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  #1  
Old 06-14-2010, 10:51 PM
Riptide Riptide is offline
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I am making a dinghy and need some professional guidance.

I've found a mold for a 10ft dinghy and I'd like to try my hand at producing (rather than buying) a car-top boat. I plan on using this mainly as a little fishing boat for my girlfriend and I using an electric or small gas outboard motor for propulsion. I will be beaching it on sandy and perhaps stony shores.

I have no experience in the lay up of anything other than open mold and light RTM tooling (it's part of my job).

The 2 things I do know are: I will not be using epoxy, and I would prefer not to chop the hull.

So that said, how should I go about the hull lay up? How many layers of what type of cloth? How about coring? - I plan on mounting a small outboard or electric motor to the transom. I'm assuming plywood would be good here.

Should I skin it with veil and VE resin and do x amount of layers with 1.5oz CSM? Or would it be best to skin with 1.5oz CSM and use some heavier cloth?

Also - What's the best method of putting seats in?

I'm so glad I found this community. I've spent the better part of my evening reading and learning. I'm not a seafaring guy, but I've always loved boats and the prospect of building one has me pretty excited.
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  #2  
Old 06-15-2010, 12:43 AM
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Landlubber Landlubber is offline
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"skin it with veil and VE resin and do x amount of layers with 1.5oz" CSM....
......nothing wrong with that, and you could simply add a layer of core mat then skin it again and it wil last for many years just like that. The core mat will not add all that much weight but will create the thickness for the glass to work as a beam structure (distance)......try 45 biax instead of the 1.5 if you really want to beef her up.....and have fun.
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  #3  
Old 06-15-2010, 11:42 PM
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tinhorn tinhorn is offline
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I'm not a boat builder, but I've manufactured and sold tons of kit car bodies (all built with VE and Parabeam 3D fabric core), and this spring I've repaired several fiberglass dinghies and canoes. So with that background, may I make some observations?

1.5 oz. sounds kinda lightweight, even with CoreMat. It's not going to take much of an impact on a rock to go right through a layer of 1.5 oz. (I'll be repairing a couple of respectable rock punctures over the next few days on what appears to be one layer of 3 oz.)

I bought a 12' ELI dinghy with the center seat removed. Without that seat, the sides are barely rigid enough to turn the boat over. CoreMat would have resolved that floppiness problem. I'm not sure a wooden seat that is attached with a few #10 wood screws should be considered a long-term substitute for a solid hull.

You do know that you should have equal fiberglass on both sides of the core, right?

I've seen a few different seat designs. The most modern is a fiberglass box that is attached to the floor and sides in order to provide bouyancy. The most common seem to be planks (or a fiberglass panel) screwed to wood blocks that have been 'glassed to the sides of the hull. Of course the front and rear seats are designed to provide flotation.

I'm still grumpy from being a day late on a set of FREE dinghy molds a week ago. If you decide to get rid of yours, please keep me on your short list of interested parties.
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Last edited by tinhorn : 06-16-2010 at 09:05 AM. Reason: Yes, of course.
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Old 06-16-2010, 09:09 PM
Riptide Riptide is offline
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Thanks for the replies, guys. I'll have a look and see what kind of core material we've got in the scrap bin. I'm assuming it doesn't need to be all that thick.

I did a bit more reading here today and came across this post:

Quote:
A simple lay up for a dinghy
1 -GEL COAT ALL OVER 2 COATS IF BRUSHING
2 -1 450 GRAM CSM TO COVER GEL AS THE FIRST SKIN
3 -1 450 GRAM CSM + 760 GRAM WOVEN CLOTH OR A FABRIC OF THE SAME WEIGHT ALL OVER . OVERLAPS ALONG THE CHINE AND ALONG THE LENGHT OF THE KEEL . DO THE BOTTOM FIRST AND LAP THE TOP SIDES AND TRANSON INTO THE BOTTOM
ALL OVER LAPS SHOULD BE 50MM OR MORE
4 -A SECOND LAYER FOR THE BOTTOM ONLY -1 CSM 450 + ANOTHER WOVEN MATT TO FINISH
WOVEN MAT HAS A 150GRAM SCM ON ONE SIDE SO MAKE THE CSM FACE UP WHEN YOU LAY THE LAST ONE .
THE TRANSOM NEEDS PLY TO HOLD THE OUTBOARD .
WILL GET BACK TO YOU WITH THAT ONE
If I did my math right 760g/m2 = ~22.5oz/yd2. That to me seems like a pretty heavy weave for the application. Or am I mistaken?

Here are some pictures of the mold and of the mold liner which is what the boat hill would look like. As you can see, there are some areas that would be a real pain to roll heavy fabric into.
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I am making a dinghy and need some professional guidance.-img_0035-1-.jpg  I am making a dinghy and need some professional guidance.-img_0039-1-.jpg  I am making a dinghy and need some professional guidance.-img_0040-1-.jpg  

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  #5  
Old 06-17-2010, 02:09 AM
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Landlubber Landlubber is offline
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...that would make a seriously strong and heavy boat...it all depends on end usage as to how much cloth you throw into it.
450+450+760 then another 450 plus (mat and CSM) of 150+300 at least, so that is 2560...silly for a little dinghy

...do away with the heavier cloths and she will still be very strong and reasonably light.....do the full layup if you do not care about weight, but it is way too heavy.
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Old 06-17-2010, 08:23 AM
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tinhorn tinhorn is offline
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My gosh, that's a pretty little boat! I'm sure glad I'm not helping you roll it out, though; that mold is full of hiding places for air bubbles.

I'd follow Landlubber's advice, except the part about CoreMat.
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Old 06-17-2010, 10:57 AM
Riptide Riptide is offline
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I agree on both counts, tinhorn. I'm sure the effort and headache of rolling all those nooks and crannies will be well worth it in the end.

What is a reasonable weight for a dinghy like this? Do you guys figure a 1.5 oz csm skin followed by a layer of 1208 double bias followed by another 1.5 csm layer would suffice? I want her to be robust enough to stand up to being beached and driven by an outboard but light enough to put on top of my car without a hurculean effort.

Thanks again for everything so far
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  #8  
Old 06-17-2010, 05:08 PM
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Landlubber Landlubber is offline
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Tinhorn....good point there, I just read your comments, the Coremat is for the bottom only, chine down...thanks for the wake up.
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  #9  
Old 06-21-2010, 09:43 PM
tunnels tunnels is offline
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If the guy is a beginer then he should be using woven glass not stiched glass !! Woven is a lot easyer to lay and fit to shapes and is more robust as well
The weight of the woven i would be looking at is in the 800 gram or a little heavyer . And it only need to be in the bottom areas where it gets dragged , banged , stood on etc etc with a strip up the bow and the whole of the transom !! KEEP IT SIMPLE !!!
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  #10  
Old 06-23-2010, 09:49 AM
SamSam SamSam is offline
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Rolling all the nooks and crannies will be a nuisance, but I see a support out the stern so apparently you can put it up to working height and rotate it as you work. That is helpful.

22.5 oz a square yard isn't much. In the US, csm is measured by the square foot, (cloth by the sq. yard) so 1.5 oz csm = 13.5 oz a square yard. Two layers of that is 27 ozs. and that would end up a fairly weak laminate.

Using a woven cloth on the dingy will present a constant challenge rolling it out. Like say you have a piece overlapping the keel and covering one side. You'll start at the keel and have to do 1 whole strake at a time. Moving on to the next, you'll have to make sure you don't stretch the cloth out of the one you just finished or you'll create long lines of air bubbles. ETC,etc,etc all the way to the top.

If you are going for light weight, you can try a minimal layup with unwaxed laminating resin, and without finishing the inside, remove the dingy and try it. Put some 4-6 oz plastic sheeting in it to keep dirt and yourself from sticking to it. If it seems to be weak in areas, clean the outside, put it back in the mold and put some more glass or core and glass here and there. Latex porch and floor enamel directly on the unwaxed resin works very well for the inside finish, waxed gelcoat is more waterproof and durable.
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