Would anyone like to design a 39 foot/12 metre Stabicraft-style boat?

That is great input Bajan - many thanks. I'm not sure what weight Tommy designs for but I will be sure to let him know what our max weights will be. Won't be pulling any treasure onboard unfortunately as the amount of paperwork alone that would be required to do that would negate it - the Egyptians explain that they invented paper (papyrus) and then the British taught them how to make everyone's life hell with it!

Less drag sounds good to me, am aiming for one of his planing designs so as we can get good speed out of it. The forefoot probably won't be able to change much due to the cabin weight but if we can skinny down the rest of it then that would be great! We might even end up with a semi-SWAFT design for rough conditions when we can't get up on the plane!

 

Have you contacted Tommy, to see if he will provide just a design, for you to build in Egypt?

Re a planing hull, be aware that cats can have slender displacement hulls, and still achieve a good turn of speed.
The Glacier Bay cats were / are excellent examples of displacement cats - one of their models is now being built in Australia -
2780 Glacier Bay https://worldcatboats.com.au/2780-glacier-bay.html

It should still be possible to have that wheelhouse forward without a bulbous bow underneath to help 'support' it - it is all a case of getting your overall longitudinal centre of gravity in the right place, along with the centre of buoyancy.
Does it have to be a 'full' wheelhouse, or could it be a more simple helm station with a 'T-Top' that has a solid windscreen, and maybe plastic side screens?
One disadvantage of having that (relatively heavy) wheelhouse (complete with heavy crew) right up in the bow is that it increases the longitudinal moment of inertia. Having this wheelhouse up forward, and heavy engines on the transom is a bit like having a flywheel with a lot of it's weight concentrated on the rim rather than near the centre - the flywheel wants to have a lot of inertia, but longitudinal inertia in a boat does you no favours. The more inertia you have, the more inclined your boat will be to pitch, although I suppose the bulbous bows will help to dampen this somewhat.

 

He is happy to do that

OK that is good to know. After seeing a couple of their videos I would have called them planing hulls as the bow lifts so much at speed - but that is just me being wrong due to lack of knowledge in this area.

We really want a full width wheelhouse that is fully lockup as there will be a lot of marine electronics going in there and we want the full width of the cabin to block spray going aft when the sea cuts up really rough like it did yesterday - its one of those places where it is nice most days but then gets really ugly sometimes. We also want to make the boat + crew available to local authorities for sea rescue work when we aren't diving (e.g. night time) as there have been some pretty big maritime disasters in the area.

Its looking more and more like we will have to go with inboard diesels after what I learnt today - either driving props or jets. I guess we could use long shafts to get the engines more towards the longitudinal centre.

 

This now raises a good question - for rough weather are we better off with a displacement hull or planing hull?

Seas here can range from bathtub smooth to 7 feet waves and swells - which are usually confused and stacked up - intervals of 3-4 seconds with waves from the NW and swells from the east is typical of the bad conditions particularly in winter.

 

@RSD, for what you are looking for a displacement mono-hull is always better. Don't hesitate.

 

Need the deck space of a catamaran though.

 

Well, you know better than anyone what you need, but I would try to think of a boat as simple as possible, even if it has a lot of deck area. You don't need a huge breadth, do you?.
Do some numbers and deduce the beam you need.

Edited: 12 m in length by 3 m beam, why you need a catamaran?

 

Going for a 12 x 5.5 m beam now - 3 was going to be too tight

 

So, I agree with you. The catamaran seems to be the most suitable option. Keep in mind, if you are going to transport large weights on occasion, that the tons per cm of immersion of the catamaran are quite low. You should consider, for those occasions, the need for a higher freeboard than normal, which can be inconvenient for divers or for a rescue boat. As always, you have to balance options that are often incompatible.

 

I've had a (non-diving) friend mutter on more than one occasion that divers are incompatible with boats full stop!

My thoughts on the occasional possible need to carry a heavy item such as a cannon is that it will be done with an appropriate reduction in the number of self loading cargoes (divers). I've calculated maximum weight of crew + divers + dive gear and that will be the max weight allowed on board as I don't want to have to further compromise the design to deal with "what if" scenario's - I would rather have less divers on for the very rare day that situation arises.

 

Divers are not incompatible with a boat, not at all, what you have to do is design the right boat for divers.
In any ship it is necessary to study all the possible loading conditions and design the shapes that achieve the best compromise among all the possibilities. For this, it is very important, essential, to write an SOR as detailed as possible. You should start there as well as drawing a General Arrangement plan of at least the non-submerged part of the ship. Or seek help to create that information.

 

What do you mean here?

Also, many outboards can be converted to jets. There are quite a few fab shops in the US. As to the dps/jet combination, I can't speak for how well it works; even though I have it, but the autopilot on my boat uses software only and adjusts the steering cylinders. But there is no throttle involvement in my simple system.

I'm not sure how you get jet conversions, dps, steering all from the same provider. But at least be aware of the fact jet conversions are possible.

Another idea would be to run with caged props..

 

What is this maximum weight please?

I am thinking that if you might only occasionally, or even very rarely, have to move something heavy it would be easier in the long run to sub-contract somebody with another vessel to do this for you?
Or have a very simple barge (it could be steel even) that you can tow with your dive boat for carrying cargo when required.

Are you still aiming to be under 12 metres length re the local regulations?
And can you remind us what is the desired cruising and maximum speeds for your dive boat? And the range required?
For a 12 metre efficient cat, carrying just the required number of divers and gear, you should be able to work with much less than the 1,200 hp mentioned in your other thread.

If you do have to use inboard diesels on shafts, then you could have the propellers in nozzles - these would act rather like cages, but be more efficient hydrodynamically. You would still have to be careful though, re anybody's feet getting anywhere near the nozzles.

 

There are contradictions in the OP's wishes that make it almost impossible to decide on one type of ship or another. On the other hand, the selection element seems to be how beautiful or ugly a certain ship looks in a photo or video, rather than an analysis of a seriously defined SOR. With all due respect I must say that I really don't see a solution.

 

Up until late yesterday we were thinking Yamaha XTO 375 or 425 hp with Yamaha's Helm master control system. Unfortunately late yesterday we learned that the local coast guard has just stipulated that no new outboards over 300 hp will be permitted - I guess someone with some 600 hp Mercury's or something similar has gone and done something stupid and ruined things for everyone else. That pretty much rules out outboards now for this project (prop or jet converted). The reference to moving engines more towards the centre was in reference to inboard diesels after we learnt of this decision.