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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Sun Jun 19, 2011 5:33 am 
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Joined: Fri Jan 14, 2011 12:56 am
Posts: 22
Holy cow! Sheathing the hull in 480gsm glass cloth was a task. Talk about thirsty - Must have taken good litres of epoxy. Prime coat with timber preserrvative in the epoxy, wet out coat to anchor the cloth, two rolled on coats over the top. Took all day. Looks good though, adds a heap of rigidity to the bottom panels.


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Wed Jun 22, 2011 8:11 am 
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Joined: Fri Jan 14, 2011 12:56 am
Posts: 22
Having sheathed the bottom of the boat I now have a question: Is it better to fill the completely weave of the cloth or leave the texture visible? One thing that springs to mind is the issue of boundary layer and drag. I know that with some aircraft it is better to have a turbulent boundary layer to reduce drag at certain Reynold's numbers. The same principle should apply to hydrodynamic drag. I read that some advanced Americas Cup yachts have textured hulls to reduce the drag, has anybody experimented with this on their boats? For some odd reason I like the look of the glass weave and would prefer not to fill it! I'm hoping there is a performance increase although I suspect it will have no material effect and it is up to aesthetics.


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Wed Jun 22, 2011 8:38 am 
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Joined: Fri Jun 18, 2004 8:14 pm
Posts: 4855
Location: Greensboro, NC
Morten can speak specifically to his design but I'll say this much: Playing with the Boundary Layer works nicely for golf balls because of thier realtively low Reynolds number not to mention golf balls rotate as they move through the air. Hulls in water will have a much have Reynolds number and the effect is tough to predict.

Some experiments have been done subs I pretty confident it's more controlled than the random weave of saturated fiberglass.

Interesting reads:

http://www.nasa.gov/centers/langley/new ... blets.html

http://www.nytimes.com/1987/02/04/busin ... ffort.html

Look up some research by Mark Drela. Here's some: http://www.rcgroups.com/forums/showthre ... 098&page=2


Filling the weave is going to make taking care of your hull easier not to mention more durable.

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-Matt. Designer.


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Wed Jun 22, 2011 1:59 pm 
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Joined: Tue Dec 21, 2004 7:55 am
Posts: 141
Location: Denmark
I would recommend filling the weave completely since it will give you much more advantages than trying to make a texture that reduces the drag. There is so little to gain in speed or fuel consumption that it's not worth the effort. Also there is very little chance it will work at all.

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Morten Olesen - Boatplans.dk
Website: http://www.boatplans.dk/
Blog: http://BoatBuildingArticles.com/

Check out my author page at Amazon and browse for boat building books: http://www.boatplans.dk/news.asp?news=amazon


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Wed Jun 22, 2011 5:55 pm 
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Joined: Fri Jan 14, 2011 12:56 am
Posts: 22
Thanks Gents! It was just an idea. Since my supercomputer has a flat battery and I can't run a simulation, it's out with the microballoons...


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Thu Jun 23, 2011 11:32 am 
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Joined: Mon Nov 12, 2007 8:12 am
Posts: 192
Location: S.E. Queensland
I was thinking along similar lines about leaving the weave half filled rather than filled. Not because of all that science stuff but because some very fast fish have scales. Which have that irregularity about them.Also because of trapped air... See I'm not as daft as I look.


I couldn't be :roll:


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Thu Jun 23, 2011 12:50 pm 
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Joined: Fri Jun 18, 2004 8:14 pm
Posts: 4855
Location: Greensboro, NC
There's some really cool science behind it. I read about some "tape" that can applied and it acts like that ribbed layer pattern. But it's never been made for public sale.

But from what is known currently, it would be hard to duplicate it for a hobby boat builder. Heck, even the production company would struggle with it currently. The USN does some stuf with it but they have a slightly bigger budget. :shock:

What I would worry about is what happens when dirt and debris get into those grooves.... would it slow you down more so than a smooth hull?

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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Thu Jun 23, 2011 6:21 pm 
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Joined: Fri Jan 14, 2011 12:56 am
Posts: 22
Don't get carried away fellas! This is an interesting topic.

You've seen the head to toe textured swimsuits the Olympic swimmers like Ian Thorpe wear? Same principal - control the flow of water at the boundary layer to reduce overall drag, a bit like sharkskin. Next time you're sitting at a window seat near the wing on an airlliner, check for the little vortex generators sticking up all along the leading edge, or on 737s all along the fin near the rudder - Same principal, keep the boundary layer turbulent to keep the air stuck to the surface to reduce overall drag. Also improves stalling characterisitics at high angles of attack or rudder deflection.

I think there's a project here for someone's masters or doctorate. Not me, I just want to finish my boat and get on the water. My project is a trailer boat that will spend more time on the trailer than in the water, so keeping the bottom clean is not an issue. I will start building up the epoxy layers and finish off with a slurry of microballons to fill and fair starting this weekend. The cool weather means a couple of days for the eopxy to harden off, so I will start working on a turn-over cradle in the meantime.


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Mon Feb 06, 2012 5:50 am 
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Joined: Fri Jan 14, 2011 12:56 am
Posts: 22
I haven't posted for a while, so a quick update. The hull is well underway to being finished. I am into the painting phase. Two pack poly all over, which is a good looking and tough paint but a bugger to apply. My advice? Mix it well, add a some thinner and let it stand for a little while before you use it. Treat your brushes as disposable!

Some photos posted here:


http://www.facebook.com/media/set/?set= ... 8096f8d206

Looking forward to finishing it, trick is not to rush to the end and cut corners. Tasks left are fitting the steering controls, wiring the lights, fuses and switches, mounting the outboard bracket, motor and throttle.

Will try to update more frequently!!


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 Post subject: Re: 14ft Motor Dinghy - Builders?
PostPosted: Sun Feb 19, 2012 12:49 am 
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Joined: Fri Jan 14, 2011 12:56 am
Posts: 22
Finished painting. Decided the lower cheat line needed to be lower down so repainted that part. The Northane Gloss Marine 2 Pack has a lovely glassy finish. Hope it survives!

Had built a small control box, which I mounted on a bracket on the port side but now have decided to simplify the whole lot and incorporate the box and bracket into one unit. Stay tuned.

I have been dry fitting some parts and am getting quite excited to be nearing the completion and getting a feel for the finished boat.

Off to the 'authorities' this week to get the boat and trailer registered. God help me.

http://www.facebook.com/media/set/?set= ... 1c8c046bb3


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