Please sanity-check my 5'6 dinghy design!

I would like to sanity-check my first design with you folks. I will be building a dinghy to go on my tiny 18ft trailer sailer in the next 3 weeks, come what may.

I have the following design constraints:
- cheap as possible
- *really* easy to build
- carries 2 (we're ~150kg)
- won't flood in rough-ish costal water
- max length 5'6
- max width 3'6

My current design is a flat bottomed pram. It is fore/aft symmetrical. It has extreme rocker: 7" deep at the centre, so that the waterline at 150kg sits at the join between the bottom and the bow and stern transoms. The sides are 14" deep, and at 150kg there is 7" of freeboard at the lowest point in the centre. The bow and stern reach 7" higher. There will be a 7" deep skeg on the back half of the hull.

I will build it from cheap 6mm ply, following the construction method of the 'Elegant Punt', i.e. external rails, glue and screw.

Does this sound sensible enough, given the design constraints? Can you see any refinements?

Here is a paper model to help you visualise:

 

A small inflatable would seems more suitable.

 

Thanks Mr. E. I am not keen on inflatables: they are wider than the spec when inflated, and not ready to go if deflated. Plus I would like to make a boat.

Is the design problematic somehow?

 

Where are you going to put it ? In the trailer-sailer ? If you tow it, it will act as a brake much beyond 3 knots.

 

It's a pram.

As for the design, if you intend to build a plywood boat you might be profited by using Calrson Boat Designer. It is free and can be downloaded from the links here: Free or Low Cost Yacht Design Software : the Boat Design and Boatbuilding Directory https://www.boatdesign.net/Directory/Software/Free_or_Low_Cost/

Good news is the download comes with lots of sample hulls to compare to. It will also do so basic calculations.

 

I am hoping to attach high on the transom, on its side with the open top facing forward.

 

Thanks Rurudyne, I will take a look.

 

It is very tiny, and any mention of "rough-ish" water, would make me nervous. But, with caution, might be OK, if only because it has a little bit of beam to help.

 

For me around 8 feet is about as small as it gets. My dinghy is 7' 11", weighs 55 lbs. can hold two without danger of capsize or sinking and is easily transported. It was easily built of plywood using stitch and glue construction. The whole boat (not counting labor) cost about $300.00

 

Mark

I'm no naval architect, but my gut feeling is your boat is too small to be safe for a tender for two. I have owned a 16' winklebrig for 10 years, and struggled with the same problem. I ended up using a small inflatable as a tender for the times I could not moor to a bank or pontoon, or anchor shallow enough to wade ashore. Have a look at Hannu's boatyard; he specialises in small, very simple boats, but even he baulks at 5 foot boats for anything other than beach toys. He's built them down to 4 foot. For a boat that size, I would say some buoyancy is vital - bags or built in.

with very small boats, (according to Hannu), much of the difficulty can also be in the tight bending radii needed. perhaps 4mm ply for some elements.

Some of the geodeisic aerolite boats may be of interest; super light and very beautiful, though of limited robustness, and a rather more technical build.

Might your desire to build a boat be a separate project from the need for a tender?

Might an inflatable SUP be a solution for the tender?

Adrian

 

A couple of people who are young and fit and nimble might be able to cope with it, but otherwise not, I'd say.

 

this is almost like a very small coracle. Make sure you wear your PFD.
I had a friend with a 17' daysailer. He carried a folding boat- worked very well. If no luck at marine stores, check out hunting stores etc.

 

That seems like a good suggestion, the folding boat. 5' 6 is a bath-tub !

 

Butcher, Baker and Candlestick Maker?

 

even bath tubs are longer than that,