Small Steel Workboat Design

You just *cannot* get 10 knots plus from a 15' WL displacement hull. Nobody that I'm aware of ever has so I really doubt you'll be the first.

My 12m sailboat - 30' WL - has a max design speed of 7-8 knots before the power required starts going hyperbolic. That design was done by a naval architect who has done literally hundreds of boats designs.

If you must have that speed with that WL, then you need a planing or semi-planing hull. Then you need to deal with the weight & stability aspects.

Alternatively you need to increase the WL to the point where you can achieve the hull speed you want at a power budget that makes some sense.

I understand your reasons for wanting to use steel but that doesn't invalidate reality. There's a reason why small heavy steel workboats that are also fast aren't built and aluminium ones are built, and it's staring you in the face.

One or more of your length, speed and material criteria has to change.

PDW

 

Of course a small steel Catamaran, or pontoon boat, could easily cruise at 10kts......

 

Sure. Pity about the load carrying requirements in the SOR, though....

PDW

 

The added complication is the OP intends to use either an 8hp or 15hp outboard as propulsion, which definitely rules out anything other than displacement speed. So you either have to go slow, or have a much larger motor. It is pretty true to say in powered craft you can have any two of Speed, Comfort, and Economy of Operation, relatively easily, but having all three is asking too much, just plain greedy in fact !

 

"Oyster punts" are a popular choice here for well... oyster farmers & waterfront contractors like this...

http://www.excelmarine.net.au/2011/2011_workPunts_Oyster.php

They were built in timber, then ply & now ali generally, possible in steel but with the weight penalty which would "tax" the payload. At a certain point of loading/power/sea state planing becomes difficult!

http://mammothcustom.com.au/photos?id=1

http://www.alltravels.com/australia/new-south-wales/soldiers-point/photos/current-photo-41907543

Jeff

 

Exactly the thing I was thinking of when I suggested an ally boat, in fact.

PDW

 

Such is the nature of humanity..............

Of course Oz needs two boats in fact....... the sports model to zip around in that doubles as the push boat when he needs to shift a barge load.......

kinda like having a really shiny utility & a bashed up trailer for the serious work.

Jeff.

 

Ok, I get it.
A steel boat of the size I need will not give me the speed I want
(Not with the propulsion I want to use!)

I've been looking at Ali planing boats, the ones I've seen over here are wafer thin, can't imagine them taking much abuse!
There also pretty expensive £1500 +
I'm also a bit nervous with a thin gauge "welded" Ali boat.

If I had enough money and time, I'd buy the Ali plate and hard rivet one up.
I don't have either though,

I may still build a 14 ft displacement from steel, I've done a few drawings.
I will post them on here later, hopefully to be demolished before I embark on another costly mistake!

I really appreciate all of your comments, both encouraging and discouraging,
All very helpful!

 

Maybe you can revisit ply, for 1/2" it's roughly 1/3- 2/5ths the weight of 3mm plate & would make a reasonably tough 15 footer, add some hardwood sponsons/rub rails & a few car tyres down the side & it's well fendered. You can probably still use your HB steel epoxy paints available & get some pace unloaded with the 15HP with the right hull form.
Jeff.

 

There's a vague possibility.... & quite unlikely in steel.... that you "could" have 2 slim easily driven hulls placed beneath a "barge with swim end" & whilst in light mode & smooth water zoom around with lowish HP & when loaded heavily use the box like barge to carry your load in a more stately manner...........................

Jeff.

 

That may well be about the only way possible ! A similar thought occurred to me, except rather than two "sponsons", I was thinking of a single, central, fairly deep, canoe shaped thing, superimposed on a flat bottomed punt, so that most of the displacement was carried by the narrow centre part. But with the weight of steel, and the extra material required, it might not work out.

 

Jeff and Mr Efficiency, thanks.
I love the sound of this!
Im a bit intimidated with the idea of designing it, my knowledge of N.A is very, very, limited. Im sure I dont have to tell you

I do enjoy a challenge though, so I will do some research.


should the sponsons be square bottomed or more rounded like canoes?
Also, at 15ft length, is there an optimum beam to length ratio for this type of design?



As far as ply and epoxy, It is tempting to use this as Im very familiar (and competent) with the materials and the build process.

I was intially thinking of building something like John Welsford's "Trover".

The problem I have had with ply/epoxy boats in my environment, is that they "don't last", the abrasion problem is so acute.

I could use several layers of fabric, and many kg's of epoxy to overcome this (to a degree) fendering etc, but that will be very expensive.
Also ive had boats crushed between barges, trapped under piers etc.
Steel deforms- not a big problem, wood cracks- big problem!

I fear that if I do build from ply/epoxy, then I will back in this situation one year from now!

 

ok, Im going to start with the sponsons (2 of them). draw them up, work out weight, size, displacement etc, then I should be able to see what weight of hull is possible to put on top.

so, im going for 15ft length

can anyone advise me please on a efficient length to beam ratio for the sponsons?

also shape? I guess for planing relatively flat wide bottom?

I know I probably sound like I want someone to design it for me, I really don't.
Just a couple of pointers on the basics to start me off would be great.

Thanks in anticipation

Oz

 

And now for something totally different......

In the far east they take a bunch of plastic pipes, like gutter down pipes, and tie them together to make very buoyant rafts with an long shaft outboard driving the contraption along.....some in Thailand seem to go at a brake neck speed with big engines.
Need more buoyancy, just add more pipes.
Form underwater hull form by stacking pipes in vee.
Alter length by ordering longer pipes, 6mtr seems standard.
Just seal ends with standard blanking caps.
pull up bow section to get nice planing section lift.
Use commercial banding machine to tightly wrap pipes.

You can even make your own long tail outboard with an old auto engine.

Good luck, and fair winds. [you'll need it]

 

I remember seeing some info on a small dinghy that used the single slimmer body underneath, with a small outboard & one up it was supposed to get up & go & when loaded up the rest took up the "weight". With steel the proposal is doomed, any narrow slipery low resistance underslung hulls simply increase the
square meters of a heavy material...... I think the returns are diminishing to negative in that material. I think some tourist barges in the Kimberly region use a similar approach successfully in ali, they do the passenger carrying in sheltered water in barge loaded style but are returned by sea for the wet/dry/off season/service at speed unloaded on sponsons.
Jeff