Custom 19' all weather, minimalist, strip plank composite 'go fast'

I know that one should not generalise, but I think it would be fair to say that as the length increases, the L/B ratio can increase as well.

Take a typical cargo ship - here is a link to a cement carrier currently loading bulk cement here -
DIEGO (Cement Carrier) Registered in Panama - Vessel details, Current position and Voyage information - IMO 7214284, MMSI 351067000, Call Sign 3FMK6 https://www.marinetraffic.com/en/ais/details/ships/shipid:406928/mmsi:351067000/imo:7214284/vesselIEGO

Her LWL is probably about 100 metres, and her beam is a tad under 16 metres, so a L/B ratio of approx 6.25.
If you boat had the same L/B ratio, your beam would be about 3'.
Yet the Diego is definitely not skinny - she can carry an impressive deadweight.

Look at navy vessels - they might have an L/B ratio of 10 or more. If your boat had this, her beam would be less than 2' - it is then tending towards something like a rowing shell. Yet the navy ships are very stable.......

 

We are really talking about a "panga" style boat here. Slender and lower power. How they rate with divers I don't know.

 

There are a lot of panga and pirogue types of dive boats here, and they work very well - but they are VERY beamy in comparison to Socal's boat.

 

I suppose 6 feet for a 20' panga ?

 

Can you modify your slip platform? My thoughts are another foot of waterline beam would really pay for itself.
Have you considered some sort of canvas bow dodger?

 

Yes, these are all things I researched. On larger military and SARs hull dimensions are very similar to mine and often significantly more extreme, as are powered racing monohulls. I can't find anyone with experience at this scale though besides myself, with the kaboat. It's counterintuitive but I found this article yesterday: https://www.gerrmarine.com/Articles/StabilityPart1.pdf It's a description of CWP before it gets very specific to sailboats... My empirical experience my inflatable kaboat comes down to CWP, that it is much more stable than expected because even though it's long and narrow, if it's edge loaded you still have the bouyant force of that entire edge keeping it stable, and since it's so long, the bouoyant force on that bit of edge really, really adds up. I mentioned that on the inflatable we sit down in the floor when underway at full speed not because the boat is unstable but because people sitting on the tubes are likely to tumble off the side... The relative weight/leverage of a human passenger on that edge is insignificant when taken as a whole against the forces acting on the boat. Apparently this is 'CWP'... it's an area function not a function of length/beam. I think again canoes are very 'tippy' when you look at CWP since the fine ends make a CWP significantly smaller, plus the weight of a person vs bouyant force created by total waterline area of the canoe is much greater on that craft than an overall larger boat. The hull of a canoe or kayak also has no weight in the bottom so COG is quite high above the waterline and more affected by the weight of a person, which is very significant against the bouyant force on the edge of the canoe.

From research, boats have trended beamier over the years for several reasons... first primary design consideration for most small boats is (1) will it keep me dry on the water? (2) is it stable enough/secure enough (enclosure) for fishing or roaming children/wives? (3) can I put a lot of people or doodads like bait wells and seats in it? Those are bottom of my list. Also, slips and boats are categorized based on length, so there are economic reasons to go beamier--more boat for less money. And then also beamier boats on paper need more power to get on plane and modern engines provide such greater power to weight ratio that what was formerly impossible is commonplace these days. There are market forces of course, too. No matter what can be said about fuel economy and speed, what percentage of the market chooses a boat that is almost certainly a wet ride in weather, has no tranverse cargo or hatch space, requires passengers to practically climb over each other if they want to move from one end of the boat to the other, seats that are practically on the floor, and is virtually impossible to stand on in open seas? This boat I'm proposing seems staggeringly narrow to most people but I've been dealing with and using a vessel for years that's about 42" wide with ONLY 18" of usable interior width since most of the beam is taken up by inflated tubes... I'm looking at something that's a huge upgrade, more than TWICE as beamy as what I'm currently using.

It would be pretty simple to make a scale model out of basswood so maybe I'll do this before going further.

I have been on a lot of pangas and they make pretty good dive boats but to my understanding they were designed as seaworthy fishing boats with large cargo capacities. Also, one note is the original pangas designed and marketed by Yamaha were not at all beamy--they were long and skinny by modern standards, closer to 4:1. To my understanding this was a function of better seakeeping for a commercial user who really needed this, and the high bow/sheerline was also a design feature so heavy nets and cargo could be handled from the bow safely. Dry ride was not a major design consideration and stability was less about how the boat felt and more about whether it would actually sink. Also, the original pangas had relatively small engines since that's what was available and cheaper in those developing markets. Advent of more powerful engines in a small package allowed big changes in hull design.

 

Your inflatable will have a higher CWP than your new design, so for the same length and width, the inflatable will have much better stability.

The inflatable will be much safer because it has those inflatable tubes - if it heels, they have good reserve buoyancy giving righting moment.
Your design has nothing like this - and it won't take much heel before water comes over the side.
I think that your design would work IF you have buoyancy tubes like on your inflatable.

You say that you are "looking at something that's a huge upgrade, more than TWICE as beamy as what I'm currently using" - but you initially said that the beam of the boat you are building is 46". And your current inflatable's beam is I think 42"? I'm baffled here.

The pangas can have a L/B ratio of 4:1 because they are relatively long - if you have a 4' long dinghy then you probably need it to be 4' wide to give you the required stability and buoyancy. It will not be very stable if it is only 1' wide.

 

Agreed on all those points regarding reserve bouyancy and righting moment, etc. I am somewhat on in denial and head scratching as to whether that amount of righting moment and reserve bouyancy is as necessary as we think.

Sorry I was conflating beam and usable interior width, re: current inflatable vs proposed design. The kaboat's effective interior width for cargo or seating is only about 18" because while the 'beam' is 42" the majority of space inside the beam is taken up by the tubes. In many boat discussions, beam directly correlates to cargo capacity. Saturn originally designed the Kaboat the way they did since they were looking for something very light/cheap which hits users who want ultra portable for no money, and at least according to them and in my experience it was also surprisingly stable (they said on the website it was inspired by Asian dragon fishing boats). They kept gravitating towards wider because most people were like WTF when confronted with kaboat.

 

Yes, I think you are in denial about this boat, big time.
And yes, righting moment and reserve buoyancy is very important and is necessary!
Comparing your Saturn KA boat with your new design is like comparing the proverbial apples and oranges - they are two very different designs.
As I said before, I think your design would work well if it has an inflatable collar.
Sure test it out first without an inflatable collar, and with your full complement of outfit items and passengers - but I think you might be disappointed by it's stability.

Re the lines plan drawing that you posted earlier - I think it would be very useful if you could use this to produce a general arrangement drawing showing where the console is, along with the fuel tank, stowage lockers etc.
And if possible, show to scale the people on board - they will make it all much more realistic.

 

The stability concerns are warranted I think, with the narrow beam, and only magnified by the intention to have it be self-bailing. I am amazed that you could get away with using something like this, as a vessel used to carry paying passengers on the open sea. There is far too little reserve of safety and stability.

 

I will do some proper calculations of all these things and make some sketches. I can't do the inflatable collar also because insurance co's freak out on those and won't insure them for commercial use to include spearing, even if the boat floats without and the actual tubes are foam filled, although a more generous inwale would be a good addition for floatation. For spearing use for us RIBs are some of the best vessels we have for our conditions. They're expensive though since the tubes don't last. This intended boat is ultralight (lighter than many inflatables and way lighter than a RIB) and sits practically at the chines with engine but no people; with a 9" mostly foam filled deck it's still per ABYC standards got enough freeboard even fully loaded (I think based on their formula it needs 4.5" or so between top of deck and waterline)--I do need to double check and write everything up, probably redo sims in the light of knowledge since gained. It will float level if swamped (this is USCG) and with the giant bilges and scuppers and amount of foam it would certainly be engineered to not stay that way for long. The electrical system (fuses, switches, batteries, lights, etc) on my inflatable are custom built to dive spec (I built them) since they spend most of their time soaked and often underwater so I'd build this electrical the same way. I think also most people looking at this are thinking BOAT as opposed to GAS POWERED SURF BOARD WITH SIDES. If rigged and finished like even a small skiff (BOAT) it is nothing I'd want to venture out in.. When I said the seats are 'low' I mean really low--like 4" high. Passengers sit practically on the floor. I sit a few inches higher. COG is very low, below the shearline. Initial stability is less relevant (we need to be able to climb over the side without it tipping over though but that's taken care of) since we don't stop in the middle of the channel and walk around fish. We are looking for an excuse to get wet the instant it stops--boat gets loaded at the harbor, zips fast to a dive spot on the frontside of the island possibly through some mild/moderate seas, tucks up into a cove where it looks good to dive and anchor, and we fall into the water and do our thing, move around the island a bit, then stow gear and blast straight home in the afternoon when the wind may have come up a bit and we will be in a combination of moderate following seas.

 

46" is very little even for a lake boat. But I have limited experience.

Send 3d model my way, buy a $299 VR headset and I can make you a presentation setup where you can poke around and get realistic feel for dimensions.

This is bit tongue on cheek but it could be quite easily done. It really is pretty amazing design tool to be able to get real spacial feel. Example here.

 

I skimmed the thread as it is rather long.

The main issue I take is you are confused about stability of your current vs intended hull.

The narrow panga was never meant to have 200 pound men flopping over her gunwhales.

What you are doing is going from a pontoon to a narrow vee, basically going from extremely good stable platform to a fishing boat that doesn't have the ability to bring in a diver from the side, nor from the back because the bottom is too narrow.

Draw up your plan and then model the diver entering the vessel. You are going to find the boat heels over badly. Sorry.

A narrow catamaran like a Skoota 20 would be better and not roll over boarding divers.

 

In design, you don't get to accomodate a diver going over the side by having all the others move to the opposing side. In fact, the design would be done the opposite. As if there are two other divers assisting the third diver who is having trouble reboarding. So when you start to consider righting, it cannot be done without considering worst case; not best case scenarios.

Same with low center by getting them all to sit on the hull. For certain, divers will stand.

I have no c.v. in naval architecture, but the plan is flawed. You put together a very good sor, but you have not met the sor with the proper hull design.

 

Here is a panga rigged for diving. They are boarding off the stern; fyi.

This can get you away from the righting issue.

Panga 28D Scuba Dive Boats https://pangacraft.com/categories/28_d_panga.shtml