The Boat Design & Boatbuilding Site
Multihull Structure Thoughts
- Thread starter oldmulti
- Start date
- Featured
Co-founders Dan Hook and Rich Daltry of RAD Propulsion of Southampton England, developed an electric outboard company that started by working out the most efficient propellers to be used by an electric motor. The logic was: “What’s the most efficient prop we can give for a given thrust and speed? And then we worked our way back up the drivetrain, from the propeller up to the electric motor”.
Because electric motors produce incredible torque—the capacity to go from zero to maximum power almost instantly—they can swing bigger propellers than a gasoline engine. The 40-kW (equal to 55 to 60 HP) outboard they were designing could easily cope turn a 13″ (33mm) propeller (and larger) rather than the standard issue 11¼” (286mm) model fitted to an equivalent petrol outboard. If correctly pitched, those bigger propellers could increase efficiency. Another holdover from conventional outboards was the cooling system, with water intakes on the leg and an exhaust exiting through the propeller, creating more drag. An electric engine doesn’t heat up as much, so it can be cooled internally by a closed-circuit system fitted with a heat exchanger. No need for raw-water intakes and the associated complications of blocked filters and corrosion and you also get a more streamlined propellers.
Daltry and Hook wanted the steering system to incorporate 180° rotation, which they achieved by fixing the top part of the outboard (the powerhead) and only rotating the lower leg. When fitted to the back of a boat, this has the added benefit of freeing up space normally needed to accommodate the arc of the outboard powerhead. In practical terms, that means appendages such as a bathing platform can come right up to or even over the engine. The next step was to hinge the outboard from the top of its front edge rather than from under the powerhead, so when it was raised it didn’t impinge on the internal space.
For the motor, they opted for an axial-flux geometry rather than the more common radial-flux technology found on most electric outboards. The axial-flux is lighter, more compact, and more energy-efficient—with a claimed efficiency of 96% compared to 80% for a standard brushed motor (or just 35% for an internal-combustion engine). Axial-flux motors are often described as “pancake” designs because they are low, wide, and round.
“The idea behind RAD was to make things very reliable and robust, highly digital, software-driven, and very autonomous,” says Daltry. “In almost all cases, our unmanned systems were very connected. Everything was connected back to one computer infrastructure where all the software could manage it. That was then connected to humans onshore. And so RAD was born of that same mindset but trying to build it in a productized set of building blocks, starting with our electric propulsion business.”
The system was designed to be powered by 400V batteries, which deliver greater power and can be rapidly recharged at a suitable fast-charging station. For their first creation, they chose 40 kW (rated at 55-60hp) as a popular mid-range size that could power most RIBs and other small motor vessels. Daltry estimates the RAD 40 fitted with a 120-kWh battery (largest Tesla battery) will run for three hours flat out (much longer at lower speeds) with a range of about 100 miles—ample for many workboat applications.
There were other, more surprising applications, such as the floating hotel used for wildlife safaris on the Chobe River in Botswana. Four RAD 40 outboards have been fitted to the large trimaran platform, one on each corner, allowing it to move in almost complete silence and get significantly closer to wildlife. Combined with solar panels and a pair of 61-kWh batteries, the propulsion system is completely self-sufficient, delivering a 100% fuel-cost savings.
“A lot of what we do uses automotive technology,” Daltry said. “There’s so much investment into electric cars, and we get to piggyback that. So, if a new battery chemistry comes in or if a new motor architecture comes out, that’s something we can fold in—and that helps to bring the price down as well. Typically, the marine industry lags the automotive industry by many years, and there’s good reason for that—obviously a lot of work goes into marinizing an entire combustion engine and you want something reliable. But we’re managing to be right on the edge of the electric vehicle technology. There’s no time lag. What’s in our drive is what’s in the current state-of-the-art electric car.”
With the RAD 40 fully launched and most of the supporting infrastructure now in place, the team are now turning their attention to broadening the company’s range of products. A 120-kW model (rated at 160hp) is currently in development for larger craft, as well as a smaller model currently known as the RAD X because its size hasn’t yet been finalized—though it’s likely to be in the 5-10 kW range, tiller-steered, and powered by a 48V battery.
Below is the specifications for the RAD 40. The jpegs show the RD 40 and applications and also the last jpeg is the new RAD 120 KW equal to a 160 HP outboard.
RAD 40 SPECIFICATIONS
Power 40 kW
Rated horsepower 55-60hp
Voltage 400V
Propeller speed 2250 rpm
Weight 98kg (216lb)
Steering angle ±90°
Because electric motors produce incredible torque—the capacity to go from zero to maximum power almost instantly—they can swing bigger propellers than a gasoline engine. The 40-kW (equal to 55 to 60 HP) outboard they were designing could easily cope turn a 13″ (33mm) propeller (and larger) rather than the standard issue 11¼” (286mm) model fitted to an equivalent petrol outboard. If correctly pitched, those bigger propellers could increase efficiency. Another holdover from conventional outboards was the cooling system, with water intakes on the leg and an exhaust exiting through the propeller, creating more drag. An electric engine doesn’t heat up as much, so it can be cooled internally by a closed-circuit system fitted with a heat exchanger. No need for raw-water intakes and the associated complications of blocked filters and corrosion and you also get a more streamlined propellers.
Daltry and Hook wanted the steering system to incorporate 180° rotation, which they achieved by fixing the top part of the outboard (the powerhead) and only rotating the lower leg. When fitted to the back of a boat, this has the added benefit of freeing up space normally needed to accommodate the arc of the outboard powerhead. In practical terms, that means appendages such as a bathing platform can come right up to or even over the engine. The next step was to hinge the outboard from the top of its front edge rather than from under the powerhead, so when it was raised it didn’t impinge on the internal space.
For the motor, they opted for an axial-flux geometry rather than the more common radial-flux technology found on most electric outboards. The axial-flux is lighter, more compact, and more energy-efficient—with a claimed efficiency of 96% compared to 80% for a standard brushed motor (or just 35% for an internal-combustion engine). Axial-flux motors are often described as “pancake” designs because they are low, wide, and round.
“The idea behind RAD was to make things very reliable and robust, highly digital, software-driven, and very autonomous,” says Daltry. “In almost all cases, our unmanned systems were very connected. Everything was connected back to one computer infrastructure where all the software could manage it. That was then connected to humans onshore. And so RAD was born of that same mindset but trying to build it in a productized set of building blocks, starting with our electric propulsion business.”
The system was designed to be powered by 400V batteries, which deliver greater power and can be rapidly recharged at a suitable fast-charging station. For their first creation, they chose 40 kW (rated at 55-60hp) as a popular mid-range size that could power most RIBs and other small motor vessels. Daltry estimates the RAD 40 fitted with a 120-kWh battery (largest Tesla battery) will run for three hours flat out (much longer at lower speeds) with a range of about 100 miles—ample for many workboat applications.
There were other, more surprising applications, such as the floating hotel used for wildlife safaris on the Chobe River in Botswana. Four RAD 40 outboards have been fitted to the large trimaran platform, one on each corner, allowing it to move in almost complete silence and get significantly closer to wildlife. Combined with solar panels and a pair of 61-kWh batteries, the propulsion system is completely self-sufficient, delivering a 100% fuel-cost savings.
“A lot of what we do uses automotive technology,” Daltry said. “There’s so much investment into electric cars, and we get to piggyback that. So, if a new battery chemistry comes in or if a new motor architecture comes out, that’s something we can fold in—and that helps to bring the price down as well. Typically, the marine industry lags the automotive industry by many years, and there’s good reason for that—obviously a lot of work goes into marinizing an entire combustion engine and you want something reliable. But we’re managing to be right on the edge of the electric vehicle technology. There’s no time lag. What’s in our drive is what’s in the current state-of-the-art electric car.”
With the RAD 40 fully launched and most of the supporting infrastructure now in place, the team are now turning their attention to broadening the company’s range of products. A 120-kW model (rated at 160hp) is currently in development for larger craft, as well as a smaller model currently known as the RAD X because its size hasn’t yet been finalized—though it’s likely to be in the 5-10 kW range, tiller-steered, and powered by a 48V battery.
Below is the specifications for the RAD 40. The jpegs show the RD 40 and applications and also the last jpeg is the new RAD 120 KW equal to a 160 HP outboard.
RAD 40 SPECIFICATIONS
Power 40 kW
Rated horsepower 55-60hp
Voltage 400V
Propeller speed 2250 rpm
Weight 98kg (216lb)
Steering angle ±90°
Attachments
If there is one thing that really annoys me in boatbuilding is sanding and fairing to get that really good finish. As Jim Brown of Searunner fame said, do you want to polish your boat to perfection or go sailing. Unfortunately to get the perfect finish it generally involves a lot of sanding. If your building a boat from a mould or using flat panel foam glass techniques you will still be doing a lot of sanding. Even with moulded parts your still going to need tape joints etc that will need to be sanded.
There have been people who have built very large home built boats and they have one thing in common they semi automated the way they faired their boat. One USA guy, Clyde, built almost single handed, a 100 x 40 foot aluminum full wingdeck power cruising catamaran. He had built the shell of the cat then came the minor detail of getting a smooth finish on the outside. After putting a chemical surface cleaner over the entire exterior, Clyde put a 4 to 6 mm layer of Bondo filler on the outside of the cat and start fairing the 14,000 plus square foot. After a very short time Clyde developed 4 different sanding machines to assist him doing the fairing. He in effect had a power assisted long board. Are success! Now we had a good looking structurally sound shell with engines installed. After 5 years of work now all Clyde had to do was shift the 100 x 40 foot cat 85 miles to the water. (Page 177 Multihull Structure Thoughts).
Last year, Viking Yachts (New Gretna, New Jersey) turned to automation, artificial intelligence (AI), and robotics specialist Viam (New York City) to help reduce the thousands of hours of manual fiberglass sanding required for its superb finishes. Conversations started in November 2024, and by July 2025 the first custom robotic block sander was installed at Viking and early-stage testing began.
“Fairing is one of the most labor-intensive and critical parts of the build process,” said Daniel Thompson-Rhodes, Industrial Engineer at Viking Yachts and the project lead. “If we can maintain Viking’s standard of finish while reducing physical strain and reallocating our skilled techs to other areas, that’s a win. This isn’t about replacing people—it’s about evolving the process to support them and the long-term quality of the product.”
Viam designed its sanding tool by learning how the best technicians at Viking did the work by hand. That meant avoiding the random orbital sanders that are good for final finish, but less so at fairing flaws out of the molded surface. The Viam tool is essentially a block sander on a robotic arm with a 63″ (1.6m) reach. It requires a skilled sanding technician to operate it, running the machine’s scanners, changing sandpaper, applying dye to the subject surface, identifying areas in need of sanding, directing whether to sand more or less, and approving when a task is complete. Contrary to the hype that AI is completely automating production tasks, this is not a technology you can walk away from and find a perfectly faired surface when you return from lunch. “People should be thinking of them as tools first, robots second,” Horowitz said. “You can instil some knowledge into it and have it repeat tasks.”
This is a step along the way to reduce a time consuming and annoying sanding tasks. It also helps enormously if you work neatly with the basic structure, eg use peel ply when rolling out glass fabric, clean up drips or lumps as you go, do a fairing coat within a few hours of a final glass layup to minimise the sanding required and if possible use clean moulds for parts so there is no requirement to finish the entire surface, just the edges that will be taped in.
The jpegs give the idea of the automated robot. The first 2 jpegs are of the 100 foot catamaran and the sanding long board.
There have been people who have built very large home built boats and they have one thing in common they semi automated the way they faired their boat. One USA guy, Clyde, built almost single handed, a 100 x 40 foot aluminum full wingdeck power cruising catamaran. He had built the shell of the cat then came the minor detail of getting a smooth finish on the outside. After putting a chemical surface cleaner over the entire exterior, Clyde put a 4 to 6 mm layer of Bondo filler on the outside of the cat and start fairing the 14,000 plus square foot. After a very short time Clyde developed 4 different sanding machines to assist him doing the fairing. He in effect had a power assisted long board. Are success! Now we had a good looking structurally sound shell with engines installed. After 5 years of work now all Clyde had to do was shift the 100 x 40 foot cat 85 miles to the water. (Page 177 Multihull Structure Thoughts).
Last year, Viking Yachts (New Gretna, New Jersey) turned to automation, artificial intelligence (AI), and robotics specialist Viam (New York City) to help reduce the thousands of hours of manual fiberglass sanding required for its superb finishes. Conversations started in November 2024, and by July 2025 the first custom robotic block sander was installed at Viking and early-stage testing began.
“Fairing is one of the most labor-intensive and critical parts of the build process,” said Daniel Thompson-Rhodes, Industrial Engineer at Viking Yachts and the project lead. “If we can maintain Viking’s standard of finish while reducing physical strain and reallocating our skilled techs to other areas, that’s a win. This isn’t about replacing people—it’s about evolving the process to support them and the long-term quality of the product.”
Viam designed its sanding tool by learning how the best technicians at Viking did the work by hand. That meant avoiding the random orbital sanders that are good for final finish, but less so at fairing flaws out of the molded surface. The Viam tool is essentially a block sander on a robotic arm with a 63″ (1.6m) reach. It requires a skilled sanding technician to operate it, running the machine’s scanners, changing sandpaper, applying dye to the subject surface, identifying areas in need of sanding, directing whether to sand more or less, and approving when a task is complete. Contrary to the hype that AI is completely automating production tasks, this is not a technology you can walk away from and find a perfectly faired surface when you return from lunch. “People should be thinking of them as tools first, robots second,” Horowitz said. “You can instil some knowledge into it and have it repeat tasks.”
This is a step along the way to reduce a time consuming and annoying sanding tasks. It also helps enormously if you work neatly with the basic structure, eg use peel ply when rolling out glass fabric, clean up drips or lumps as you go, do a fairing coat within a few hours of a final glass layup to minimise the sanding required and if possible use clean moulds for parts so there is no requirement to finish the entire surface, just the edges that will be taped in.
The jpegs give the idea of the automated robot. The first 2 jpegs are of the 100 foot catamaran and the sanding long board.
Attachments
Skip Johnson
Senior Member
- Joined
- Feb 12, 2021
- Posts
- 365
- Reaction score
- 207
- Location
- Lake Tenkiller, Ok, usa
I'm in the Jim Brown finishing school.
This is a little primer for those who have purchased plans for EG a power catamaran that has a maximum of 2 x 200 HP outboards on the transom. The cat is a proven design and performs well. The builder starts to build the cat and as it nears completion new 350 HP outboards come on to the market not too much heavier or expensive, so they are purchased. He upgrades the transom strength a little and installs his 2 x 350 HP outboards. Initial tests runs at slower speeds are OK but when full throttle runs are done the cat becomes difficult to control especially when there is any sort of waves.
Outboard motor manufacturers made the jump from the 250-hp range to 350 hp and 400 hp or more, and consumers’ appetite for speed pushed builders to install triples and quad outboards on transoms. Soon a new problem appeared—legacy hull designs had not been updated, and many were unable to handle this new outboard power and higher speed. The old bottom-up solution to creating a new model to accommodate additional horsepower was to take a bare hull, lengthen the running surface, add a few lifting strakes, provide a place for built-in trim tabs, and you were good to go. That worked well until we bolted 1,600 hp on the transom, at which point many existing hull forms became untenable and downright scary at the helm.
At 45 mph most late-20th-century high-speed designs perform well in moderate sea conditions. But at 60 mph, unfavourable characteristics appeared, especially when traversing oncoming wakes or navigating power-on tight turns. At 90+ mph the hull is clearly either able or unable to handle the increased power. There is no cover-up for an unruly, poor-handling boat at that speed. Newton’s Third Law says “For every action there is an equal and opposite reaction.” The faster the boat, the greater the forces it is subjected to by the water and air it encounters, and the less forgiving it is of design or handling missteps. At high speed the stakes are higher, the tolerances tighter, and the old add length and hope process doesn’t work.
In high speed offshore power racing catamarans are going through a design revolution that has required very accurate computer modelling of the water aerodynamic interface to help control the water and air flow around the hulls and tunnel at the higher speeds possible with more power. EG the modelling found that above 100 miles/hour water in the tunnel at bridge deck level goes backwards in some conditions due to air compression and the hulls need some steps to help release that pressure. This was reducing top speed by about 5 miles/hour.
Translation. Build a design as specified or speak to a designer if you want to make changes. Adding extra power can result in a difficult to handle or dangerous boat if not done well.
Outboard motor manufacturers made the jump from the 250-hp range to 350 hp and 400 hp or more, and consumers’ appetite for speed pushed builders to install triples and quad outboards on transoms. Soon a new problem appeared—legacy hull designs had not been updated, and many were unable to handle this new outboard power and higher speed. The old bottom-up solution to creating a new model to accommodate additional horsepower was to take a bare hull, lengthen the running surface, add a few lifting strakes, provide a place for built-in trim tabs, and you were good to go. That worked well until we bolted 1,600 hp on the transom, at which point many existing hull forms became untenable and downright scary at the helm.
At 45 mph most late-20th-century high-speed designs perform well in moderate sea conditions. But at 60 mph, unfavourable characteristics appeared, especially when traversing oncoming wakes or navigating power-on tight turns. At 90+ mph the hull is clearly either able or unable to handle the increased power. There is no cover-up for an unruly, poor-handling boat at that speed. Newton’s Third Law says “For every action there is an equal and opposite reaction.” The faster the boat, the greater the forces it is subjected to by the water and air it encounters, and the less forgiving it is of design or handling missteps. At high speed the stakes are higher, the tolerances tighter, and the old add length and hope process doesn’t work.
In high speed offshore power racing catamarans are going through a design revolution that has required very accurate computer modelling of the water aerodynamic interface to help control the water and air flow around the hulls and tunnel at the higher speeds possible with more power. EG the modelling found that above 100 miles/hour water in the tunnel at bridge deck level goes backwards in some conditions due to air compression and the hulls need some steps to help release that pressure. This was reducing top speed by about 5 miles/hour.
Translation. Build a design as specified or speak to a designer if you want to make changes. Adding extra power can result in a difficult to handle or dangerous boat if not done well.
Attachments
bajansailor
Marine Surveyor
- Joined
- Oct 27, 2007
- Posts
- 4,064
- Reaction score
- 1,908
- Location
- Barbados
It baffles me as to why people want to put so much horsepower on the back ends of their boats.
I saw a video yesterday of a large (maybe 45') centre console monohull with six outboards (each around 350 or 400 hp) on the back.
And in addition to the cost of operating these beasts, think of the maintenance costs of six big outboards.
Do people buy these boats mainly for pose value perhaps?
I saw a video yesterday of a large (maybe 45') centre console monohull with six outboards (each around 350 or 400 hp) on the back.
And in addition to the cost of operating these beasts, think of the maintenance costs of six big outboards.
Do people buy these boats mainly for pose value perhaps?
The L7 trimaran was designed and produced by Michael Leneman (now deceased) from Multi Marine. The tri has been featured before and is:
Length: 23′ 6″
Beam: 16′ 6″
Folded beam: 8′ 4″
Mast height: 33′
Sail area (main & jib): 370 sq ft
Weight (empty): 1300 lb
This item is about the build technique. Mike looked for an inexpensive way to construct a small trimaran. He decided to combine a fiberglass molded “pan” with plywood/glass/epoxy topsides.
For the floats, the builder starts upside down with the deck and then installs trapezoid-shaped, bulkheads. The okoume marine plywood topsides, which have already been fiberglassed, are then installed onto the deck. Next, the butt blocks are put in and the fiberglass pan is epoxied in place. Once that is done, the whole outside of the hull is fiberglassed. An amateur builder can make this float in less than 40 hours.
The main hull is done a bit differently. The full bulkheads for the main hull are built first and placed on a strongback. Then the fiberglass pan is glued to the bulkheads. The butt blocks are glued to the pan (not the topsides, as in the floats). Lastly, the topsides are put on. After everything is set, the builder turns the hull right side up, levels the sheer, and puts on the decks and cabin. The cockpit floor, decks, anchor locker floor, cabin side seats, and lazarette floor are all flat and are pre-made with foam, glass, and plywood. There is basically an entire mid-height sheer web that runs through the entire main hull.
For the main hull decks, cockpit floor, anchor locker floor, and seats are a combination of thin plywood, styrene foam, and glass. High-density styrene foam is not usually used in custom boat construction because polyester resin eats the foam, and styrene foam is susceptible to pressure dings. However, WEST SYSTEM Epoxy works great with styrene foam, and bonding a thin layer of plywood to the top face of the composite panels eliminates the pressure ding problem. A composite panel made this way (ply-styrene foam-glass) is very stiff and light. The best part is that the cost is about 1/3 that of a standard urethane foam-cored, glass composite panel.
The X-arms are one of the coolest parts we came up with. They are fiberglass pultruded I-beams. The best part is that you can cut the sheer web of the beams and bend the caps down to make a nice looking, tapered, outside shape to the beam. The beams look good, they are pre-made, they can’t corrode, they are strong, you can paint them any color you want, and they are inexpensive.
A simple recipe of how to build a workable trimaran from cheaper materials. Mike L7 trimaran design had retractable cross beams to allow trailering. The jpegs give part of the idea.
Length: 23′ 6″
Beam: 16′ 6″
Folded beam: 8′ 4″
Mast height: 33′
Sail area (main & jib): 370 sq ft
Weight (empty): 1300 lb
This item is about the build technique. Mike looked for an inexpensive way to construct a small trimaran. He decided to combine a fiberglass molded “pan” with plywood/glass/epoxy topsides.
For the floats, the builder starts upside down with the deck and then installs trapezoid-shaped, bulkheads. The okoume marine plywood topsides, which have already been fiberglassed, are then installed onto the deck. Next, the butt blocks are put in and the fiberglass pan is epoxied in place. Once that is done, the whole outside of the hull is fiberglassed. An amateur builder can make this float in less than 40 hours.
The main hull is done a bit differently. The full bulkheads for the main hull are built first and placed on a strongback. Then the fiberglass pan is glued to the bulkheads. The butt blocks are glued to the pan (not the topsides, as in the floats). Lastly, the topsides are put on. After everything is set, the builder turns the hull right side up, levels the sheer, and puts on the decks and cabin. The cockpit floor, decks, anchor locker floor, cabin side seats, and lazarette floor are all flat and are pre-made with foam, glass, and plywood. There is basically an entire mid-height sheer web that runs through the entire main hull.
For the main hull decks, cockpit floor, anchor locker floor, and seats are a combination of thin plywood, styrene foam, and glass. High-density styrene foam is not usually used in custom boat construction because polyester resin eats the foam, and styrene foam is susceptible to pressure dings. However, WEST SYSTEM Epoxy works great with styrene foam, and bonding a thin layer of plywood to the top face of the composite panels eliminates the pressure ding problem. A composite panel made this way (ply-styrene foam-glass) is very stiff and light. The best part is that the cost is about 1/3 that of a standard urethane foam-cored, glass composite panel.
The X-arms are one of the coolest parts we came up with. They are fiberglass pultruded I-beams. The best part is that you can cut the sheer web of the beams and bend the caps down to make a nice looking, tapered, outside shape to the beam. The beams look good, they are pre-made, they can’t corrode, they are strong, you can paint them any color you want, and they are inexpensive.
A simple recipe of how to build a workable trimaran from cheaper materials. Mike L7 trimaran design had retractable cross beams to allow trailering. The jpegs give part of the idea.
Attachments
This is a quick reminder of composite cored (fiberglass sandwich) construction “strength” characteristics. In composite construction the outer fiberglass skins are normally loaded in compression while the inner fiberglass skins are loaded in tension EG for a hull side or deck.
Using these principles, some calculations were done to find the comparative data shown in the table below. The data shows how adding balsa core to a composite panel affects different properties of the composite panel in cored composite laminates.
In the table, 1t represents the thickness of a solid fiberglass laminate. For the comparative data, the thickness of the fiberglass laminate is the same and the overall composite’s thickness was increased by adding a balsa core at varying thicknesses. As expected, when considering the I-beam model, as the thickness of the composite increases, the stiffness significantly increases. Using balsa as the core material to increase the thickness of the composite, you can see that the weight increase is very minimal in cored composite laminates.
When calculating the strength of the composite, you have to consider that the core may fail in shear before either of the fiberglass skins would fail. It is clear to see that the fiberglass laminates for the 4t sample would take about 9 times the load of the 1t laminate before it would fail. However, the overall cored composite of the 4t sample would only be able to handle about 2.5 times the load of the 1t laminate before the core would fail in shear.
It is important to consider this because the core could have failed, and there would have been no visual indication of this failure when looking at the top and/or bottom skins. This is just one of the things engineers need to keep in mind when designing a composite structure, and now you can too with your knowledge of cored composite laminates.
Another possible failure mode that can happen in cored composite construction is crushing the core. This is most common in areas where fasteners are installed and the load is not distributed appropriately. This is why backing plates are critical when installing bolts through cored laminates.
Finally with modern foam cores and carbon fibre skins there are other issues such as water permeability as you may fully wet out the unidirectional carbon but not fully seal the surface of your core because there is insufficient “excess” resin to fill any gaps. Also, EG bulkheads may have theoretically strong enough structural carbon skins on them but if the EG bow bulkhead is subject to side loading from diving into a wave there may not be sufficient “stiffness” in the carbon skins to prevent skin buckling. Early carbon fibre masts were very thin skinned, the masts were prone to failing in compression and need additional wall thickness to prevent buckling.
The jpeg below is a view of various types of composites.
Using these principles, some calculations were done to find the comparative data shown in the table below. The data shows how adding balsa core to a composite panel affects different properties of the composite panel in cored composite laminates.
In the table, 1t represents the thickness of a solid fiberglass laminate. For the comparative data, the thickness of the fiberglass laminate is the same and the overall composite’s thickness was increased by adding a balsa core at varying thicknesses. As expected, when considering the I-beam model, as the thickness of the composite increases, the stiffness significantly increases. Using balsa as the core material to increase the thickness of the composite, you can see that the weight increase is very minimal in cored composite laminates.
When calculating the strength of the composite, you have to consider that the core may fail in shear before either of the fiberglass skins would fail. It is clear to see that the fiberglass laminates for the 4t sample would take about 9 times the load of the 1t laminate before it would fail. However, the overall cored composite of the 4t sample would only be able to handle about 2.5 times the load of the 1t laminate before the core would fail in shear.
It is important to consider this because the core could have failed, and there would have been no visual indication of this failure when looking at the top and/or bottom skins. This is just one of the things engineers need to keep in mind when designing a composite structure, and now you can too with your knowledge of cored composite laminates.
Another possible failure mode that can happen in cored composite construction is crushing the core. This is most common in areas where fasteners are installed and the load is not distributed appropriately. This is why backing plates are critical when installing bolts through cored laminates.
Finally with modern foam cores and carbon fibre skins there are other issues such as water permeability as you may fully wet out the unidirectional carbon but not fully seal the surface of your core because there is insufficient “excess” resin to fill any gaps. Also, EG bulkheads may have theoretically strong enough structural carbon skins on them but if the EG bow bulkhead is subject to side loading from diving into a wave there may not be sufficient “stiffness” in the carbon skins to prevent skin buckling. Early carbon fibre masts were very thin skinned, the masts were prone to failing in compression and need additional wall thickness to prevent buckling.
The jpeg below is a view of various types of composites.
Attachments
The following is a link on how to build a FAO grass roots L7 style trimaran.
openknowledge.fao.org
Fishing Boat Construction - Building a hybrid banca
Small trimarans with ski-like planning floats are rare, Samba is not conventional, but it claims to be a narrow skiff with wide hiking wings, to keep both floats completely out of the water to minimize skin friction and wave/form resistance. Peter L Thompson of Thompson Designs (Canada) has created this little boat and you can find the article about Samba in Multihull International May 1994.
Samba is 16.45 x 15.75 foot with a dry weight of 98.5 lbs and a displacement of 295.5 lbs. The 18.8 foot mast carries a 107 square foot fat head mainsail. This could be a windsurfer mast and sail with a minimal of rigging. The mast may need some reinforcing due to the higher righting moments. The length to beam on the main hull is 12.3 to 1. The daggerboard draft is 2.5 foot. The floats are basically low volume planks that are designed for planning at higher speed. The floats also can act as hiking seats to help stability in moderate to higher wind speeds.
The construction of this tri would require 3 or 4 mm light weight plywood with light framing and stringers. The cross beams appear to be 80 mm aluminum tubes with waterstays. The tubes would need to have thin walls (EG 1.2 mm) to meet the dry weight requirements.
No idea of performance but it would be fun to find out. The 2 jpegs give the idea.
Samba is 16.45 x 15.75 foot with a dry weight of 98.5 lbs and a displacement of 295.5 lbs. The 18.8 foot mast carries a 107 square foot fat head mainsail. This could be a windsurfer mast and sail with a minimal of rigging. The mast may need some reinforcing due to the higher righting moments. The length to beam on the main hull is 12.3 to 1. The daggerboard draft is 2.5 foot. The floats are basically low volume planks that are designed for planning at higher speed. The floats also can act as hiking seats to help stability in moderate to higher wind speeds.
The construction of this tri would require 3 or 4 mm light weight plywood with light framing and stringers. The cross beams appear to be 80 mm aluminum tubes with waterstays. The tubes would need to have thin walls (EG 1.2 mm) to meet the dry weight requirements.
No idea of performance but it would be fun to find out. The 2 jpegs give the idea.
Attachments
HelmutSheina
Senior Member
- Joined
- Dec 25, 2025
- Posts
- 284
- Reaction score
- 93
- Location
- New South Wales
Many years ago a company I worked for received an order for a 20' x 8' power cat with 2 x 200hp Mercury two strokes for commercial fishing in the Southern Ocean off the Australian coast. Being commercial it was under government survey and the inspector was not happy about the power, however our manager was a persuasive type and had it approved.
We fitted the biggest available pitot speedo for water trials and the needle stopped at 70mph but the boat kept going, somewhere around 75mph. The owner wrote it off after swamping it off a wave at full throttle. The cuddy was a lovely wedge shape and she submarined.
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making any potentially dangerous or financial decision, always employ and consult appropriate professionals. Your circumstances or experience may be different.
When making any potentially dangerous or financial decision, always employ and consult appropriate professionals. Your circumstances or experience may be different.
Advertisement:
