2 or 3 speed hull design

I'm curious, would it be possible to build a hull for maximum range at 1.0 or 1.1 v/l ratio (for crossing oceans) AND be able to sprint out of harms way as a planing hull? I've seen the HYSUCATs, and would like to get feedback on a trimaran (HYSUTRI?), or another hull design. A tri could cruise on a small diesel in the center hull and have big block chevrolets with jet drives in the outer hulls. The idea would be to waddle along at single digit speeds for maximum range, then roar over the horizon when the Somali pirates show up.

Would this vessel have to be a hydrofoil or multihull, or could a monohull with a hinged transom of sorts convert a double ended full displacement hull to a squared off planing hull?

 

Displacement-type cats probably the most likely candidate for a boat that can operate efficiently over a wide speed range. You may need to lay down a smoke screen for the pirates, though.

 

Thank you for the reply!
I like the defense idea, and displacement cats. I've sketched (in AutoCAD) a semi-submersible with water cannons. I'm curious about purpose built hulls and even retrofitting existing planing hulls. First hand experience with a planing hull at displacement speeds proved to me that just pulling back the throttles does not save much, if any, fuel. Puget Sound sea states vary from glass smooth to snotty, close spaced white caps. If a vessel could be designed that could accommodate a wide range of speeds, say a quick blast from Bellingham to Stuart Island in good weather AND a 2 or 3 week vacation to Princess Louisa and further... I wonder if there would be a market for a boat such as this? In smaller sizes, fast, economical, and shallow draft...in larger sizes this idea could cruise quick locally, and slow for passagemaking.

Any thoughts as far as the transom extension? Is it enough to change the stern between planing and full displacement? It would have to be almost flat bottomed, like a dory maybe?

 

If such things were easy, or practicable, or affordable, they would have arrived on the scene by now. Transom extensions might make some theoretical sense, but in practical usage, transitioning from one mode to the other, on water, would likely be problematical. Remember those swing-wing military planes of a few decades ago ? A complex idea that seems to have been largely abandoned.

 

Very good point. Simple seems to work better. Hydrofoils sound great, until one hits anything more solid than a duck and somersaults into oblivion. The idea of a passagemaker that had a "daytime" speed and a "nightime / heavy weather speed" with nearly equal nmpg appeals to me, I imagine making a 10 day passage into a 7 day. More and more containers are lost at sea, so a metal hull is probably a good idea. A trimaran would have room for an engine in each hull, max range in the center engine, max speed with all 3 engines, and something in between on 2. It sounds good until factoring prismatic coefficients (and other formulas naval engineers have a total grasp on) for different speeds. Displacement cat's and tri's seem to be able to break the rules mono's are bound by, is there a v/l ratio range that very thin hulls are capable of?

 

One way to do it would be to hull a bit like a Handy Billy that at a bit less than hull speed would have her transom out of the water and present an excellent full disp hull form to the water. But because of her rockered bottom w more power she'd pitch up, squat a bit and have a submerged shape much more like a planing hull and go gracefully at speeds much higher than disp. Gracefully that is excluding the fact that her bow would be ungracefully high in the air, subject to winds that could make a mess of directional stability.

 

As far as I've understood there isn't really a maximum (displacement) speed for long and narrow hulls. At some speed it is however more efficient to use a proper planing hull instead. From boat data I've seen, and my own calculations using Michlet, long and narrow hulls are used at speeds up to around 20 knots for a 12 m long boat. These long and narrow hulls are also very efficient at low speeds. If you go for planing multihulls you will sacrifice efficiency at low speed for a higher top speed, but the low speed efficiency might be adequate for your needs anyway.

Power cats are more common than power tris. I guess that this is because it is easier to make a practical boat with a catamaran hull than with a trimaran hull.

Erik

 

Thank you Easy Rider,
I'll have to do a little research on the Handy Billy, I'm not familiar with the design...the idea of rotating the hull is brilliant! It may call for a straight or reverse sheer...for forward visibility. I prefer a slow cruise so, for me at least, a high bow wouldn't happen very often. I look forward to meeting Handy Billy!

Erik818,
A v/l approaching 3.2 is very good news! I do have a fondness for long, narrow hulls. Gardens TLINGET comes to mind. I'm interested in the cat vs. tri comparison. Would a narrow, deep hull have any advantage over a wider hull with less draw? I like the idea of shallow draft but would not be opposed to something like a SWATH. I'll have to look up "Michlet,"

I'd like to say thank you again, this is an incredible site. The opportunity to ask questions I've had for years to knowledgeable people is indeed a privilege...Thank you so very much for your time and consideration on my meanderings.

 

Mr. Efficiency,
Thank you for your responses. I'm wondering...whatever is an aerostatic lift vessel? Sounds like an interesting, out of the box idea. I can certainly appreciate that!

 

If you google 'air cavity ships' you get the idea. A wise man once said the most efficient way to go through the water is not to go through the water, but the air instead, with as little of the boat in the water as possible. I have had a bit of change of thinking about this recently, and will shortly be testing a new idea via some scale models, if it's got legs I'll post some news. The boat I have experimented with is unfortunately a bit too heavy to get a good response from, with the air cavity size it has. I can now see a better way to increase that, but not in the existing boat.

 

Rubenova,
Michlet is a software for predicting power vs. speed for long and thin hulls. The software is written by Leo Lazauskas, who is world famous at least on this forum. Michlet is based on theories for wavemaking that is applicable for long and thin displacement hulls and consequently is most relevant when wavemaking is the major resistance component.

Long and narrow is always more efficient than short and wide. More draft doesn't hurt efficiency very much. Optimum L/B ratio for minimum resistance is about 20:1, which means a suboptimal hull from most other aspects. The basic route to get a maximum efficient hull is:
1) Make it as long as you can accept.
2) Make it as narrow as you can accept.
3) Give it the draft needed for the displacement you need.
If you want to beat the mythical "hull speed" restriction for a displacement hull, you need a L/B ratio better than 6:1. Aim for 10:1 and hope for better.

The performance of any boat, and especially a multihull, goes out the window if you overload it. An efficient boat is long, thin and light. It's not very difficult to suggest efficient hulls. A greater problem is to make a useful boat from them.

Erik

 

From the thread, I am leaning t'ward a catamaran with hyper narrow, deep hulls. Is there any way a trimaran can carry more (or the same) with less drag? I wouldn't count out a mono-hull Tlinget or Otter types either (people in the Northwest will know what I'm talking about). My search is for the hull(s) design that goes the fastest with the least power, then I can work on a compromise of accommodations, tankage, complexity, initial cost vs long term cost, etc. Are there any other designs out there? Please note I don't want to reinvent the wheel, use "fantasy" fuels, or spend 40 million for a 1 percent boost in range. My limited exposure to naval architecture fuels my thirst for knowledge. thanks all in advance for your kind insight.

 

There is no miracle design that will do everything better than all others. Ultra narrow catamaran hulls might be attractive for their minimal wave-making resistance, but cause hobby-horsing running into a sea, as well as being very sensitive to fore-and-aft weight distribution.

 

Rubenova,
The subject of efficient power boats interests me, especially in the 10 – 20 knots speed range.

For a displacement less than a ton or two, a short planing hull is probably best. I estimate that the boat that fits my requirements has a displacement around 4 tons, so I’m looking at long and narrow hulls used in displacement mode.

If fuel efficiency is the only criteria and length is not a restriction, a long and narrow monohull is the answer. One hull is more efficient than 2 or 3 hulls. If you also want a useful boat, a monohull has to be considerably longer than 12 m to provide an acceptable beam. As I see it, 4 tons is too little displacement for such a boat. The data LWL = 20 m, BWL = 2 m, draft = 0.5 m, displacement 12 tons approximately describes the minimum useful displacement for a monohull (for speeds around 15 knots).

A catamaran is an option. The platform connecting the two hulls needs a clearance of half a meter or more to the water, to avoid waves slapping into the platform. Anything you build on the platform will have quite a high CoG so you will need a minimum total beam for the catamaran of 3 m - 4 m for stability and to avoid excessive wave interference between the hulls. A catamaran also needs two motors which increase cost and complexity.

A conventional trimaran has stabilizing amas that extend outside the sides of the hull proper. This arrangement complicates handling in harbours so I’ve ruled out a conventional trimaran for myself.

Another option is a stabilised monohull. If two stabilizing hulls (amas) are used there is effectively a trimaran under the boat. The clearance to the water for the hull spanning the distance between the main hull and amas still needs to be around half a meter with resulting high CoG. For adequate initial stability, the total beam needs to be more than 3.5 m for a 12 m long boat. Such a configuration is in my opinion an attractive option for a 4 tons boat.

Another variant on the stabilised monohull theme is the displacement glider, also called box keel boat. The box keel provide maybe 80% of the buoyancy, provide standing height in the forward cabin and can accommodate engine, fuel, water and technical installations.

I’ve calculated the power requirement (with 70% propeller efficiency) for a 4 ton boat:
a) catamaran, two hulls 0.8 m * 11.5 m, total beam 3.8 m
b) trimaran, 0.8 m * 11.5 m central hull and 0.3 m * 6 m amas, total beam 3.5 m
c) displacement glider, monohull 2.2 m * 11.5 m with box keel 1 m * 11.5 m

The draft is approximately 0.4 m in all cases and initial stability acceptable. Length and beams are at the waterline.

A cat is more efficient, but my interest currently lies in the "displacement glider" type hull. My hope is that such a boat can be made in the spirit of the beautiful power boats designed by CG Pettersson during the first half of the 20th century. See enclosed pictures.

Erik