GF18+?

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beardman3
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GF18+?

Post by beardman3 » Fri Mar 25, 2005 6:57 pm

as usual I'm dreaming about what boat to build and I recently started looking at the GF series of boats. I live in Phoenix so I really only have access to lakes unless it gets hauled to CA or Mexico. I like the GF18 a lot. Especially when looking at the Blue one with the pretty sun unbrellas :) That setup is really close to what I'd like to do. My first question is can it be made bigger? I think I saw a thread about adding extra beam width and it was suggested that would make the ride a lot more rough. I think I've seen that all the plans can be extended in length 10% without requiring any work? If my math is right that would put it about 19'2". I'm not sure why but something over 20' sounds really cool. Is something like that possible or would it require a major rework?

Does anyone have any cool ideas for seating for 6-8 people comfertably without losing to much utility. Like I said I like the layout of the "Blue" GF boat (sorry can't remember the screenname for the person that built it). I've been toying with the idea of a bench seat in back but I like the nice mini casting deck it has now. I thought of building a bench that could fold flat into a deck but I think I'd be better off leaving seat building to the pros.

I know the extra power topic is a popular one but assuming an extended design was used could a 50 or 75HP motor be used. I only ask about the 75 because smalloutboards.com has them cheap.

Is there anything that can be done to the bottom of the boat to improve handling at speed? I know this isn't a speed boat but I like to keep my options open :) I was thinking along the lines of runners or something on the bottom. I am a total newbie to boats so again if I'm using the wrong terms please correct me.

Thanks,

Matt



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UncleRalph
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Re: GF18+?

Post by UncleRalph » Fri Mar 25, 2005 7:57 pm

beardman3 wrote: I think I've seen that all the plans can be extended in length 10% without requiring any work?
Without requiring any work? NO.

This question gets asked so often there should be a FAQ addressing it. I will give my 2 cents worth. It easier to scale up a design in all dimensions than it is to scale it up just in length. I will explain why. There are basically five main components you get when you purchase plans. They are:

1. Overall shape of boat – In traditional design this is represented in the Table of Offsets. In Jacques it is shown in the frame and/or station drawings.

2. Scantlings – This is the thickness of wood and fastener dimensions in traditional designs. In Jacques designs it is going to include the wood thickness and the fiberglass lamination schedule.

3. Developed hull panels – These drawings show the 3-dimensional hull panels, unrolled and laid flat in two dimensions. These allow you layout and cut out the hull panels flat on the floor. In designs where you don’t get this (other designers) you have to set up all the frames/moulds and hold a big piece of plywood against it and mark the panel shape.

4. Nesting drawings – These show the layout of the plywood pieces on standard plywood sheets to get the most efficient use of materials.

5. Building directions – Jacques plans include building directions that specify what order in which to do things.

This is not meant to be and advertisement for Jacques, but many designers only provide the first two items in this list. Now lets look at changing the design.

Changing just the length-
If you only change the length of a design, items 3 and 4 become useless. If you keep within 10%, the commonly accepted rule is that the scantlings (item 2) do not need to change. You can use the frame information (item 1) by simply increasing the spacing of the frames. The developed panels cannot simply be scaled up in one dimension. They are the 2-dimensional representation of a 3-dimensional shape. There is no dimension on a developed panel drawing that corresponds to the length dimension of the boat. You will have to revert the traditional method of marking the plywood from the setup frame structure. The nesting drawings will be of no use.

Scaling in all 3 dimensions-
If you scale the design by the same percentage in all directions; you can use, with some modification, all the information you paid for except the nesting drawings. The frames (item 1) can be resized by increasing all dimensions. The developed hull panels (item 3)can now be increased in size by scaling all dimensions. You will have to figure out how to lay these increased size panels out on the plywood sheets (item 4). You will use more plywood.

I apologize for the lengthy post, (and maybe this is WAY more information than you wanted) but it seems to be a misconception that increasing the length of a design is a trivial task. It can be done, but you are going to be redoing a lot of the information that you paid the designer for.

Ralph

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Post by beardman3 » Sat Mar 26, 2005 12:05 am

makes perfect sense to me. I was actually thinking after I left work that increasing in 3D would probably be the better idea. I guess the real question is then at what point do I need to increase the "Scantlings". I'm assuming at some point just adding 10% to everything will not create a boat as strong as the original design.

sorry what I ment by no work was no redesign work from Jacques and his crew.

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Post by UncleRalph » Sat Mar 26, 2005 12:16 am

beardman3 wrote: I guess the real question is then at what point do I need to increase the "Scantlings".
I think that is where the 10% comes into play. If you don't inrease it more than 10%, there is enough safety margin in the design such that the scantlings don't have to be increased.

Ralph

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Post by Steven » Sat Mar 26, 2005 11:16 am

The one in the picture with the seat and console far back on the starboard side has performance issues do to having so much weith to the back of the boat. You have to respect the weight balance to achieve proper performance.

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Post by beardman3 » Sat Mar 26, 2005 12:33 pm

I was leaning a little more towards a CC setup with a bench seating setup in the same location he uses. That should get me to about 5 decent seating spots without using up to much floorspace. Going CC would alow me a bemini top as well which I don't care to much about but when its 115 and the suns reflecting off the water I'd probably change my mind in a hurry.

anonymous

Post by anonymous » Sat Mar 26, 2005 1:42 pm

For carrying 6-8 people, using a 75 hp outboard and having a hull over 20 feet long I think you should be looking at a more substantial design rather than trying to expand the GF18. To me, those requirements sound like a Panga 20 or 22 would be about right.

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nort
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Post by nort » Sat Mar 26, 2005 4:16 pm

How about the houseboat 20 and don't build the house. You will have a big roomy hull about 20 feet long. You can set it up inside however you want it. Use the planing hull scantlings and you can use your 75 hp motor.

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Post by NorCalBob » Sat Mar 26, 2005 6:27 pm

nort wrote:How about the houseboat 20 and don't build the house. You will have a big roomy hull about 20 feet long.
I was going to suggest the same sort of thing, only with the XF20, building it without the tunnel. That would be very similar to an enlarged GF.
Building: GV17 Built: D15

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Post by LIVEWIRE » Sat Mar 26, 2005 10:22 pm

i dont even want to be on a 25 footer with 6-8 people.
people make boats get smaller and smaller in a hurry.

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