Twin hull sternwheeled landing craft

Beam dimension

Ok, this is a distress signal May-Day May-Day. I have numbers form my initial 20' beam to a 30'beam from an esteemed board member.

Be advised, I don't know the first thing about building anything that floats, but I am doing it, anyway. If this board functions as it supposes to be, I can get a little guidence here and provide feedback on what actually happens.
On the machine gun forums, you will find me. In that world we share thoughts, dreams, accomplishments, things for sale and all kinds of personal support.

I came to this forum because I saw a brain trust and I have more questions than answers to a technical task into which I have thrown myself. My instints told me to build tw hulls 61 feet long and 60 inches in diameter with 10 foot bulkheads and hatches.

This is most fun that you can have with your clothes on. But it's only fun if it works.

Is it improper of me as an ignorant newcomer, non achitect boat builder to ask for the brain trust here to give a little guidence?



The responses so far have awesome, by the way. I've heard everything from 'Atta Boy" to 'Forget about it' .

The last one suggested a beam half the length. But keep the load centered. That's the best advice I have heard yet. Keep the load centered.

Don't let me do anything stupid?

At a 20 foot beam she looks skinny to me.

Ryland

 

How far have you actually got with this hull build ?

The big issue probably isn't how wide you want it - its probably how wide the joining structure has to, or can be designed to, support t he hulls, and what components in the hulls themselves are available to provide support the joining mechanism.

You said early on that the reason for two hulls was for dissasembly - so here you have limitations already, as the joining structure has to be capable of being moved or being removed, or whatever you have in mind.

The wider the joining assembly, the more robust it will have to be. Resisting say a half metre wave on 60' hulls with a 30' gap, will require quite a bit more 'muscle' than say a 10' gap.

Basically, if you have to ask structural questions on an open forum, you are lacking the very necessary professional advice for a project of this size. You need a good engineer to calculate the loads and design a suitable structure, both for performance and safety reasons.

Bear in mind, that you will probably want to insure this considerable investment. Without a professionally supported design, this may be tricky.

 

Thanks

Thanks for your time and good advice. I am not a store-bought structural injuneer, but just about everything I have ever designed has worked flawlessly. I stick to some instinctive guidelines and test at every juncture. I do employ some eggheads from time to time to crunch the numbers, as I will do on this adventure.

The hulls are finished, delivered and set in place. I sat with an engineer at lunch today and did a drawing on Napcad.....found that my 60" hulls will have a 22" waterline with 8000lbs of structure added and 8000lbs of cargo. good news, actually as a 30" waterline is my design limit. Each ton of weight will raise the waterline by 1/2 ". That equates to a fair amount of wiggle room in design and cargo weight.

Participating on this forum is an effort to follow one of those guidelines.......There's no advantage to ignorance.

Thanks again,

Ryland

 

Well, you can't say that since you are using round tubes. You are talking about displacement, which means x amount of boat weight displaces an equal weight of water. Water weighs apprx. 62# a cubic foot. Each 1/2 " deeper your hulls sit will displace more and more water, as each 1/2 " will be wider. That will continue until your round hulls are 1/2 submerged, and then each 1/2" submerged will displace less and less water as the hulls become narrower.

You have to figure out the volume of the outside of the hulls to figure out how much they will submerge with x amount of weigh. You have to count all the weight including the hulls.

As a simple example, if you had a 1' square box that was weightless, it would take 62# to submerge it flush with the water surface, 31# would submerge it 1/2 way, or 6". If the box weighed 10#, it would take 52 more pounds to sink it all the way, or 21# to sink it 1/2 way . (21 + 10 = 31)

 

Neatly sidestepped. My message that the width of the supporting beams is well down the list of design requirements, and certainly well after after joining calculations, doesnt seem to have recognised.

For a start, what is your design for the hull joins ?

If we allow two main beams, we have ~ 1 tonne per join to contend with. If the joiners are demountable, then perhaps the number, size and type of bolts will be involved in the calculations. Certainly, the structure of the hulls will be a big component. If you hulls are already built - have you placed the internal stiffeners in any calculated locations ?

If we also allow for differing loads on the different length supporting beams, then you have torsional loads to calculate, especially if you then have to load some sort of decking structure, let alone engines and large paddle wheels.

I personally dont think that "instinctive guidelines" will cut it on a craft of this size. Way overengineered solutions will probably be as big a worry as way underengineered solutions, if you have trailerable considerations to consider.

The fact that you have only just now calculated the bouyancy of the hull after actual manufacture, leads me to think that you have not yet considered any engineering solutions to joining things together. Its not the process I would personnally prefer.

 

sidestepped

Sorry I left that impression.

My waterline to weight ratio is based on 22" loaded which is 8" below the centerline. Certainly the displacement changes as the waterline comes up, but not by much and any error is a positive one, as the hulls are indeed round.

You made good sense regarding over-engineering. Out of ignorance I do just that from time to time. But where gravity is a factor, (aviation, boatbuilding), the penalty for overbuilding can be burdensome, to say the least.

My attach points will have doublers and will be strategicly located where a bulkhead is welded inside the hull. I have access to the inside, and can easily reinforce as required.

Someone, (was it you?) suggested an overhead beam structure for added rigidity. An upper deck would certainly result.

I have had good results with 'space frame' structures in geodesic domes that I have built. The rigidity is great compared to the weight. And any yard ape with a mig welder and a band saw can make some strong and light-weight structure. We have a big galvanizing plant nearby that can handle large workpieces.

I am an amatuer boat builder, to be sure. Please don't take anything I write as disrespectful or dismissive. The learning experience here makes this project worthwhile. There is no profit motive. I'm no fat rat, either. I'm a very succesful worker bee, looking to get out of the machine gun business and into something with a calmer amusement quotient.

Ryland Fleet

www.tanstaafl.biz

 

I have done what you have done many a time, but not on something so big. The problems that be far worse. A major problems is if you want charter or run any kind of commercial operations you will need insurance and/or approval from some government agency, they don't like home-builts specially without engineering design/approvals. Something to think about now. Trust me they can get very anal about the smallest thing, better to check the requirements first. They can range from electrical wiring, hull thickness, to the toilets, to the filters, to materials, to buoyancy/stability analysis.

 

I'm not building this for any reason other than to amuse me, the journey and the destination. But you make a good point about commercial regulations. And there is no need to limit the vessel's capability if a few compliance items need addressing up front. What you said has value.

I stumbled into this forum because forums worked so well in the military arms business. Can the number of posts here equate in some way to credibility?

I'm incredible here. (that is, not credible) I have yet to be of value to the forum. So far, I am a taker, one that receives. I appreciate the minds here that will humor me, and at some point, I will be the expert in some field that will be of value to the forum. Like machine tools, (Expert!) or God willing, boat building. Anyway, thanks.

Ryland

"Expert Since 10 AM"


Thanks.

 

Umm, no.

Yup, me, too. (I'm fascinated by your project simply because I'm a paddlewheel nut.)

 

Ryland, did you ever come up with a deck plan for your landing craft yet? By chance I ran across an old thread for a boat that's about the same size. It's a houseboat vs. landing craft concept, but may be worth getting in touch with the person who built it (Matt D.).

http://www.boatdesign.net/forums/boat-design/houseboat-pontoon-19935.html

 

Thanks. Now that I have a loaded waterline, I will establish the beam dimension and configure the deck/connection scheme. I know it needs to be wider than I have it laid out now. Tomorrow we will move the furniture. Somewhere around 25' outside is where I think the happy spot will be. I have a swing bridge down river that opens to 28 feet.

Ryland

 

A 25ft beam is a bit wide if you ask me unless you never plan to trailer it down the road (will be as wide as the entire road). Another factor is handling such a wide craft in high winds & currents. If under-powered you'll have a rough time putting her where she needs to go. Fuel efficiency & range are other considerations (wider = less fuel efficient = less range).

 

width/drag

I'm not sure I understand how width makes it less efficient. The contact of the hulls with the water won't change with beam dimension, aside from a minor effect from more deck materials. Unless I just don't get it.


Ryland

 

Hydrodynamically, the drag will be the same. Aerodynammically, with the boat being wider it will have increased drag caused by your wide deck structure and any objects on top of the deck. It's basically creating a lot more surface area the wind can catch.

If you're not sure how it will perform after being rigged out I would suggest posting some sketches and get some input. Include some info on the type of propulsion and the area the boat will be operated in as well (river, lake, intercoastal waterway, etc.). It might save you some $$$$ on wasted materials and address safety concerns during testing.

 

Wider makes it more stable but more finicky about weight distribution. you can disassemble to move by truck. The wider the heavier also.