Long thin hull for motor boat with bulb keel

[dustman]

When I saw this for the first time it all became clear to me:

 

[Ilan Voyager]

Scull hulls are not a good example. The thinnest are the 8 rowers, on that you fart or cough carefully. Good on only mirror like water, as waves over 4 inches will be a drama. I have rowed in a 8 for a few years.
When you want a solo sea scull boat the optimum found by many designers is about a 18 to 20 feet, between 2.5 to 3.5 feet wide hull. The LiteRace 1X is 20 feet long, 2.5 feet wide and weights around 35 kg (77 pounds). With this scull you can row safely at sea but with an excellent speed. As you can see on the the boat is deceptively simple and could be DIY made with 3 mm plywood and glass. Note that the stern is open so it can't be filled by water.
20 years ago I designed and made a similar one for me.

So we'll make a SOR for a usable slim motor boat, going at the highest possible hull speed with the minimal power. Thus optimizing efficiency of the power and fuel consumption.
You can go very long and rather thin. After all a cargo ship of 300 meters, 320000 metric tons needs only 110000 hp to go to 20 knots, it's only 1/3 of hp by metric ton. But it's a bit expensive and cumbersome for a leisure boat, and definitely not trailerable.
The other requirements are :
A minimum of habitability. Ie at least good seats and no cramped legs. With place for the drinks and sandwiches.
A minimum of seaworthiness, as even on lakes you can have suddenly difficult conditions (ask the people navigating on the Great Lakes, or in Switzerland...). So a correct stability is primordial.
A minimum of place for the engine, fuel tanks, and security items for a day boat.
A minimum of strength without going to exotic expensive materials.
A minimum of engineering or brain's juice. That eliminates ballasts, bulbs, keels as sources of drag.

I would add rather cheap and trailerable thus having a maximal length of 12 meters and a maximal width of 8 feet on the trailer. It's a leisure boat you want to take anywhere.

The first thing to do is to study the hydrodynamics of a slim boat going at a usable maximum hull speed. As many others you'll find that above a ratio of 15/1 the gain is marginal and that 12/1 is a good ratio for a small motor slim boat going at speeds not too close to the hump "wall" where you need more power to go over the hull wave and start planing. let's keep the 15/1.
For a 12 m long that gives a LWL width of 0.8 meters or 80 centimeters wide enough to get a correct transversal and longitudinal rigidity of the hull with the least weight, and to get enough volume underwater for the expected displacement. Besides there is enough place for the engine inboard or outboard, tanks to get a good range and a minimum of amenities.
The second thing as you have not intention to die by drowning you want something seaworthy. Good movements longitudinal and lateral, not a submarine nor a roller capsizing suddenly.
Hydrofoils are interesting but need some speed, if by circumstances the speed is insufficient they are useless. A bulb keel has no damping effect on the roll, it's just efficient for the maximal righting moment at a very high and not very comfortable angle of heel. It's heavy, adds useless weight and drags a lot and basically on a slim hull you get a pendulum. Furthermore a deep keel add problems in the waves plus structural problems to solve.
After examination of possible safe and simple solutions without stabilizing computers you'll arrive at the Polynesian solution: multihull.
The catamaran is a good one for an habitable boat but implies 2 hulls, with the weight and drag going with.
Remain a trimaran, or better said "assisted monohull" in our case, with 2 small amas just flush over the water to get the minimal weight and drag.
As its a motor boat 60% of the displacement amas are sufficient. Sailboats are a very different problem and need big amas.
The rolling and heeling is damped statically and dynamically at a small angle around 5 degrees, they can be the support of hydrofoils, the profiled beam is a supplementary safety volume and a place for lots of material.
There are other design details with the amas (planing or no, pivoting or no with hydraulic shock absorbers), and the shape of the main hull for dampening the longitudinal movements (have a look at the pics of Ilan Voyager and you'll understand the solutions used by Nigel Irens), but this post is long enough.
There are not miracles in NA, and none boat is universal. The good boat is the one that meets the intended goal.

 

[bajansailor]

I think that the original poster SkinnyPaul has given up on this thread - the last time he checked in here was a week ago.
He originally mentioned that he wanted to mount an antenna array on this long skinny hull, but he did not elaborate further.
However to our benefit we have had some wonderful thread drift, like Ilan's and Dustman's posts above.

 

[Ilan Voyager]

Bajansailor I knew that SkinnyPaul has left the thread , but I felt, reading some posts, that a little reminder of an example of the intellectual process of the design of a boat was needed...

 

[rnlock]

Most of the single sculls I've been on were narrower than me, not counting the riggers. And no, I am not 3 feet wide even now, and I was skinnier when rowing single sculls. Not sure how they'd be in the ocean, of course. I've been thinking this is likely to be a robot boat, so there will be no crew to fall off if a wave washes over the boat. That probably allows for a much lower deck, and possibly less concern about initial stability. It's possible that the antenna array will tilt LESS when a sea is running if there's less initial stability.

 

[dustman]

I just had the thought that if the hull had a round bottom and the antenna was very tall and heavy the boat may always lean to one side or another once disturbed. Would be pretty easy to build a simplified scale model to get some idea of how a vessel like this would behave.

 

[Ilan Voyager]

Robot boats are perfectly known as a lot have been made for scientific research and military purposes. Buoys drift with the current, robot boats are directed or are kept circling to stay in the same zone.
There is no need to make then skinny. Furthermore you have a big bunch of electronics intruments, motor and batteries plus eventually solar panels to install and that asks for volume and surface. Simply the robot boat has to be designed to be self righting and you can get a self righting boat without the need of a keel. It's simply a matter of low center of gravity when upright and volumes above the deck so the boat has a very low stability when inverted and will get upright at the minimal external solicitation, thing over abundant in sea. Rescue boats are the perfect illustration of self righting boats and none has keels. The Coast Guards rescue boats have even pretty high towers.
You may use a keel and ballast but that will add weight (so less batteries) and parasitic drag.
Rowing skiffs are made for rivers or lakes on mirror flat waters. The main problem for the skiffs is the longitudinal movement of the weight of the rower when the seat is sliding. The long oars help a lot for the lateral stability.
On sea it's a very different matter as you have waves and a need of minimal stability specially if you have to climb inside the boat after some havoc. Most one person sea sport rowing boats (with sliding seats) for nice weather are around 65 to 75 cm, a few 90 cm, with a length around 4.5 to 5 meters. These are empiric proportions known since more than one century, and confirmed by calculation. Open stern are mandatory so the boat empties the water immediately, it's a matter of security.