FL16
05-08-2008, 04:40 PM
I would like to share my design ideas with you. As I have no experience neither in dinghy design, nor in dinghy construction every comment as basic as it may be is welcome.
I am currently sailing a RS800 (http://www.rssailing.com/fleetsnew.asp?fleet=RS800&selection=Details%20and%20Spec). It is designed for big sailing waters, waves are no problem but sailing on small inland venue with light air but loads of shifts and gusts is “challenging”.
So the focus of this design should lean more to the latter conditions.
The steps could be like this:
1: Dream up a shape
2: Verify it by some computations (at least buoyancy…)
3: Build a kind of prototype
4: Test if it survives…
Probably the design will not make it further than step 2 but it is fun and interesting anyway.
Thoughts that went into the design so far:
To keep cost for the prototype low I would like to reuse spars, sails and foils from the RS800. Carbon mast is good, foils could be a little bit on the small side. Regarding sails I would prefer one full length batten maximum instead of the RS800 main sail design so I do not need to force them to pop in light air.
As I do not have the possibility to place a hull somewhere inside for weeks of laminating, sanding and painting and I really, really hate work like that (too lazy???) I started wondering if it would be possible to build a dinghy from coated plywood. At the home depots here you get these sheets covered by a brown plastic layer, some are smooth no both sides, some are “non slip” on one side. They are used for example for car trailers.
A “hard” restriction is the fact that it shall fit into our trailer so it needs to be smaller than 2 X 5 m, some “safety margin” is welcome.
Intended crew weight is 120 – 160 kg, many classes claim they can carry 160 kg but it should do it happily not like a 29er that makes you feel like you are sinking in light air.
The buoyancy distribution should allow to keep the stern out of the water just by sitting face to face (helm windward side, crew leeward) behind the shrouds, crew fighting the jib on the narrow foredeck while helm needs to press his body against the boom to keep crew weight at centreline is not desired!
Other ideas to increase crew comfort would be off boom sheeting and cleats for Cunningham/kicker right at the mast to keep the cockpit floor uncluttered, maybe with Musto style T-shaped control lines so they can still be accessed while hiking. A rigg designed from scratch would sure have a gnav, the RS800 still features a vang.
The lines of a hull should be in a way that sailing slightly heeled to leeward to keep boom on the leeward side by means of gravity does not ruin the flow completely, e.g. by immersing a wide and flat stern. Furthermore the hull shape should provide some reserve stability at this point unlike some “box shape” design that go over right away as soon as they are heeled slightly. A light weight carbon mast will definitely help here!
However if breeze get’s up it is o.k. if the design is slower than comparable dinghies but it should still be somewhat “controllable”, e.g. no “nosediving” whatever this means for the hull shape.
To keep it simple I excluded trapezes, racks, lifting foils and kites. A pole for the jib is an option but needs to be fixed to the hull as I do not want to put “new” forces on the RS800 mast. For the same reason I intend to keep the distances of mast, forestay, shrouds and lowers the same.
“Influential” designs are RS800, 505 (both sailed myself) and Merlin Rocket, NS14, RS400 (some pictures/data from the web).
Can you propose other designs that are worth a look? I have to admit that I do not like scow designs very much, though.
Starting point is a hull of 4,8 m length, 1,8 m width and 50 cm height. This is lengthwise somewhere in the middle of RS400 and 505, both known as good weight carriers. A shorter hull would save weight but I guess it would make it more difficult to get a nice flow at the stern at low speeds without crew on the fore deck.
A sailing ready RS800 weighs in at 110 kg. Adding 160 kg crew weight gives 270 kg. The weight for the aluminium racks can be saved but I will for sure not reach the low base weight of a foam core GRP hull so I need to add some margin. What about 300 kg buoyancy if immersed to static water line?
For safety reasons and being used to it I prefer a self draining design, i.e. false floor. Combined with absence of trapeze wires this poses the question of minimum (vertical) distance between false floor and deck. Do you think 25 cm are o.k. for somewhat “comfortable sitting” on deck? How wide should the deck be if it is a straight horizontal surface and not specifically shaped like a 505 tank? I started of with 30 cm.
Next question is if the room under the side decks should be closed by a wall at the inside to act as a buoyancy compartment or stay open. While I like the centreboard being close to the water surface when capsized the extra buoyancy should give increased stability when the boat is heeled. Furthermore righting could be difficult it a lot of water rushes from one room under the sidedeck across the cockpit floor to the other side making the boat “wanting” to capsize to the other side right away? What do you think about this?
So how does the design look like now?
The hull basically results of three panels, a flat bottom and the side walls. A Flat bottom would make building way easier, I do not know at all if it is possible to come up with a decent hull shape like this.
Fore deck and side decks as well as front half of the bottom panel are strictly horizontal. The second part of the floor panel rises from 50 cm below deck to 30 cm below deck. The bow is vertical so both start at the same point of length.
The false floor is 25 cm below deck so the height of the stern is 5 cm. This allows to mount a bung on each side to drain the hull.
The idea is to have a inner “skeleton” of which the member that runs all the way on the centreline creates a independent buoyancy compartment on each side.
The “skeleton” further consists of bulk heads under the mast foot, the shroud fittings and wherever else needed, especially so the crew does not break through if they step somewhere on the cockpit floor or deck
Front floor panel and fore deck would both feature a more or less triangular shape to ensure a fine entry. The back part of the boat is a more roundish shape. Maximum width of the floor is 80 cm while maximum width of the deck is 180 cm again which should give a nice flare of the side walls and a maximum waterline width of 130 cm.
Mast is deck stepped at 200 cm from bow, the false floor starts right behind. At the stern the hull narrows down to 60 cm at bottom panel and 120 cm at deck height would give the same flare angle as at maximum width. Perhaps too narrow in regard to righting moment down wind when the crew moves aft, though.
Using similar shroud positions as the RS800 forces a minimum beam of 150 cm at half length.
Bottom and side panels could extend 1 or 2 cm aft of stern to ensure the water flow splits of the hull nicely.
The initial design in the attachments got the widest section pretty far aft (about 70 %). Due to keeping the angle of the side panels constant at 45 °C this moves the rise in the bottom panel pretty much to the stern as well.
Mast should be further forward to allow crew move forward but that would mean a smaller jib.
So please tell me what you think? How weird is it?
I am currently sailing a RS800 (http://www.rssailing.com/fleetsnew.asp?fleet=RS800&selection=Details%20and%20Spec). It is designed for big sailing waters, waves are no problem but sailing on small inland venue with light air but loads of shifts and gusts is “challenging”.
So the focus of this design should lean more to the latter conditions.
The steps could be like this:
1: Dream up a shape
2: Verify it by some computations (at least buoyancy…)
3: Build a kind of prototype
4: Test if it survives…
Probably the design will not make it further than step 2 but it is fun and interesting anyway.
Thoughts that went into the design so far:
To keep cost for the prototype low I would like to reuse spars, sails and foils from the RS800. Carbon mast is good, foils could be a little bit on the small side. Regarding sails I would prefer one full length batten maximum instead of the RS800 main sail design so I do not need to force them to pop in light air.
As I do not have the possibility to place a hull somewhere inside for weeks of laminating, sanding and painting and I really, really hate work like that (too lazy???) I started wondering if it would be possible to build a dinghy from coated plywood. At the home depots here you get these sheets covered by a brown plastic layer, some are smooth no both sides, some are “non slip” on one side. They are used for example for car trailers.
A “hard” restriction is the fact that it shall fit into our trailer so it needs to be smaller than 2 X 5 m, some “safety margin” is welcome.
Intended crew weight is 120 – 160 kg, many classes claim they can carry 160 kg but it should do it happily not like a 29er that makes you feel like you are sinking in light air.
The buoyancy distribution should allow to keep the stern out of the water just by sitting face to face (helm windward side, crew leeward) behind the shrouds, crew fighting the jib on the narrow foredeck while helm needs to press his body against the boom to keep crew weight at centreline is not desired!
Other ideas to increase crew comfort would be off boom sheeting and cleats for Cunningham/kicker right at the mast to keep the cockpit floor uncluttered, maybe with Musto style T-shaped control lines so they can still be accessed while hiking. A rigg designed from scratch would sure have a gnav, the RS800 still features a vang.
The lines of a hull should be in a way that sailing slightly heeled to leeward to keep boom on the leeward side by means of gravity does not ruin the flow completely, e.g. by immersing a wide and flat stern. Furthermore the hull shape should provide some reserve stability at this point unlike some “box shape” design that go over right away as soon as they are heeled slightly. A light weight carbon mast will definitely help here!
However if breeze get’s up it is o.k. if the design is slower than comparable dinghies but it should still be somewhat “controllable”, e.g. no “nosediving” whatever this means for the hull shape.
To keep it simple I excluded trapezes, racks, lifting foils and kites. A pole for the jib is an option but needs to be fixed to the hull as I do not want to put “new” forces on the RS800 mast. For the same reason I intend to keep the distances of mast, forestay, shrouds and lowers the same.
“Influential” designs are RS800, 505 (both sailed myself) and Merlin Rocket, NS14, RS400 (some pictures/data from the web).
Can you propose other designs that are worth a look? I have to admit that I do not like scow designs very much, though.
Starting point is a hull of 4,8 m length, 1,8 m width and 50 cm height. This is lengthwise somewhere in the middle of RS400 and 505, both known as good weight carriers. A shorter hull would save weight but I guess it would make it more difficult to get a nice flow at the stern at low speeds without crew on the fore deck.
A sailing ready RS800 weighs in at 110 kg. Adding 160 kg crew weight gives 270 kg. The weight for the aluminium racks can be saved but I will for sure not reach the low base weight of a foam core GRP hull so I need to add some margin. What about 300 kg buoyancy if immersed to static water line?
For safety reasons and being used to it I prefer a self draining design, i.e. false floor. Combined with absence of trapeze wires this poses the question of minimum (vertical) distance between false floor and deck. Do you think 25 cm are o.k. for somewhat “comfortable sitting” on deck? How wide should the deck be if it is a straight horizontal surface and not specifically shaped like a 505 tank? I started of with 30 cm.
Next question is if the room under the side decks should be closed by a wall at the inside to act as a buoyancy compartment or stay open. While I like the centreboard being close to the water surface when capsized the extra buoyancy should give increased stability when the boat is heeled. Furthermore righting could be difficult it a lot of water rushes from one room under the sidedeck across the cockpit floor to the other side making the boat “wanting” to capsize to the other side right away? What do you think about this?
So how does the design look like now?
The hull basically results of three panels, a flat bottom and the side walls. A Flat bottom would make building way easier, I do not know at all if it is possible to come up with a decent hull shape like this.
Fore deck and side decks as well as front half of the bottom panel are strictly horizontal. The second part of the floor panel rises from 50 cm below deck to 30 cm below deck. The bow is vertical so both start at the same point of length.
The false floor is 25 cm below deck so the height of the stern is 5 cm. This allows to mount a bung on each side to drain the hull.
The idea is to have a inner “skeleton” of which the member that runs all the way on the centreline creates a independent buoyancy compartment on each side.
The “skeleton” further consists of bulk heads under the mast foot, the shroud fittings and wherever else needed, especially so the crew does not break through if they step somewhere on the cockpit floor or deck
Front floor panel and fore deck would both feature a more or less triangular shape to ensure a fine entry. The back part of the boat is a more roundish shape. Maximum width of the floor is 80 cm while maximum width of the deck is 180 cm again which should give a nice flare of the side walls and a maximum waterline width of 130 cm.
Mast is deck stepped at 200 cm from bow, the false floor starts right behind. At the stern the hull narrows down to 60 cm at bottom panel and 120 cm at deck height would give the same flare angle as at maximum width. Perhaps too narrow in regard to righting moment down wind when the crew moves aft, though.
Using similar shroud positions as the RS800 forces a minimum beam of 150 cm at half length.
Bottom and side panels could extend 1 or 2 cm aft of stern to ensure the water flow splits of the hull nicely.
The initial design in the attachments got the widest section pretty far aft (about 70 %). Due to keeping the angle of the side panels constant at 45 °C this moves the rise in the bottom panel pretty much to the stern as well.
Mast should be further forward to allow crew move forward but that would mean a smaller jib.
So please tell me what you think? How weird is it?