[PropGunOne]

Well the leap got interesting today. Had the plug perfect and ready for CSM two days ago. Went out to glass this morning and discovered... low spots, high spots, small cracks in the filler... all kinds of stuff.

What happened was about a 35-degree temp swing over the last 24 hours. Didn’t consider the thermal expansion abilities of 2-part foam. The amount of movement is minimal... almost imperceptible, but it’s enough to throw fairing all of of whack. Basically the areas between the bulkheads dropped down about .25mm. We could fill back in and re-sand, but then when temps rise again everything is off once more.

Trying to figure out the smartest move forward: build up a CSM “exoskeleton” on the plug to give us a hard, (hopefully) unflexing surface, then fair out from there and prime/polish, or cut the losses, call it an interesting experiment, and foam-sandwich our way through it.

The pic above is of that hard chine from before much sanding or shaping was done. Ignore the roughness. Tried to get an angle that shows the curve in its most extreme form. Really not as harsh as it looks. In the foreground you can see the upwards sweep. That sweep then straightens out until it hits a point about even with the transom, then it takes a very slight concave shape towards the back of the sponson. Think it’s an artifact from CAD conversion, and instinct tells me it should be a long, straight shot back, but when you sight along it it looks aerodynamically “correct,” if that makes sense. Almost like the shape itself should trim the chine downwards. Hard to explain. Like the anhedral shape on the back of an F-4.

Still debating what to do going forward.

 

[Guest]

How long did you allow foam to cure before cutting and shaping? The stuff shrinks on it’s own w/out thermal changes! I would throw a layer of 10oz cloth and a layer of 1 1/2oz csm on it then fair it out. The cloth will resist sag better than the csm.

 

[Guest]

Whatever you do, your gonna have to move fast from fairing to mold making if temps are giving you this much of a variance!

 

[PropGunOne]

How long did you allow foam to cure before cutting and shaping? The stuff shrinks on it’s own w/out thermal changes! I would throw a layer of 10oz cloth and a layer of 1 1/2oz csm on it then fair it out. The cloth will resist sag better than the csm.

Over a month at this point. Certainly had a full cure to it. Seemed like every time we sanded we’d ind more high or low spots, but overall it was getting to be about perfect. That was through constant, slowly rising temps. Saw 80’s on Tuesday, back to 40 this morning. Cloth is a good idea, mat certainly has no real shape-holding ability.

 

[firecat1981]

This was one of the reasons DuckNut was alluding to building a functional prototype first then splashing off of it. You would get the exact same results plus one hull.

I've played with foam shaping similar to what you are doing and learned the pitfalls of using the 2lbs foam. It can regularly expand and contract no matter the age, so it's not suitable for a skin backing, it needs to be braced well. Because you essentially have no real core, atleast one you can rely on, you need to build up a glass skin stiff enough to stand on its own.

Thin cloth will flex under the weight of the mold layup later on. Your best bet I think to get a truly fair plug that won't buckle and ripple, is several layers of glass. Maybe a few layers of cloth topped with csm at a minimum.

 

[CurtisWright]

Icarrumba... Maybe rent a portable HVAC system for your Garage? Keep the temp and humidity constant long enough to re-fair and make a mold?

 

[el9surf]

That or wait for more consistent temps in a couple months

 

[jonny]

Ah the joys of skiff building. You live and you learn. You can get plenty of issues even if you do a cold mold one off. When you pull those off the stations. You can get a lot of movement in the hull. Once the interior support is removed. There are pros and cons to both. I personally like the cold mold approach. Due to the ability to work out issues. Before committing to a mold. I wonder if a different fairing compound would help. Something like that bondo with glass in it. That might have a bit more structure to it. Maybe enough to hold the foam in place.

 

[DuckNut]

@PropGunOne - any progress being made?

 

[DuckNut]

Also, on your website you are selling plans. Are these Chris's plans or your plans?

 

[DuckNut]

Abandoned?

 

[KurtActual]

Saw that this was popped off the mold.

Really interested in an update.

https://hogfishdesign.wordpress.com/2020/04/28/captive-skiff-coming-alive/

 

[PropGunOne]

Hey guys sorry... forgot I left this thing unattended for as long as I did. I have a habit of short/long term abandoning projects and then getting mad enough at them to attack them at a ridiculous pace.

The plans I own, but Chris did all the drawing. I just commissioned and went to him with the ideas. He wrote it up awhile back. The build is on the ‘gram... @PropGunOne. Also updating the blog. I’ve stayed away from here because honestly I’ve been too dang tired from working on it. Back at work (real work) temporarily so I’m catching up on my internet duties. Plan was to get it near finished, then start a thread with a FAQ section leading it off so no one has to search through 14 pages for answers. Give me a day or two and I’ll get one running. Minus a ton of fairing she’s not all that far away from being a functional skiff.

 

[bryson]

Really cool lines on that skiff. Glad to see you back at it -- looking good!