Another computer aided drawing of a boat shaped object

[lewisboats]

I would like some informed opinions and CONSTRUCTIVE criticism on the following set of drawings. No it is not a "design" in the sense that I am going to go out and build it tomorrow but it IS an excercise. I have not had much experience with multi-hulls (except my noodled canu) so I would like some feedback on the various performance aspects between the two hull shapes, sponson/ama positions (too deep/not deep enough, too far back/forward etc), cockpit location, choice of rig etc. I would also like an explanation of WHY you think what you think about it and WHY your proposed solution should be used and to what benefit. Frankly I don't care if someone thinks it is ugly...that has nothing to do with the performance so is irrelevant. I would like to thank one and all who reply for thier input.




I was looking at the Triak design ( http://www.triaksports.com/paddling/ ) and thinking that that design going to the EC would be rather uncomfortable to use over an extended period and perhaps I might give a try at one that I might be more willing to use. I oriented it more towards the sailing aspect and a bit less the paddling side. There are two iterations for consideration here...a shallow Vee version and a Flat bottomed version. For all practical purposes the two hulls are near identical but I actually think the Vee'd version would be as easy to build and with the filleted vee running the lenght of the middle...actually stiffer. First the Vee version which I originally drew. The hull would be a composit of build methods...the bottom would be of 4mm ply, the chine strake would be of 2-3mm and the sides and most of the deck would be SOF to reduce weight. The sponsons would be of 2 or 3 mm ply bottoms and SOF for the rest. Framing would be of cut out plywood joined by Cedar Stringers...typical SOF style. The boat would be built top down so the bottom would be stitched and filleted to the lowest stringer after the skin was attached. This should provide a light stiff structure that has a bit more resistance to sub surface nasties like coral.

The cockpit is significantly larger and deeper than the Triak and in wet weather could use a skirt to keep water out. The boat is wider and would be correspondingly a bit more difficult to paddle but...with a removable seat and a good bed roll to pad the bottom there would be sufficient room to sleep comfortably nestled in the hull under a small boom cover or simply a tarp. In order to keep the boat innards open, a single high aspect pivoting leeboard would be used for lateral resistance (heel would not be much of a factor so a single board should suffice). Steering could be foot operated, push/pull operated or hand operated via a handle bar affair mounted at the forward edge of the cockpit and cabled to the rudder quadrant. For sail power and ability to handle spars easily I propose either a form of Gunter rig or perhaps a high peaked Sprit or Lug sail and a fractional Jib. The mast should not exceed 12 ft in length and would be stepped on the deck, mounted on a ball and socket with a forstay and shrouds. The forstay should be led back to a small winch near the cockpit so the rig can be lowered without having to try and get forward (not really practical). The sponsons would either unbolt or fold up...depending on how stiff the support arms needed to be to stave off twisting in a seaway although the actual CB of each is near the middle of the arm/sponson joint area. The drawing is crude and the actual connection would span from the edge of the sponson to the centerline. Also...there is no top to the sponsons in the drawing...obviously there would be in reality.







Design length : 16.878 [ft]
Length over all : 17.375 [ft]
Design beam : 3.020 [ft]
Beam over all : 8.750 [ft]
Design draft : 0.375 [ft]
Midship location : 8.439 [ft]
Water density : 63.989 [lbs/ft3]
Appendage coefficient : 1.0000
Volume properties:
Displaced volume : 7.917 [ft3]
Displacement : 0.226 [tons] 505 lbs in salt
Total length of submerged body : 16.125 [ft]
Total beam of submerged body : 8.713 [ft]
Block coefficient : 0.1503
Prismatic coefficient : 0.7419
Vert. prismatic coefficient : 0.5968
Wetted surface area : 42.697 [ft2]
Longitudinal center of buoyancy : 6.380 [ft]
Longitudinal center of buoyancy : -12.767 [%]
Vertical center of buoyancy : 0.249 [ft]
Midship properties:
Midship section area : 0.662 [ft2]
Midship coefficient : 0.2026
Waterplane properties:
Length on waterline : 16.125 [ft]
Beam on waterline : 8.713 [ft]
Waterplane area : 35.376 [ft2]
Waterplane coefficient : 0.2518
Waterplane center of floatation : 6.451 [ft]
Entrance angle : 11.861 [degr.]
Transverse moment of inertia : 56.680 [ft4]
Longitudinal moment of inertia : -1429.1 [ft4]
Initial stability:
Transverse metacentric height : 7.408 [ft]
Longitudinal metacentric height : -180.26 [ft]
Lateral plane:
Lateral area : 4.118 [ft2]
Longitudinal center of effort : 7.745 [ft]
Vertical center of effort : 0.224 [ft]





And here is the Flat bottomed version...another 95 lbs displacement for the same draft but I really don't know if it is needed. Unless you are carrying a lot of dense stuff, there isn't really a whole lot of volume for the extra weight.




Design length : 16.878 [ft]
Length over all : 17.375 [ft]
Design beam : 3.020 [ft]
Beam over all : 8.750 [ft]
Design draft : 0.375 [ft]
Midship location : 8.439 [ft]
Water density : 63.989 [lbs/ft3]
Appendage coefficient : 1.0000
Volume properties:
Displaced volume : 9.345 [ft3]
Displacement : 0.267 [tons] 600 lbs in salt
Total length of submerged body : 16.127 [ft]
Total beam of submerged body : 8.713 [ft]
Block coefficient : 0.1773
Prismatic coefficient : 0.7218
Vert. prismatic coefficient : 0.7001
Wetted surface area : 44.177 [ft2]
Longitudinal center of buoyancy : 6.492 [ft]
Longitudinal center of buoyancy : -12.072 [%]
Vertical center of buoyancy : 0.226 [ft]
Midship properties:
Midship section area : 0.803 [ft2]
Midship coefficient : 0.2457
Waterplane properties:
Length on waterline : 16.127 [ft]
Beam on waterline : 8.713 [ft]
Waterplane area : 35.594 [ft2]
Waterplane coefficient : 0.2533
Waterplane center of floatation : 6.419 [ft]
Entrance angle : 11.864 [degr.]
Transverse moment of inertia : 56.721 [ft4]
Longitudinal moment of inertia : -1423.5 [ft4]
Initial stability:
Transverse metacentric height : 6.296 [ft]
Longitudinal metacentric height : -152.10 [ft]
Lateral plane:
Lateral area : 4.095 [ft2]
Longitudinal center of effort : 7.782 [ft]
Vertical center of effort : 0.224 [ft]

[PAR]

The chine "band" seems to offer very little to the design, particularly the main hull. The outrigger arms, though stylish, seem dainty. What's the logic in carrying the displacement so far aft?

[The Loftsman]

Hi,
Can see you have put a lot of work into this and well done, but it may be worth your while taking a look at what the Island peoples of most of the small islands in the world use when it comes to doing very similar boats, they after all have came to the best optimum design through trial and error (sometimes fatal) and they go with the tried and tested method.

[NoEyeDeer]

If you're serious about sailing this I'd make the amas longer and use a fore and an aft crossbeam. This will give much better stability and less drag when pressed.

I'd raise the amas so they are just kissing the water surface when lightly loaded. They don't need to be immersed unless you are leaning on them. This would give a fair reduction in wetted surface when just paddling in good conditions.

I'd not split the crossbeams into port and starboard halves. A continuous beam right across is going to be stronger.

Also, if you're relying on a trimaran arrangement for stability, you don't need much in the way of beam for the main hull. You could get a reduction in wetted surface by using a narrower and deeper main hull. This would also allow a better paddling position since you wouldn't have as far to reach.

In short, if you want to build a sailing tri I'd be looking at how they are put together and why.

[ancient kayaker]

Definitely raise the amas - then I think you may have to move the cockpit forward to raise the transom clear.

If you haven't already done so, check that the paddle clears both the amas and the crossbeam - looks kinda close to me; with a sweep stoke the paddle is further out than a normal drive stroke.

Sail handling from way down low in the cockpit can be difficult, bring all lines within easy reach, provide stowage and maybe consider a seat that can be raised.

Oh, you want reasons - all the above based on experience!

[Anytec1210]

Cool design idea but I think paddeling it would be quite hard. From the middle of the cockpit there is only about 3 ft to the rear beams and that´s not enough if you where thinking about paddeling it like a kayak.

[Squidly-Diddly]

and that was just from casual observation of kayaker's normal 'natural' strokes on flat water.

Even at 4' I'd think it would risk an irritating occasional clank.

[michael pierzga]

Why have two beams and two floats. Alot to pull thru the water and keep on a straight course. Why not one beam and one float always above water when stroking , symmetric geometry...always paddle one side.. reverse direction bow for stern, a Proayak... Could be interesting ...a Biyak instead of a triyak

[ancient kayaker]

Originally Posted by michael pierzga

Why have two beams and two floats. Alot to pull thru the water and keep on a straight course. Why not one beam and one float always above water when stroking , symmetric geometry...always paddle one side.. reverse direction bow for stern, a Proayak... Could be interesting ...a Biyak instead of a triyak

Been there, done that - problem is -in an average kayak - you can't get off-center far enough in a kayak cockpit to balance wind force with the float to windward.