Avon Ribcrest

[dancrane]

Daft project I know, and I don't care. If it doesn't make sense or it's too much work, I'll drop it by midsummer. Possibly much sooner.

I could just go ahead and do it and present photos of the finished job, but I know little about working with ply so I'll benefit from comments along the way.

I have an old Avon Redcrest and an old Redstart. The project is to give one of them a removable plywood RIB-style hull to make the most of my Tohatsu 3.5.

It's tempting to use the smaller 8ft Redstart which I think holds pressure better. Plus, the Redstart is missing the loops for the standard outboard bracket so cannot otherwise take an engine, and being smaller, may go faster with a hard hull beneath.

Three years back I thought how to attach the outboard to my Osprey dinghy without damaging or altering the stern, which wasn't built to carry an engine. I created a solution using 1.5mm stainless sheet (with butyl rubber padding) to spread the engine's weight and thrust rather than damaging the only available contact area, above the transom.

If there was a clever bit, it was using 90° stainless exhaust sections to turn the pipes supporting the weight, into verticals that hold the wood bracket.

I admit, I don't know how clever that was because I don't know if 1.5mm-thick 304 exhaust pipe is strong enough for that job. Your views are welcome.

I had to scrap the Osprey before trying the o/b on it, but I want to repurpose the pipe elbows & bracket to connect the engine to the ply hull for the Avon. I think any other connection would involve lots of solid wood. Below is the bracket, now upturned because the pipes will be inside the flat top of the rigid hull.

I'm thinking I'll drill out the rivets holding that sheet metalwork to the exhaust elbows, and rivet on a new stainless sheet to sheath the ply it will connect to.

What I don't know is the exact shape the ply hull should take. I guess it needs a flat top (rising forward, so the hard floor includes the raised bow of the the rubber dinghy). That central 'deck' can include a robust portion under which the engine bracket pipework can be bolted.

Then (I'm speculating) I could screw or glue pre-shaped, pre-cut 20mm softwood planks to the underside of the plywood 'top', along their long edges, so their bottom edges provide the underwater curves of the hull...and a skin of Robbins 5mm Super Elite (or something cheaper) can form the outer skin.

But what shape should that outer be? Somebody recently said on the forum that flatter is better for speed, but shouldn't I incorporate some V? How much V? Is it okay with a pronounced V forward, becoming flatter aft?

And should the hard hull be squared-off at the point where the engine attaches, or should it reflect the round-tail shape of the dinghy?

Actually I'm going out to the shed after I post this, and by the time I come back I may have decided to forget it...unless anyone thinks I should go ahead?
.

 

[Sea Change]

I'm no naval architect but as I understand it, with no vee you'll have no directional stability, and the boat is highly likely to spin out. It will also slam in to every wave which will be very uncomfortable and possibly dangerous.

I'm not sure how you would add a vee shape without cutting away the existing floor of the dinghy, which would seem a bit of a shame.

You can pick up a tatty RIB quite cheaply... I got an old Rover 310 for about £150 a few years ago. That might not be much more than what you're proposing to spend on ply, epoxy, and hypalon adhesive.

 

[dancrane]

That's a really good point.

I had the opening post on my laptop since the weekend, trying to work out if I even had the energy to raise the question.

I'd much prefer an old Avon Rover but despite watching outlets, I've never seen one in a useable condition, anywhere near as cheap as yours was.

But the materials for my DIY-bodge-plan would indeed be costly, in money and in wasted summertime.

There was also the fact that I already have Avons cluttering the garage, and I'm reluctant to get another, but I certainly could sell as well as buy.

Yep, I think you persuaded me. I'll defer the conversion plan till I've given up hope of finding a Rover.
.

 

[Sea Change]

That's a really good point.

I had the opening post on my laptop since the weekend, trying to work out if I even had the energy to raise the question.

I'd much prefer an old Avon Rover but despite watching outlets, I've never seen one in a useable condition, anywhere near as cheap as yours was.

But the materials for my DIY-bodge-plan would indeed be costly, in money and in wasted summertime.

There was also the fact that I already have Avons cluttering the garage, and I'm reluctant to get another, but I certainly could sell as well as buy.

Yep, I think you persuaded me. I'll defer the conversion plan till I've given up hope of finding a Rover.
.

If you can come up to Skye you can have mine!

 

[dancrane]

PM sent.

 

[Daydream believer]

Find some online dinghy plans & convert the bottom parts to suit your required shape by some computer trickery enlarging or shrinking as necessary

 

[alahol2]

if you turn the blowup seat of the Redstart or Redcrest through 90 deg you get a rudimentary keel. You can also try putting an old oar along the centre of the floor, under the seats, to again create a kind of keel. Both assist with engine propulsion.
If you want a project, maybe fabricate a more shapely wooden keel to sit on the floor wedged under the air tanks...

 

[johnalison]

Since the floor is not needed for buoyancy, it wouldn't do any great harm to split the floor down the centre to accommodate a V hull, so long as you seal the join enough to stop your feet getting wet(ter). If your project fails, it might make it hard to sell the Avon later though.

 

[dancrane]

You can try putting an old oar along the centre of the floor, under the seats, to again create a kind of keel. Both assist with engine propulsion.

Interesting idea, Alahol. I took time to make close-fitting floorboards for both the Redcrest and Redstart, and I've seen someone use a "pool noodle" as a way to form a soft central keel under the hard floor of a dinghy.

What I can't say is whether the benefit to the underwater form, of floorboards, will be lost or made less good by "re-rubberising" the floor when a false keel is set up along the centre, so the floor is only rigid to the users' feet. I mean, the part of the hull that contacts the water (and whose flexibility reduces the boat's performance) was at least rigid under floorboards, but won't be if a central 'keel' lowers the sides away from the floorboards, like the walls either side of a tent's ridgepole.

 

[OCuea]

If you have so many ordinary avons, how about buying the plywood floor panels or making them and cutting down the centre line and adding a bit of copper pipe foam cladding diwn the centre line to create a bit of a V keel? Not perfect but may work and save buying another Avon.

 

[dancrane]

Thanks. I actually made floorboards for both boats. I really need to try the motor on one of them with the rigid floor, before I change anything else.

 

[Refueler]

Thanks. I actually made floorboards for both boats. I really need to try the motor on one of them with the rigid floor, before I change anything else.

With the designed boards ... was OK ... but when the two piece ply floor was fitted - the dinghy was transformed ... no longer was standing in it like walking a tightrope .. the fabric floor under the ply still flapped when boat was moving across the water ... but it felt more surefooted. I never thought it needed a 'keel'

Wife and I rode some pretty bad weather in harbours getting ashore and the ply floor made such a difference ...

But boy is it heavy ... once that floor is in - its a permanent 'in water' dinghy .. I could cut a lighter floor - basically the water under the dinghy would support it ...

 

[Boathook]

Years ago I had an avon redstart (the bigger one out of the 2) and installing the ply floor made a big difference as to how the dinghy went / behaved, etc.

Putting the boards in was another matter and nearly impossible to do it on board, so they weren't used to often I seem to remember.

 

[Refueler]

I have to sort a leaking valve ... it has the later sprung loaded valves - but one seems not to seal very well .. hope a bit of soapy water will wash whatever is making seal not 'seal'.

Another hard transom inflatable I have : Bush ... is excellent with inflated floor etc. - but that has a leaking seam at bow and also a non sealing valve ..

The one inflatable I have that does not leak - is a copy of the Soviet 2 man military dinghy .. but that has no way to mount an engine .. maybe an E-motor could be ... but certainly no fuel motor.