Logic in different length hulls

Morning

Hopefully a quick question. If anyone has links for me to read up please throw them my way.

What is the reason / advantages/ disadvantages of the hulls being of different lengths in a tacking outrigger like Ontong Java? In the video and some of the write ups it is claimed that improves stress on the deck. Just trying to understand. Also, do the canoe shaped bottom of that sail better than the submerged section of production catamarans like Lagoons etc for performance?

Thanks

 

Yes, absolutely.

 

Hi BlueBell

Are you referring to reduced stress on deck? If so, I can understand that smaller pontoon will reduce the stress on deck but does it have any more complex reason such as dynamics/diagonal loads etc, as in more reduction in stress than expected from a smaller pontoon?

 

Outriggers (ama) tend to have L/B>>10. This means it is effectively wetted surface only. So maximum beam with minimum drag compared to equivalent righting moment gives greater power. So it pays to make the length of the outrigger as short as needed to support the overturning moment. Wooden Boat had an article several years ago on the different sized Malay Archipelago craft.

 

Thanks JEHardiman. I will look for the article. L/B>>10 meaning a very narrow and skinny hull right. How can that have a maximum beam? And if the outrigger is made as short as needed, won't it make L/B smaller? I am obviously missing something.

Also, when you say outrigger, does the above apply to crafts like Ontong Java with much fatter hulls than for eg: proas?

I am looking for design considerations, opinions, performance anecdotes and plans for something like the boat above - to see if it is feasible to build a smaller version of the same. Before I contact the people / yard who may have plans available, I want to read up on the feasibility of something akin to Ontong Java on a smaller scale - around 30ft range.

Most of the google searches for double canoe outriggers and tacking outriggers, apart from two sites shows up very different things.

Thanks

 

That's because Ontong Java isn't actually an outrigger, it's a catamaran. It does have different length hulls, but that's more or less nominal. The difference is mainly in the overhangs, if you look at the waterline it's a different story. OJ2 has its dimensions given as 71 and 57ft, that's a 14ft total difference. The hulls are double ended, so it's 7ft at each end. The different stem/sternpost angle on the two hulls is mostly responsible for this difference, the actual waterline lenght difference is much less.

Unless you plan on using hollowed out logs for the centerline there is no advantage. If you do then then the reason becomes obvious, you don't need to find several identical logs. The islanders had to chip everything out with stone and shell adzes, less work was important.

One last thing, OJ doesn't have "canoe bottoms" it's a deep V hullform. There are plenty of opinions about this hullform on the forum.

 

Makes sense. I wondered why it is considered a tacking outrigger too as the hulls are pretty similar in form and displacement/submerged portion. However, my knowledge isn't enough to give me an answer there.
So, for a catamaran, there isn't much of an advantage building unequal length hulls.

I have been reading about the deep vee vs submerged lateral foils on this forum and indeed, plenty of opinions. I looked at the shape of the hull but misinterpreted it as a flattish bottom like a canoe. I rarely saw any flattish bottom catamarans. I am guessing for a reason.

With my limited knowledge of naval architecture -
For a given length of a boat, a catboat of New England type shape will offer most interior space. However, they are not suitable in an ocean. So, I thought how about a catboat style short and wide body with little submersion and stabilized with two foils as a tri or two of the hulls with deck as a catamaran. However, I haven't seen any like that, perhaps for a reason. Are they not safe, or slow, or some other obvious reason?

Thanks Rumars

 

It's not considered a tacking outrigger, that's just what the designer calls it. He also calls it polynesian inspired, when in fact it's two traditional west african canoes lashed together. The rig is more indonesian inspired, and so on.

Most production cats today have some sort of U bottom, with daggers or short keels added. Deep V's have gone out of fashion a long time ago, but they do have their followers.

The lack of seaworthiness in catboats has nothing to do with their form, it's a function of their internal ballast and low freeboard. A L:B ratio under 3 doesn't mean you can't make a blue water boat. In fact the trend slowly goes from "pizza slice" (wide stern) to "loaf of bread" (scow bow). They have not yet gone to 2:1, but 2.5:1 is availabe, pro buildt, as a kit, or as plans.
If your interest is maximum internal volume monohulls then you must go french:
Accueil - Afep Marine https://www.afep-marine.com/revolution29-bateau-aluminium-presentation.php

Sailscow, l'alternative entre mono et multicoque – En 2024, cette année sera la bonne ! Acceptez nos meilleurs voeux de voileux :-) https://sailscow.com/

Rocket 800 – David Reard architecte naval https://davidreard.com/rocket-800/

 

They look quite interesting. However, I must come clean! I am looking for a catamaran close to 40ft range, that I can build in a reasonable time frame, if such a thing exists.

I built a dinghy before and know exactly what people say that it takes longer than you think. It did.

I know that it will still take a lot of work and buying a second hand cat will probably come out cheaper. However, there are not a lot of catamarans out there that are close to 40ft, for anything under $150k USD.

My thoughts were, something like harry proa/cruising pacific proa vs double canoe like the OJ.

Or a catamaran with very simple hulls without much built in to them other than them giving a platform for the covered deck to simplify the building - not using hulls means, needing higher amount of space on deck, more windage but hopefully simpler build.

 

It's good to come clean, people can actually steer you in the right direction, like for example this models.
Sailing Catamarans - Mirage - 11m hard chine open deck cruiser https://www.sailingcatamarans.com/index.php/designs-2/5-catamarans-over-40ft/179-mirage
Sailing Catamarans - Mira - 10.65m deep V hull open deck ocean cruiser https://www.sailingcatamarans.com/index.php/designs-2/5-catamarans-over-40ft/181-mira
Kohler is redesigning his site at the moment, so I'll put a plan vendors link for one of his bigger designs: DUO 1000 Plans https://duckworks.com/duo-1000-plans/

If you can keep your build at Ontong Java standards of fit, finish and comfort it's possible to keep costs and timeframe reasonable. If not, well, there are plenty of examples out there to compare with.

 

I am still doing a lot of reading up to see if I do want to build or work/save up and buy. Despite building one boat, I am not put off from building another, although it is obviously a much bigger project.

It is interesting to see the scow boats making it to cruising world. It seems, they are a midway between multihulls and monos in a way, but, how do they behave if one wishes to cross either of the ponds in one of them?

 

I can't work out why scows are coming into fashion in anything outside classes that have big rigs and short overall length limits. Just take one of those cruising scows and draw out the bow lines while leaving just about everything else the same. You will end up with a boat with a conventional bow that will cost not much more than the scow to build but will go faster, pound less and have more space on deck and down below.

Look at it the other way around; take a conventional modern boat like say a J/109. Cut the front 6' off and reposition the forestay. You have now ended up with a "cool" modern scow which may be faster for its length but will be slower in reality, have less space, pound more and not cost much less.

We can apply the same thinking to just about any short scow type. If you extended the bow of an Optimist pram the boat would be faster, more seaworthy and more seakindly for very little extra money (although given the Opti's role, the pram is fine). Conversely if you cut the bow 3' off a Laser 4.7 to make it a "cool" scow, you would make it slower and less comfortable. My first ever boat was a scow Moth. Late scow Moths were about half the weight of a Laser, carried an 8m fully battened sail instead of a 7m dacron one, and could power up their sophisticated hull with the leverage of wide hiking wings - and they were still all-round slower than a Laser because they were shorter.

The basic appeal of scows seems to be that currently, many people judge boats only by how big or fast they are for their LOA, which is a very artificial measurement. We all know that to make the fastest and bigger possible boat for the LOA you just load it up with big sails, lots of beam and lots of righting moment - but it will normally end up slower and less comfortable than the same amount of RM, SA, beam and space in a skinnier, longer package.

The funny thing is that some people are now thinking fat bows are great when it was just a couple of years ago that the hype was all about wave-piercing bows which are right at the opposite end of the spectrum.

 

That's what Sales, Hype and Marketing is for...to make you feel you have something new, unique and different.
Who said anything about...facts!

 

"I can't work out why scows are coming into fashion"

First, there is an Iron Law: the sailboats imitate the sailboats of the most famous regattas.

Second, many sailboats were a hydrodynamic disaster when sailing Downwind in big Waves, and with the Scow shape the Yaw control and Pitch control is made easier.

To put it in a depressing way: in a world dominated by inertia, laziness and ignorance, the Scow is a magnificent solution for sailing Downwind with Big Waves: the bow does not sink.

 

The hydrodynamic Lift Force depends on the width, Beam (b), of the Foam/Spray Squared.

On the other hand, it is a bow that is more resistant to sinking from a hydro-static point of view.

And to top it off, it does not dig into the back of the wave ahead or into the valley: let's say it adapts better to the roughness of the terrain.