[windcol Posts: 24 Status: Offline]

Ian, good day,


Thanks for that, it's just the sort of guidance needed to move me forward with the jumble of ideas that have been in my head for around 50 years, some have dimmed over this time and others been swayed by experiences along the way! If i am to get this project off the ground this year all info such as you have provided is welcome, Thanks again


Colin 

[Ian Brooks Posts: 3598 Status: Offline]

Hi

When we planned the Otter project we looked extensively at the available core materials, both experimentally and theoretically. The theory looked at the cost/performance ratio of the finished sandwich structure. There are better core materials than we used, but what we chose gave the best cost/performance. It's not the cheapest nor the most expensive but it is the best for a reasonable price.

Key properties for a core material are shear strength, ductility and delamination resistance. We've seen in the testing of the Otters that our choice was well justified, there's been no delamination, the ductility provides damage tolerance when whacked by a boulder or the like  and the hull has stood up to some pretty ferocious conditions.

Foam composites are the way forward! Lighter, stronger and more cost effective than pretty much anything else.

Ian

[John Robertson Posts: 5272 Status: Offline WWW]

I tested a load of core materials years ago but, like everything, it's always a compromise involving a variety of conflicting requirements.  Polyethylene cores are quite good but difficult to repair (and quite expensive).  Bottom line is that the hull only needs to be stiff enough to do the job - any additional stiffness just adds cost with no usable performance benefit.  The best designs are  always just "good enough" otherwise it's a bad design because it cost more than it should to produce.   In theory using stiffer cores could reduce the structure but, in reality, it's quite difficult to reduce a box hull as there is no conventional internal structure in the first place.

Heavy isn't a good idea as many have found to their cost.  It increases mass and cushion pressure which, in turn, negatively impacts on almost every performance parameter.  Playing around with the design calculator is an easy way to see this in action.

[windcol Posts: 24 Status: Offline]

John, Thank you for your thoughts, I am familiar with composite foam build techniques, aka boat building, two brothers in that industry! but am surprised that more use of stiffer cores , nomex or similar have not become more apparent particularly around structural areas. didn't you yourself look at nylon honeycomb once?
I have looked at Sevtecs designs and superwedge but wonder if a craft being built a little heavier would actually be a bit more controllable.
Of course simple build methods are important for the home builder but a little time taken to learn a new skill is always worthwhile is my philosophy.
I shall look at the designer as you suggest and put my rusty CAD skills to use to see if i can garner a working plan in the not too distant future hopefully.
Thanks once again all thoughts gratefully accepted.
regards.   

[John Robertson Posts: 5272 Status: Offline WWW]

First place to start would be the Hovercraft Designer on this site - that'll reliably get you to a working platform with no risk.  You'll need a rough spec for what you want the craft to do and what you want to use it for.

As to composites - probably more than half the self-build craft out there use foam core composites (with good reason as its the lightest and strongest way to build a hull - certainly for the home constructor).  Welded ali is beyond most peoples capabilities I suspect.  By today's standards, the Tiger 12 is pretty noisy (maybe a bit unfair as it was designed in the 1960's!).  Diesel is a personal choice - the installation and maintenance issues can be quite significant and the fuel savings not as great as you might expect (the fuel is heavy so for a small craft it can all be a bit marginal when combined with the extra hardware needed).  Care is needed if using car engines as they aren't rated for continuous power use (usually a de-rate of at least 50% is needed to ensure reliability and longevity).

[windcol Posts: 24 Status: Offline]

Just starting to think about building a craft that has been on the drawing board in my head since i was 15. Now too long in the tooth 65, and having just finished a five year barn conversion in sunny west wales want something to fill the time etc.
Apprentice at Fleetlands all those years ago and was involved at one stage in the overhaul of the ishtu SRN 5 from Deadalus also spent some time drifting around various locations around the solent poking about around Hovercraft. Never made the dream of becoming full time hovercraft pilot but have flown helicopters, and fixed wing aircraft including some of the modern Boeings having eventually settled into a career, as a licenced aircraft engineer.
Why am i telling all this? well i am pretty handy at working in most materials, but am wondering why i am not seeing more modern designs in the craft being produced, why no composite builds with welded alloy Hulls (vis Tiger 12 one of the quietest commercial craft built) and maybe foam or kevlar/nomex decks. and why no lightweight diesels being used such as hondas tec engines.
Thought please would help to sort my head full of ideas into something i can work with to get my project off the ground so to speak. [size=78%]    [/size]