38' power Trimaran: Kurt Hughes Design?

What you are doing, is making a lot of generalizations about boats that can be designed in many different configurations. Just because some multihulls have fine bows doesnt mean they all do, to highlight but one example of this. Same goes for high super structure, and just about everything else you've written above. Most of it is completely irrelevant to the number of hulls a boat has.

The design of the tri in question here, is a full displacement hull without question. It will easily exceed hull speed with little horsepower because;
Its very light for its length, ie has a favourable displacement length ratio,
And,
Has very slender hulls, ie has a high length to beam ratio.

None of this is magic, its very well documented subject matter. I'd suggest doing some research to better help your understanding.

A hard ride is caused by proportionally large surface areas which are closer to horizontal rather than vertical and the amount of lift they can generate, which is also a function of vessel speed and the resulting accelerations which are felt as 'hard'. A slender round bilge hull such as this is rarely described as hard riding as they generally cannot generate any hydrodynamic lift, but rather bouyant restoring forces only which produce more gradual or 'soft' accelerations. Only small portions of the total area is close to horizontal. This is completely opposed to a low dead rise planing hull which will blow your knees out in anything but calm seas.

 

All cats have a high superstructure, because the wheelhouse is on top of the high bridge deck, not down inside the hull. I can't think of a low windage Cat design, unless you've got a Polynesian outrigger.

Equally, all high performance Cats have slim hulls. It's a self fufilling reality of low drag.

Regarding round bilge hulls, if they are so soft riding, can you give me an example of a planing hull that has one?

 

This is turning into a comedy...

Consider 2 vessels of equal length, 1 a cat and one a mono... let's say the cat is double the beam. They both need to carry 100pax. The cat can just fit all seats on 1 level. The mono therefore needs 2 deck levels to seat the same no of people. Unless you put them in the bilge with no windows, the cat will have the lower profile design.

Have a think about the last question above and reread my last post, the answer is already contained within.

 

The mono can put all the passangers down inside the hull, the Cat would have to put all the passangers higher up at deck level. Ergo, the Cat has higher windage......

Regarding round bilges, it's because the surface area is distributed over a shallow curve that would make them a very bad choice for a fast boat. Deep vee hulls with large deadrise absorb the shock of hitting waves very well.
Hence I was asking if the hull profile on the Tri was a suitalbe choice for a vesel that cruises at 12 kts, which is very fast for a displ. hull of this lenght!!!

 

Hi, long time lurker and reader of this forum.

I happen to have been looking at this power trimaran lately and am very interested in the design. I would like something similar but out of plywood with all developable surfaces.

I have a question regarding the Ama's. Why does the KH design have the ama just above the waterline? Does this lead to problems with ride and bouncing as the ama goes in and out of the water? All the other power trimarans I have seen such as Adastra and Earthrace all have the ama slightly submerged and supporting some weight.

 

To address the problem of docking and boarding a tri please look at the Dragon Fly boats from Denmark and with swing wing floats they solve the problem.

 

Milo less submerged surface and drag. When the boat leans only one float is needed to stabilize the other comes up and causes less total drag. In a sailing tri you often sail as a cat with two hulls in the water one out on big boat that can be 6-9 ft up. many times only one hull in water then you have to be careful.

 

Hi Milo,

I was looking at a super mono-displacement hull a few weeks ago on another forum. http://passagemakerlite.com/ The conclusion was that the design was really too flat bottomed to be seaworthy.Greenline boats manufacture as similar boat, and in a recent magazine review they came to the conclusion that it was no more econmomical than a standard semi-d boat at 7-8 kts, and less economical at 15kts!!!http://greenlinehybrid.com/
Their conclusion was that it would mske a very good boat for sheltered bays, rivers and inland waterways.

....Hence my interest in the Tri concept.
The Question: is this a seaworthy hull design?

 

I agree with you, I don't think docking is going to be a problem.
What's wrong with trampolines?

 

That's too big a generalization!! all cargo ships are flat bottomed!!

The problem with docking is not when tied up but actually coming alongside and tying up in the first place. Try to imagine coming into dock. You approach at 45deg. The first point of contact is the outrigger bow, which is someway aft of the main hull bow and is low and very narrow. But this is where someone has to stand with the warps to jump ashore without you hitting the mainhull into the dock first.

I know my wife would find it "challenging" in fact I know she would refuse to do it. But it is easy on a wide catamaran foredeck. Or maybe its just that I am not very good at manouvering.

This is a big thread drift, the OP asked about a specific boat which did not have swingwing outriggers

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

yes that's true, but cargo ships don't cruise at super displ. speeds!

Here's a super displ. Cat design, round bilge with fine entry bow sections: design speed up to 20kts..,http://www.powercatsnz.com/public/index.cfm?fuseaction=app.showdesign&ID=162

A good comparison to the tri ?

 

RustyB, your generalisations are completely out of control... its like your drunk or something when weighing up the factors that pertain to suitability to task or simply knowing when your comparing apples to apples.

Here take this for example; both are 100 pax ferries;




Now tell me which has a lower profile design?

Its a completely irrelevant consideration, as both vessels have completely different set of requirements and any number of hulls can be designed with any number of decks depending on the arrangement the owner wants to best suit their need! You simply cannot make these kinds of generalizations where one set of design parameters always holds true for a catamaran or trimaran or monohull etc. Boat design is always about a set of compromises which achieves set of requirements, nothing more and nothing less.

You still fail to understand this, as you ask "a good comparison to the tri?"

Why would you compare the 2 in the first place?
Were they both designed for the same purpose to achieve the same set of requirements? Clearly not, so why would you even WANT to compare them?

Your logic makes no sense at all...

 

You are comparing a twin deck monohull to a single deck Cat.....

We can make generalisations about particular design concepts that will hold true for every vessel in that specific class, be it Tri's, Cats or Mono's taking into account hull speed/ length/ beam etc etc...

The super displ. Cat from NZ would make a good comparison against the super displ. Tri.....surely ?



The Tri:
Length overall: 38'-0" (11.6 m)
Length at waterline: 37'-6" (11.5 m)
Beam: 16'-11" (5.16 m)
Draft: 1'-6" (0.46m)
Weight: 4,125 lb
Displacement: 5,278 lb (2.394 kg)

The Cat:
Specifications

Builder Various
Displacement 4,840 kg
Draft 0.53 m
Nominal Length 10.20 m
Length Overall 10.20 m
Length Waterline 9.81 m
Beam: 4.55 m

Boy that Tri is light...

 

I must be missing something as I don't see the KH design as "flat bottomed" at all.

On the KH site the side view and section view, other than the flair for more interior volume, look like a fairly normal sailboat hull bottom.

Is there another reason the KH design is not seaworthy? I agree the amas are very small but that is because he wants it to remain trailerable. I don't care about trailering so the ama could be enlarged for more buoyancy.

Now docking is another story. I have no experience with a tri of this design but I think RW has it right, it would be tricky.

 

I think you are right to say it's more like a sailboat hull profile.
Is that suitable for a power design?

just got an email from the designer Kurt: