newbie do i need to add boyancy

hi all i have been restoring a fibreglass dinghy have added top rails all round boat and have added middle seat and rear seat but as i have added weight do i need to add more boyancy now its a single skin and at present only has a boyancy chamber built in at the front the seats i have added are box seats any advice would be great thanks

 

Pictures would be most helpful, as would the make, model and year of your boat for reference. In an attempt to offer an answer, yes, you probably do need some buoyancy, but how much depends on what you've done. Pictures and a better description would be helpful.

 

If its a dinghy the fastest way to find out is to get it in shallow water, dump it over and play a bit. The answer should become obvious.

 

re do i need to add boyancy

hi thanks for advice though am still unsure what to do i have added a pic of how boat was when i got it but dont know what type it is or how old she is i havnt taken pics of it how it looks yet as still working on it but this is what has been done so far original seat removed all round top edge wood has been added to inside 2x2inch just off center towards front box seat is added ply hight of seat 1ft width 1ft goes from side to side and is hollow at moment at the back seat added ply again depth 10ins width 15ins and joins to both sides and is also hollow which i was going to fill with foam and middle seat as storage other than back seat should i add foam to inner floor and part of the inner sides
many thanks

 

You don't have to fill the seat boxes with foam, but it would probably be a good idea to make full boxes not just a thwart, at least the furthest aft seat anyway.

 

I used to manufacture fiberglass dinghies many moons ago, built well over 1000 of various sizes. All our boats had bow and stern bouyancy tanks/seats as shown in your photo which was enough. If you choose to add more add it high,not in the bottom.As Par said you dont need to put foam in the seats as long as they are properly glassed in and are indeed airtight,air gives the most floatation for the least weight.That said,i would add foam in the form of styrofoam peanuts which can be had for free from businesses that receive a lot of packages. In my experience people tend to wear away the glass on the forefoot where the boat first hits the beach and the aft end of the keel where they pick up the bow and drag the boat up the beach,these just happen to be the locations of both the bouyancy tanks,it would be nice to think it wont happen but ............,foam is cheap insurance with insignificant weight in this quantity. Dont use the polyurethane A/B foam,it absorbs water when the hard skin wears away.
Another thing worth doing is to layup thick kevlar wear patches in these two areas,you can buy kevlar felt from Raka that works well for this,it works better than metal 1/2 oval because you can make it as wide as you need and it needs to wrap up the sides of the keel.
Steve.

 

Hi Maidtracy, since youre on that big island , you might pick up the MCA guidelines for floatation. Perhaps on the Internet ? Over in the colonies they use USCG floatation recommendations. This is a copy, might help you with your floatation placement and volume.

http://www.jamestowndistributors.com/userportal/pdfs/SafetyStandardforBackyardBoatBuilders.pdf

 

look here http://newboatbuilders.com/pages/flot1.html and here http://newboatbuilders.com/pages/flot_calcs.html

A boat that size needs only about 2 cubic feet of added flotation. Part of that you already have in the bow. If you add a 1/2 cubic foot on each corner at the stern it will probably be enough. Make two fiberglass boxes, 12" by 12 x 6 " and glass them into the corners. You can leave them as air chambers or as someone suggested fill them with styrofoam peanuts. Altenatively go to a hardware store and buy some insulation foam. It comes in 2 inch thick sheets, or maybe you can get some scrap from a construction site or a house being built. Cut six rectangles 12 x 12 by 6". Glue three together ( 2 each ) and glas them into the rear corners. But use epoxy resin. Polyester resin attacks polystyrenes foams (styrofoam).

 

cant really tell how long that is from the photo, but looks similar to my 13' little fishin boat I'd built up from a real basket case of a "traveller" 2+2 40hp fish-n-ski runabout.
I went as far forward as possible/comfortable to reach the tiller of my littler 20hp outboard with the rear bench, hatches behind that making a rear casting deck (under which is 6 gal fuel on 1 side and other gear under the other side). the bench itself is "vented-closed", but stuffed with 2 liter bottles for floatation was cheep effective.
the other seat runs lengthwise up the center for passenger position to help trim it.
(hinged top, it's also the livewell)

having seen many small boats with lengthwise bench seat up the center, it made sense to me for trimming weight forward/aft, where the passenger sits centered kinda like a jet ski, and rides dryer too. from the pic, also cant tell if the dingy is a planing hull or a displacement type hull, but either way, seating that allows for balancing forward/aft would probably be advantageous, but it's also completely different from thwart benches.
I kinda prefer the bench up the center for less balancing issues left/right, and being less inviting to try to carry more persons than a small boat can reasonably handle.