Norwegian sailing pram by Kenchington – Model Shipways – 1:12

[Kenchington]

Begin Step 7: Assembling bottom of boat

 

With the two bottom planks nicely curved, I could get a better look at how they fit on the transoms, which has led to a change of plan.

 

Last night, I intended that the two of them would meet at the centreline of the bow transom, with a  triangular fillet inserted between and abaft. This morning, being able to hold them curved over the moulds while examining them through a magnifying glass, I discovered that the planks are way too narrow to span the bevelled flat on the transom. I don't fancy trying to adjust the other bevels, then fit the next strakes, adjust again etc. etc. So I have fitted the bottom planks to match the bevels, leaving a gap between. Instead of a fillet hidden between and behind, I will insert a stealer (to borrow a term from plank-on-frame planking) to fill the gap. That wouldn't work in full-size, nail-fastened lapstrake construction but, in the model, all that will be visible will be the butt end of an extra plank, between the bow transom and the keel plank.

 

The space to be filled won't be properly visible until the rubber bands come off, but you can see the size of it:

 

Again with the benefit of curved planks to facilitate checking, the junction between bottom planks and stern transom revealed its own problems. Again, the bottom planks are far too narrow. For anyone following this log before building their own pram: Make the two saw cuts in the transom further from the centreline than the laser bevelling marks suggest!

 

Too late for that in my case and I don't fancy placing little spacers alongside where I made the cuts. But at least the stern transom is large enough to give scope for adjusting the other bevels, so that is what I will do.

 

Also, despite carefully matching the bevel marks where the bottom planks must go, the step up to the bevel for the keel plank was still not high enough -- which would leave the gap between transom (plus knee) and keel plank that others have found. Again: If you are following this log before building your own pram: Make the two saw cuts in the transom deeper than the laser bevelling marks suggest, then bevel down to the greater depth! But be warned, the extra depth needed is probably no more than a tenth of a millimetre.

 

My solution, at this stage, will be to sand the bottom planks, from the toe of the stern transom knee to the transom itself, until their outboard face is flush with the bevel that awaits the keel plank. Not an advisable option in a boat that has to face wind and waves but fully viable in a model.

 

With that much decided, it was time for glue. It is, however, important to let the planks take up their natural curves, as those define the longitudinal shape of the hull (while the moulds and transoms set the transverse shape). I put a rubber band around the forward mould (with clips to stop it jumping off), slid the two bottom planks under, then moved them forward and back until they only had a little overhang beyond the bow transom, checked that they were sat nicely in the angle provided for them on the mould ... and glued them to the transom with outer edges aligned to the edge of the bevel. Three minutes finger pressure, then put bands on.

 

Left the glue to set while I made a coffee, then slipped a band over the after mould. I lifted each side of that, while settling the plank into place, so that the wood could take up its own curve, then clamped everything nice and tight.

 

Gluing to the stern transom only added one complication: I had marked the position of my balsa block when it held the knee in what I thought was the right place but that proved a wasted effort. The trouble is that downward pressure on the transom, while the glue sets, causes a bend, which changes the length between mould and transom. It was necessary to first bring the planks down to the transom with the latter in its proper position, hence with as little down force as possible, then move the balsa block until it pushed the knee up ... and then adjust the block's position until the toe of the knee is exactly flush with the outboard faces of the bottom planks (which can be seen well enough in side view).

 

While dry-fitting, I had found that rubber bands around the stern transom tended to force the bottom planks up over the saw-cut steps. Solution was to put pressure on the glue using a piece of scrap and hence a downward, not wanted, force.

 

The net result, as the glue sets, looks like:

 

Edited Sunday at 10:15 PM by Kenchington

[Kenchington]

Step 7 completed

 

After giving the glue plenty of time to set, bands came off. The tiny bit of infill went into the bow -- in the end, a linear fillet over the knee, rather than a triangle (easier to shape and less obvious in the finished model:

[There: Managed to get the image-size control to work!)

 

Sanded the bottom planks near the stern:

And then turned back to the keel plank. I slipped it under the rubber bands on the moulds, held down one end to its transom, centred that, then traced along the edges of the plank, marking the bottom boards, so that I had mark for where to apply glue (and a guide to placing the keel plank -- though that is mostly done by aligning centre marks at the transoms). Then the elastic bands had to come off. At that point, the bottom boards wanted to move around on the moulds and needed clamps to make them behave.

 

In this pram, as with most lapstrake boats, the strength and stiffness of the hull is almost entirely in the plank-to-plank connection -- glue, in the case of the model. Makes me a bit nervous but this one went straightforwardly. The instructions say to glue from the bow transom half way to the forward mould, then do the rest as a second operation. I did it in three bites instead, with 5 minutes setting time between them, each time working the tip of the glue brush into the narrow gaps where the joint was already glued. Cleaned up the limited amount of excess glue while it was still wet, took care to avoid gluing planks to moulds ... and that was about it. I used clothes-pegs for most of the clamping, with something more aggressive where the keel plank had to be held to the transom knees (or spaced was limiting), while the ends were tight to the transoms:

After giving the glue ample time to set, clamping and bands came off. Then it was time to trim the excess length of the planks, with saw, snips and sanding sticks. 

 

At last, after all the preparation time, I have the beginnings of a boat!

All of the effort to keep the two ends tight has paid off too:

 

Next up: Garboards

[druxey]

That looks good, K!

[Kenchington]

Thank you, sir!

[Kenchington]

Gains and Things:

 

A digression here, while I figure out next steps and maybe ask for some advice.

 

With the garboards and all higher strakes, the pram kit's instructions call for a "gain" to be shaped in the inboard side of the lower edge of each end of each plank. Admittedly, explaining a lapstrake "gain" in a few words and one picture is challenging but I was lost -- and I know others have been before me. Part of my trouble was that I learnt lapstrake (or I should say "clinker") construction in the UK, where the terminology seems to be different, so I had never heard of a "gain". (Oddly, Chapelle didn't mention them in his "Boatbuilding", only noting that the bevel of the lower strake has to transform into a rabbet as it approaches stem or stern.)

 

I learnt the theory in the UK, I should say, as I have never built a clinker boat at full size and never want to try. (I have rowed and sailed various of them, for a time even owned and maintained one, but that's not the same as building.) And that long-ago learning drives me to digress from my digression ...

 

I've seen lots of accounts of clinker/clencher/lapstrake boat construction but the only one that could teach me how it is done was Eric McKee's wonderful little "Clenched Lap or Clinker", published by the National Maritime Museum (the UK one), more years ago than I care to count. Druxey drew MSW's attention to that booklet near ten years ago but nobody seems to have followed up. Long out of print, it can still be picked up on eBay or through AbeBooks.

 

The centrefold of McKee's booklet was printed on card stock and showed everything needed to build a 1:15 (or maybe 3/4"-to-the-foot) half-model of a 10ft workboat (with a very clever arrangement for setting the moulds in position). I could have sworn that I built that model as a teenager but, from the publication date, I see that I must have been on vacation from my undergrad university at the time. The little thing has been kicked around on bookshelves ever since, battered, bruised and ignored, but it is still with me:

Filthy inside, breaking up, with oars that are an utter embarrassment -- but I see that I fitted it with a grating for whoever sat in the stern sheets, along with a scoop bailer and a painter on its own eyebolt. (I had quite forgotten those touches.) And, before anyone disparages the state that I allow models to fall into, that little half-boat has circumnavigated our watery world. Went around in shipping containers, along with much household furniture, but it went by sea, which is more than I have done.

 

Aside from drawing attention to McKee's booklet, my reason for posting (other than as an excuse for showing off!) is to suggest that a 1:12 full-hull wooden version of that same boat would be an excellent next-step after the Model Shipway's dory and pram. With 9 strakes each side and rabbets at keel and stem, it would add another level of skills. Maybe that's a hint to one of the kit manufacturers?

 

Or do I have to scratch-build from McKee's strake drawings?

 

 

Edited yesterday at 05:17 PM by Kenchington

[SaltyScot]

I must say, Trevor, I have the bug now. The CEO was in here just now and, having asked me a few days ago to think about building what in essence would be a good old fashioned lifeboat, she saw your blog here and said, "See, there are some out there who are building them too." I would like to build the James Caird form Shackleton's expedition in 1912, but not the one with all the rocks etc. in it, just the plain old lifeboat. We will see. I guess what I am trying to say is you have inspired me, sir.

[Kenchington]

I had this posting all ready to go some hours back, then my internet link went down. Fortunately, MSW has remembered my draft, so I can finish up my original digression, then catch up on other messages:

 

The pram kit instructions call for a gain at each end of each plank. I now understand that a "gain" (in at least American nautical English) means a rabbet in the end of a lapstrake plank. I have modified a drawing found for me by Google into something clearer than the kit instruction's photo:

1 is the inboard face of a starboard strake where it nears the stem. 2 is the end of the plank, to be embedded in the stem rabbet. 3 is the end of the bevel on the outer, upper corner of the strake, shaped to receive the next strake -- though, as Chapelle made clear in his "Boatbuilding", that bevel would have had to transition into a rabbet this close to the stem. 4 is the lower edge of the strake. The gain is the sloping rabbet cut into the nearest corner of the plank. (A scribed line corresponding to the upper edge of the next strake below is just visible, extending from the end of the cut.

 

Except for the shape of the bevel at 3, so far so good. I understand the shape of a lapstrake gain. With Chapelle's aid, I can even get my head around its normal purpose:

This is my crude drafting of the junction between two lapstrake strakes, seen in section, as they should be (maybe could be?) on a typical round-bilged boat with a straight stem -- lower strake in brown, upper in tan. The left-hand diagram shows the junction somewhere amidships. In transverse section, the boat is curved there and that curve appears as an angle between the strakes. The upper, outer corner of the lower strake is bevelled, so that the strake above can sit flat and firm. So far, so good. That's the way that the pram model is built, amidships. 

 

With a straight stem, however, the boat's transverse section must transition into something near to a vertical line, as the planking approaches the stem. Continuing to bevel further and further (not shown) would reduce the planks to paper-thickness. Going the other way and mounting the upper strake on the outboard face of the lower one (middle diagram) works if you are putting clapboard onto the side of a barn but, used at the bow of a boat, it leaves a wide hole (blue) for the water to pour in.

 

So the solution is to trim the last 6 inches or a foot of the lower, inner edge of each strake into the form of a gain, while transitioning the bevel on the upper, outer edge of the strake below into a mirror image. Then the two fit together, where they reach the stem, as a form of lap joint, while presenting smooth outer and inner faces (which fit neatly into the stem rabbet). The hood ends of the planks are nailed to the stem, so the removal of wood does not involve an unacceptable loss of strength. Nice, if not always well explained in print.

 

Our pram, however, does not have a stem but rather a bow transom, which changes things because the angles between the strakes are carried to the transoms -- to the stern transom in most lapstrake boats, to both transoms in the case of a pram. In the case of our pram, the angles are almost constant from bow to stern.

But if the structural arrangement was maintained throughout (top left) we would get a water gap again. There seem to be three ways around that.

 

#1: With a lot of work, we could joggle the edge of the transom, so that one corner of each strake sits down in its own recess. On our pram, the junction between bottom plank and keel plank is handled that way, but I have not tried to illustrate it here.

 

#2: We could develop the bevel until it fines away, at the transom, into a knife-edge (top right). Then the next strake could sit on both the transom and the strake below without trouble.

 

#3: We could cut off the edge of the lower strake, perpendicular to the bevel, cut a gain in the upper strake and match them up. That's the version that the kit instructions call for. (And they call for the gain to be cut in 3/64 stock!)

 

What the instructions do not say is that it is necessary to match each gain to the shape of the strake below. They do say to cut away no more than 1/3 the thickness of the wood but that gives no account to how much of the thickness of the other strake remains after its bevelling. I would welcome any advice or comment but I suspect that each gain needs to be individually shaped to fit with the strake below. And that's doubly interesting because the kit planks are so narrow that some transom bevels will need to be adjusted. So we have the shape of the bevelled lower strake, the shape of the upper strake's gain and the shape of the transom, all of them rather fluid, yet needing to be matched into one unit with no gaps. Interesting challenge!

 

 

Separate from all of that, previous build logs have suggested practicing cutting gains on scrap. I'm going to start with scrap a lot thicker than 3/64 and see how I progress. But I just might end up sanding the transoms down until the lower-strake bevels form knife-edges.

 

Your thoughts and comments would be much appreciated!

 

 

Trevor

 

 

[Kenchington]

2 hours ago, SaltyScot said:

I must say, Trevor, I have the bug now. The CEO was in here just now and, having asked me a few days ago to think about building what in essence would be a good old fashioned lifeboat, she saw your blog here and said, "See, there are some out there who are building them too." I would like to build the James Caird form Shackleton's expedition in 1912, but not the one with all the rocks etc. in it, just the plain old lifeboat. We will see. I guess what I am trying to say is you have inspired me, sir.

It seems that I have inspired someone. Not sure whether you or your better half!

 

I do enjoy large-scale models of small, open boats. Every piece of wood in the prototype's structure can be (maybe: has to be) represented in the model, so the builder gets to experience the structure of the boat and not just its shape. That can be done with small-scale models of large ships but it is very, very demanding -- way outside my skill level.

[palmerit]

I was also completely lost by the “gain” too. 
 

@Bryan Woods pointed me to the@modeller_masa build of the Pram that had some drawings. 
 

I didn’t understand what I was doing and why until I had it done (kind of) and it made sense. 

 

[Kenchington]

Thanks for that, Palmerit! I'll likely put steel to basswood planks this evening and I'll need that bit of reassurance.

 

Trevor

[SaltyScot]

13 hours ago, Kenchington said:

but I suspect that each gain needs to be individually shaped to fit with the strake below.

 

That is what it looks like to these inexperienced eyes as well, Trevor.