Efficiency: Hull interference vs length on catamarans

Wow, that's very little variation. what's all the fuss about hull spacing? My thought is that hull spacing is an issue for cats with fat hulls and high wavemaking, and that issue diminishes the longer and thinner the hulls get in relation to displacement. What were the characteristics of the hulls tested?

 

The issue I see with these hulls is the turbulence created by the square edges.

 

The hard chines would be rounded with a 5 or 10cm radius and stay below the waterline so shouldn't give turbulence.

I added a graph for the monohull (~trimaran) graph of the same 15m hull with 4t displacement. It's between 25% to 45% more efficient. It has more draft and "half" the total hull beam, so this doesn't tell you too much.



What this might tell you is that for a given target displacement you need vary the length and beam until you find a sweet spot. For example 15:1 could be more efficient than 20:1, or you might be too long if your weight is very low. Unfortunately prelimina doesn't allow scaling the length and width of a hull to test this quickly, so you need a separate obj file for each variation.

 

Here is the graph of hull interference of a catamaran. At low speed there is a lot of dip and bumps. When it dips, there is wave cancellation and it is beneficial. When it goes up, the interference is adverse and there is added drag. At Fn 0.5, the hull separation is pronounced but at Fn 0.7 it collapses into unity. there is still some interference but there is very little effect on hull separation.

 

Here is the graph of wave making resistance. Coincidentally, it is almost the same graph as the cat interference. Always try to design when there is a dip. Designing around the bump or peak is where the wave making resistance is greatest. Find the sweet spot. Image from Principles of Naval Architecture.

 

Here is the graph of the interference factor. Basically a zoomed in version of graph 1 but expressed as a factor .

When it dips, the interference factor is negative and is deemed beneficial. When it goes up, it is adverse and considered detrimental. when it is at zero, there is no interference. Image from PNA.

 

That could work .

 

So, I am awake now. Aren't you all glad? Our friend in Japan is winding down.

Is it going to be difficult to build this boat under 4000#? I think there will be some challenges.

The likely reason we don't see such boats is they are impractical. Not simply that the hulls are not a livable space, mine are only a sleeping bunk, but that the boat becomes more of a day boat. If you build the hull of a 30'er really light, it is still coming in say 1000#, which allows 2000# for cabin, beams, engines and fuel...freshwater, etc. just 20 gallons of water is 160#, but I suppose you could spend 4k on a watermaker and reduce the tank, etc.

It will be tough to achieve. Not saying impossible. Wood's lovely 20' Skoota is kind of a dreamy little version. Someone loved it so much they paid to haul it thousands of miles. There are a few videos of it.

I'd like to try to see where my hulls land in all these charts and graphs.

On this matter of clean exits, turbulence, displacement, semi-displacement.. What makes a boat a displacement boat versus semi-displacement? If my boat achieves 20 kts, would I be better off with a sharper or rounder hull? I have always assumed sharper exit, but was on the fence on rounder versus sharper sides.

Finally, and this is an official hijack, I have to answer a question about turbulence now. I have a very $$$ transducer mounting that I am worried about. I'll start a new thread.

 

The term semi-displacement is more applicable to monohulls, I'd say, they are typically characterized by the maximum waterline beam being well aft of amidships, and a reasonably flat stern section.

 

Just compute your Froude number or convert Taylors number (speed to length ratio) to Froude and look at the chart. You will see if your boat is a hill climber, or a slider. This is the part where if you are at the peak, you are the worst and when at the pit, you are in a sweet spot.

 

Here is one from Watson & Gilfillian. A correlation between the Cb and the shape of waterline. At a glance, you can see if the boat is a pig to push.