Looking for a modern 18' design

[Ghard83]

Hi all,
Newbie poster. I'm looking for plans for an 18' similar to the Bowdidge Marine Design Sea Strike. I like the hull shape, layout, power, range etc. It fits my needs perfectly. The only problem is that it's been designed for plywood and not foam. I believe he is rightfully insistent that ply core is used (makes sense, that's what it was designed for and suits most builders). No issues, but I was hoping to eliminate timber. Bowdidge seem to have the most modern designs that fit my needs, so I'll happily buy those plans if I can't find something that is designed for foam composite instead of using ply as a core. Does anyone know of a similar design?

 

[TANSL]

I don't see it as feasible for plans for a plywood hull to be valid for the same hull in FRP. The weight difference is so great, which would require resizing the submerged volume, that is, changing the shapes somewhat. It is certainly possible to design that boat for FRP, maintaining the appearance of the non-submerged part and changing the rest to adapt it to the new weights and their different distribution.

 

[Ghard83]

Hi Tansl,
I completely agree and understand the reason why. I'm looking for something that was designed for foam core.

 

[TANSL]

Of course, you can create a design that appears identical (the "visible" area) to the plywood one, but in FRP.
You could also calculate the weight that would need to be added to the plywood design to build it in FRP without any other changes.
Oh, and sorry, welcome to the forum.

 

[fallguy]

So, the main issue with building a planing hull in foam is the hull to bottom and hull to deck interfaces must be made a certain way, or the foam at those interfaces will crush under pounding loads planing boats will encounter.

And while it can be overcome with proper work; the work involved extends the hull build time greatly and becomes complex.

So, you don’t see foam plans for small craft like the Sea Strike because of this limitation. That said, it can be overcome with stronger foam or all glass interfaces, but plywood wins here easily. Build speed is tremendously different and exponentially so for amateurs.

A plywood hull, properly built with all penetrations overbored, filled with epoxy properly, and redrilled and sealed is very hard to beat.

 

[Ghard83]

Thanks Fallguy. That's a good insight and pretty helpful. Every day is a school day - I've learnt something.

 

[TANSL]

There are ways, not as attractive but more practical, to make the deck/side joint.

Except for the cost of the mold, which can also be made in less expensive ways, FRP has some advantages over plywood, although, in fact, for such small boats the advantage is less.
The first thing to consider is the need to complicate things with sandwich construction. It's better to go straight to monolithic, hand-laminated construction.

 

[BlueBell]

Ghard,

Indeed, welcome to the Forum.

Good for you to look for an existing ( hopefully proven ) design.

Foam can be a challenge to build / design with.

 

[fallguy]

There are ways, not as attractive but more practical, to make the deck/side joint.

Except for the cost of the mold, which can also be made in less expensive ways, FRP has some advantages over plywood, although, in fact, for such small boats the advantage is less.
The first thing to consider is the need to complicate things with sandwich construction. It's better to go straight to monolithic, hand-laminated construction.

Can you explain the last sentence a bit?

Does monolithic, hand-laminated refer to solid frp on a mold?

 

[montero]

I can't see any disadvandages of full foam hull . Also use of plywood design is possible .

 

[TANSL]

Can you explain the last sentence a bit?

Does monolithic, hand-laminated refer to solid frp on a mold?

Solid or liquid, is that the question?
Well, it's very likely that you understand solid to mean the same thing I describe as monolithic.
We can leave the mold aside, if that's okay with you, as that's a different matter, and I don't think it influences whether the laminate is "solid" or not, or whether it's done by hand or by infusion.

 

[fallguy]

I can't see any disadvandages of full foam hull . Also use of plywood design is possible .

Foam is harder to build and takes longer and is probably not lighter in 18-19’ boats unless you deviate from good design.

 

[montero]

I haven't made much with foam, but the pieces I did make were groundbreaking for me. I don't see any advantages to using plywood. The weight gain is enormous.

 

[fallguy]

I haven't made much with foam, but the pieces I did make were groundbreaking for me. I don't see any advantages to using plywood. The weight gain is enormous.

This is incorrect. You’ve not done the homework to make such a statement.

For smaller vessels, a minimal amount of stiffness and a minimal amount of abrasion resistance requires considerable amounts of glass. For hand lamination, this doubles with the resin. Vac bagging can reduce the resin amounts, but overall, plywood is hard to beat for certain hulls.

I did a ply/foam comparison spreadsheet once and was surprised at the results and I realized the only way to come out ahead for a small dinghy was vacuum and minimal fairing.

The deviation to carbon fiber can be done, but ironically, carbon elongation properties align better with plywood. So a carbon foam hull is lighter, but the skin can fail rather easily to elongation (think bumping a dock).

 

[montero]

I have two hulls on the way (actually three because one sail mini cat ) and I will use minimal (IMO) amount of fibers and eventually I will strengthen them up if something will brake.