Flat bottom/vertical sides/flat deck/ flat everything

[ancient kayaker]

I have to say, whatever its sailing characteristics might be, I strongly support Ron's attempt to come up with a simple, easy to build hull.

Rick, interesting point about chines. I have a home-built 12' canoe and a paddling buddy of similar age and ability so we can compare it with my 13' plastic kayak which is supposed to be fast according to its manufacturers. Up to the canoe's hull speed it is by far the easiest to paddle, despite being shorter. Doesn't matter which of us is in which boat. Only if we put our backs into it does the longer, smooth-hulled plastic boat pull ahead, and not by much. At any level of effort we are likely to want to sustain for more than a few minutes, the chined canoe runs away from the plastic smoothy.

In this case, I suspect the reason is the shape of the stems. The plywood canoe has very sharp stems, the plastic kayak stems are rounded, presumably because it is rotomolded.

 

[Guest625101138]

Actually canting keels open up a whole new arena for monohulls that has not been explored as far as I can see.

If you just want to make fast passages in an outright racer I think the best monohull is long and really slender using a deep canting keel.

I have sketched the attached example that would be capable of 20kts with 20kts apparent at 45 degrees. It has a total displacement of 4 tonne, length of 20m and WL beam of 0.84m.

Downside is poor accommodation.

There would need to be some good structural design for supporting and operating the canting keel.

The wing sail is simple to give an idea of the area required. It could be fractional rig with conventional sails. In this case mast support would need some additional structure.

We have seen extreme cats and tris but I think monohulls have a long way to go in getting the fastest machine.

Rick W

 

[Timothy]

I really never got the canting keel thing . Seems to me it is not a keel at all but just another way of moving ballast in the most mechanically disadvantaged way possible. As I understand it you need another foil for lateral resistance so there is no surface area reduction..If the arm that supports the arm along with it's attendant bulb is above the water(pacific proa) the same righting moment is achieved with less drag and if you switch the bulb for a float(atlantic proa. catamaran) then you save weight as well and if you add a float to the other side (trimaran ) then you can tack the boat conventionally without moving the ballast. Perhaps more work should be done on a process or configuration that would allow ballast or better yet buoyancy to be moved port to starboard . Sorry to be of topic .

 

[ancient kayaker]

The canting keel requires much force and must be moved a significant distance to generate heeling moment and so power must be available. I doubt deck apes would be sufficiently quick to respond to a dynamic situation.

Twisting the lower extremity of the keel should do the same job once underway, and would need much less power. The keel doesn't need to be literally twisted of course, slots and flaps would do the same job with even less force, now perhaps within the ability of an individual. It seems so obvious that surely this has been tried?

 

[peterraymond]

Timothy said: "...If the arm that supports the arm along with it's attendant bulb is above the water(pacific proa) the same righting moment is achieved with less drag and if you switch the bulb for a float(atlantic proa. catamaran) then you save weight as well.."

Actually, underwater drag is much less than hull drag. No hull shape can approach the efficiency of a streamlined underwater body. If you look at the small size of the bulb it may even make intuitive sense. I will admit that getting the righting moment from a canting keel adds the drag from the keel as well as the bulb, while the connection to a second hull would be out of the water. But, I think the added drag of the canted keel and bulb together would at most equal a full second hull. Compared to a catamaran, a bulb the same weight as the hull would give equal all-up weight. The disadvantage is that to get the same righting moment you need a draft at least equal to the width of a catamaran. In light air you don't need full righting moment and I think you are ahead of two hulls, forgetting for the moment that ultimate racing speed is not the goal.

The disadvantage of completely out of the water movable ballast is that you have to actively balance the boat, or widen the beam to provide form stability. Even with underwater ballast the angle of heel of this boat would change a lot as wind strength went up and down.

If you are looking at knots per dollar, it helps to only build one hull, not two or three. Much easier to dock too. I think a canting keel could be cheaper that a second hull. If you go to any multi-hull design you are back to the capsize problem also.

My main problem with an aft mounted mast is structural. The hull can be lighter and structurally more efficient if the mast, the source of overturning moment, is right next to the keel, the source of righting moment. On the other hand, the safety aspects of being able to sail under all conditions without ever leaving the cockpit is very appealing.

I think you should set a limit on how many unconventional things you are going to include in your boat. Each adds risk. If you want to check off every box you probably have to assume that you will be building more than one boat.

 

[Guest625101138]

The canting keel requires much force and must be moved a significant distance to generate heeling moment and so power must be available. I doubt deck apes would be sufficiently quick to respond to a dynamic situation.

Twisting the lower extremity of the keel should do the same job once underway, and would need much less power. The keel doesn't need to be literally twisted of course, slots and flaps would do the same job with even less force, now perhaps within the ability of an individual. It seems so obvious that surely this has been tried?

Terry
Once you get down to such slender hulls they do not require big forces.

The hull I have sketched will do 20kts with 2.8kN drag. The overturning moment to achieve this, in 45 degree apparent wind, with the wing shown is around 28kNm. It could be less with a convention fractional rig. You can safely handle this torque with a 5" steel shaft on the keel. Fitting the hydraulics into the narrow hull might be another matter.

We have seen extreme multihulls but I believe we are a long way from seeing the extreme with monohulls. The leap is simply too far from existing technology but the canting keel opens up a whole new arena. Existing hulls are still driven by the old concept of big volume.

You can get outstanding performance with a boat that is still self-righting. You do not need to be constantly trimming because it will still reduce power and stiffen up with some heel the same as any monohull. The canting keel just gives the ability to keep the sail upright without any form stability in the hull.

I looked at introducing some form stability but the hull drag goes up dramatically so I think there is nothing better than having the metacentric height at the water level in conjunction with a deep keel. For practical reasons you might need a bit more beam to fit keel mechanisms into. Alternatively they could be faired items externally or something like a ring bearing incorporated into the hull.

Rick W

 

[ancient kayaker]

Rick: your numbers seem high: I converted to Imperial and 2.8 kn = 630 lb-force (conversion factor x224.8 according to my tables).

The second para in my post was not about form factor or self-righting, it was about getting righting moment from the forward motion of the keel through the water using differential lift at top and bottom of the keel to create a righting moment. However, a quick calculation suggests the keel drag would increase about x6 for typical sail and keel geometry which would explain why I haven't heard of it being done.

 

[Guest625101138]

Terry
The boat displaces 4t so weight is 40kN. The drag to weight is only 0.07. That is very low for a boat doing 20kts. Doubt that you could build a hydrofoil that would do better.

My analysis was not comprehensive it was intended to make the point about things that are easy to build but will go fast.

This shows the latest wisdom for fast jet boat:
http://www.boatdesign.net/forums/at...-propulsion-ddx-seajet-050823-n-7676w-103.jpg
This is one of the places using Leo Lazauskas's Michlet/Godzilla software.

Rick W

 

[Earl Boebert]

I hope I'm not out of line by bring up model yachts, but in the context of simple, fast boats Harold Kethman's 1954 Delta design might be of interest. Class rules were 600 sq in sail area, no other restrictions. Like all free-sailing racers, it was optimized for windward work and had an effective draft limit imposed by the need to be able to sail anywhere on the pond.

This thing was a killer, regularly beating Marblehead class boats (also 50" long, but with 800 sq in of sail) by wide margins. A look at the heeled waterlines gives you an idea why.

Anyhow, FWIW.

Cheers,

Earl

 

[dskira]

I remember a 70' version of the Star, built by an affecionados of Star. A desaster. Dosn't mean your flat every where project will not work.
It seams, and I can be wrong, that flattie after a certain size, get in trouble.
But worth to try, and hope your project will work well.
Anything worth to experiment inset of following the corporate plain vanilla trend in boat design.
Good for you.
Daniel

 

[Crag Cay]

Reputed to be very fast and fun.

 

[Manie B]

There is so much work in building a sail boat that the extra bit of work to build a multi chine hull is really not a problem. First time builders often get tricked into thinking that slapping a **** hull together will save time and money - no way, please believe, been there and got the "T" shirt

 

[Guest625101138]

Reputed to be very fast and fun.

Looks like an absolute flyer. Maybe need one leg longer than the other on the wind but it would go like a rocket on all points.

I have found that a single hard chine gives very little away to a rounded chine on my little boats. The reduced beam with the single hard chine makes up for the slightly higher wetted surface. In fact the flat bottom planes more readily so top speed is better. A case of simpler and probably better.
http://www.boatdesign.net/forums/at...1d1245477506-pedal-powered-boats-p6190093.jpg

Rick W

 

[dskira]

There is so much work in building a sail boat that the extra bit of work to build a multi chine hull is really not a problem. First time builders often get tricked into thinking that slapping a **** hull together will save time and money - no way, please believe, been there and got the "T" shirt

I think it is very well said.
I also been there, and it's true that the building time is almost the same.
Even a single chine with a very open V will make a great amount of difference.
But as I said, better to try something than to fell asleep contemplating a of the mold floating thing.
About design, it is better to have one persone wrong, than a committee right.
Daniel

 

[ancient kayaker]

I am currently building a small flattie, but looking forward as usual to the next build which will likely be (as usual) a bit bigger. I am considering a rounded hull for that one, which might be prettier and more comfortable, but there are factors like reduced stability and reluctance to plane to consider in addition to the more complex build. Perhaps the flat bottom morphing forwards to a vee is the solution; best of both worlds. Or maybe a flat bottom with bilge planks like my canoes.