What differences do wider or narrower tunnels make?

I'm interested in learning more about what differences it makes having a wider or narrower tunnel on a power catamaran?

As an example, (and keeping it simple) -

All designs are for a 12 metre aluminium catamaran -

Design A - 5.5 metre beam, each hull is 2 metres in width, with a 1.5 metre wide tunnel

vs

Design B - 5.5 metre beam, each hull is 1.5 metres in width, and a 2.5 metre wide tunnel

vs

Design C - 5.5 metre beam, each hull is 1 metres in width, and a 3.5 metre wide tunnel

Please edumacate me!

 

An Australian academic Leo Lazauskas produced a program Michlet which amongst other things analysed the interaction between hulls, he was a frequent contributor to Boatdesign.

He has not logged in since 2017.

Here is a manual to Michlet which might be of interest. http://www.oscae.org/content/courses/shipdesign/michman_933.pdf

IIRC there are threads on this site which discussed this topic.

 

Well, for starters, narrowing the hulls will reduce displacement unless you made them deeper. What that does to the flow through the tunnel is outside my pay grade, but I'm guessing it might make steering more difficult; C worse than B, B worse that A.

 

Its worth noting Michlet is freeware, is not hard to use and give meaningful results.

You have two variables: tunnel width (hull spacing) and hull width, those have independent effects.

Richard Woods did tank testing of catamaran hulls and found that wider spacing has lower drag. He also says wider spacing reduces pounding.

Narrow hulls reduce drag but also reduce load capacity. There's two kinds of drag - wave making and wetted surface (total surface area). Narrow hulls reduce both, assuming the load is appropriate.

So design A will have the best load capacity, and design C will have the lowest drag, assuming the hulls are not overloaded. Realistically the hull width (and therefore tunnel width) will be determined by the load you need to carry, assuming fixed beam and length.

 

Your query is a good one, but it is a problem of apples and oranges or carts before horses. You cannot compare in this fashion because the hulls are different.

For the same hulls, a wider bdeck will generally weigh more. And so, finding the hull size and required displacement is driven by the demand of the bdeck, not vice versa. The bridgedeck loads and space needs and speeds/safety drive the hull design. This was a problem with my boat. The bdeck got too heavy with double bottom, and well built internals. But the bdeck drives the design, not the tunnel. And designing the hulls and plopping a deck on is not good design.

The more important issue for cats if you ask me is the bdeck clearance. Of course, a very high bdeck on a very narrow set of hulls and/or narrow tunnel results in a less stable boat, so the issues are connected.

Anyhow, wise designers may use rules of thumb to establish typicals. But start off the design by determining the requirements of the bridgedeck first. Once you know the space and weight of it, then the speed requirements, then the hulls should be fairly easy to spec, not the other way around.

Now, that said, I am not a Naval Architect. I am a person who experienced what I consider to be a design error, where the hulls tried to drive the requirements of the bridge and not the other way.

 

Many thanks! A ton of good info there!

Wider spacing reducing both drag and pounding sounds like something worth aiming for! Length and beam are pretty much fixed - length by the various rules and laws and so forth, beam because deck space is king with a dive boat, I wouldn't want to go narrower than 5.5 metres beam, and while wider might increase deck space - there comes a point where I'm guessing that too wide causes more problems than just ugliness.

Option C - one metre wide hulls is something I would prefer to avoid - one of my pet hates is an engine space with no room alongside the engines to allow you to access anything on the engine - last catamaran that I worked on had these enormous Cummins engines that were over 600hp each on a 12 metre aluminium catamaran and trying to do anything in the engine space was a challenge for anyone who wasn't a gymnast or contortionist - even doing the basic daily things like checking the oil level was a challenge.

Option B or something around that might be a good compromise. Interestingly the boat I linked to in the other thread () appears to have hulls about 1.5 metres and a tunnel about 1 metre wide (just visually scaling knowing that it is a 4 metre wide beam), so having a boat of about the same length and hull widths but with a much wider tunnel (2.5 metres instead of 1 metre) should give less drag.

Hopefully Option A won't be needed!

 

Interesting. I can understand what you are saying, and I guess it is like a bridge - the longer the span then the stronger (and usually heavier) the superstructure of the bridge needs to be.

Taking that a bit further (again with a fixed beam) - there could possibly be situations where going to wider hulls and shallower draft might be able to be done with little to no penalty as the bdeck will weigh less as a result of the shorter span between the hulls - I guess that is what really expensive software is for!

Do you know offhand the hull and tunnel widths for your boat? That might be interesting information in the context of this thread.

Totally agree with you on the bdeck clearance need - that was about the only thing that was decent on that catamaran that I last worked on - pretty much everything else would get a grade C- and then as if there wasn't enough problems some clown mounted a ridiculously heavy crane on in the starboard rear corner of the deck which left the boat with a permanent list to starboard.

 

I will get you some numbers later today.

In your example, the wider hulls will weigh more. Simple physics, ceteris parabis, and so they will have more waterplane and more drag. Then in rough seas, wider hulls will also pound more and cut less. So, the problem with your query is you are sort of trying to eliminate the most important part of the hull design.

Am I working around a definitive answer for bow wake interference? I suppose. But the tunnel width also drives bow wake drag. There is a lot of good reading here. I need to rest.

Wave interference in catamarans https://www.boatdesign.net/threads/wave-interference-in-catamarans.64814/

 

And, if you want my best advice, see if Ad Hoc will consult a review of a design or draw your design.

 

Well, assuming the same displacement for each options, the the following will occur:

B will sit deeper in the water than A, and C will sit deeper in the water than B.
Thus C has the deeper draft.

Thus propulsion comes into to play and depth of harbours etc.
Inboard/props/waterjets etc...

Then the wet-deck draft is a major factor.
If the distance from the floating waterline, at full load, is "not much", than none will be pleasant in a slight to moderate sea.
But option A would likely be the best. One gains a benefit from a "cushioning" effect of the hulls closer together, with minimal wet deck clearance.

Resistance, option C is the best, as the hull separation plays a major influence in resistance, wider apart the better. In addition, the demi hull being more narrow, the residuary resistance is lower, as the resistance is proportional to B^2.

Thus, as always there is no "optimum"/ideal solution, as many always seem to think...they each have their merits.
Thus what is you want to do with the boat.

 

This is good advice, it's best not to build a catamaran without professional input. I've seen it done more than once and it rarely ends well.

 

Here's a 11m boat with a 0.77m beam. It uses outboards, so that gets around the engine clearance issue as well as other fit out / maintenance issues with inboards. The lightweight construction enables the slim waterline beam, and together they allow for good performance and fuel economy with modest horsepower.

Because the hulls are slender they are more sensitive to load placement, which could be an issue on a dive boat. One forum member here built a modified skoota with the CG too far forward and unfortunately found that out the hard way.

SKOOTA 36
LOA 11m
LWL 10.75m
BOA 4.9m
BWL 0.77m
Empty Displacement 3800Kgs
Loaded Displacement 5900Kgs
Engine HP ) 2 x 60hp
Cruising Speed 10 - 14 knots 17 knots max
Fuel Consumption 10 mpg at 10 knots
(varies according to load)
Headroom 2m
Berths 1 single, 2 double


Sailing Catamarans - Skoota 36 comfortable live aboard cruiser or day charter https://www.sailingcatamarans.com/index.php/designs-2/6-powercats/265-skoota-36

 

May I correct you here?

It is best to find the best professional, or at least a very competant professional. Even building when one assumes the professional has done a good job can go wrong.

I recommend a design and then paying someone to review the design. When I built my boat, I saw designs for $3k, and some for $10k. I bought my design, not knowing much about what that meant.

Oftentimes, the poorest critic of a design is the designer himself.

Anyhow, a couple thousand dollars for a review would have improved my outcomes and shortened the work window, etc. I do not regret the boat I built, but it needed a review/audit by an independent third party.

I don’t know the industry specifics on whether this is done or not; perhaps Ad Hoc can shed light on it.

 

My hulls are about 33” wide midships at dwl.

The tunnel is 116” wide.

The center to center distance is 12’6” or 150”.

LWL is 35 feet; back of engine is 3’ further, all +/- 1”

116+33 is 149, so pretty close on the math check.

My hulls are narrower than need.

My boat has not been sea trialed well. Here on the Upper Mississippi, the biggest wave we see is a 4’ wake.

 

The speed of the boat is a primary factor on the influence the tunnel has. A 10 knot boat will have little sensitivity to the dimensions (within reason), but a 90 knot boat will.