Bruce Roberts
Yacht Design

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Bruce Roberts-Goodson

Gwenda Roberts-Goodson


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The following is excerpted from Bruce's forthcoming book "CHOOSING A CRUISING POWERBOAT". You may print this out for future reference.

Power boat hulls are divided into three main types namely, Displacement, Semi-displacement and Planing hulls. Each hull type can have many sub-types which are closer to one or other end of the spectrum. Considering each hull configuration in detail will reveal its benefits and disadvantages. Your choice will be influenced by your intended usage and the size of your wallet.

The size of your intended power boat will also be a factor in your choice of hull type; for instance if you are considering a large power boat (large is a relative term) then you will be less likely to choose a full planing hull. Large fast planing hulls require large expensive engines and use large amounts of fuel and operating costs are high. Below are the outlines of the qualities of the various types of power boat hulls.

Starting at the Displacement end of the range, these were the first to be developed and to go back to the beginning of time; the original log canoe and even the ark (as far as we know) were all displacement hulls.

These heavy displacement hulls include such craft as tugs and deep sea trawlers. If you study these boats in profile you will notice that the stern rises above waterline. The mid section of the hull is very full and the midsection is deep in the water. The chine and buttock lines will reveal the full bellied shape usually present in this type of hull. The heavy displacement hull has to be able to carry great loads and in the case of tugs, be able to get a great grip on the water in order to do its job properly. The "hull speed" of this type of vessel is generally less than that of other types.

This Roberts designed Trawler was built
using the 'Ezi-Build' one off-fiberglass technique.
Other types can be built using the same methods.

These hulls include most regular work boats, general fishing boats and the pleasure boats where speeds of 1.34 times the square root of the water line length (or less) are sufficient to fulfill their operating requirements. For instance let us consider a 40 ft / 12.19 M, LOA. motor cruiser with a waterline length of 36 ft / 10.97 M, the square root of the waterline is 6 so multiply this by 1.34 and you arrive at a potential speed of just over 8 knots / 14.8 kmh. This is an economical speed for this vessel taking into account power required and fuel used to drive the vessel at "hull speed". Medium displacement vessels can only exceed the 1.34 rule by adding excessive amounts of power. If you already own an engine that has more horsepower than required to fall within the 1.34 calculation, then consider building a longer hull or one that employs semi-displacement hull characteristics. In a medium displacement hull, the V at the transom is usually fairly flat with anything from 3 to 7 degrees being the norm.

This PCF 40 is a typical full displacement Trawler type hull. This hull is also used as a working trawler.

Once the most economical speed is achieved, it takes a considerable amount of power to make a displacement hull go faster. When this type of hull is over driven then the stern will drag in the water and usually create a large stern and bow wave. The boat may reach such an extreme bow high, stern down angle, where water could come in over the stern and swamp the vessel.

Displacement hulls should not be driven much in excess of their "hull speed". For vessels ranging in size from 30 ft / 9.1 M to 60 ft / 18.3 M waterline length, you should consider displacement hulls if your speed requirement is around the 6 to 12 knot mark respectively. For higher speeds consider Semi-displacement or Planing hulls.

Trawer Yacht 485
This is one of our new 'Voyager' passage making offshore Trawler Yachts. This boat can be built as a full displacement or semi-displacement vessel.

See TRAWLER YACHT pages for more details on these type of vessels.

TY485-D1.jpg (21682 bytes)

One important factor is that Displacement and Semi-displacement hulls are generally considered better ‘Sea Boats’ and are more suitable for serious offshore cruising than the planing hull type. As with heavy displacement hulls, medium displacement hulls are not so affected by weight as the semi-displacement and planing hull types.

As the names suggest these hulls fit neatly in between the displacement and the planing hull types. The stern of the Semi-displacement hull is lower and designed to be always below the water. The hull can be round bilge form but is generally of the 'Hard chine' type. These hulls have less fullness than a full displacement hull. The chine line runs aft with a small curve from where it enters the water and on back to the transom. The hull sections are moderately Veed.

This Waverunner 38 is a typical semi- displacement powerboat. This version
performs best at speeds 8 - 14 knots

The semi-displacement hull will out perform the displacement ‘Hull Speed’ rules and will accept additional power and convert it to additional speed however there are limits to this benefit. Generally speaking for vessels with 30 ft 9.1 M to 60 ft / 18.3 M waterline length, you should only consider Semi-displacement hulls if your speed requirements do not exceed 12 to 18 knots.

As you have seen with displacement hulls additional power is wasted, however with semi-displacement hulls often the extra power may be utilized to advantage. If you already have access to a certain size of engine; or you already own the engine(s), then this factor may assist you in making the decision as to which type of hull best suits your situation.

As with Displacement hulls, Semi-Displacement hulls can be driven harder, but at the expense of greater fuel consumption and again the stern will tend to dig in at higher speeds. Existing semi-displacement hulls can be made to achieve extra speed with the same horsepower by adding trim tabs or planing wedges at the stern. The trim tabs and the wedges will be fixed after trials are completed to establish the best angle. In no case should you try to improve the performance of your hull in this manner without the assistance of professional advice.

If you are building a Semi-Displacement hull, you should try and keep the weight to reasonable levels. The Semi-Displacement hull is a good weight carrier but it takes additional power and fuel to get the best out of an over weight boat of this type.

Finally this is the type I would personally choose when planning to undertake extended cruising, that is cruising that regularly involves cruising distances of over 100 miles from home base.

The planing hull is recognized by the straight run of the chine and buttock lines from midships aft. The chine and the bottom of the hull V will generally run parallel to the waterline. The V in section will generally be constant from just aft of midships to the stem The angle between the baseline and the bottom of the V will be in the range of 12 to 20 degrees at the transom. As with other types of hulls there is a great range of planing hull variations. Usually there is a planing strake or flat at the chine and often several planing stakes on the bottom of the hull.

This steel Waverunner 44 was built in Oman as a Fisheries Patrol vessel. This is one of many
examples of this design. The hull is shown hereto illustrate the planing flats of the chine that are necessary to help these larger steel boats up onto a plane.

You will often hear the terms 'Deep V' or 'Moderate V'. These terms are meant to convey the amount of V at the transom and in addition to this they do express two different types of hull. A true 'Deep V' hull will have 20 to 24 degrees of V at the transom while a Moderate V hull is one with around 20 degrees of V at the transom. The area in between 16 and 19 degrees can be described either way by the particular designer or builder of the particular boat. Suffice to say that a hull with a V at the transom or 20 degrees or over can be safely classified as a deep V and in my opinion should not be described as a long distance or passagemaking cruising powerboat.

When deep V hulls were introduced they were touted as the last word in planing hull design. These hulls do perform well at high speeds in rough water which is one reason that they are so successful as racing powerboats. Deep V planing hulls, depending on the particular design, can be driven at speeds in excess of 50 knots, however most are designed to cruise at speeds between 30 to 35 knots. Modern computers can accurately estimate the power requirements and speed expectations of all hull types and are especially helpful in the case deciding the power needed for individual planing hulls.

Planing hulls are very popular, they make great pleasure boats if you are prepared to install sufficient power and pay the larger fuel bills. Planing hulls do not like being operated at low speeds; they throw a most unfriendly bow wave. Planing hulls are not the best of sea boats especially in severe conditions. For local and coastal cruising it is worth noting that a planing hull may allow you to get home before the bad weather arrives. If your type of cruising lends itself to the advantages of a planing and if the disadvantages including high cost of operation do not bother you, then by all means consider this type. In this case a moderate V hull is recommended. On no account select a planing hull if you intend to operate your boat in the canal systems of USA or Europe. These hulls are not suitable if your cruising area is restricted to low speed operation.
Almost all planing powerboat hulls are of single chine configuration and most have 'chine flats' or 'planing chines' and occasionally 'planing strakes' that assist with getting onto, and maintaining the planing attitude. It is my opinion that chine flats are desirable on all planing craft. Intermediate planing strakes may not be worthwhile on boats intended to perform at less than 30 knots. Planing chines or flats, will start with a small or no flat, at, or near the bow and the width of the flat will gradually increase, until it reaches its widest point somewhere just aft of amidships and maintains this width through to the stern. The efficiency of the 'chine flat,' may be improved by canting it downward by say 2 to 4 degrees throughout its length.
Hulls that are intended for Passagemaking will most likely be of the round bilge hull form. It is possible to design a semi-displacement round bilge or round chine hull but the type is more suited to the chine hull configuration. Round bilge can be used for any displacement type hull especially those that are to be used for long distance voyaging. One area we are exploring is the design of steel radius chine power boat hulls; our ideas are at the developmental stage and I believe that this idea is worth further investigation.
We have developed new Radius Chine steel techniques that produce beautiful round bilge steel or aluminum hulls. These boats are easy to build and exhibit all of the advantages of both chine and round bilge hull forms. These radius chine hulls represent a new development that has been made possible by the technology offered by modern computer assisted yacht design. These soft chine hulls have the capacity to offer the soft ride that we have often desired in a round bilge or chine hull form, but seldom achieved. The design criteria of these hulls is meant to eliminate the firmer ride offered by a chine hull and also helps with the installation of active anti-roll fins that minimize the rolling that is a less desirable feature of most round bilge powerboat hulls. Through our wide experience in using the radius chine hull form in our sailboat designs, we have established that this hull form is ideal for displacement "Passagemaking" Trawler Yachts.

These boats are becoming increasingly popular and come in a variety of hull configurations. It is possible to design displacement, semi-displacement and planing hulls to be used with the CATAMARANamaran concept. Semi-displacement power CATAMARAN offer promise as comfortable, roomy and economical cruising powerboats and we are investigating this concept and already designing boats in this configuration.