David and I have been batting ideas backwards and forwards. I have been frantically busy since Christmas - it's hard work, living a life of leisure, so I've had a bit of catching up to do:
D: I think you’d better remain flexible over the ballast, for the time being. Yes, you could get a foundry to cast it in iron, or even lead, if you make a wooden pattern ( a plywood box made to the right shape, with shrinkage allowance added). Yes, you could have a box welded up to the right shape (⅛” copper or bronze sheet would be brilliant, if it were affordable) and melt lead into it, a bit at a time - lead is 1.5 times as heavy as iron, so that could be 100m thick. But the ballast is an add-on, so the question of inside or outside needn’t be agonised over at this stage. However, I’d like to have 10 - 20% inside, to correct the longitudinal trim after launching.
D: My idea with the outboard is that it would be mounted on a mini bulkhead about 500mm forward of the transom, and would pivot up out of the water in the normal way, but between twin rudders, set far apart enough not to foul the prop. There would be just a small U-shaped slot in the bottom of the hull, just big enough to let the motor leg drop into place, with the cavitation plate just below the hull. The motor head would remain forward of the transom, and could be covered, or partially boxed in, and out of the elements, as you say. The prop would protrude a little aft of the transom, but not as much as a normal transom-mounted motor. What size had you in mind? 6HP to 9.9HP, I guess. I had the Volvo Penta 7.5HP saildrive with the Honda engine, in my Sadler 25, and that was enough.
A: Outboard motor arrangement Now that I understand it, I like your idea of the outboard and prefer it to Arne's suggestion: the further into the boat it is, the less likely it is to cavitate. I was thinking of a 6hp four-stroke. From all accounts that would be substantially more powerful than the 7.5 hp sail drive that you had on the Sadler. It would be completely impossible for me to lift anything larger (I doubt I could lift this too readily, which is why it has to be semi-permanent). Yamaha seem to have a good reputation. You can get a second-hand one 'never used in saltwater' from a reputable dealer for just over $900. A brand new 6hp four-stroke seems to go for about $2,500, which is a gulp until you think of the price of a diesel engine.
D: Motor - I know nothing about outboards, and their brands. At random, I looked at Suzuki, and saw that the 9.9 4 stroke high thrust had all the bells and whistles one could desire: 4 blade 10” prop, 12A alternator, electric start, power tilt even. Probably have to sell your soul to the devil to afford it, though. Some of the problems with inboard diesels are soluble: PSS drip free shaft seal (I fitted an early one of that type to Ivory Gull and i think it’s still OK), keel cooling, using your grounding plate: two 4ft x 1ft x 1/2in steel plates, spaced apart a little. The diesel engine itself’s OK, it’s putting salt water through a strainer, impeller, heat exchanger and wet exhaust that’s not OK. Fifty million workboats and canal boats can’t be wrong. But financially, the outboard wins.
A: Certainly whatever outboard I would go for, I would choose the high thrust type. I think 6 should do it and don't want the complications of alternator, electric start or power tilt. I couldn't include KISS in the name, but maybe she should be called Sib-Lim-Kiss! Yes, the problems with inboard diesels are soluble. But I've seen some of those drip-free shaft seals fail: a terrifying thought. Yes, keel cooling, but it's a lot of extra work. Yes, 50 million work boats and canal boats can't be wrong. On the other hand, price apart, there is a huge extra layer of complexity with an inboard engine: gear box to be bought and matched, control cables, tank and extra lines, electrics, to say nothing of the space that it all requires. Plus, of course at least one extra hole in the hull. And all for what? Something that I use about one trip in six. No, it makes neither common nor financial sense to me.
D: You could have twin rudders, each with their trim tab. There can be a crossbar linking the tillers of both rudders and trim tabs. I’m keeping the transom vertical partly to make things easy in that department, with no funny angles in the linkages. I imagine a vane turret like the one you have, mounted on top of the transom with dyneema cords down and across to the tab tiller crossbar. Easy-peasy. Easier than my servo linkages.
A: Twin rudders I am growing into the idea of the twin rudders and ditto trim tabs. I can see these working very well on a stiff little boat like you have designed. And redundancy, too! Seriously though, I have of course, had experience of twin rudders linked by a cross bar, in Stormalong. They work well, but it's not so easy to get the feel of it as with a tiller, on which, essentially, one just rests two or three fingers and merely suggests what it should do. I would always be a bit more cloddish on the helm, but you can't win 'em all.
D: I had in mind to put the crossbar close to the rudders, so that the cockpit is not taken up by it. Then you just have the two tillers to grab hold of (one at a time!) or stand between. If you want to lift the tillers to clear the cockpit for entertaining, the bar has to be aft of the hinge anyway.
A: we need to think a little more about the tiller/crossbar arrangement. (A detail, I know, but this is part of the fun.) I steer from the windward side of the boat. I don't see how I could do that holding one of the tillers. I dare say that I'm not looking at it properly, but you know me and visualising in three dimensions :-(
D: Think of Brent Swain and his origami steel boats much bigger than Sib-Lim. Not a problem, to bend three sheets of 12mm ply that have been scarfed end to end, and there has turned out to be very little twist and hollow in the bow area. The bottom panel, though, would need two layers of 6mm around the stem, running up further to form the bow transom.
A: Stitch and glue Right, of course, you scarf the panels together first. That makes so much sense. I don't see the bow panels adding too much grief and I'm sure if I shop around, I can find 5-ply 6mm. The building method would be very familiar: Badger and China Moon. 24mm for the bottom is bullet-proof - Badger was designed for less - and we may, yet, have my big steel shoe on it.
D: Here’s the development of the panels that Freeship spits out. You get all the offsets, to draw them out. You can make these up, flat on the floor, and then drop them on top of the bulkhead and longitudinals assembly. The topsides come out of a 4ft sheet nicely, but the middle panels will need a bit adding on, to make up the width. After assembling all five panels, I’d add an extra bottom panel, to make up the thickness to 24mm, then fair it in.
A: Gosh! It's a terribly clever program, that Freeship. They will be big pieces of wood to handle, but I should be able to rope in the occasional helper :-)
D: Yes, you’ll have to get a team of coolies to lift the hull panels into place, but that should all go fairly quickly. You may need some number eight wire to stitch them! I like Dave Zeiger’s 3/32” copper sheath for the bottom panel, so you wouldn’t necessarily have any steel outside. One steel plate doesn’t add much ballast, and it would take an awful lot of steel plates to make a tonne; it doesn’t seem reasonable to go that way. However, no need to deal with that now.