Aluminium dinghy design

JSYmartini

JSYmartini

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Hi People

I currently have a naval architect engaged to design a 2.4m (8ft) aluminium dinghy/harbour tender which I plan to build one for myself and perhaps build more if there's demand locally.

They've just been testing hydrostatics this morning and have come back with a max payload figure of 180kg for stability and freeboard. No doubt it would be able to handle a bit more and this is with a built in safety margin.

But looking around at other plastic or grp tenders on the market, 180kg does seem a bit weak, I just wondered what others thought, could this turn out to be useless? I'm a bit concerned it could be really unstable with a bit of weight on board but without looking at the underwater profile it's difficult to comment.

Any thoughts or comments welcome
 
on 8ft, that is only two people without bags.

I can see the incentive of building it yourself, but commercial ali dinks have a proper factory with controlled conditions to ensure weld quality - unless you have too ?

If you really want to go ali, then I'd imitate the design of a RIB but with the chines that ali dictates at that scale. A couple of long tanks running fore and aft will give structural stability as well as load capacity and reserve buoyancy.

My 9ft rubber dinghy carries 4 people, and two dogs, and doesn't feel overloaded. You will have to go some pretty large steps forward with your NA to beat that !

I remember reading an article years ago on explosive forming of aluminium to produce curved hulls for dinghies. Perhaps that's an idea !
 
JSYmartini said:
I currently have a naval architect engaged to design a 2.4m (8ft) aluminium dinghy/harbour tender which I plan to build one for myself and perhaps build more if there's demand locally.
Click to expand...

There are various aluminium dinghies on the market already. 180-200kg would seem to be the right sort of max load for an 8ft version.
 
Hi Sarabande

I've been in the welding and fabrication trade since I left school 22 years ago so if weld quality is going to be an issue then I've got serious problems :o

Inflatables are on another level regarding payload and cant really be compared to anything else. What we're looking for here is a tender that will sit on a pontoon for it's entire life without any kind of maintenance at all. Lets face it, harbour tenders are way down near the bottom of most of our "to do" lists and are generally pretty neglected. A hypalon inflatable wont last forever and will often need pumping up before use and, like the pvc sport yak's etc, UV will take its toll soon enough.

A well built ali dinghy should handle the rigours of being outdoors 24/7, put up with all the bashing around on a crowded dinghy pontoon and even be tied to the back of your car and dragged up the slipway or beach.

The tanks idea is a good one but it increases the size of the boat (or decreases internal volume) and would add greatly to materials and labour. Foam buoyancy tanks will be fitted under the thwart seat and corner seats aft.

A slightly larger boat would be nice but Jersey Harbours have a 2.4m size limit on the dinghy pontoons so although half the boats down there are at least 10ft I couldnt really market anything bigger for that purpose.
 
pvb said:
There are various aluminium dinghies on the market already. 180-200kg would seem to be the right sort of max load for an 8ft version.
Click to expand...

Thanks, this is a stock design thats just being tweaked to suit my needs/preferences, nothing really radical.
 
glad to know that you can weld up, down, sideways, fillets, grooves and butts ! I wasn't casting nasturtiums :)

Have you seen this page ?

http://www.wavedancer-yachtdesign.com/html/Aludink_build.html

Ok, it's a bit longer, but you can see where you might add some permanent tanks under or as part of the side decks.

I'd also add good eyes for carrying loops, and bridle plates for towing, and a waterproof, lockable, locker for e.g. flares and outboard spares. There are some neat designs by PBO's Dick Everitt for capturing oars so they don't go walkabout.

Corrosion ? What grade / spec ali?

Finish ? Painted or "au naturel" like some Ovnis?

What about insolation heating up the thwarts and sides in full sunlight ? Could be too hot to use sometimes.

I'd still go for flotation tubes well out to port and starboard to give form stability, and possibly the option of an inflatable collar to act as a fender and flotation device.

Imitating a wooden dinghy's architecture does not develop the benefits you get from using ali.

Best of luck and keep us informed on how you get along.
 
Thanks for the link, I've not seen that one before.

Mine would be quite similar but a bit more robustly built and weighing in at around 50kgs. Surprisingly they reckon it'll take up to 15hp as well.

Also she will have a 30ish mm dia tube welded to the top of the gunwales for strength and rigidity as well as to lose the sharp edge.
 
You could always add fenders strung longitudinally round each side just below the 30mm ally coaming for reserve stability, flotation
I suspect that in the battle of the dinghies all rubbing and chaffing each other wedged against a crowded pontoon, this one will 'win':D
 
Ali dinghy

I have an ali dinghy now after years of using a f/g dinghy. Mine is about 10ft long but seems to be about the same weight as good the old 8ft f/g. Ali dinghies are very popular in Oz I would say much more so than RIB.
I have on occasions had 5 people in the 10ft ali although it is a short row in shallow water. It will occasionally ship a bit of water if there are waves with a big load. Perhaps OP load rating is very conservative.
I do find that after lying in the midday sun in summer up side down that it can be too hot to lift with bare hands and you have to sit it in the water for a bit to cool down before you sit on the seats in thin shorts.
Certainly ali is the way for leaving outside all the time. But here you do need a stout chain and lock for security. good luck olewill
 
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