1981 Dolphin 20' Commercial

Questions about boat repairs with our resins and fiberglass: hull patches, transoms and stringers, foam, rot etc.
fallguy1000
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Re: 1981 Dolphin 20' Commercial

Post by fallguy1000 » Thu Jun 20, 2019 2:48 pm

Bigger stringer inside all the way to hull on pad.


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MSRiver
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Re: 1981 Dolphin 20' Commercial

Post by MSRiver » Thu Jun 20, 2019 4:39 pm

I mean 3 layers of 1708 over foam is pretty much the way the original stringers were made just different glass they used back then. If they just didnt do a poor job of laying them up id imagine they'd still be good and i wouldn't be dealing with this. I'm not saying its just as strong as wood but they work and proven and they never rot, several big names of boats use the same concept. Aquasports are the same way, newer boats are also doing a more intergrated system with just flotation foam and glass but essentially the same thing. That being said you do need more glass with just glass stringers since that is your strength but thats the beauty of using todays glass is strong, wood you dont need as much glass since wood it self is strong, and like you said its really just to seal it. As long as you have a good shape (hence this big box beams) for glass to resist forces theres nothing to say glass stringers are just as strong as wood, its just a matter of getting the right shape and thickness.

And i assuming when you say all the way inside meaning the distance between the new stringers will be closer together than original width of them. With that should i split the difference for the small stringers. Or when you say pad you mean like along a fin on the bottom of the hull?

fallguy1000
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Re: 1981 Dolphin 20' Commercial

Post by fallguy1000 » Thu Jun 20, 2019 6:11 pm

Pad is just for plywood. Avoid building the ply stringer directly onto the hull.

Three layers of 1708 is nowhere near as stiff as a piece of 3/4 ply. Nowhere.
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Fuzz
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Re: 1981 Dolphin 20' Commercial

Post by Fuzz » Thu Jun 20, 2019 11:20 pm

Both of the ways you are talking about will work. As you say foam with glass will never rot and lots of builders do it that way. Just keep in mind you will need a lot of glass. three layers of 1708 will give you 1/8 inch of glass.
Go to this site for more glass thickness info
http://fiberglasssupply.com/Product_Cat ... ators.html

fallguy1000
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Re: 1981 Dolphin 20' Commercial

Post by fallguy1000 » Fri Jun 21, 2019 8:09 am

I think there is some confusion here.

The only practical way to build an open stringer is to have a male form and form the glass over it. It would be setup for release. That male plug could be made from say a 2x10 cut to fit.

Otherwise, you could use polystyrene foam and radius the edges well and leave it in. But polystyrene foam offers zero to the strength and will delam under stress. It is there as a form only and would require lots of glass; like I said 5? layers of 1708...

The best solution really if you want the no rot choice would be corelite board. It is about 200-300 a sheet. You can get it cut to 2'x4' panels (4) to reduce cost of shipping. Or, a size more suited to your needs. Say you need 13" depth, cutting to 24" would be silly.

The cheaper solution would be roseburg marine ply.

To laminate them up from 3/4", use a 1/16 trowel and thixo mix with epoxy and cab and hit both sides of the join after the stringers are cut to fit.

The plywood would be best scarfed, but can be butted if you use more glass than less or xtra glass over the join.

The corelite board could be formed to shape and the top edge radiused and the joints butted. Then a layer or rwo of 1708 and some tabbing and you'd be good. Just make sure to allow an opening for glue on the bottom.
My boat build is here -------->

viewtopic.php?f=12&t=62495

Fuzz
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Re: 1981 Dolphin 20' Commercial

Post by Fuzz » Fri Jun 21, 2019 1:14 pm

Lay up two layers of 1708 as long and tall as your stringers. Put them in the hull and scribe to shape. Glue/tape them in place and fill with foam. Then get serious about laying glass over the form you just made.

cape_fisherman
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Re: 1981 Dolphin 20' Commercial

Post by cape_fisherman » Fri Jun 21, 2019 2:54 pm

In your situation (cost, availability of materials, and timeliness) I would not be against using 2bys for your stringers. Spread them out for floor stability & to help close that 29" gap. Epoxy & glass will keep them watertight (do a good job & watch for pin holes).

Weight savings is a wonderful thing, but you will do nothing on a 20' boat that goes beyond negligible. The most weight savings people do on boats like this is removing waterlogged material. What you put back in is typically not much different than original. Yes it can be lighter...but on a boat this size the savings typically aren't worth the cost.

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Re: 1981 Dolphin 20' Commercial

Post by MSRiver » Wed Jun 26, 2019 12:42 pm

Ended up taking out both stringers so that when i put the new ones back in im making a bracket to assure they are bot parallel to each other once i put them in. Surprisingly lost part took just leveling the boat then just deciding the height i wanted my floor to be. Ended up making the back a little higher than original so that i could get a little more floor space up front before it ran into the chines.

I've got the two big stringers cut, just need to epoxy them together and last little bit of trimming before they go in, from there ill get the dimension of what the two outside 3/4" stringers will be.

Quick question, Since i plan on laying new glass on the hull, should lay the new glass before i put the stringers in or after? Also i guess once putting them in i just need to lay a fat bead of glue down first so that they get set in then i can lay glass on them. Which btw i saw i the aquasport rebuild they glass the stringers out the boat first then just tabed them in, seems a lot easier but it may just be because they used the corelite type board for them.

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Re: 1981 Dolphin 20' Commercial

Post by Fuzz » Wed Jun 26, 2019 4:28 pm

If it were me I would glass the inside of the hull before adding the stringers. Also I wouls set my stringers on a pad of epoxy glue and let that harden. Then I would pull nice fillets and glass over them and onto the stringers.

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Re: 1981 Dolphin 20' Commercial

Post by MSRiver » Wed Jun 26, 2019 5:39 pm

Yeah i think would be the way to go. Now i just have to order some more epoxy gonna give the 1:1 marine epoxy a try since its its alot cheaper than westsystem, which ill use on the stringers. Aslo you say a pad meaning like 1/4" thick almost like a rectangular box let that harden then lay another bead down to put stringers on that and fillet and glass? I guess im saying a two step of epoxy glue to stringers to sit on.

Oh btw i liked the idea of adding like a 2-3" PVC down the middle for draining / stringer with 3 layers of 1708 over it. I'm also planning to move the gas tank a little more forward from where it was (to help balance it out and not have all the weight in the back), and add a under deck storage in front of the gas tank. For one i wont have any other storage unless i put a bench up front but kind of like it being open like Whodey's and also could serve as a fish box.

And while im planning ahead, im trying to figure out the best layout for chase tubes, gas lines, and draining. I've seen a few where they they use 2" electrical conduit, 2 down between the two stringers 2 down the inside of main stringers and Im' guessing at like a good 3" from motor to CC. Another issue im going to have is from where the original gas cap was, which is in the front 1/3 of cap and would be very noticeable not to mention have the gas line exposed. One problem is that it has to be in the front since the intake and vent for the tank is in the front, but dont know a good place to mount it to where it will be out of the way unless i put it on the floor but that just doesnt seem smart. Maybe put it in the front bulk head and let it u around under the floor?

Any thoughts or more place to look feel free to share.

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