SWATH Transormers

[Dave Gudeman]

I've seen two different companies talking about boats that convert between SWATH configuration and shallow-draft catamaran configuration. Although one of the web pages is sketchy, it looks like they both start with a shallow-draft catamaran with an extremely high profile and then use water ballast to lower the boat into a SWATH with low profile. Here are the web pages:

http://www.stabilityyachts.com/

and

http://www.sealevelscuba.com/Website...kton/Swath.htm

I have some questions about this. Anyone have answers?

First, isn't that backwards? Don't you want high water clearance in the SWATH configuration (which is used in rough water) and low profile in a high-speed shallow-draft configuration (which is used in smooth water)? This design seems to give you low clearance in the SWATH configuration and a high profile in the cat configuration.

Second, are there any boats with a similar concept but that use mechanical reconfiguration rather than balast changes? I'm thinking of, for example, a shallow-draft trimaran with bulb-like outriggers that can be mechanically lowered to raise the center hull and transform the boat into a SWATH. If there are no such boats, is there a good engineering reason why not? -- I mean other than the obvious (and uninteresting :-) ) expense and weight of the lifting mechanism.

Third, I've read a couple of brief mentions of using hydrofoils with SWATH hulls. How would that work? Wouldn't you have to lift the entire hull from the water and then some, leading to a very high and unstable center of gravity?

Fourth, (only peripherally related, but as long as I have your attention) is it really a good idea to beach a 50-foot+ foot boat? Besides the difficulty of getting it off the beach, isn't the weight of a large boat a lot more likely to lead to hull damage on rocks?

[Ad Hoc]

Dave

to answer your questions

1) You're getting confused with the reason for the raising and lowering. The SWATH aspect is having the water level in-line with those thin struts. ie a small waterplane area. But, the problem is SWATHs are very very draggy hull forms. So to go faster, one needs convention multihulls, hence raising the hulls so that the water-level is as it would be if 'normal' catamaran hulls, ie the waterline is along the length of the hull and not on the struts.

2) define mechanically...since how else can one raise and lower the entire vessel, statically, other than by using ballast? Unless you are thinking of a complicated "lift" type system (or gearing) for raising and lowering...heavy, and prone to failure.

3) These work by using a single hull SWATH (there are variations), with little outriggers which have hydrofoils on. These provide transverse stability. Some Proa's have been designed this way too.

4) That is your call and your reason for doing it. If the hull structure is designed to take the beaching loads, then you can do what you like. Personally i wouldn't, not a 50 footer.

[Dave Gudeman]

Originally Posted by Ad Hoc

Dave

to answer your questions

1) You're getting confused with the reason for the raising and lowering. The SWATH aspect is having the water level in-line with those thin struts. ie a small waterplane area. But, the problem is SWATHs are very very draggy hull forms. So to go faster, one needs convention multihulls, hence raising the hulls so that the water-level is as it would be if 'normal' catamaran hulls, ie the waterline is along the length of the hull and not on the struts.

Thanks for the reply. I understand the reason for the two hull forms, I'm just thinking that this way of doing the transformation leads to severe compromises. When in SWATH configuration, presumably you are in that configuration because the water is rough and you want to smooth the ride. But in that configuration the profile is low so you are more likely to get pounding. When in catamaran configuration, presumably the reason is that the water is smooth (so you don't need so much clearance) and you want to go fast (there are other reasons, but this is a big one). In that configuration you want low wind resistance, but the configuration gives you high wind resistance because it raises the profile. Presumably this design requires some compromise between clearance in the SWATH configuration and profile in the cat configuration. And it seems to me that it has to be a rather severe compromise in one way or the other (or both) since the difference in depth must be on the order of three feet or so.

Originally Posted by Ad Hoc

2) define mechanically...since how else can one raise and lower the entire vessel, statically, other than by using ballast? Unless you are thinking of a complicated "lift" type system (or gearing) for raising and lowering...heavy, and prone to failure.

Actually, I was thinking of a simple lift-type system. :-) What I mean is a system that actually changes the length of the vertical supports of the outer hulls. So your answer is that it is not practical because it is more prone to failures than a pump? Presumably this is largely a matter of cost, right? If you are willing to pay enough, you can get whatever level of reliability you want.

[Ad Hoc]

Originally Posted by Dave Gudeman

But in that configuration the profile is low so you are more likely to get pounding.

AH ok, what you are referring to is the freeboard and the raft, or bridging structure clearance. Yes if this is too low, one can get slamming/pounding of the raft structure in big seas. BUT, for this to occur on a swath, the seas must be large (what sea state are you intending on going in?) and/or the clearance low. Since the motions are benign and not violent.

If this were just a normal catamaran, then the clearance under the raft should be a min of 6% of LWL and at the bow 10%LWL.

Originally Posted by Dave Gudeman

Actually, I was thinking of a simple lift-type system. :-) What I mean is a system that actually changes the length of the vertical supports of the outer hulls. So your answer is that it is not practical because it is more prone to failures than a pump?

Correct. Also costly and much heavier to install. SWATHs are extremely weight sensitive. Additionally. The ballast system for raising and lower performs the other principal reason for having ballast on board; to ensure the LCG is in the correct location.

Up here (Alaska), beaching is commonplace - especially amongst seiners, typically 58'. I sometimes operate landing craft, and I have to say it was unnerving when I first started. Though they are designed for it, they can be damaged from perching on a rock. The key to beaching the small boats that are typically FRP is to look at the bottom first and not lite on rocks. There is no nap in a gale like one when entirely aground for a tide.
Aground is the place to be when this happens!; http://www.ktuu.com/Global/story.asp?S=11603071 . They said it sustained over 150kts for the better part of the day!