Calculating draft and other complications

[HUM469]

Ok, well I have been checking out the posts here for about a day, and so far have managed to miss the information that I came to find. Well to be honest I have found a couple of helpful bits, but I have a project which is rapidly becoming so complicated, I doubt there is anything that would specifically apply to me, so I hope a few of you are willing to provide some input.

The complicated part comes from the fact that I am not building a boat. While I have sailed a lot, and know how to opporate a boat, I can fix systems and such, I know nothing about how to design a hull. But I am building an expedition RV. Confused yet?

I started out intending to build a trail capable RV that I could explore Alaska as well as central and South America by land similar to the way I explored their coasts by boat a few years back. The machine is being completely designed and built from the ground up as I could not find anything close to what I wanted, and I thought I was all set. The trouble came when last week a friend pointed out the latest amphibian machines out there (the Gibbs Technology site which talks about working on any scale), and there is even an RV (Terrawind I think?) that is capable of doubling as a house boat. Now I had already designed the chassis to be completely sealed as I wanted to be able to ford something like 48" river crossings, but the more I think about it, the the more I want to motor on the occasional river or lake.

The things I do not know then; first and most importantly, how to figure out what draft will be? If I build a system similar to the Gibbs setup (and I have plenty of room in the wheel wells as already designed for suspension travel), I would have about 18 inches or so from the bottom of the hull, to the bottom edge of the door. Will I draft less than that? The truck is going to be about 33 feet long, by just under 8 feet wide. Based on the current shape though, not all that length would be in the water line at 18 inches, I think the length at water line would be something closer to 25 feet.

The second question I have is stability? Assuming an 18 inch draft, I would have at least 9 feet above water on an 8 foot width. To complicate matters even more, much of the underside will be closer to 5 feet or less as an unbroken width because of the wheel wells and their need to accomodate 44" X 12" truck tires. Will these breaks affect the stability at all, after all, there will be 8 of them? A possible solution is the idea of pontoons, the terrawind mentions having them so that the slide outs work. On mine, nearly the whole side will slide out, so I could even have the inflatable pontoons mounted to drop out from under the slideouts. This would give a beam of about 12 feet, would that help, or even work? At least one thing I have going for me is that the weight is almost all low. I do not have a sure weight yet as I am still in teh design phase (something like 25,000lbs dry), but the design is such that all the drive train, batteries, tanks, and other heavy items are in the bottom 2.5 feet of the chassis. The upper body we are building out of composites with honeycomb structures for both insulation and weight savings. The goal is to have the center of gravity at or near 36 inches off the ground, (or 12 inches above the bottom of the hull).

The final question is what about hull shape? I understand that a sharp V or center ridge helps in cutting through the water and improves inline stability. I also understand the bare basics of planing. My interest is not to go all that fast, and I know that it will be even less likely if I am having to do the pontoon thing, but would I be fine with a flat bottom "swamp boat" type, or should I look into "V"ing the bottom a bit more. After all, I want it to run efficiently, and be able to motor up stream at least in smooth water on rivers like the lower Colorado, Amazon, and some of the Alaska rivers. Of course here is where shallow draft will have to be balanced with stability. But then again if it got too shallow, I could always lower the wheels....

Sorry for the long post, but if a few people could point me to the best resources, share their opinions, or otherwise give me some rules of thumb to help, that would be greatly appreciated.

[Tad]

Hum....

If you are serious you need a naval architect.

If you have a box 33' by 8' by, say .9 (reduction for wheel wells) = 237.6 sq. ft. Weight of 28,000 pounds / 62 lbs cubic ft for fresh water = 451 cubic ft.
451/237.6 = 1.9' Draft.

Tad

[Toot]

I'll just wish you the best of luck as I don't know enough to help you and I also think it's a rather silly idea. (But silly can be good sometimes, so more power to ya! Lots and lots more power! You're gonna need it to push that thing if you are even contemplating getting it to plane. Heck, the very thought that you mentioned it gives me pause...)

[HUM469]

Thank you Tad, that simple calculation tells me a lot about the idea already. I think that being nearly 22" means a lot of modification to the machine's current design. Not that I would not do it, but it gives me a more rational basis from which to consider it. I am still in the process of trying to shave weight as well, so it allows me to at least ball park it when I get closer to a final design. I know full well that the best bet is to higher a skilled pro, but I wanted to get an idea if it would be possible without impacting other mission parameters before putting out the money.

And speaking of money, is that part of the reason you think it sounds a bit silly Toot? No, I am not offended at all, and not intending to argue, but I am curious if you think the whole thing is daft, or just the fact that I might want to make it float. A few years ago, I would have thought the whole thing was nuts. I hated all the big, expensive RV buses that would hold me up on the highways all across the country here. I was "superior" in my boat out in the ocean where I was not impeding anyone. But then I saw the European take on the RV in the form of Action Mobile in Austria, and somehow it just stuck with me. But all the same, I wasn't so sure I wanted to spend the million and a half that they wanted for one, even though they could not gauruntee that I could import it to the US successfully. All the same though, I have no intentions at all getting it up on plane unless that is the only way to go up stream on smoother rivers. I mentioned planing only because I understand the difference between hulls designed for displacement vs. plane travel, but no, even with a jet drive, I would expect to plough on through at a couple knots against the current, a couple more with it.

[HUM469]

I meant to ask Tad, do you only calculate the area of the "foot print" if you will, and not the actual area of the hull? After all, a pair of planes for a shallow V would add up to a surface area than a single flat bottom, correct?

[Tad]

Hum,

I was calculating the volume of a cube, assuming a vertical sided box and not knowing one dimension. Solve for x and all that....

At a given weight (= displacement) a vee bottom will float deeper than a cube.

Tad

[HUM469]

Thank you again Tad, you have been a major help in equiping me to determine if I want to invest in professional help. As intriguing as the idea is to me, I am not yet sure if it is worth it. Now to sit back down at the drafting table and see if I still like the idea after a bit of work here...

[Ike]

Definitely hire a professional, or go to libray and do a lot of study. You need to understand the basic principles of displacement, Wetted surface, immersed volume, static stability and so on. By the way, what you are talking about is certainly not imossible, it's been done before. Saw one on TV just a few days ago. The guy didn't start from the ground up though. He converted an existing military vehicle. Speaking of that, one of the Coast Guard units I worked at had a big truck that would run through 3 or four feet of water. It wouldn't float by any means but all the necessary electrics and mechanical parts were sealed, and it had huge ballon tires.

So what I'm saying is, you can do it, but you're going to need some help with the boat part.

[Seafarer24]

Heh, my almost-stock '89 Jeep Wrangler would go through 3-4' of water with little more than a snorkel and a sealed distributor. Personally, I don't advise going through water over neck-deep.

Anyhow the pontoons on the Terra Wind look like they are there to provide stability- not floatation. They also look like they take a little time to set up. Extention and inflation needs to happen before entering the water.

[HUM469]

Ike, I agree about the professional part. If I do go this route, I certainly will. The last thing I would need is to sink in the middle of the Amazon, 800 miles from the nearest help. I myself went to school for engineering, but I did not study anything naval. I am sure I could understand it, and possibly make a decent design after a time, but this is something I don't want to risk. I also know all about forging, I have built something like 30 4x4's of extreme capability, and have forged more than 4 feet on several occasions. I agree that I would not recomend deep water forging, but forging capabilities were planned from the first. I only became interested in floating when I saw the Aquada and Terra Wind videos.

Seafarer24, looking at the Terra Wind closer, I see what you mean. I am not concerned with the set up time per-se. After all, the point of the trip is to take my time. However, based on the projected weight at this time, it looks like I am going to draft too much, therefor requiring me to move the doors up higher, and things will get rather complicated from there. Either I will need a different pontoon setup that does provide some floatation, reduce the weight, or give up on this particular idea. Moving the doors up will have more impact than I would like it to, so if that is what it comes down to, I will give up. I should have some more numbers on equipement weights in the next couple days.

Should I go forward with it, anyone care to recommend a good naval architect or designer to help me with this idea?

[CDBarry]

A key element of swimming or amphib vehicles (there is a difference - the former are less capable and generally require special prepapration to enter the water), is to look carefully at entrance into the water, especially from unprepared banks. This is a classic ship launch and unless properly done the vehicle will downflood prior to rotating from the shore trim to the floating trim.

[HUM469]

CDBarry, would you elaborate please? I believe I get what you are saying, but I must confess a certain lack of knowledge or experience in this way. All I am familiar with are the amphibious vehicles, but past and present and it seems that they all drive right into the water and float away. Some clearly better than others, but my thoughts are to opperate almost identicle to the Aquada with Gibbs type technology. After all, they at least claim that it can be scalled up to almost any size. It looks to me like the Terra Wind also just drives into the water, although I cannot seem to find any video anymore. The Gibbs videos are very clear however, on how their technology works, and I like it.