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Power Boats for Ultimate Conditions

Boat Handling vs. Boat Design

Copyright 2003 Michael Kasten

A question always arises among power boaters with regard to running off versus heading into the seas...

Per conversations with professional power boat skippers, and per my own experience, most of the usual "head into it" philosophy needs to be heavily qualified by including relevant data about the boats in question, about the weather conditions, the route, and so forth.

In particular, several power boat design attributes are relevant... most of which are also applicable to sailing craft.

Designing a Power Boat for Running

Here are a few basic concepts with regard to power boat design, and how a design can be optimized from the very start (while still on the drawing board...!).

* Bow bulbs and / or a deep forefoot will encourage adverse behavior when running, including "steering by the bow" and will encourage the possibility of severe broaching.

* Directional stability is encouraged by the same things in a power boat as in a sail boat, i.e. modest beam; approximate symmetry of hull form fore and aft; an "easy" bow profile; adequate depth of keel aft; drag to the keel overall (i.e. shallow forward, deep aft). In other words, the approximately "square" underwater profile of many popular "trawler yacht" types is very unfavorable in this regard.

* Once directional stability has been maximized, steering control is affected by depth of rudder. Deeper is better. The deeper part of the rudder is the most effective, so should be wider. Rudder foil selection and overall rudder design are significant factors. For example, an end plate at the bottom of the rudder will increase its effectiveness substantially.

* Under power, the location of the propeller, its depth, its diameter, and the reserve power available are significant factors affecting control of the ship. In particular the propeller should be well below the ship in order to keep it in the water at all times.

* Stern design will have a big effect on safety with regard to taking on water astern (being pooped), therefore on safety. A modest counter stern with aft-raking transom is ideal for reserve buoyancy. Conversely a "sugar scoop" stern having a swim platform with an opening to the cockpit is usually the worst combination, even though it can be successful on racing sail boats where the tactic is often to outrun the seas or to more or less stay with them.

* The shape of the topsides affects reserve buoyancy. In the trough of a wave, extra reserve buoyancy in the ends is a benefit, i.e. in the bow upon reaching the trough, and in the stern when being lifted by the next wave. A higher prismatic coefficient inherently increases buoyancy in the ends and is therefore favorable.

* Running off will place the vessel in its position of least stability and greatest exposure to the elements for a longer time period at the crest of each wave. There is less hull immersed at the crest of a wave, and the boat changes direction vertically which momentarily reduces its mass thereby temporarily reducing its righting moment. Combined with exposure to the full force of the wind at the crest, these three factors conspire to create the position of greatest vulnerability with regard to stability.

* Therefore adequate reserve stability is desirable as is a low profile. This does not imply that there should be excessive initial stability, too much of which will encourage the vessel to be thrown around by the seas. This does imply that a large roll moment of inertia is favorable, therefore distributed weights (transversely). In general, this implies that there should be modest beam combined with adequate freeboard and low deck structures. For further information on the question of beam please see the web article: Beam vs. Ballast.

* For the sake of comfort and to prevent excessive roll motions, some means of roll attenuation is extremely favorable. It is conceivable that paravanes may not be usable in ultimate conditions. Some sail area will be a benefit for steadying, even though it is likely that no sail would be set in very high wind conditions. Even without the sails, the presence of the mast(s) will increase roll moment of inertia, therefore steady the ship. It is possible that active stabies would be optimum here, and would ideally be located aft of amidships in order to minimize their effect on steering / slithering down the waves. For further information please see the web article: Roll Attenuation Strategies.

Pitch Attenuation

Aside from possible benefits to larger craft (having a WL over 80 feet) in terms of propulsive efficiency, bow bulbs are decidedly not favorable in running conditions. Another common justification for bow bulbs is reduction of pitching motion.

If pitch attenuation is of paramount concern, one can achieve the same or greater effect by proper longitudinal distribution of weights; by not concentrating weights in the ends; by increasing boat length; and by reducing the ratio of beam to length, i.e. by creating a longer and more slender overall shape.

Collectively these strategies will enhance directional stability and steering control, will reduce pitch substantially, will enhance propulsive efficiency, and will provide greater overall speed potential. If displacement is not increased, then it is probable that the vessel's initial cost will not be increased.

The Question of Running Off vs. Heading Up

Given a vessel that has been optimized for running and given adequate sea room, running off can be a much more comfortable tactic, will be much easier on the ship and crew, and need not present an overly large danger of broaching.

The skippers I have consulted on this (depending of course on all of the above factors) will often prefer to turn and run with the weather rather than bash and trash the ship and crew by continuing into it. As should be evident, "the turn" is a hairy moment and needs to be timed exactly right...!

I cannot even imagine heading into the most severe of the steep breaking seas that I have run with. Even though yes we did broach and had the masthead in the water, the boat did not break (steel) and we obviously did survive.

If we had attempted to head into those seas we would have been rolled over anyway, very probably more frequently and with less favorable results, and we may even have been pitchpoled. There is no question that we would have been far less comfortable, and in fact may not have survived...

With Regard to the Stability Curve...

It has become extremely obvious over the last several decades that a simple consideration of the "range" of positive stability is far from being the whole picture, and can even be misleading.

The stability curve is quite instructive though, and is very much the basis for any rigorous analysis of stability such as within the IMO offshore stability criteria. The IMO criteria have been under development since the early part of the 20th century, as chronicled in the various publications of the FAO with regard to fishing vessels. The FAO research was refined over the years and was eventually adopted as an international standard.

It is interesting to note that a power vessel has the unique ability to have zero region of negative stability, given that there are usually larger deck houses and often greater freeboard than their sailing brethren. Ideally, power boats should have house structures engineered to the same structural standard as the hull. For offshore use, hatches, doors, windows, and ports should all be robust and water tight. Additionally, downflooding points such as engine room vents should be located as high up as possible.

With superstructures designed as described above it is realistic to claim credit for house structures, windows, hatches, doors, etc. when calculating the stability curve. In fact, assuming this standard of engineering there is every probability that the majority of typical power vessel hull and house configurations will not have any region of negative stability. While this is a desirable attribute, it is not the whole story...

I believe it is desirable as well to pass the IMO offshore stability criteria, including the IMO extended weather criteria. The IMO has set forth various methods to analyze the "character" of the stability curve in order to model the inherent "righting energy" present, and compare that to pre-established criteria. To pass the IMO criteria, the "range" of positive stability is considered, but there are a number of additional analyses of the stability curve, each of which has a pre-established threshold.

With the exception of one power trimaran prototype, I am not aware of any Power Vessel among my own designs that does not have 100% positive stability throughout the righting arm curve, nor one that doesn't pass the IMO offshore stability criteria. Of course this assumes that superstructure, windows, and openings remain watertight.

The IMO minimum GMt for Fishing Vessels is an additional criteria that I superimpose on the basic IMO criteria for offshore power vessels. It is more conservative than the "basic" IMO criteria for GMt. Most small craft (under 100 ft) will exceed the minimum GMt requirements by a generous margin. Too great a GMt is undesirable however. It is usually an indication of poor large angle stability (as with a multi-hull).


In many cases during the design process there will be requests for violations of offshore "common sense" that may compromise or lessen the ability of a vessel's structure to withstand a capsize, that may possibly increase the likelihood of capsize, or that may in some other way affect general seaworthiness.

Examples of the first are large picture windows subject to breakage; of the second the presence of bow bulbs that may induce broaching, or low freeboard that may reduce the stability range, or overly heavy superstructures that raise the CG; of the third the presence of an integrated swim deck with cockpit access open to astern that may invite the seas aboard.

Even though these are common power boat features, they are not conducive to ultimate survival... While any one of them may or may not be problematic, taken together in combination they can be dangerous.

It should be mentioned that within the US, there are no standards or regulations having to do with stability criteria for recreational sail or power boats.

With Regard to Comfort...

There are many attributes that contribute to comfort. In terms of rolling behavior it is desirable to have a relatively modest GMt for the sake of comfort and sea-kindliness. This must not be taken to an extreme since it is also highly desirable for an offshore passagemaker / trawler yacht to pass the IMO offshore stability criteria, including the IMO extended weather criteria. For a more in-depth discussion, please see the article on Roll Attenuation Strategies.

Ideal Hull Form

From the above, if safety in ultimate conditions is paramount and it is desired to optimize a design for the best behavior while running as well as for survivability, the ideal passagemaker will have:

It should go without saying that the vessel's hull and superstructure should be designed to withstand punishment and that windows, hatches and doors should be heavy duty and water tight.

Except for the temptation toward incorporating a bow bulb, all of the above apply equally to sailing craft, although of course there will be different criteria applied with regard to optimum prismatic, keel depth, rudder design, etc.

Given the above design parameters, the net result will be a vessel that is more spacious due to her length; more gentle in terms of rolling accelerations; more economical to operate, and one which will reach her destinations more safely and more quickly. Such a vessel will not only travel faster, she will do so at less cost, and in greater comfort, with greater ultimate safety, and with considerably greater peace of mind...

For blue water voyaging, whether power or sail, is this not how a true passagemaker should be...?