What differences do wider or narrower tunnels make?

Another alternative is to simply build a boat that already has a track record.

If you're building the first example of anything there is always a chance that things can go wrong. If you design your own boat and you don't do your homework the odds are far worse.

 

Sorry everyone - just catching up now after a busy 24 hours playing volunteer firefighter - which included at least two incidents where people reminded us that nobody calls the fire department after they have done something smart...

Will reply to the various posts now.

 

So basically avoid wider hulls than necessary as they

• weigh more
• have more drag
• will pound more and cut less in rough seas

So hulls no wider than necessary. OK that helps define things more. Given that the overall beam is fixed, and that a wider tunnel/spacing further reduces drag and pounding (as per @waterbear in an earlier post above), then there is a double benefit in this case to keeping hulls no wider than necessary.

 

Will do! With everyone's input etc I think that the way to go is becoming clearer all the time now.

 

A Skoota 36 was also built wothout accounting first for bdeck plans and the boat also had bow extensions AND sponsons added aft.

The issue is the hulls were designed without full considerstion of bdeck plans.

The builder/owner, for example, wanted to live onboard as the title of the vessel suggests and so he has a small washer/dryer onboard. But the hulls are too small for such accomodation, etc.

In my case, the bdeck, for example, should have never been a double bottom structure with hulls as is. But there is no practical way to vee the bdeck then, etc.

 

I think Ad Hoc will offer the best reply.

One way to achieve the goal you stated above is to design the hulls with some reserve in the shape.

My boat was drawn far too close to her loads or even under to be fair. And I wet bagged 90% of it and had low resin ratios.

The thing to do well is to determine the difference between displacement and loads. If, for example, you plan to dive and recover artifacts; how much extra do you need for hauling more load. A good designer will cover this detail.

 

I guess it will be a bit of back and forth between hull width and propulsion as some forms of propulsion will require larger engines which will potentially require wider hulls etc etc.

One of the requirements is that the vessel will need dynamic positioning. The "easier" forms of propulsion to get an off the shelf solution for DPS include water jets, outboards, and sterndrives. Waterjets seem to be out as they are just far too expensive unfortunately - unless I can locate a good cheap solution. Sterndrives such as the Volvo Penta IPS system are a better option than outboards for a number of reasons so hopefully the hulls will end up suiting these. I'm going to rule shaft drives out as the prop can't be raised at all in shallow water etc.

 

I definitely don't want to go that way - it would end up being an expensive experiment that is ultimately unfit for the job - so I will get it all drawn up and properly checked.

 

Sorry left this part out of my reply.

It seems to me that having too little clearance in the tunnel is something that should certainly be designed out at the design stage before it is discovered to be a problem when the boat is sea trialled - I definitely want to avoid problems that are designed in and can reasonably be foreseen - especially as the prevailing wind howls down the Red Sea in winter and motoring into it is never much fun - so I don't want to make it an even worse experience - day after day after day.

It is sounding more and more like hull width will be determined by the engines required - to drive the hull width...

As I mentioned I hate engines that are a really tight fit (or hulls that are so narrow that you can't easily work on the engines) so I will be wanting hulls wide enough to do basic work on the engines.

 

I guess in a way a dive boat is similar to a workboat in that it may or may not have a load on board. Because the divers on this boat will be doing technical diving with a LOT of equipment - I am working on a maximum load of divers and equipment ("the variable load) of 2000 kg - if that sounds a lot but you are unfamiliar with tech diver then take a look at this photo - and then add an underwater scooter weighing another 28kg

 

If only the Red Sea was that pleasant - some days it can be like glass, but in the winter months it can and usually is as rough as guts - anywhere further south than Safaga can be a very unpleasant experience even in a 150 foot dive vessel.

 

My plan is to come up with a rough design that a naval architect can then turn into a good design - in the last 48-72 hours I've made a fair bit of progress thanks to the help of everyone on here.

 

Some reserve in the aft half will be important as that is the area where the load will be changing, the forward half should have very little change. One good thing about a wreck as old as this is that we aren't expecting really heavy objects to be found - the wreck sank about 1250 years before the first known existence of cannons etc.

Time for me to crawl into bed and get some much needed sleep!

 

RSD,

Pretty sure I've said it before but I'll say it again:
If you want to avoid unexpected delays, costs, frustrations and inconvenience...
go with a proven design that meets your SOR.
Otherwise it's guess work, second guessing, mistakes, added costs, unpleasant compromises,
delays, hardship, and ultimately disappointment.

How many years will your planning drag out for with no boat?
Pages and pages of idle chitty chat on a public forum where you never know what the quality
or perspective the advice your getting is coming from.

I have an international dive group leader with a huge client base I'm waiting to introduce
you to but at this rate he may be retired before you're ready to entertain his business.
I'd sure like to see you get the ball rolling.
What's the hold-up?

I hope you had a good sleep.

 

Starting with an SOR, it may be that a catamaran is not the right type of boat. The first post shows wide hulls with a very small tunnel. Catamarans make sense when the hulls are narrow and the tunnel wide to get a lot of deck area. The problem is that the narrow hulls can't carry as much weight as a monohull for the same length. A group of divers with gear can get quite heavy.