What differences do wider or narrower tunnels make?

I had assumed wider hull spacing would cause more slamming, much like Gonzo's drawing. But I don't have a lifetime of ocean sailing experience on a wide variety of cats like Woods does, so I will assume he's correct. I don't perceive a lot of hot air coming from him, so there's no reason to be skeptical.

 

Gonzo's drawing makes sense because on some days you could be in a situation where wave height and spacing etc makes that possible. I would imagine that having a large-beamed trimaran with a larger demihull in the middle might be preferable to having a catamaran with the same large beam - from both a seakeeping perspective and an engineering perspective as the middle hull significantly reduces the span size(s).

 

There is a lot forgotten when you make a simple, sort of deceptive two dimensional drawing to attempt to support a failed argument.

First of all, Woods says a wider hull slams less, ceteris parabis. That is, all else equal. So, you cannot draw two boats and two different sized waves, for heaven’s sake and claim the wider one slams more. The bow of a boat slices through a wave to some degree and also lift occurs, generally.

When you a running into big head-seas(or any, really), the waves part, but both waves then combine somewhere further back and are actually higher. So, a wider boat’s bow wakes meet further back. This nullifies two dimensional drawings in the discussion. The wave is highest where the two bow wakes meet. This is why, ahemm, width matters.

This touches on Froude’s business, but it doesn’t seem necessary to me.

Now, ceteris parabis, in some nasty headwaves, discounting the bow wake matter entirely, the same height bdeck in a wider cat is gonna probably be more subject to wave striking. This is why Woods goes further and suggests avoiding nacelles and also to vee the bottom.

As for the trimaran; it has a high operating cost because in most places; you cannot trailer it or keep it on a slip and so the vessels are typically moored. This would be a lot of hassle for a dive boat. Loading gear and pax would require an additional boat. I cannot speak to the stability of the trimaran as I have zilch, zero, nada for experience. But early on when I sought a slip here, it was ruled out to cost of slip. The way slips work, is a slip is sort of sized proportionally, so a 24 foot wide slip is designed for say a 100 foot vessel, or only side tying is allowed, etc.

 

I think the slamming refered to waves encountered and not bow waves. At the same wave height, a narrow boat will roll and ride the wave, when a much wider boat may be in the trough of two waves and hit the crest with the deck. I did that with a 34 foot cat sailing off New York. The whole boat was submerged. Admittedly, I push boats really hard for fun.

 

RSD,

There's also the consideration of a raised centre hull which only comes into service during "slamming" wave action.

So much of this topic depends on so many variables including: sea state, direction of travel in that sea, payload variability and placement, and the amount of time spent under each, varying condition. Very difficult to theorize about. Once again, it's all about compromise and application / need ( SOR ).

 

My boat data..as promised, for semantics.

loa … let’s use 35 feet, never sure if to include engines, it was less before bow extensions, if I disclude them, I get a bit of a brake upwards from 4%

bdeck clearance midships
17”

4%

bdeck clearance forward
43”

10.2%

zoom in and you can see a bit of angle for wave splitting, also, the foredeck is webbing so if we hit a wave hard; it doesn’t impact the boat as much

needs bottom paint, coming in the next two weeks

 

As always, it depends....
Ostensibly this is adding a '3rd bow'...and helps to prevent bow/deck diving of catamarans.
Look at the large wave piercing cats.
Concomitantly it helps to reduce the effects of slamming.

Slamming is simply a function of the wave amplitude to wet deck clearance. Reduce this, and the proposenity to slam increases.
A Vee'd or curved wet deck just reduces the effects of a slam, does not prevent it.

So one would have to ask the question first, does it slam and then why and is it a problem, and then...is mitigation required?.....before automatically assuming there is a problem and a fix.

 

All our tank testing and sea trials in such sea states also correlate with this effect.
Which is why, 'sometimes' a narrow width between the hulls and low wet deck clearance can be "beneficial", but it depends, upon many factors.

 

I agree. "It depends" is the most common phrase in engineering.

 

Trust me when I say that there simply aren't things like slips or boat ramps or even moorings in most places in Egypt - the boat will be swinging on the pick every night.

 

Those bows really look like they were designed by Edward Scissorhands!

 

Some good logical thoughts there @Ad Hoc