Monomaran

[Guido]

Hy guys,
I'd like to have your opinion about some example I've found on the web of power monomaran or monotrimaran as those in pictures attached.
They're three examples of a single bow and double (three in one case) stern. To me they're interesting (especially second and third ones) because they seem to have an external aspect as a classic power monohull but with some of the advantages of a multihull (stability, fuel efficiency?). More specifically, my interest is in their fuel efficiency and seakeeping capabilities thinking about them as semidisplacement boats with a very good slenderness ratio (but how would you calculate this ratio in this case? as a catamaran or as a monohull?).
Thnaks everybody

[Doug Lord]

If you search under the "boatdesign" and "sailboats" subforums you can probably find at least one other thread on this design-I remember seeing it but don't remember where....

like this one by you: Mochi Craft Long Range 23 Hull

[Guido]

Doug,
I've posted my questions in a new thread because in the other threads I haven't found any specific technical answer to my doubts on these kind of hull and their use at tipical semidisplacement speed (Fn=2) and on their ratio calculation (i.e. slenderness ratio). Have you got any answers to' my questions?

[Doug Lord]

Sorry, I don't. Good Luck....

[TeddyDiver]

Originally Posted by Guido

More specifically, my interest is in their fuel efficiency and seakeeping capabilities

Very poor IMO...

[Guido]

Teddy,
on which technical arguments is your opinion based on?

[TeddyDiver]

Much of wetted surface (poor fuel effiency) and large metacentric height (short rolling period)
BR Teddy

[catsketcher]

I don't have any technical data about these particular hull shapes but the idea of a section of hull with negative rocker has never been done well.

An aussie cat designed with such a rounded upsweep quaters hull should have gone about 15 knots with the diesels it had in it but could hardly get over 8. The expensive moulds were never used to make the production run expected.

In seaplane floats there is a step on a hull to reduce wetted surface. Any yacht and dinghy sailor knows that rounding the transom licks water up the transom and its slow. Consider a double ender transom like a Perry - Valiant 40 - nice displacement boat but that stern was never meant for high speed. In Australia there was a similar yacht I saw called Boomerang. On her rounded transom the owners had put a little fairing in to break the water away from licking around the stern.

If you want to do it - make a model and tow it. I would be incredibly surprised if the bottom two were to work at all. As for the top one there was a monohull into a cat in Sydney in the early 80s - as slow as a dog on a chain.

cheers

Phil

[Guido]

Teddy,
I agree with you that metacentric height and wetted surface could be an issue, but it should be the same problem as a standard powercat, or not? Where I'm wrong?

[MechaNik]

The first hull bares some resemblance to Austal Trimaran ferry's.
Although it doesn't look as though the tunnels will clear the water promoting excessive drag.
The other two look as though they should be air cavity hulls in the rear, with perhaps the vee upfront to soften the ride where an air pocket won't stabilize.

I too am fascinated by such designs and other slender hulls, but it seems that they are not possible on there own. They require active stabilization to reduce some unfavourable motions and slow roll rates down.

The Australian company http://www.one2three.com.au/ seem to be making ground in this field.

[TeddyDiver]

Originally Posted by Guido

Teddy,
I agree with you that metacentric height and wetted surface could be an issue, but it should be the same problem as a standard powercat, or not? Where I'm wrong?

Somewhat but not quite.. It's like one tries to get the best of both worlds (mono/multi) but instead gets the worst..
Better to have either widish flat bottomed mono or a normal cat ( both have the same metacentric height with less wetted surface compared to monomaran) first one with better cargo carrying capabilities and latter one with better speed or fuel effiency with less cargo..
BR Teddy

[Guido]

MechaNik,
I'm not convinced yet by different negative answers I've received till now. Probably because I'm too fascinated by these hulls and I'm not a yacht designer (so I can be justified ). But I'm an engineer and till now I couldn't find really strong technical arguments that could "destroy" my dream.
I've looked at your interesting link, but I couldn't find the image of blu yacht you've attached to observe it better (it seems not having slenders so "separated" from main hull). Where did you take it?!
Thanks everybody.

[Guido]

Relating to my question about Length/Dispacement ratio for a "monomaran" like the third one of my attachment, am I right starting from the following formula which I've found at this link (http://www.multihulldynamics.com/new...?articleID=174 ? In this article is cited as follows: "the ratio of displacement of the hull to its length (divided by 100) cubed, or (D/2*(0.01*Lwl))^3. The two (2) in the denominator is to divide the displacement into the two hulls of a catamaran"
If that's right, in the Length/Displacement ratio for a monomaran can we consider a coefficient as 1,6-1,7 instead 2 in order to take into consideration that in this case we don't have two bows but only one? Or what else?
Thanks everybody

[MechaNik]

Guido,
The blue hull yacht was taken a while ago from a site on the net. It has two slender outside hulls.
The word monomaran doesn't sit right with me. Essentially the successful hulls I have seen out there are literally stabilised mono hulls. The outside hulls are there for stability and possibly can't provide very much lift. A vessel at speed would be looking at having only the necessary amount of arma in the water to reduce drag.
This is unlike a power cat that shares it's displacement equally over two hulls and a little more like a sailing cat that might look at lifting most of the windward hull at speed.
I am clearly no designer either but have gone out my way to ride on several different ferries in rougher weather. The tri motion is far more to my liking over a cat.

[daiquiri]

Hi Guido

This is my opinion about this subject: of the three boats in the picture, only the first one (Ferretti, I guess?) is imho done in a rational way, with the use of slender hulls and no submerged negative rockers.

The other two are imho at risk of increased viscous pressure drag due to concave aft part of the underwater volume (as catsketcher has also noted). If the manufacturers of the latter two hulls claim a reduced resistance with respect to more traditional shapes (mono, cat or tri) it might be true (but I want to see the facts) just for a very narrow speed range, where the hull shape has possibly been tuned to reduce the wave drag. At all the other speeds I would expect either the wave and/or viscous pressure drag to increase due to unfavorable interference. The friction drag will surely be higher due to the increased wetted surface (when compared to a monohull).
So, summing it up, I'd expect the total drag of the latter two vessels to be higher at all speeds different from the design speed.

Regarding your question in the post #13, the displacement-to-length ratio as defined in the Multihull Dynamics' article is imho not very useful in case of irregularly-shaped hulls like the former two in your picture, because the viscous pressure resistance (due to surface complexity/irregularity) plays imho a significant role here.

The way this parameter was used in that article is also questionable, and in fact the resulting graph is extremely scattered, providing very little useful information to the reader.

Cheers