Questions bout the Interceptor 's Deep V hull design.

How fast will it go?

A very important question. The interceptor has a hard chine, deep V planning hull, from a design point of view somewhere between say a Sunseekert or Fairline (planning hulls) and a Nelson or Aquastar (semi displacement) combining elements of the two different hull forms.

  The buttock lines are  semi displacement  in form, (i.e they rise up towards the stern). There are a couple of reasons for this, a) to reduce buoyancy in the transom so that she doesn't go down by the head at rest or when traveling at displacement speeds. (This is due to the inevitable f/wd LCG of the cab forward design). b) to give good handling in following seas, big wide deeply immersed transoms cause the boat to yaw, especially at displacement speeds. 

   The Interceptors underwater sections are deep V in form with a midships deadrise of at least 22 degrees. Most Other hulls commercial hulls are medium V hulls.   Deep V hulls inherently require more power than shallower V hulls to give the same performance,  This is due to the angle of the planning surface to the water, a flat planning surface provides the greatest lift and speed. You could therefore say the Interceptor would be slower than, as an example a Bullet, which is probably one of the fastest commercial hulls around, but the story doesn't end there, The interceptor has a very slippery, low resistance hull form, it has a reverse main chine and multiple spray rails which give additional lift to the hull and reduce wetted surface area.  These design features of the Interceptor allow her easily driven hull  to give close to or similar performance as a  lower deadrise and cathedral hull types, of course it will all change the moment you encounter a head sea, when the deep V hull design will leave everything else behind.

What about economy? Especially when fully loaded.

Obviously the faster a boat is with a given power, the faster will be her cruising speed and as a result the better her economy. But things start to get complicated when you begin carrying heavy loads. In the 90's many skippers advocated the theory of turning a large diameter propeller, and their reasoning was spot on. When a boat is heavily loaded which is often the case with 12 anglers and a ton of fuel, small diameter props lose there effectiveness. On the interceptor  I designed the prop to run in an Elliptical tunnel to reduce shaft angle and allow her to swing a 25 - 28 " prop. Of course there is a limit to propeller size as to large a propeller will, because of the massive torque cause the boat to twist herself resulting in excessive heel when running. Shaft angle is also critical, to much and efficiency drops. 

Will she slam?, what will her sea keeping be like?

The simplest way to determine weather a boat will slam is to look at how much dead rise the hull has, and more importantly how much deadrise there is where the hull will be hitting waves when at speed! Medium V deadrise hulls have typically 14- 15 degrees deadrise amidships,  Cathedral or Gullwing type hulls have similar deadrise but much wider outer chines, obviously these hull are not going to cut cleanly through a wave, but will bump over it with high acceleration forces imparted into the hull and crew.  The Interceptor has a deep V hull of 22 degrees amidships and 48-58 degress at the bow. Traditionally semi displacement hulls have fairly steep deadrise at the bow and similar deadrise amidships, which is what makes them so good into head seas, Whilst its impossible to make a boat that wont slam in head seas without being seriously compromised in other areas, the Interceptor by virtue of her deep V hull gives a very smooth ride into head seas with little tendency to slam. 

Sea keeping! How will she handle?. In designing the Interceptor I faced the same problem that must face every designer, how to make her good into head seas but also good in following seas? The two just don't go hand in hand. Fine sections forward with a deeply immersed fore foot are best in head seas, but wait till you try to run downwind, especially at speed, on this course she will tend to bow steer. Cathedral hull types are very good in following seas.  Why? just look at how much buoyancy there is in their bow, it is hard for them to  dig theirr bows in. But that same virtue of a full bow is what makes her not so good in head seas. There has to be a compromise somewhere! so in designing the interceptor I gave her fine sections on the forward waterline (47 degrees)  but with lots of flare to the bow, the stem is well raked and the forefoot is cut away, the reason being that when she buries into the back of a wave in surfing conditions she will bury only to a certain point, that point being the upper deflection rail and where the bow flares out rapidly. At this point the force of the wave will be spread over a large area, the bow wont be deeply immersed and consequently wont act like a fulcrum tending to swing the stern around.  But probably the most important factor in how a vessel will handle in following seas is her dynamic stability. This is different from static stability, when a vessel is at rest. Dynamic stability comes into play when, as example the boat is running downwind in a following sea and begins to surf on the crest of a wave. At this moment the bow and possibly half the boat will be sticking out over the crest and unsupported. The boats effective waterline beam will in this moment effectively be halved. If the boat does not have enough stability a yawing motion and heel will be introduced possibly resulting in a loss of control and broaching. A high GM figure in excess of 1.2m ensures the craft will maintain her course without yaw in this scenario.

What about head seas? Well she's still pretty fine up there but remember she's been designed for higher speeds, how much use is a deep fine forefoot when your running at planning speeds and that part of the hull isn't really impacting with  seas, more important is the deep V sections aft of the fore foot, to amidships, as this is the area that's going to be hitting the seas when you come off a wave, and this is where the Interceptor  hull design pays off.

O.K. So what about stability? I've heard deep Vs aren't that stable.

This probably comes from light displacement sports boats where the chine is often not in the water at rest or only at the transom area. A flat bottom boat is probably the most stable initially, but in some ways more uncomfortable as there will be very fast acceleration forces in her initial roll,  resulting in a snap roll that in some ways is more tiring than a larger slow roll. The Interceptors V sections flatten off at the transom to 18 degrees which helps stability and importantly  her chine is underwater  right up to the shoulders, further more  the chine's themselves are reverse angle, causing the chine to literally grab the water and prevent rolling, also there's a full length keel, this is very important in giving a vessel good static stability. And remember that because of the deep V sections, it is possible to position the engine right down in the hull lowering the Vertical center of gravity, one of the most important aspects of stability. That last point brings us to the next question important to you angling guys. 

How fast will she drift?

Very important for angling boats, but once again its an area that's a bit of a catch 22 situation. The slowest drifting hulls are deep heavy displacement craft, there weight and the fact that there's so much of them under the water means that the wind doesn't move them much, so if a really slow drift is required, go for a displacement boat!  But that's not what you want. Boats that travel fast across the water do so because there's not much of them under the water. And planning boats drift fast because again, there's not much of them under the water, especially at the bow. In addition, the fact that they have quite high sides causes a lot of windage. The Interceptor has quite high sides, mainly because I wanted a flush deck, and  safe bulwarks! but the deep V sections  and larger keel mean that there is more grip on the water, especially at the bow resulting in a slower drift. Again you can see that itís all a compromise.

What about dryness? will the interceptor be a dry or wet boat?

This is an area that has been given a lot of consideration. Obviously hard chine's make for a dry boat and hulls of the semi displacement form without chine's are very wet boats, on these type of hulls, even when there is a knuckle incorporated, they are still wet,  but even some craft with chine's are still wet. The shape of the bow plays an important part, bluff bows tend to push spray forward where it is caught by the wind and blown back over the boat, or if the chine is to low then it isn't so effective. We have carried out a lot of research by observing video footage of the chine and spray pattern in waves and calm water, observing the effectiveness of the chine and trying to establish the most effective position for them. This research showed that on our type of craft, angling boats, where your carrying a heavy load at an economical speed and with everyone invariably moved forward to keep out of the weather the boat is not at an ideal attitude or angle of trim. Going faster would improve matters but is often impractical, motion in a seaway becomes uncomfortable for anglers and who can afford the diesel costs at 22 knots plus! At lower speeds, the standard main chine's position  becomes less effective. In a head sea straight on the nose its generally ok with spray being thrown out and away, but when your taking weather on the shoulder its often horrendously wet. The reason I felt was that the main chine was, at lower speeds ineffective as it was mostly immersed. Therefore on the Interceptors I incorporated a larger secondary cine or spray deflection rail 280mm above the main chine to stop water riding up the hull. A further design feature was to slightly down angle all chine's and rails to actually deflect spray down rather than out, where it is just caught by the wind and thrown back on the boat. A further point was spray rails. Bringing them right up and around the bow  mimicking the chine actually  makes matters worse as invariably they are to small where they fair in to the bow, causing spray to be only partially deflected, thrown out and  caught by wind, much better to make them parallel and  bring them forward only enough for effectiveness at high speeds, this allows the main chine to do its job properly.

What about beam?

All Interceptors have a good beam to length ratio of around 3.2, but you have to ask yourself this question. Do I require the maximum sea keeping ability, speed, and economy or the absolute maximum deck area. Once again it's a compromise. A very wide beam gives a greater deck area, but not the best sea keeping or performance, the wider the beam, the less deadrise which equals  slamming and more power to push through the water. I felt that for a charter angling boat sea keeping and economy combined with an adequate beam were the more important design elements.

What about weight?

I make no excuses, the Interceptor is a strongly built hull. She is built to Lloyds special service craft rules for scantlings and panel size and thickness. These are the most stringent standards you can build a commercial hull to. This results in a hull that can, within reason, be driven hard in rough weather with out fear of structural damage. You only have to look at the video footage and photos to see what Interceptors and Wildcats are capable of. Of course the penalty of the strength is weight. But you have to decide what you want your boat for. If is sitting around in marinas or fair weather sailing, then your probably better off going for a lightly built pleasure craft. But if what you need is strength and seakeeping, then the Interceptor has to be your choice. One other point to remember is that at sea, size and displacement matters. The bigger the boat, the better she will be in rough weather. And, all other things being equal, such as VCG, freeboard and trim, the heavier displacement hull will have better seakeeping in rough conditions. As a proven example, of all the Interceptors ever built, the pilot boat, with its heavy 18,000 displacement has the best sea keeping due to her heavy weight and huge stability, as opposed to the standard 10,000-12,00kg displacement of the workboat versions.

So there you go, a few thoughts about boat design that I considered important when I designed the Interceptor.

Frank.