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The 80' Power Trimaran
'TROIKA'
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Perspective Aft Starb'd | Perspective Fwd Portside | Perspective Fwd Low | Perspective Above Port | Perspective Above Starb'd
Copyright 2011 - 2013Michael Kasten
The Concept
The idea for this design is to achieve high efficiency under power in a seaworthy trimaran, and to have an on-deck pilot house that would include an ample saloon and galley.
The most efficient hull form is always a long, slender shape with easy lines. Thus it is natural to consider a trimaran, where the center hull can be light, long and slender in order to minimize wave-making resistance at speed. With a trimaran, the outer hulls can be designed so they are only immersed sufficiently to provide stability, but with sufficient reserve buoyancy for safe ocean travel. With this, the outer hulls contribute as little as possible to the overall resistance under way.
Layout: The trimaran hull is quite narrow inside, making for a limited number of options for the layout below. Thus, we have provided a pilot house that extends beyond the center hull sides, and ample length in order to carry the house gracefully. Outboard of the house is a side deck on each side with an open wood grating to allow it to shed water, but still be firm under-foot. The "arms" that bridge across the hulls are designed for maximum strength and to house light weight items for easy access to the side decks.
The amas are quite small, and should be kept water tight with a secure hatch. Each outer hull could become a small 'kid's cabin' so they would have their own 'private vessel' to command...! Alternately, though it is best to keep them as light as possible, they could be used for light weight storage, say for fenders, lines, sails, etc.
Appearance: Despite many of my designs having a rather traditional appearance, I actually like 'spaceship' styling on boats, such as the well known 'ILAN Voyager' or the 'Cable & Wireless.' With that in mind, I have allowed a little of that wildness to enter the picture here. But I also wanted to step away from the "airplane" type of styling for the house structures that has become so common these days. Here, the house is upright, allowing full use of the interior space, and an excellent opportunity for windows all around.
Sails: Having been conceived as an efficient motor vessel, sails are considered primarily as an emergency get-home strategy. However, there is no doubt that sails can contribute well to the overall efficiency of the machine. The synergy of motor-sailing has to be experienced to appreciate. The rig for this purpose can take a variety of forms, from the Polynesian sail types, to junk sails, to possibly a cat-ketch configuration. Mast placement is always problematic to the interior layout, so here they would ideally be deck stepped, allowing the masts to also be easily lowered.
Kites...! If a sail rig and masts were not desired, wind propulsion is still quite possible using kites...! This is not at all far-fetched. A good kite system it probably the perfect get-home rig, and offers excellent propulsion, especially down wind or on a reach, however they also can be made to work to windward. This technology is actually well developed among kite buggies, kite surfers, and even for large cargo vessels.
Keels: For good performance whilst 'get-home' sailing or motor-sailing, dagger-boards or centerboards are planned. With some ingenuity, they can be located so they do not interfere with the accommodations.
Materials: The structure in this case is planned for light weight wood-epoxy throughout. However this shape is equally well suited to the use of a light GRP structure. For a snapshot of our thinking about this kind of option, please see my article on Composites.
Materials Considerations
Given the type of vessel in question we can definitively say a few things about the materials of construction:
- Building a vessel of this type in steel would be completely out of the question, due to its weight, which would quickly defeat the purpose of the vessel - i.e. to be light and lean.
- Aluminum alloy would be an excellent choice for the entire structure, being strong, stiff, and light. However for the shapes being considered here, there would be quite a bit of forming required. Not at all impossible, merely labor intensive.
- Cored FRP construction does offer several advantages, among them strength, longevity and freedom from general maintenance. Composite construction of any kind will always be more costly in terms of materials than any other type of structure. However, a one-off mould can be easily carved out of foam using a 5-axis router, directly from the 3-D NURBS surface model. With that, labor cost would be minimized. In order to achieve the best laminate quality, we prefer vinyl ester resin or epoxy resin. With these, the logical fabric would be Kevlar or another high strength fiber such as E-glass or S-glass.
- There is little point with a cruising vessel to use high-cost exotic fibers such as carbon fiber. While light and stiff, carbon fiber is quite expensive, thus it is ordinarily ruled out for this kind of boat.
- Cold-moulded wood-epoxy construction is also strong and light in weight, offering a fairly quickly achieved structure for the hulls, the beams and the house using internal web frames and stringers.
A light weight interior is equally important. In order to achieve this, I like to make use of honeycomb panels for joinery flats wherever it is practical and easy to do so. One such material brand is Nida-Core, possibly the most cost effective among the HC panels. Nida Core panels have thin plywood skins bonded to a phenolic resin honeycomb core. This produces very stiff and light weight interior joinery with more or less the same labor cost as with standard plywood construction.
Hull Form
The amas (the outer hulls) should not be deep in the water. Opinion varies widely on this, with some advocating that the amas be completely clear of the water with the vessel upright. In my view, the amas should just "kiss" the water when at rest with the vessel lightly loaded. Thus, the amas would never be entirely free of the water unless rolling in a sea, or if a sail rig were provided and the vessel were under a press of sail.
A monohull or a power catamaran can be designed to carry enormous cargo, however a trimaran cannot tolerate being overly burdened by the accumulation of extra weight, which most cruising vessels seem to suffer from. Vigilance will therefore be required in order to preserve the inherently excellent performance that such a long, light and lean vessel has to offer.
Opinion is also divided on the best fore and aft location for the amas. In this case, the amas have been located quite far aft so that they can provide added buoyancy where it is needed most while under power. In other words, in the aft location the amas help the stern of the vessel resist squatting at speed. In this location they also help support the weight of engines and the house structure. The aft location also minimizes any tendency to trip, and provides the maximum tracking ability.
Naturally, if sailing ability were to receive greater emphasis, the amas would be located farther forward in order to maximize sailing stability. However, even for sailing the amas would not be moved beyond amidships, as that would introduce the potential for tripping as well as potential steering anomalies.
Power & Range
A trimaran allows the use of an efficient single engine, and there is no need for roll attenuation devices to complicate things...
Power could be via a diesel engine located below the Pilot House in the main hull, or could be diesel-electric. All systems must be kept as light as possible, however this need not preclude the use of simple battery power, using solar and wind assist. When you consider eliminating the weight of an auxiliary diesel generator and the fuel to run it, this option begins to make fairly good sense..! In this case much more emphasis would be placed on the vessel's sailing ability.
Being long, light and narrow, the trimaran will be capable of exceeding its theoretical hull speed by quite a bit. Although displacement speeds as high as twelve knots would be possible, higher speed always requires considerably more power. On a WL of around 70 feet, in order to achieve the best combination of range and speed under power, the sweet spot will be a cruising speed of between nine and ten knots.
Sailing Option
If it were desired to turn the 80' Troika into a full-on sailing machine, the primary change would be to the amas, making them slightly larger and placing them farther forward for the best stability and balance under sail. The 80' Troika is a perfect candidate for using a modern fully battened ketch rig, such as that which we have outlined for the 50' Sonja prototype.
Summary
The power trimaran has excellent merit for long range cruising. A few of the reasons for that statement are as follows:
Monohull power passagemakers expend considerable expense on roll damping, which in this case is just not an issue. Compared to active stabilizers, the amas would be silent and efficient. Compared to paravanes, the amas would be very simple, would require no tending, would provide extra space and extra buoyancy, and would not present nearly the amount of drag induced by paravanes.
There have been some very impressive voyages in power trimarans of late. The whole concept has engendered a completely new vessel type: the ILAN (Incredibly Long And Narrow). An internet search for ILAN will turn up quite a few interesting precedents for such a vessel, and illustrates the practicality of the concept. With an ILAN type the hull materials must be light weight in the extreme, therefore the use of more exotic materials and construction techniques should be assumed.
The adventure with the 80' Troika design has been:
- To create the best combination of economy and efficiency under power
- To introduce a unique and interesting aesthetic style
- To create a shape that lends itself to GRP or Cold Moulded building methods
I think those design objectives have been admirably met...!
Click for Larger Image
Perspective Aft Starb'd | Perspective Fwd Portside | Perspective Fwd Low | Perspective Above Port | Perspective Above Starb'd
Similar Designs...?
We have developed various prototype designs for multi-hull craft, including catamarans and proas in a variety of sizes. For more information, please inquire.
Please see the AVAILABLE BOAT PLANS web page.
Home | Intro | Our Design Process | Stock Design Info | Motor Yacht Designs | Sailing Yacht Designs | Prototype Designs
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