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Hull thickness fibreglass boats.

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    Posted: 01 Nov 2019 at 2:41pm
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Platinum
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I was wondering if anyone on the forum could satisfy my curiosity about the typical hull thickness used on the bottom of GRP boats in the 5-6m range. 

If you have ever drilled holes in your boat in the bottom (not the sides) I would be interested in knowing how thick the layup was. 

I have plans to build a larger one out of ply/epoxy and I am trying to run some comparison calculations. 






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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2019 at 8:43am
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Titanium
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Dont understand the comparisons calculations.
 The construction of glass over ply is totally different to glass and stingers.
 ie a log will punch a hole between stringers far more easy than punching thru glass laminated ply.
And as to lateral differences , that very much depends on stringer type, radius etc and bulkheads.

Bottom line, if u have plugs/ moulds, or the time and deep pockets to make them then you can go glass...and pop a few extra hulls.
 If not.. just go ply,  even then why glass over ply? its just extra weight, bigger power required more gas.

Think you are way over thinking...
just choose 1 of the very reliable , historically proven methods and go for it.

 PS as to thinness of a glass hull, that will vary as to where you take the measurements and size of boat.
 IE around bulk heads, stringers if in the bow or stern etc.
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Post Options Post Options   Likes (0) Likes(0)   Quote neil_cb125t Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2019 at 6:53pm
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This is a quote out of the seaforce 530 ute review on this site

The planing section of the hull features a keel with bonded alloy protective strip and a single planing strake on each side. The bottoms are 10-12mm thick and above the chines the ‘glass is 7-8mm thick. 

may help
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 02 Nov 2019 at 9:13pm
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Thanks Neil. That seems really thick for a 5m boat.

Steps:
 I have plans. Do intend to follow them mostly, but like many that build I'll be making some mods here and there, and scaling the size up a bit. If I wanted a cookie cutter boat I'd buy a glass tub or a tin can...

Hull material is 6mm ply core glassed with biaxial glass 400g both sides. I was toying with the idea of reinforcing it a bit. 

I'm not so much concerned about the hull on the water, but more on the roller trailer that it will likely end up on, especially on some of the roads around here. In the USA most use bunks. 

My calculations and the information I have puts this material at better stiffness and strength to a 6mm chopped mat/woven layup. So I was wondering how thick New Zealand glass boat bottoms would be. 

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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 03 Nov 2019 at 9:22am
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Titanium
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What size boat planing?
Yes Personally I agree about your concerens on a roller trailer.
The old keel trailer hulls are constructed different to the modern rollers.
 Of concern with older (well designed in particular thu that may sound strange) keel boats on roller trailers is  they are designed to take the weight of the boat on the keel, and the side rollers simply stablise. ie without the boat strapped down, the side rollers should still be able to be turned by hand.
Hence you will find the bottom of the hulls of the well designed older keel boats rather thinner than the 1/2" 
(and doubt if modern boats are that thick either. if they where they would be far heavier than even the old heavy brands)

Not long back, new tow vehicle, different hitch height etc. had to re position the boat on the trailer forward about 3/4"
I did not .. forgot, to re adjust the 2x wobblies up front. I put a tiny 2" crack about 2 to 3"inside chine on one side.
We are looking at around 5 to 6mm thickness. This was just outside the chine lay up area where the glass is a little thicker on and each side of the chine.

The biggest issue trailering ply/ glass boats, and to some extent alloys (latter cracked seat and transom welds.) Is a stape is put across the boat and tied down.
What this does is not just hold the boat down, but any force in doing so, and any force going over a bump, transfers to pulling the gunnels together...Squeeze the sides in.
Something a boat design is not designed for.
Hence way many boat designs are critised for "cracking top back of transom" or "gelcoat stress cracks around rear seats"

A boat is held down in such a manner it is not squeezed together..  but rather tie downs over the transom, and bow, and/or bow eye tied down (be it winch or rope on bow)

 With non keel boats, the areas (in many cases the lines) where the boat sits/is supported on rollers will be designed with reinforcing to spread those loads.
 The smaller, lighter the boat/ dingy the less weight and distance between bulkheads etc so less required in the hull.

So the 1st consideration if concerns are trailer support is
1/how its straped down
 2/ Weight/length
3/Design of bulkheads etc to match trailer , or trailer to match bulkheads etc.

Which then makes issues of hull thickness rather a moot point...ie taking to extreme, if bulkheads and rollers in the right place the hull could be made of ducttape.


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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 04 Nov 2019 at 10:01pm
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Platinum
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There is that. Still 20kg extra in the build phase may make life easier later on. 

The particular design I am looking at is in the 5-6m range. The designer originally specced it at 18' and then scaled up and down a foot. I have the plans for the 17' but scaling to 19' not an issue.The designer recommends another layer of 400g both sides of the bottom for people using larger loads or motors near the top end of the range (115hp). 

That would bring the bottom to 8mm. 10mm at the keel as the bottom glass overlaps there. 
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 04 Nov 2019 at 10:03pm
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The basic rule of thumb scantlings I had read on FG boats was basic skin thickness 1mm per m of length. Add 15% on hull bottom and another 10% for planing craft. That and a stringer frame system. I have the more complex formula from Elements of boat strength but these are close.

https://www.gerrmarine.com/Articles/BoatStrengthIBEX.pdf

Seems most NZ boats are in that ballpark. 

I know I'm overthinking this, but the Maths is a hobby for me. Some guys like to tie flies. I like to calculate boat hull specs. 





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Post Options Post Options   Likes (0) Likes(0)   Quote Schampy Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 2019 at 6:19am
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One way. 6mm ply with 400g double bias are you building a Mark Bowdidge Design?
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 2019 at 8:05am
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Titanium
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20kg extra in the build phase may make life easier later on.

Like requiring more power for the hull to work right?
Or extra weight not needed when hauling the boat around?

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Post Options Post Options   Likes (0) Likes(0)   Quote Big -Dave Quote  Post ReplyReply Direct Link To This Post Posted: 05 Nov 2019 at 12:19pm
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Unless a boat builder, build his own trailers so that rollers are directly under internal supports, then I doubt there is any real planning of roller position... You buy a trailer from Voyager for example, it is simply sourced for the boat length.. Maybe with light and heavy duty options.
you can't fix an idiot with duct tape, but it does muffle them for a while...
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 09 Nov 2019 at 9:38am
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That's true Big Dave. When I get around to fitting a trailer, I may be in a position to get  a custom trailer and adjust the rollers to fall under internal stringers and frames. At least I will know exactly where they are, and the layup schedule. Not something most can say. 

Then again I may be stuck with whatever trailer I can get cheap. 

Scampy: The plans that I have are from Bateau. I bought several as they are fairly cheap. FS17,OB17,LB22, PG20. 

I've had these for some time, Bowdidge didn't have any designs in that size that I liked at the time. His Broadwater 5.1 especially would appeal to me now though I didn't see it back then. 

One thing in favor of Merhten's plans are that there are many documented builds on the forum, Bowdidge's forum is only to those who have purchased plans so it's hard to tell how many builds are on it and how well they are documented. They are also cheaper plans though many of the finer details are scattered through the forums and website. Merhten's plans have all been downloadable for some time: a big plus factor.

Bowdidge plans generally have higher HP requirements, deeper Vs but look very sharp. I prefer a slower speed, cabin rather than CC (South Island based) and lower HP requirements, preferably with the capability to run at any speed (no hole to get out of.) Filling a 200l underfloor tank is going to be a non starter for me. 

Both have similar scantlings and strength. 









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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 09 Nov 2019 at 10:20am
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Titanium
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Modern wobbles can be adjusted sideways , up and down, and given the rollers themselves are free swing, adjustment to boat construction is quite variable.

Which also makes positioning of stabilising a boat on a keel trailer under stringers, bulkheads, chines also variable.
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 09 Nov 2019 at 2:14pm
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Originally posted by Steps Steps wrote:

20kg extra in the build phase may make life easier later on.

Like requiring more power for the hull to work right?
Or extra weight not needed when hauling the boat around?



The extra layer of glass is recommended for the 19’ version if powered with larger motors or heavy loads. The 17’ version will work with a 25-50hp. 19’ 50-115hp. Weight in all cases well below a single skin glass build.
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Post Options Post Options   Likes (0) Likes(0)   Quote Steps Quote  Post ReplyReply Direct Link To This Post Posted: 10 Nov 2019 at 9:24am
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Titanium
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My reference was a thread a while back....a ply dingy build, extra layers of glass/ resin , built like a heavy battleship

A std 17' glass cabin boat, based on gross weight on water will be nicely  powered for salt water with 115hp...
50 will be badly under powered...
 May as well build a displacement hull.
 And when comes to displacement hull powering I know next to nothing
 Im assuming your designs are plaining hulls/
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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 11 Nov 2019 at 11:05pm
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Dinghy: 200g cloth both sides of bottom. One layer of 400g biaxial on the seams, 6mm Okoume plywood. Total hull weight 35kg. Including launching wheels and motor 50kg.

 I have had to make the odd repair here and there to the areas I did not glass. When I needed to trailer it 1500km with the outboard lying down inside. The skeg wore a dent in the side despite being wrapped up and tied down. 

 After two years of use I'd conclude that the boat is not as overbuilt as you might think. If I kept it near a mooring yes or lived in Auckland yes. But given the wear and tear that it endures bouncing around tightly strapped down to a poorly sprung trailer over unsealed roads to Lake Coleridge no. 

I would save a bit of weight second time round building that dinghy just from better glass work. Maybe 5kg not more, unless omiting glass. The published weight was with no bottom glass, only taped seams with 300g cloth tape. That was never going to work for me. 






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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 11 Nov 2019 at 11:26pm
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Just to give an idea, this is an example of the FS17.

 

This boat has an advertised hull weight of 143kg. The plans make clear that this is for the 'basic' version. Glass only on the bottom to halfway up the sides, no gunwales, open layout 30hp tiller steering.FS17 console skiff open layout


 That was the original program of the boat. Builders see the design but almost nobody builds it without modifying things. That adds weight. In this example you can go to a bit above 700kg displacement before the waterline reaches the scuppers. More if the builder raises the sole. 

Nevertheless that boat is powered with a 60hp. The boat can take it but isn't designed for high speeds. Runs better at 25mph. The hull form is a warped bottom. High deadrise at cutwater, almost none at the transom. 

I may not build this one, but if I do I'd be scaling it up the 10% allowed and putting a little cuddy at the front. And adding glass everywhere exposed to wear and tear. That is an option and a good one imo.




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Post Options Post Options   Likes (0) Likes(0)   Quote OneWayTraffic Quote  Post ReplyReply Direct Link To This Post Posted: 12 Nov 2019 at 5:37pm
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My point being that it is important to distinguish between extra weight that is useful and weight that is not. Only the builder can judge which is which. In the case of my dinghy that’s maybe 50/50. If I were building it again I would not change much.
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