weighed yourself recently?
Since my last article I arrived at the boat to find my well used and abused Honda 9 outboard had been stolen.
OK maybe I could have been more security conscious even though I am in a marina, but this must be weighed up by the damage that a determined thief possibly stealing to order can inflict on a plywood interior. Also, the cost of adding £500 outboard to the insurance policy. One must write it down to experience. Still a nice comforting pamphlet on being the victim of crime later and we are back at square one. We need power.
This has had me seriously thinking, what power- how much do I need, when do I need it? What might push a Dawncraft 25 easily along the cut on a warm sunny day may struggle uphill on the river Avon in a bit of a breeze – Actually that Honda 9 was far too little for the task - it was even known to have stopped dead trying. So, is it horse power? Bloke down the pub swears his 15 will take him anywhere, apparently horse power doesn’t really exist – it was a marketing tool by James Watt to graphically illustrate the efficiency of his new steam engine in a force that people could equate to or imagine, because in theory 9 horses on the hoof should get a Dawn craft properly bumbling along.
It's actually the twisting force that a horse can produce known as torque and manufacturers will produce a torque curve for all their engines as another marketing tool. Interestingly my trusty electric back up is rated as 1lb per foot. In my case 55 which is about maximum for a 12 volt motor. Electric motors produce nearly all their torque at zero revs – that sounds crazy but ever got a drill stuck? However, all this doesn’t matter until we know how much power we need and for this we need to work out our displacement, how much water we push out the way.
Ok brace your self – there are a number of calculations and methods you could use, none of which I can get my head around or are remotely practical to any one but a naval architect – but I assure you are well quoted in any yacht club bar. I found an online calculator which neatly came out as 2450Lb or basically a ton, I say a ton but that doesn’t include me, Mrs W, any guests, weight of engine and half the junk on board - all of which will increase our weight and thus displacement and thus require more power.
Basically, I don’t know what we weigh any more than any other boat owner and without this I can’t calculate engine size other than recommendations and best guess on what is available.
We could go back in time and assume that when Dawn Craft made it they had all this scribbled out with the aid of a slide rule - According to an original on line brochure Dawn Treader naked or without junk weighs 32 CWT or 1.6 tons So much for the on line calculator then!! Originally mine came with an in-board petrol BMC driving the Z drive - That had to be 30hp plus to produce enough torque to drive that prop, the diesel I took out was 25 hp. Suddenly who ever nicked my hard-worked Honda 9 is in for a shock – It's properly worn out.
So what did Dawncraft originally use? – I have researched as much as possible, the brochure says a range of engines to suit your needs. Hmm may be that is salesman's talk for a range of engines to suit your pocket or ones that they had arranged a decent discount for using from the supplier. It would also appear from the brochure that outboards were for the cheaper end of the market- this quote from the original brochure for a 25 with out-board well built in where the inboard would be states ” you’d think this boat was an inboard craft as she cruises past with no engine in sight”.
I need to to digress here for a moment, during the research for this article I found a number of surveyor reports on sunken 25s of this design, the sides of your engine bay are the same plywood that the inboard is built from, there is a small glass fibre strip of about 5 inches at the water line. I am not saying that this was wrong, I have sailed plywood boats across the North Sea but 30 years of weather, water and being forgotten about has turned many of these to mush, suddenly with a few people on board the glass strip is under water allowing ingress and sinking. Be warned.
Back to the issue in hand we can now look at 1970s out-board engines which were nearly all two strokes. Devil engines that can do more environmental damage than the Exon Valdez, noisy, smelly, won't tick over, too fast for the cut – or so the bloke down the pub said. OK I will admit that I have an old seagull forty that is possessed by evil spirits that want to take your soul to Davy Jones locker, but they reigned supreme for 30 years. Lighter, fewer moving parts etc the later models used petrol injection systems making them very fuel efficient and no oil residues. As for the noise, you measure this in decibels, it would seem the level of noise from the two strokes is not appreciably more than 4 strokes – it is the pitch of the noise, for those with Music O level 4 strokes are some where around D and two strokes F sharp to A. So why the change to 4 strokes? Basically, it was the European craft directive environmental emissions that forced engine manufacturers to develop better and lighter 4 strokes.
Before I am keel hauled for even thinking about replacing my engine with a modern 2 stroke, did you know that 2 stroke has nothing to do with the fuel it burns and that most large ships use diesel 2 strokes – its to do with engine design and the all-important torque curve. The twisting effect on the propeller.
Let’s look at hulls and displacements a moment. We have a displacement hull, it stays in the water pushing its 1.6 tons out the way , in fact we can measure this in 1lbs per foot required from 1 to 6.7 knots the latter being known as hull speed, it doesn’t matter what extra power you have, the weight of water being pushed out the way becomes too great to go any faster. So, a thirty-horse power engine will get Dawn Treader up to 6.7 knots in seconds but won’t go any faster - The propeller will slip cavitate and consume itself in angry bubbles.
The next hull shape is a planing hull- this climbs out the water on its own bow wave leaving less hull, less drag and thus more speed, the extreme being racing boats of which the prop is the only thing in the water- this is where two strokes reigned supreme. Fast revving engines that spin the prop at obscene speed producing tons of wash whilst pulling water skiers – I have checked, and water skiing doesn’t seem to be banned on the canal, it's covered by the 4-knot limit. There is an in-between hull called semi displacement, the hull will climb out the water and go faster and many estuary cruisers use this to gain speed , whilst producing wash . Some of these have arrived on the canal to spend their lives pottering. It is this hull shape and limit to speed which is why I would never take a Dawncraft to sea, though people do.
So, there was no point in the early Dawn crafts having huge powerful 2 strokes as they couldn’t go any faster if they wanted to - they just need torque to get their great lump of fibre glass moving and more importantly staying moving. All this before we even start calculating the pitch of the prop - too fine like a electrical machine screw and it will whizz round without going forward at any speed, too course like a coach screw and it will take some real effort to turn it – the calculations for this in itself is worthy of A level maths and we all end up with a compromise , fine enough to get us moving without stalling but coarse enough to give some speed.
OK so what have I done - found the cheapest old outboard I could just to keep the boat moving for a few months because I have lost interest in any petrol, diesel or internal combustion engine- there isn’t one that does not burn or lose a bit of oil from their very nature- they are technology that has come directly from the steam engine - they need water cooling impellers which always pack up at most inopportune moments and not the kind of thing you can readily fix on board. They are heavy, and I am getting too old to lump them about and the nearest service agent to me is getting to be a weekend away trip.
Over the years being an ex ag engineer, I have fixed most things from old diesels to ratty old sea gulls and I am bored of them. Nope I am in my shed building and designing an electric propulsion kit relying not on electric trolling motor which even two together won’t push the boat any faster because they are not designed to. But a proper dedicated motor. Ok so I lose some range, just like electric cars but the technology is there, even You Tube has programmes on charging them as you go along, a sort of hybrid unit. It is no coincidence that nearly all commercial boats are diesel electrics as is the HST 125 train. The benefits of this type of propulsion system have been understood for a long time - they just haven’t been available to us. I read with interest Contessa the sailing boat manufacturer are installing similar power units as standard. The internet is full of companies who can supply and help fir or design your own system. Torquedo make a dedicated powerful electric outboard and If I had the money, I would have bought one straight away. We are at a pioneering change in technology and like all new ideas it always starts off expensive and needs trail blazers prepared to buy into the idea to keep it moving – I had and still do ( I whizzed out tonight on it) one of the first ever electric bikes , people laughed their socks of at me 12 years on and it’s still going strong. The bloke down the pub still swears at his 15 - I tend to swear at them.
This little 12-volt 55lb thrust has got me home – albeit slowly more times than it ought to have. You can see how fine the pitch is on the prop- which means it takes little bites of water not propelling the boat any great speed- because of this two together would make little difference.
The LED indicator shows which way the prop is turning in this case forward and speed - more speed the more power it will use – I have seen an installation where this bit has been taken off – the wires extended, and this becomes the control in the cockpit – I will be using a 24 volt for my project.
Engines aside boat life continues with the usual winter moss growing in the channels which get over whelmed when it rains. - hence the drip trays which still work.
I have made the tape removable , giving it a good wash through – I have now started using an anti mould spray see if we can stop the green from coming back.