Predicting Hull "Ride"

[jimh]

Readers have often asked for evaluations of the "ride" of a particular boat, or for comparisons between two boats and their "ride" characteristics. Usually there is no proposal included in the request for "ride" information for a basis to determine the "ride."

Prediction of "ride" characteristics can be made by assessment of the hull design in a rudimentary manner by determining the ratio of the hull length to the hull width. The general trend is for hulls that have a higher length-to-beam ratio will exhibit better "ride" characteristics on the basis of how much area they present to a wave coming from dead-ahead.

Another possible metric is the general hull design. Hulls with a V-hull bottom are usually considered to exhibit better "ride" than hulls with a flat bottom when encountering waves from dead ahead on the basis that a V-hull will slice or carve into a head sea with little flat surface while a flat bottom will present more blunt entry and a flat surface of much greater area. The metric for the degree of sharpness in the bottom of the V-hull is the deadrise angle.

A third metric is the overall length of the boat. Wind driven waves tend to be generated with short wavelengths, that is, a short distance between wave crests or between wave troughs. As a boat hull increases in length, the boat will tend to be able to span the distance between the crests or troughs of two consecutive waves, and this feature is inferred to reducing the up-down pitching of the hull when encountering head seas.

With these general assumptions, the "ride" of a longer boat with a sharp V-bottom and a relatively high length-to-beam ratio will tend to produced a better "ride" into head sea than a shorter boat with a rounded or flatter bottom and a relatively low length-to-beam ratio.

As for the quality of "ride", so far the consideration has only been for travel into head seas at moderate speeds. There are other sea states or orientations in which the assessment of the "ride" will be different. For example, when drifting without any propulsion power in a beam sea, a boat with a high length-to-bean ratio and V-hull bottom with sharp dead rise may exhibit annoying roll characteristics compared to a boat with a low length-to-beam ratio and a flat bottom.

Another situation where the "ride" may differ is in making headway in a large following sea. Boats with a very sharp entry and very sharp V-hulls may exhibit a dangerous tendency to bow-steer when accelerating down the face of a large wave that has just been overtaken, causing the heading of the boat to suddenly veer off to port or starboard, causing the boat to lie abeam to the large wave and be subject to possibly being rolled by the wave overtaking the boat.

As for weight, I do not see that more weight is always a positive influence. Certainly more weight tends to require greater force from the waves to raise the hull out of the waves, but when the hull drops back into the waves, more weight increases the impact with those waves. Weight is not to be ignored, but more important is where the weight is located. To reduce fore-and-aft pitching in waves when traveling a displacement speeds, the general trend is to keep as much weight as possible out of the ends of the hull, and putting as much weight into the center of the boat. This will reduce the pitching moment.

For speeds where the boat is on plane, an extremely light bow can be a problem, as interaction with waves will force the bow to rise higher due to the less bow weight. For stability when on plane, the hull weight distribution may favor the stern but not leave the bow too light..

About the best observation of relative "ride" I have personally observed occurred many years ago in the open water of Georgian Bay--a bay that is so large some think of it as the sixth Great Lake. I was aboard an OUTRAGE 18 heading south into head sea and wind. The "ride" was not comfortable, as the boat speed was set to try to maintain the hull on plane, but each head sea encountered was jarring. While enduring the discomfort of this mode of travel, a larger boat on an opposite head passed close abeam. The other boat was the 108-foot Burger motor yacht CHANTICLEER owned by the Evinrude family. It was traveling downwind, downsea, in displacement mode, with almost no observable motion from the seas, while the owner was enjoying a late breakfast being served on the stern deck. If there were ever a demonstration of the effect of length, length-to-beam ratio, and hull design occurring, that was the moment.

Regarding Boston Whaler boats, the initial hull designs with rounded bottoms like the OUTRAGE 21 or the 16-foot hull were often noted for the very "hard" ride characteristics into head seas. The evolution away from those hull designs and to moderate dead rise V-hull designs of the OUTRAGE 18, 20, 22, and 24 (later 25) boats, was seen as significant improvement in "ride". Those OUTRAGE hulls are not optimized for slamming into head seas, but they are fantastic for tracking downwind in a following sea.

One of the most exhilarating days of boating I have ever had was a run south in the Atlantic Ocean from Ft. Lauderdale down to Miami on a sunny but blustery day with very large following seas in an OUTRAGE 25. The hull tracked fabulously as we climbed the back of 10-foot seas and then accelerated down the wave fronts. I was steering and found the hull "ride" just wonderful. There we six aboard, and those sitting in the aft end of the open cockpit never caught a drop of seawater spray as the hull carved a foamy path through these seas. It was a remarkable demonstration of the Boston Whaler OUTRAGE hull. Of course, later we returned north in the Atlantic Intracoastal Water Way (AIWW) and its much better protected waters.

A further thought: most prudent boaters do set out to sea when there are ten foot head seas to be encountered. But many boats often have to return to port in following seas when the weather changes, particularly often in large seas that build at inlets as the water depth decreases and opposing currents may stack up in-coming waves to greater height and shorter wavelengths. The ability of have good "ride" in those characteristic may be the best "ride" characteristic to have in a boat.

ASIDE: for a very nice retrospective on the Evinrude's CHANTICLEER see this reprint of a 2007 article at

https://www.powerandmotoryacht.com/mega ... fords-love