Patterning the cabin trunk.

[Bowdrie]

If the new deckhouse is to be ~ 2m long x ~1m high will it look anything like the original, if not, how could the original be used as a male mold?
Be careful with radii for side-to-deck/side-to-top, and top camber/inboard side slope.
It's easy to make a structure that's hard to walk/sit on for the 90% of the time the boat's upright.
Deck molding has got to be one of the most infuriating jobs on a custom boat.

[Matt Johnson]

Originally Posted by GILow

Yeah, it’s the true honeycomb stuff. We’ve been working with it for two years now and we are getting ok results. Wouldn’t want to be building a foiling race boat, that’s for sure, but it serves our purpose.

Good point about the strip technique, I’ll keep that in mind.

I think you're using something with a scrim top layer like Nidacore. He's talking about just honeycomb without any top layer to adhere too - amaid or aluminum honeycomb which require prepeg to stick to the ends of the cells.


If you like the shape of the current cabintop, I'd just pull a mold off that and work from there to stitch it to whatever height/width you want.

[GILow]

Originally Posted by Matt Johnson

I think you're using something with a scrim top layer like Nidacore. He's talking about just honeycomb without any top layer to adhere too - amaid or aluminum honeycomb which require prepeg to stick to the ends of the cells.


If you like the shape of the current cabintop, I'd just pull a mold off that and work from there to stitch it to whatever height/width you want.

Yes, “pulling a mold” off the existing trunk is what I’m trying to achieve. I’m looking for techniques for doing so. The simplest option is to use the trunk as the mould, but it would be very awkward. Ideally I could reproduce the curve some other way.

As for the honeycomb panel, ok, it seems like there are many different sorts. Ours is a pvc honeycomb, available in a few thicknesses but we mainly use 20 mm. It has two faces that are like a very light cloth skin with microscopic holes. You could almost dispense with the fibreglass layers altogether and just apply resin to the faces. It wouldn’t be all that strong, but it would be kinda waterproof.

[GILow]

Originally Posted by Bowdrie

If the new deckhouse is to be ~ 2m long x ~1m high will it look anything like the original, if not, how could the original be used as a male mold?
Be careful with radii for side-to-deck/side-to-top, and top camber/inboard side slope.
It's easy to make a structure that's hard to walk/sit on for the 90% of the time the boat's upright.
Deck molding has got to be one of the most infuriating jobs on a custom boat.

This deck house is going to be a lot lower profile. Around 600 to 700 mm high, but yes, close to 2 meters front to back. The sides should be flush with the existing trunk, visually seamless. The height makes it perfect for holding onto if you need to go forward. (In the thread linked above about custom hard dodgers Snowpetrel makes a good case for using the dodger as a hand hold.)

Almost a cabin trunk on top of the existing cabin trunk.

At best you could maybe sit up inside it, but that’s not the plan, it’s more like one of those Japanese hotels where you sleep in a drawer. We’d slide in when under way to be close to the helm and cockpit. I’ll be able to see over the top of it when standing at the wheel, my partner will have to look around the side. We got the idea from our Swanson 42 where the deckhouse turned out to be one of its greatest passage-making assets. I live in it when under way.

[GILow]

Originally Posted by s/v Jedi

I recommend you watch the episodes of the Duracell Project on Youtube. He has been doing exactly the same stuff except using foam core.

Go back a little in his episodes because the later ones he is so proficient that he doesn’t show the process in as much detail as earlier on.

He does the joggle sticks too.

Aaaargh. I watched a lot of his stuff early on, but kinda gave up after he butchered the interior. Turned that massive boat into a rabbit warren.

I’ll suck it up and go back for another look, thanks for the tip.

(Incidentally, that foam core he used really added to the project cost. I’m pretty sure he is stuck using epoxy. One of the great things about the pvc core is it allows you to get away with polyester and vinylester where the epoxy isn’t needed. I’m not sure why he didn’t use it?)

[Matt Johnson]

Originally Posted by GILow

Aaaargh. I watched a lot of his stuff early on, but kinda gave up after he butchered the interior. Turned that massive boat into a rabbit warren.

I’ll suck it up and go back for another look, thanks for the tip.

(Incidentally, that foam core he used really added to the project cost. I’m pretty sure he is stuck using epoxy. One of the great things about the pvc core is it allows you to get away with polyester and vinylester where the epoxy isn’t needed. I’m not sure why he didn’t use it?)

The type of structural foam used in boat building can be used with any resin type. It is foam used for insulation (stuff you get at a hardware store) that can't be used with esters resins.

There's a ton of info out there on when/where to use different coring materials. Using foam core for the interior/exterior is a very common approach on boats where weight and strength (without using exotic materials) is a priority.

[Matt Johnson]

Originally Posted by GILow

Yes, “pulling a mold” off the existing trunk is what I’m trying to achieve. I’m looking for techniques for doing so. The simplest option is to use the trunk as the mould, but it would be very awkward. Ideally I could reproduce the curve some other way.

As for the honeycomb panel, ok, it seems like there are many different sorts. Ours is a pvc honeycomb, available in a few thicknesses but we mainly use 20 mm. It has two faces that are like a very light cloth skin with microscopic holes. You could almost dispense with the fibreglass layers altogether and just apply resin to the faces. It wouldn’t be all that strong, but it would be kinda waterproof.

If you can't wax the trunk to pull a mold, you can place a plastic sheet down and lay fiberglass directly on this. Once cured, you can either use the fiberglassed piece for a mold or add your honeycomb and glass the bottom skin for a finished piece. It will endup slightly heavier due to needing to use thickened resin to bond the core in place, but it won't add much.

[Bowdrie]

Ah yes, from looking at some pics of a Swanson 42 I can now visualize what you're after.
Given that type/shape/proportions, I'd have referred to it as a "Doghouse" rather than a "cabin trunk", but that's of no matter.
Given that style, I'd probably saw off the existing structure in the area you wish to rebuild and make a new male mold.
Deciding on the radii for the side-to-deck and the side-to-top would be personal preference.
The side-to-deck should match the existing trunk that extends forward, that's no big deal.
The side-to-top should be such that there is a definite "line", if you will, (imagine an installed eyebrow molding/trim piece,) for your eye to follow.
That "line" should not be perfectly straight, it should come close to matching the sheer of the deck in that area for a better appearance that is obtained by varying the height of the side pieces.
That is why you don't want too big of radii at the side-to-top, it leaves no distinction of shape.
The fore-and-aft centerline of the top can be straight.
Once the side-to-top radii were chosen then a bunch of matching "corner parts", if you will, can be cut out, also a bunch of side-to-deck parts.
Then with "top parts" cut to the desired camber and the straight "side parts" the pieces can all be assembled into the chosen number of "frames" that are spaced out.
Where that demarcation "line" is between side-and-top you'll cut a notch and fasten in a flush batten, same with centerline down the top.
That "demarcation" should not be in the center of the radii, it should fall a little above where the side starts to curve-up over the radii that blends into the top, (your eye will see that, you can fuss with a batten to line off the notches).
Now you've got this "skeleton" that you can smoothly cover to have your male mold, using thin enough plywood that will take a reasonable bend.
Additional notched-in battens may be necessary to eliminate a "dishing" of the plywood between the frames, that will be seen as the structure goes together.
You start from the top centerline working out to the forward end and towards the sides, stopping where the radii become to tight.
Same on the sides, starting above the side-to-deck radii and working up to that for-and-aft transition "line".
Now you use closely fitted "strips" to fill-in the fore-and-aft area to cover the corners and exposed areas where the radii is/was too tight to bend the plywood.
Now the work starts; A gallon size of "Bondo" and some flexible sanding boards, (made from thin plywood,) and a lot of fairing/sanding.
When you're all done you'll have a beautiful male mold to start your glasswork over,
When the entire thing is finished, inner glass/core/outer glass, you just remove all those parts you put together in the beginning.
Of course now you're still faced with the exterior fairing/sanding.
As I said earlier, deck molding is infuriating, but the construction of the mold is really not as complicated as my trying to explain it.

[GILow]

Originally Posted by Matt Johnson

If you can't wax the trunk to pull a mold, you can place a plastic sheet down and lay fiberglass directly on this. Once cured, you can either use the fiberglassed piece for a mold or add your honeycomb and glass the bottom skin for a finished piece. It will endup slightly heavier due to needing to use thickened resin to bond the core in place, but it won't add much.

We really like this idea. Not worried about weight, strength is more of an issue, so making a fibreglass piece directly off the cabin side is fine, we will incorporate it into the new construction for more strength.

I had a small alarm bell about the curve radius until I realised that the piece we create becomes the outside face of the pvc honeycomb core. No problem, we let the mould piece cure, then we wet the outside face of the pvc and press it into the fibreglass mold. Perfect bond and no added weight, provided we do it before the fibreglass mold piece has fully cured. (No problems there, it’s pretty cool here now.)

[GILow]

A quick and very dirty google found something remotely like what we are trying to build. With the caveat that this photo is of a much more modern boat so has much sharper lines. (Who doesn’t love a VDS?). Ours has much softer, more traditional lines.

But what I think this conveys is the relative proportions. We want something quite low and sleek, not at all like the Swanson in that regard. From outside it would appear that the boat has a pilot house. (And yes, we did consider making that modification, but it got too hard very quickly.)

(Please excuse the bad photo edits, but I figured I should erase any names etc that might cause confusion.)

[GILow]

Originally Posted by Bowdrie

Ah yes, from looking at some pics of a Swanson 42 I can now visualize what you're after.
Given that type/shape/proportions, I'd have referred to it as a "Doghouse" rather than a "cabin trunk", but that's of no matter.
Given that style, I'd probably saw off the existing structure in the area you wish to rebuild and make a new male mold.
Deciding on the radii for the side-to-deck and the side-to-top would be personal preference.
The side-to-deck should match the existing trunk that extends forward, that's no big deal.
The side-to-top should be such that there is a definite "line", if you will, (imagine an installed eyebrow molding/trim piece,) for your eye to follow.
That "line" should not be perfectly straight, it should come close to matching the sheer of the deck in that area for a better appearance that is obtained by varying the height of the side pieces.
That is why you don't want too big of radii at the side-to-top, it leaves no distinction of shape.
The fore-and-aft centerline of the top can be straight.
Once the side-to-top radii were chosen then a bunch of matching "corner parts", if you will, can be cut out, also a bunch of side-to-deck parts.
Then with "top parts" cut to the desired camber and the straight "side parts" the pieces can all be assembled into the chosen number of "frames" that are spaced out.
Where that demarcation "line" is between side-and-top you'll cut a notch and fasten in a flush batten, same with centerline down the top.
That "demarcation" should not be in the center of the radii, it should fall a little above where the side starts to curve-up over the radii that blends into the top, (your eye will see that, you can fuss with a batten to line off the notches).
Now you've got this "skeleton" that you can smoothly cover to have your male mold, using thin enough plywood that will take a reasonable bend.
Additional notched-in battens may be necessary to eliminate a "dishing" of the plywood between the frames, that will be seen as the structure goes together.
You start from the top centerline working out to the forward end and towards the sides, stopping where the radii become to tight.
Same on the sides, starting above the side-to-deck radii and working up to that for-and-aft transition "line".
Now you use closely fitted "strips" to fill-in the fore-and-aft area to cover the corners and exposed areas where the radii is/was too tight to bend the plywood.
Now the work starts; A gallon size of "Bondo" and some flexible sanding boards, (made from thin plywood,) and a lot of fairing/sanding.
When you're all done you'll have a beautiful male mold to start your glasswork over,
When the entire thing is finished, inner glass/core/outer glass, you just remove all those parts you put together in the beginning.
Of course now you're still faced with the exterior fairing/sanding.
As I said earlier, deck molding is infuriating, but the construction of the mold is really not as complicated as my trying to explain it.

Still digesting this lot, suffice to say you’ve got me thinking about demarcation vs blending and the risks of each.

[Bowdrie]

Originally Posted by GILow

Still digesting this lot, suffice to say you’ve got me thinking about demarcation vs blending and the risks of each.

That boat you pictured has no "demarcation" so to speak, just a continuous molding, the upper line of the windows is the visual reference from side to top.
Of course, it doesn't have any sheer either, it's one of those boats designed with a straightedge and a compass.

[Bowdrie]

Traditionally, (with an end-on view,) the camber of the house top is a section of an ellipse, and at the roll over "cut line" of the ellipse where it meets the house side is where the trim molding or drip rail was, and the upper edge of the "windows" were below that.
More modern construction tends to forego such visual niceties.
My use of a "demarcation" line is where that trim molding/drip rail would go.

[GILow]

Originally Posted by Bowdrie

Traditionally, (with an end-on view,) the camber of the house top is a section of an ellipse, and at the roll over "cut line" of the ellipse where it meets the house side is where the trim molding or drip rail was, and the upper edge of the "windows" were below that.
More modern construction tends to forego such visual niceties.
My use of a "demarcation" line is where that trim molding/drip rail would go.

Yes, I think you are right to mention this because, as I noted, we have a traditional boat and that picture is NOT a traditional boat at all.

From what you are saying, I think the sides of the deckhouse will need to slope inwards at the top slightly more than the cabin trunk for a workable aesthetic?

(For the record, I am a great believer in these various design rules, y problem is I don’t actually know the rules.)

[Spot]

I am confused, where is the new trunk going? Could I trouble you to draw over the top of this picture?

Having it taper in slightly more than existing with a generous curve at the corners/seams/edges seems right to me.