A Fast Offshore Cruiser - 7. Design philosophy.

This chapter will explain the choice of hull form and will describe in detail the ideas that drive the chosen hull form and make it the best choice. The final section explains the selection of the enhancement of stability method.

The traditional offshore cruiser is heavy, deep and slow. This may be unfair to some cruisers but this type of boat does seem to make an effort of sailing. A slow boat maybe what is wanted, but do you want to stay at sea for longer than really is necessary. The knowledge that you have the ability to outrun bad weather, catch the tide, catch last orders etc. must be a comfort when you know that it can be done without a particular increase in the amount of effort needed.

"Traditionally, cruising boats were built to be strong and, because of the limitations of build materials, that meant heavy. The very backbone of the boat, the keel, needed to run the length of the boat for strength, hence the long keel. Today those same limitation no longer apply because modern materials can be both strong and light" (7).

There are some arguments for a long keeled hull form that supporters will always try to state, which are more than likely true for some of the boats. One of these arguments will be the ability to track in a straight line with minimum or no attention to the steering, a very useful characteristic. Why shouldn’t a fin keel yacht do the same? So long as the boat is designed with this in mind e.g. a balanced hull and rig, this will not be a problem. This very characteristic is always a downfall for a long keel yacht when it comes to close quarters manoeuvring.

Today many of the one-off and small number production cruising boats, particularly from around America are long water line, low displacement serious cruising boats which have completed many offshore passages in complete safety.

Another place were the idea that heavy is best has been confronted is in the single-handed racing yachts like the Open classes. In essence these are just very extreme, powerful, large dinghies but at the heart of it they do have the same goal as a cruising boat. They want to get from A to B in the shortest amount of time. They have had problems, some very serious, like the range of positive stability and keels falling off in the middle of an ocean. But these are the boats that are on the limit of design so a cruising boat for the normal sailor will never be that extreme, at least not yet. But ideas and lessons learnt from them should be thought about carefully as to how they could be advantageous to a cruising boat.

These are the reasons for my choosing a long waterline, relatively light displacement hull form.

The main reason for a long LWL is to achieve the higher speeds that can be attained. There are also other properties that make it desirable.

Firstly a long water line, even with the same BWL, will reduce the angle of entry and it is likely the amount of flare, forward, above the DWL will also reduce. This may seem to reduce the buoyancy forward but it is actually increased due to the fact there is about a meter (in the case of 7seas 1524) extra length, on the DWL, that will bring in reserve volume lost by the low angle of attack.

When driving into waves the low angle of entry and reduced flare mean that the progression of buoyancy into the wave is much gentler, therefore reducing the impact of increasing buoyancy the wave has. This reduced impact will reduce the pitching of the boat of the boat in head seas.

If the boat does start to pitch the longer LWL will have a greater resistance against the pitching because of having a greater longitudinal water plane inertia about the LCF.

As said before the long LWL will have a gentler increase in buoyancy as you move along the hull. This reduces the rate of the pressure increase around the bow. The bow wave is produced by an increase in pressure of the water. The reduced pressure increase means the bow wave will be smaller, giving lower resistance and less spray will be produced.

Having a narrow bow and minimal flare could produce better characteristics in terms of spray and green water coming on deck, so long as you except that water will come over the some of the deck. A boat with flare hitting a wave, up wind, will dig in and the flare will push the water/spray sideways, back into the wind, and upwards. Once the wind has got hold of the spray it will blow it back over the deck. By this time it will be far enough out and up for the wind to blow it across the back of the boat, giving an unhappy wet crew. In the same situation on a boat with minimal flare the spray will not be given as much sideways energy from the boat digging into the wave. The spray will therefore move upwards quicker, reaching gunwale height in a shorter distance aft along the hull. When the wind gets hold of the spray it is still blown back and across the boat but it should still be far enough forward to miss the aft end of the boat. Boats of this type do show these properties with the spray going off to leeward before it reaches the mast. If you will not accept water getting on deck then you will need very flared forward sections or a bluff bow; both of these will have a detrimental effect on the other bonuses produced by this hull form e.g. wave riding characteristics.

With a long LWL the BWL will probably reduce an amount, to achieve a similar displacement without producing a very flat hull form. So long as the initial transverse stability is not effected to badly by this it can have some benefits. When the hull is moving, the water going past will speed up, due to the curvature of the hull and having to move sideways. The local pressure of the water will reduce causing the hollow to appear amidships, this hollow reduces the transverse stability.

With a narrower BWL, and small angle of entry, the moving water is not speeded up as much as before. The reduction in the increased water speed produces a reduced reduction in the pressure amidships so that the size of the hollow is less producing relatively greater transverse stability.

The narrow BWL also helps when driving into waves. As the wave rolls past the extra buoyancy it produces moves along the boat. As the boat is narrower it will have to sink lower into the wave to produce the same level of extra buoyancy to give the same heave. This gives a better ride; the boat will not pitch and heave as much producing a softer ride.

The combination of the long water line and narrow BWL produces a high L/B ratio; this is a long narrow water plane. Things that are long and narrow are more likely to have better dynamic directional stability with the characteristic that the faster the hull travels through the water the more stable it will become.

A shallow forefoot, rather than a deep V, when digging into a wave will not give the wave the same lateral area of the hull to get hold of to push the bow off. It may be argued that a deep forefoot will provide more resistance to this side force from the wave but to do this there needs to be lateral area below the energy in the wave which is try to push the hull sideways. This energy in large waves will exist some way below the surface.

As the angle of entry is smaller there will be even less room right forward, but there will be more room in a cabin forward as one in the same position on a normal hull. A bulkhead can be fitted forward of the cabin making the bow stiffer (slamming) and providing protection against collisions with semi submerged objects. On a traditional style bow the forward cabin tends to end above, if not in front of the forefoot, having a watertight bulkhead here does not serve its purpose, the semi-submerged container will hit the hull after the bulkhead. With a forward forefoot the container will hit forward of the bulkhead.

The transom has very little clearance above the DWL. This is a compromise between light wind and heavy wind / motoring performance. The long LWL will reduce the resistance at the higher speeds, allowing the boat to go faster for a given power, great for motoring. The same is good for sailing in winds that will drive the boat at speed. The problem with the stern wave climbing the transom should not be to bad so long as the run off the hull is smooth and fair with a sharp cut off, except possible at the lowest speeds, as it will break away and move aft of the hull.

Inspiration for the hull design came from Steve Dashew’s ‘Offshore Cruising Encyclopaedia II’ (3) and many of the boats in the parametric study.

Stability enhancement of sailing yachts has come to the fore in the last few years thanks to single handed racing craft like the Open 50’s, 60’s and Mini transits. These boats have used various systems including salt-water ballast in wing tanks, canting keels (both transversely and longitudinally) to achieve greater righting moments.

Canting keels disregarded early on, on the basis of the amount of lost space in the cabin to rams etc., other reasons against include the issue of taking the ground and hitting it at speed. Though there are designs being built that do claim that they can take the ground due to the fact they have twin-canting keels, but this seems very complicated and fragile.

Using salt water to produce extra righting moment is a very good idea for a racing or large yacht. It allows you to remove the extra weight when it is not required, say in light wind conditions. It is also possible to increase the weight of the boat dramatically by filling both tanks, for extreme weather conditions. On small yachts the volume that is taken up in the interior will be noticeable.

Another way of having water ballast is to use the yachts fresh water supply. There is still the same problem with lost volume but this can be counteracted by saying that the space that would have been filled with the fresh water can be used for stowage of other items. As the fresh water is now being used positively it makes sense to carry more. This will increase the range of the boat, even allow luxuries such as showers etc. Fresh water ballast in wing tanks is a good idea for a cruising boat, so it will be used in this design.