Long thin hull for motor boat with bulb keel

[mc_rash]

In my mind a boat with the dimensions (L 20', B 1', D 1') could be designed if especially the stability is proper calculated and a keel will have to be fitted (depth and size of the keel according to the stability calculation).

 

[mc_rash]

If it will be a good design I don't know, but I think it can be done.

 

[TANSL]

Of course it can be done, another thing is that it works well.
To give it stability, the keel will have to be a ballast keel and do you know how heavy it is to carry fixed ballast, by pedal, across the oceans? Well that...

 

[Robert Biegler]

Am interested in a design of a long thin motorboat hull (20:1 ratio) using a deep bulb type keel perhaps held by another high ratio aerofoil. I wanted a super stable design but ultra low drag. Can a seasoned mariner explain the downsides please?

I think your problem is similar to the optimisation of a kayak or surf-ski. Greater length reduces wave making drag, but increases skin friction drag. Start with power and displacement, then optimise length to minimise the sum of wave-making and skin friction drag. If you want no form stability from the hull, that might make the optimisation of the bulb keel an independent problem. You know what torque you need to balance your antenna array, so play with lever arm and weight. Make sure to sweep back that keel, so it has a better chance of shedding weed, ropes, and nets.
You can find a paper on optimising a surf-ski here: https://www.surfski.info/images/sto...ization of the length of a surf ski kayak.pdf

 

[Skinnypaul]

Let's see. At a 20:1 ratio, a 40 foot boat would only be 2 ft wide.

A bulb keel of maybe 2 ft would be more than sufficient. Going deeper only adds more drag.

The only reason for a much deeper keel would be to counter balance something heavy and high up, such as a big antenna or maybe even a sailing rig.

I think your problem is similar to the optimisation of a kayak or surf-ski. Greater length reduces wave making drag, but increases skin friction drag. Start with power and displacement, then optimise length to minimise the sum of wave-making and skin friction drag. If you want no form stability from the hull, that might make the optimisation of the bulb keel an independent problem. You know what torque you need to balance your antenna array, so play with lever arm and weight. Make sure to sweep back that keel, so it has a better chance of shedding weed, ropes, and nets.
You can find a paper on optimising a surf-ski here: https://www.surfski.info/images/stories/2012/05/Length/Optimization of the length of a surf ski kayak.pdf

Thanks very much Robert, great advice!

 

[bajansailor]

A gentle reminder @Skinnypaul re my questions in my earlier post #14 - answers to these might help to clarify things a bit.

How tall does this antenna array have to be?
I see that you are in London - where will the boat be located? On a river, or the open (North?) sea, or ??

Re how you are worried about it capsizing if it is a catamaran, it sounds like it will have to be able to survive some pretty rough seas?

 

[baeckmo]

Google "plank on edge boat design".

 

[philSweet]

Even the most extreme plank on edge cutters only had L/B a bit over six. They were called "six beam cutters". Rona 1892 is probably the best known, and still survives. But these hull forms carried huge ballast in very thick keels, and they didn't have strut keels back then, so a very different hull form due to material strengths and the fact that they were sailboats.

 

[jehardiman]

Even the most extreme plank on edge cutters only had L/B a bit over six. They were called "six beam cutters". Rona 1892 is probably the best known, and still survives. But these hull forms carried huge ballast in very thick keels, and they didn't have strut keels back then, so a very different hull form due to material strengths and the fact that they were sailboats.

Yeah, the only modern power vessels that even get close to 10:1 are Ore Boats and the old Liners; even English narrowboats are rarely 10:1. But the 22m Dragon boats and ~40m Maori war canoes (waka taua) get near 20:1.

 

[mc_rash]

 

[bajansailor]

But the 22m Dragon boats and ~40m Maori war canoes (waka taua) get near 20:1.

I will raise you a rowing eight, which typically have an L/B ratio around 30.
Empacher Racing-Eight- Empacher Bootswerft https://www.empacher.com/en/products/racing-boats/empacher-racing-eight

They are terribly unstable though when not underway - when at rest with the crew on board they rely on the blades of the oars to act like stabilising wheels.

(Apologies, this is drifting far away from @Skinnypaul original question. Although I am still hoping that he will offer a reply to my questions above).

 

[dustman]

A monohull that long and skinny would have extremely poor initial stability, even with a deep ballasted keel. The keel wouldn't add any righting force until some amount of heel is achieved. I believe this vessel would be pretty rolley within a few degrees. A catamaran would likely be as (or more) efficient but much more stable without the need for keels or ballast.

 

[Ilan Voyager]

Am interested in a design of a long thin motorboat hull (20:1 ratio) using a deep bulb type keel perhaps held by another high ratio aerofoil. I wanted a super stable design but ultra low drag. Can a seasoned mariner explain the downsides please?

A 20/1 monohull with bulb keel is a pendulum. Good luck on such an unstoppable roller, oscillating at the least solicitation, specially waves, look for orbital movement of water in waves, you'll understand the problem of a deep keel on a very narrow hull, The wind on the superstructure will be another misery. Besides there are serious problems of habitability unless a platform on the hull. Thinking well you'll have a problem to install an inboard and even an outboard.
If the boat is small (10 meters for example) you'll have structural problems.
Finally a good serie of calculations of drag and hump speed before planing, shows that after 12/1 there is little gain, and after 15/1 there is no valuable gain.
Lot of NA have done these calculations that for sail and motor boats with the same results, so the rare motor trimarans have main hull ratios around 12/1.
The slim motor monohulls are around 6 to 8, some 10/1 specially warships and that's the max as the metacentre becomes delicate to place if you have superstructures.

The good solution has been found a long time ago on 1986 by Nigel Irens : trimaran. The first one is Ilan Voyager by Nigel Irens. Main hull 17/1 at LWL. 28 knots with a Yanmar 4 cylinders 240 hp. Navigation round Britain : 2200 miles at 21.5 knots mean speed without refueling. The boat is based at Cabo Verde Islands working as water taxi and has crossed to United Kingdom without problem.
The boat has some small drawbacks: the stern lacks of volume, the short amas are immediately at planing speeds so probably planing hulls shallow V would be more indicated.
The boat is very smooth, seaworthy, and very efficient.
ILAN - Nigel Irens Design https://nigelirens.com/boats/power-boats/ilan/

Indecently simple. Low tech ; wood, epoxy, fiberglass.

 

[rnlock]

Depending on the speeds involved, hydrofoils might be the low drag solution, and could also steady any tendency to roll. At least until they collected some seaweed.
I suspect 20:1 is a bit more than optimal, though single sculls are awfully skinny, so maybe not. I also suspect that what's optimal depends on speed, size, and sea state. If this is some kind of robot boat, a bit of rolling may not be all that important, and, when underway, the fin will probably damp out the rolling. Especially if it has a little trim tab on it. I'm not sure what the sweet spot for fin depth vs. drag is. Does a shallower fin mean less drag, even if you have to use more ballast? How would rough seas affect the compromises?

 

[rnlock]

BTW, I think "wakeless" launches are above 20:1
Still Water Launches http://www.stillwaterdesign.com/pages/launches.html

There is no such thing as "The good solution..." until the problem is narrowed way down. Under some circumstances, I'm sure those trimarans are very efficient. Others, not so much.