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Old 05 July 2004, 14:53   #1
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RIB Design - your help wanted!

Right guys, heres one for you to get your teeth into!

I'm new here so please be kind!

I presently own a small Flatacraft Force 3GT with a yam 25hp. I'm finding it just a tad too small now so me and my mate are thinking of designing and building our own RIB.

Right basically we're thinking that we can get a tube and the kit to fit it from Olmec or http://www.geminiindustries.co.uk/ribtubes.htm (does anyone have any views of this company? Their tubes seem to be very reasonably priced [1160 for a 5m with plans for a plywood kit...although I wouldn't use these plans, just the tube]). I'm then thinking we can either make the hull entirely of GRP (but have to make moulds etc so cost and time is higher) or do something like build a plywood shell (say 3-4mm thick on the hull) and then sheath with GRP to make a hull thickness of around 6-7mm? I think this may be easier and simpler, what do people think? What about foam sandwhich construction?

I would like a RIB of around 5m in length with an internal beam of about 1.2m. I also think a deadrise of around 22deg and capabilities to take a 100hp engine. Our aim is to build a light, strong, dry, practical and very fast boat (hhmm, i'm sure thats what everyone would like!).

Anyway, I'm looking for help and suggestions on anything about the design and construction/constructing of a 5m RIB...deadrise, straight or swept tubes, width, depth, consoles, console arrangement, anything you can think of that you think might help. I am however at the moment most interested in the design and construction of the hull and how the tube is fitted (anyone have any hints, tips and info on how this is done?)

I have looked at a few ribs and might base my design on something like the Salcombe Flyer 4.8m RIB. This apparently does 50knts with a 75hp on the back! Does anyone have this boat? If so what is it like? Would you be willing to send me some pictues etc?

Any help would be much appreciated! Thanks in advance.

Cheers

Tim

PS If this project gets seriously underway then I may well be posting lots of messages asking for advice and help!
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Old 05 July 2004, 17:13   #2
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Tim, couple of thoughts, you could find an old rib that requires a retube which would be a good and quite easy start point. Or how about approaching some of the rib manufacturers for just a blank hull ? Ocean Ribs in Portchester near Fareham spring to mind. Or even just rent the mould for a fortnight.

Worth keeping an eye on boats and outbaords. What ever you do keep us informed and take lots of photos. Think 100 hp on a 5m rib will be very unstable because of the shear engine weight, might be better to stick to 60-70 which will be hairy enough.

http://www.boatsandoutboards.co.uk/

Pete
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Old 05 July 2004, 19:43   #3
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Petes right, and the materials alone will cost more than a decent second hand hull!
Also, think about the weight building a RIB in ply - gonna be a bit heavy isnt it?
Good luck with whatever you do and keep us informed!

Nick
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Old 05 July 2004, 19:59   #4
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A high bow and large diameter sponsons are a must for a dry ride.

I doubt that a Salcombe flyer that does 50kts with a 75 will fare well in rough seas. I'm guessing it has a very shallow V to accomplish these kind of speeds and is nowhere near the 22 degrees deadrise that you are thinking about. this would result in it slamming into waves rather than cutting through them and in fact will actually be slower in a choppy sea.

A hull design that would be worth thinking about is a variable deep V hull, deep at the front and then flattening out towards the rear. This would still cut through the waves and give a comfortable ride whilst maintaining good top end performance. The hull of my new boat is based on this principle and so far is performing brilliantly, comfortable, dry, and very fast. Deadrise angle is 23 degrees. I can send you some pics if you want.

One question for you though. if you're going to build the boat yourself and keep the costs down, why only 5m. Surely it wouldn't cost that much more to go for a 6m or even 6.5m.
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Old 05 July 2004, 20:55   #5
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Quote:
Originally Posted by Nick Hensel
Petes right, and the materials alone will cost more than a decent second hand hull!
Also, think about the weight building a RIB in ply - gonna be a bit heavy isnt it?
Good luck with whatever you do and keep us informed!

Nick
Remember F1 racing boats are made from ply!!!
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Old 05 July 2004, 21:37   #6
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Go for it. The satisfaction will be worth the work but the work will be considerable.

I had a similar idea but, because of personal circumstances, decided to give up on it. I've regretted it since. However, I'll give you a few thoughts.

6-7mm including 4mm ply isn't thick enough. I doubt too that you'll get an even enough surface to mould onto using that thickness of ply.

You'll be limited to a warped flat sheet, (In fact, at that thickness, most of your sheets will come pre warped! ) although you can join panels to improve the design. That's not to say you can't produce a very satisfacory hull from sheet.

My own plan was to build the formers in ply (which would be removed after the hull construction) and sheath these in strips of wood about 15 thick x 25 - 40mm wide. I had planned to use cedar or spruce. Spruce is a bit heavier but is has give, it's also tough and it glues well. I reckoned this method would allow the making of compound curves and good detail could be achieved because there would be plenty of thickness to work with. After sheathing it should give excellent ridgidity. Two substantial logitudinals inside the hull and a couple of good sized knees, which are joined to the sides of the longitudinals, to support the transom. As well as the usual resin/mat sandwich at the joins, there would be coach screws to give a mechanical pinning rather than relying solly on the resin to wood bond.

If you have the ability, I hope you go ahead. I'm sure it will be a rewarding experience.
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Old 06 July 2004, 13:21   #7
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Thanks for the comments guys. Pete7, I do check boatsandoutboards.co.uk all the time but I don't often see anything suitable or cheap enough. I am doing this on quite a budget (although I would be sharing with a mate) but thanks for the suggestion! I am constantly keeping a look out.

Nick, I guess it could well be heavy but I'm not really sure. As codprawn says many racing boats are made of ply! I was thinking along the lines of having a very thin skin of ply purely to allow me to layup some GRP on it and to provide a little strength I guess. I don't really know if this is a feasable idea though. I won't do it if it'll weigh too much!

swginn, thanks for your hints, I've noted them down! I think the flyer rib has a deep vee hull, but it is designed by a ex powerboat racer. The Salcombe Flyer range of boats seem to get good reviews. There must be someone on this forum who has a Salcombe Flyer rib?! Some pics of your boat would probably help so if you could send them that would be great! Thanks. Also the thing with the length is that I am limited by the size of my garage. Also 6m needs bigger engines etc and so more expense!

jwalker, thanks for your comments too. If 6-7mm is not thick enough, what is? The hull will be supported inside by ribs and bulkheads etc.

Does anyone else have any ideas? Any annoying little things that could be put right if you were to build your own rib? Things that would make life a little easier? anything!

Cheers

Tim
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Old 06 July 2004, 14:56   #8
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An issue, which is particularly relevent since you are looking at a highish power/weight ratio is the hull design, both the form & structure. Designing a good hull from scratch is a MASSIVE job. Even taking ideas from other boats for the hull form is risky because, every tiny aspect of the whole boat interacts and has consequences on other areas. Varying things even by as little as 1/4" can make a very big difference in the hull's performance. And to reiterate, this is especially so given the performance you are aiming for.

2ndly, very few people really understand structures, and the loads that are transferred through a boat. Even the pro's get it quite seriously wrong from time to time. You can just build it thicker and heavier to make up I suppose, but that's a downward spiral, and 1 reason of doing it yourself is to try and do it better than the competition.

Of course, it can be done, but be realistic on your goals and aims, speak to the right people who know about designing hulls & structures, and know who to just smile at and ignore.

If you're serious, get a copy of Glen-L's (or similar) books:
http://www.glen-l.com/books/fgbb.html
http://www.glen-l.com/books/bb.html
For an understanding of the work involved in building, there are various different ways you can prototype a hull from scratch in both grp & tree
without having to create a plug & mold. I like these 2 books particularly because they give a good overview of a large number of techniques.
It might also be worth looking at his designs. Although they look very dated , you might be able to take an existing V-hull design lock stock and barrel and adapt it to your needs. I like the Bandido personally.
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Old 06 July 2004, 14:59   #9
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I reckon 6-7mm could be thick enough, depending on if you have the structure behind it to support it and if the surfaces are skinned in GRP.
Just my HO.
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Old 06 July 2004, 19:30   #10
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Quote:
Originally Posted by slimtim
If 6-7mm is not thick enough, what is? The hull will be supported inside by ribs and bulkheads etc.
Well, if you used your idea of 4mm ply, (I take it you're are familiar with 4mm ply, it's very flimsy.) that would leave only 1.5mm layup maximum inside and out. Simply not enough. My previous 6.5 was about 10mm thick along the top edge of the hull. Compared to fibreglass, thickness to thickness, ply is light.

Don't do bulkheads, at least not ones which are in contact with the running surface.
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