would take a high boat with a low cg over a low boat with a high cg.

stone beach said:

To the op's question..is it useful...imo no

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IMO it depends on what ratio is being considered. Length to beam is very important to me and other elements of the visual essence of a boat can be misleading. Looking at the specs can make you go "oh I thought it was long and skinny" whereas it may look otherwise. Even the beat to death hull speed that most take to be religion can be mostly thrown out w an unusual length to beam ratio.

In short ratios have more and more meaning depending on how much you know about them and how familiar you are w boat design and boats in general. Studying the ratios is one of the fastest ways to learn about boat design.