Preparing for GT27 build - questions

Power Boats only. Please include the boat type in your question.
Post Reply
wkisting
Frequent Poster
Frequent Poster
Posts: 49
Joined: Tue Jul 10, 2018 11:24 am
Location: Augusta, GA

Preparing for GT27 build - questions

Post by wkisting » Tue Jul 10, 2018 7:44 pm

Hello,

I'm very close to decided on building the GT27. I've built three boats before (sailboats), but the GT27 will be the largest I've built by about 6' length. My previous builds were strip built and stitch and glue, but I've never used foam sandwich as recommended for the bulkheads and superstructure of the GT27. I searched the forums but did not see these particular questions asked or answered in the past.

1. The plans spec 1/2" foam between 1/8" ply skins (or fiberglass only on the outward facing sides), but since this will be a houseboat, I'm wondering about insulation and the potential for condensation. Is the 1/2" foam sufficient to discourage condensation, or is it permissible to increase the thickness of the foam to, say, 1"? My assumption is that would actually increase the strength of the resulting panels, but I wanted to ask since I've never tried this before.

2. Regarding the 1/8" ply faces/skins of these sandwich panels: Is the resulting skin at that thickness strong enough that one can epoxy on stringers to support seats for example? It seems the 1/8" ply might be too weak, but perhaps the resulting foam sandwich is much stiffer and stronger than I realize.

3. On another thread, someone asked about reinforcing the cabin roof to support four adults as a sundeck. The designer seemed to confirm the cabin top would need to be reinforced in that case. I probably won't use ours as a sundeck, but I'm interested in the reinforcement as I intend to roof mount solar panels and may need to periodically go on the roof to service those panels. How much reinforcement would be required to the spec'd foam sandwich, and/or how is such reinforcement added (inside the sandwich, thicker foam/skins, or additional battens underneath)?

Thank you for your time to answer these questions.
Last edited by wkisting on Sat Jul 14, 2018 1:17 pm, edited 1 time in total.


Wes
---------------------------------------------------------------------------------------------------------
My build thread is here --> viewtopic.php?f=12&t=63644

fallguy1000
Very Active Poster
Very Active Poster
Posts: 1741
Joined: Tue Jan 12, 2010 12:25 am

Re: Preparing for GT27 build - foam questions

Post by fallguy1000 » Tue Jul 10, 2018 9:03 pm

These are all designer questions.

I have not heard of anyone using ply on foam in conventional builds; you might want to double check that.

The inch thick foam is going to come in at much higher cost. You would be better off insulating the ceiling with polystyrene.

Check the $$$ end of the story.
My boat build is here -------->

viewtopic.php?f=12&t=62495

wkisting
Frequent Poster
Frequent Poster
Posts: 49
Joined: Tue Jul 10, 2018 11:24 am
Location: Augusta, GA

Re: Preparing for GT27 build - foam questions

Post by wkisting » Wed Jul 11, 2018 9:13 am

Yes, my understanding was that they prefer designer questions to be posted to the forum in lieu of email. I'm hoping Jacques will respond.

The GT27 plans specify ply and foam construction for the superstructure to save weight. I figured if I'm going to do that, it makes more sense from an insulation standpoint to increase the thickness of that foam core, as otherwise, I'll have to devise some additional skin (added weight) to cover over the polystyrene on the interior. The plans specifically state not to use polystyrene as the core material, but Divinycell instead, which makes sense to me structurally. I haven't checked the cost difference, so it may be sticker shock as you say, but I wanted to start with what is best with regard to structural performance and weight savings. My rough estimate is that this will be approximately $20K to build (including motor and trailer), so an extra thousand, for example, isn't a big deal in that grand scheme.

I appreciate your reply. I'm a little surprised by the relative lack of GT27 build logs or pictures. I'm wondering how many are out there who can comment on the design performance and/or thoughts about the build.
Wes
---------------------------------------------------------------------------------------------------------
My build thread is here --> viewtopic.php?f=12&t=63644

Jeff
Site Admin
Site Admin
Posts: 4055
Joined: Fri Apr 29, 2016 12:17 pm
Location: Vero Beach

Re: Preparing for GT27 build - foam questions

Post by Jeff » Wed Jul 11, 2018 11:56 am

wkisting, Jacques is away for a few days but he will be checking the Forum. Give him a little time and he will respond back to you!! Jeff

fallguy1000
Very Active Poster
Very Active Poster
Posts: 1741
Joined: Tue Jan 12, 2010 12:25 am

Re: Preparing for GT27 build - foam questions

Post by fallguy1000 » Wed Jul 11, 2018 1:42 pm

Ask Jeff for pricing on divinycel inch versus half inch.

Before you assume using boat foam for insulation; get that dose of reality.

I spent 12k on foam 12mm

Had I gone to full inch; it would have been 24k.

Then, the entire boat does NOT require insulation.

Rethink it a bit.
My boat build is here -------->

viewtopic.php?f=12&t=62495

fallguy1000
Very Active Poster
Very Active Poster
Posts: 1741
Joined: Tue Jan 12, 2010 12:25 am

Re: Preparing for GT27 build - foam questions

Post by fallguy1000 » Wed Jul 11, 2018 1:47 pm

If you compare buildings in MN to a boat; the attic typically gets twice the R value of the wall.

You could consider decking and rooftops in full inch for both less trouble building and better insulation.
My boat build is here -------->

viewtopic.php?f=12&t=62495

wkisting
Frequent Poster
Frequent Poster
Posts: 49
Joined: Tue Jul 10, 2018 11:24 am
Location: Augusta, GA

Re: Preparing for GT27 build - foam questions

Post by wkisting » Wed Jul 11, 2018 5:39 pm

Yes, I will check cost, of course, but I still want to know the designer's opinion on the "ideal" solution as I think through the build I'm undertaking. No problem waiting on him; it's not a rush. Probably a month out from ordering supplies, as we still have several months of brutally hot weather here in Georgia before our nicer, cooler, late fall weather begins.

I don't think I was clear enough in my original post, but on the GT27, it is only the upper cabin and roof (i.e., the deck up) that is spec'd to use foam sandwich construction. It's not the much larger surface area of the hull below, which I will indeed insulate using polystyrene and/or spray foam. The seat lockers, etc. will cover much of it, so on the lower hull, there really won't be much area that needs a second skin to hide the polystyrene. For that, I'll use epoxy coated 1/8" ply and that should keep the weight down plenty.

Appreciate all the responses so far. And I will check on cost. FallGuy1000, your 12K-20K numbers startled me, but I assume you're talking for foam sandwich construction over an entire build (hull also) and of a fairly large vessel, no?
Wes
---------------------------------------------------------------------------------------------------------
My build thread is here --> viewtopic.php?f=12&t=63644

fallguy1000
Very Active Poster
Very Active Poster
Posts: 1741
Joined: Tue Jan 12, 2010 12:25 am

Re: Preparing for GT27 build - foam questions

Post by fallguy1000 » Wed Jul 11, 2018 5:53 pm

Corecell M for a 32' catamaran. Also the most expensive foam I am aware of...

The thicker foam would never be specified for a 27' hull and I see we are not talking the entire hull.

JM will help more than me. I thought you wanted to spec the entire hull in one inch.
My boat build is here -------->

viewtopic.php?f=12&t=62495

wkisting
Frequent Poster
Frequent Poster
Posts: 49
Joined: Tue Jul 10, 2018 11:24 am
Location: Augusta, GA

Re: Preparing for GT27 build - foam questions

Post by wkisting » Wed Jul 11, 2018 6:11 pm

In case there are other GT27 potential builders who are wondering, here's a quick follow up with a cost comparison of 1/2" H45 Divynycell as spec'd in plans vs. 1" for increased insulation (using the first price I came across on Google, merrittsupply.com):

Per the cutout drawing on the plans, 11 sheets would do roof, cabin sides, and deck. An additional 8 sheets would do frames/bulkheads (Actually, I think I overestimated this by a sheet or two, as substantial portions of these frames/bulkheads can be omitted when opting for the stiffer foam-sandwich construction).

So, if we pretend the choice is between 11 or 19 sheets depending on whether I include the bulkheads:

19 sheets of H45 in HALF INCH = $2,356 ($124/sheet) or... 11 sheets of H45 in HALF INCH = $1,364 ($124/sheet)
19 sheets of H45 in ONE FULL INCH = $4,104 ($216/sheet) or... 11 sheets of H45 in ONE FULL INCH = $2,376 ($216/sheet)

It's not an insignificant difference, but my guess of $1K difference nailed it on the 11 sheet option. There would be a little more cost savings from buying two sheets of 1/8" in lieu or each single sheet of 3/8" but scarcely enough to matter (~$100). Bigger problem is I have no idea how to determine whether the added insulation is necessary, and if it is, whether the extra 1/2" would make a substantial difference. Anyone know the R-value of H45?

Of course, it matters how much weight is being saved exactly. 3/8 Okoume is approx. 28 lbs. a sheet. A foam sandwich of two layers of 1/8" Okoume plus one layer of H45 foam is about 23 lbs., but then some of that must get added back by the thickened epoxy used to bond everything. If I go to 1" thick H45, the weight goes up to 28lbs. (plus thickened epoxy to bond), so it's a null on weight savings, but there's a boost to structural rigidity. Not a clear or easy decision...

Given the weight issue, maybe it will come down to cost just like FallGuy1000 said. The 1" foam is tough to justify the added cost if there's no weight savings. My priority is to save weight for trailerability, so the 1/2" Divinycell may be the way to go after all.
Wes
---------------------------------------------------------------------------------------------------------
My build thread is here --> viewtopic.php?f=12&t=63644

wkisting
Frequent Poster
Frequent Poster
Posts: 49
Joined: Tue Jul 10, 2018 11:24 am
Location: Augusta, GA

Re: Preparing for GT27 build - foam questions

Post by wkisting » Thu Jul 12, 2018 1:02 pm

After mulling it over more, I've pretty much settled on sticking with all ply (no foam sandwich) construction and then just insulating with polystyrene. Unless I'm miscalculating, the weight savings with 1/2" foam sandwich would only amount to about 200 lbs., but there's a sizable cost increase. I had assumed the weight savings would be quite a bit more, but I don't think my estimates can be too far off the mark, so in that case, I'd rather stick with the more familiar construction method of building everything from plywood and then using polystyrene or spray foam for insulation. The reinforcements for the ply roof will actually be welcome in that case, since they can be enlisted to tie in a second skin to conceal the insulation (probably a very thin cosmetic layer, like doorskin or maybe even vinyl, so that weight doesn't increase much).

Happy to entertain other suggestions if anyone wants to venture advice.

Any reason there aren't really any GT27 builder threads? Any one know of a build blog or website out there for the GT27? Would love to know what previous builders have thought of the design in performance.
Wes
---------------------------------------------------------------------------------------------------------
My build thread is here --> viewtopic.php?f=12&t=63644

Post Reply

Who is online

Users browsing this forum: No registered users and 2 guests