1970 Seafari conversion to CC

[cdavisdb]

With your set up, stringers plus so many bulkheads, I don't think hull flexing will be a problem. Still, the possibility of hard spots makes it a good idea to spread the load on the hull around the base of the bulkheads, with some sort of pad. I used 4 layers of 1708 plus some thickened epoxy, some guys have used foam triangles, not sure exactly how those went together.

[afishin82]

Originally Posted by cdavisdb

With your set up, stringers plus so many bulkheads, I don't think hull flexing will be a problem. Still, the possibility of hard spots makes it a good idea to spread the load on the hull around the base of the bulkheads, with some sort of pad. I used 4 layers of 1708 plus some thickened epoxy, some guys have used foam triangles, not sure exactly how those went together.

Pretty sure she wont be flexin!! 4 layers to tab em in?

[Bushwacker]

Originally Posted by afishin82

so its strong enough without em will it be stronger with them. I already cut em and am ready to install em. I would think they would be neccessary down the center cause its quite a span! Ive already done most of the work

It's also heavier and more expensive than necessary! That's why you should use this excellent forum and ask questions BEFORE you start cutting and fabricating!

I see that you're also adding a bunch of potential hard spots in the bottom at every one of those bulkheads that may cause cracks in the bottom where the relatively softer panels will tend to bend about the rigid bulkhead! The standard way of installing bulkheads is to avoid concentrated loads by inserting a trapezoidal shaped foam spacer between the hull and the bulkhead. This will allow loads from the bulkhead to spread out and gradually transition into the thinner softer panels. In his "Elements of Boat Strength" book, naval architect Dave Gerr recommends that the spacer width at the hull surface be about 8X the bulkhead width. If the spacer makes an angle of about 45 degrees with the bulkhead, it will also make it easier to lay in fiberglass tabbing between the hull and bulkhead.

[afishin82]

Originally Posted by Bushwacker

It's also heavier and more expensive than necessary! That's why you should use this excellent forum and ask questions BEFORE you start cutting and fabricating!

I see that you're also adding a bunch of potential hard spots in the bottom at every one of those bulkheads that may cause cracks in the bottom where the relatively softer panels will tend to bend about the rigid bulkhead! The standard way of installing bulkheads is to avoid concentrated loads by inserting a trapezoidal shaped foam spacer between the hull and the bulkhead. This will allow loads from the bulkhead to spread out and gradually transition into the thinner softer panels. In his "Elements of Boat Strength" book, naval architect Dave Gerr recommends that the spacer width at the hull surface be about 8X the bulkhead width. If the spacer makes an angle of about 45 degrees with the bulkhead, it will also make it easier to lay in fiberglass tabbing between the hull and bulkhead.

Ok so I at least need bulheads where the tank is if I cut them to accept a triangle piece of foam between them and the hull and stringers will that do it? I put a 45* bevel on all the seams and I guess an added support underneath attached to the bottom of the floor to ensure there is no flexing in the floor.

[afishin82]

Originally Posted by Bushwacker

It's also heavier and more expensive than necessary! That's why you should use this excellent forum and ask questions BEFORE you start cutting and fabricating!

I see that you're also adding a bunch of potential hard spots in the bottom at every one of those bulkheads that may cause cracks in the bottom where the relatively softer panels will tend to bend about the rigid bulkhead! The standard way of installing bulkheads is to avoid concentrated loads by inserting a trapezoidal shaped foam spacer between the hull and the bulkhead. This will allow loads from the bulkhead to spread out and gradually transition into the thinner softer panels. In his "Elements of Boat Strength" book, naval architect Dave Gerr recommends that the spacer width at the hull surface be about 8X the bulkhead width. If the spacer makes an angle of about 45 degrees with the bulkhead, it will also make it easier to lay in fiberglass tabbing between the hull and bulkhead.

Do I need the foam between the stringers. And I was told to attach the floor using 5200 seems like that is not the way to do it. Can I use thickened resin and if not wat? I saw a guy used a piece of 2inch foam and roven woven. That would be a lot easier than this trapazoid deal!

[Bushwacker]

Originally Posted by afishin82

. . . 4 layers to tab em in?

4 layers of 1708 would be about 1/4" thick and serious overkill for your application, although Connor was trying to fix a loose stringer/cracked hull problem, so very appropriate for his situation. If you study the laminate schedule and detailed stringer tabbing sketch in the Boating Magazine article on the 20' Seafari, you'll see that Moesly used the equivalent of 2 layers of 1708 to install key structural members, i.e., the stringers. For the unnecessary bulkheads, I think one layer would be plenty.

The only bulkheads in my Seafari are the one just forward of the gas tank, and the one under the bunks, just forward of the head. Don't remember what the fuel tank bulkhead looks like, but from the attached pictures of the forward bulkhead, I'd say it's tabbed to the stringers with one layer of heavy roving. Note that there's no tabbing at all around the keel. Forum member Old Blues Player said the tabbing had come loose on his, which is why I took these pictures. Maybe his boat had a quality problem/"Monday or Friday build", but my boat has seen some very hard use on numerous Bahama trips, and those tabs still look good.

You might want to install some limber holes in those bulkheads so you don't build in a bunch of water traps! Any water that gets in those spaces needs to be able to run back along the stringers where it can get to the bilge pump thru the limber holes at the back of the stringers.

[afishin82]

ok did some research and form what i could tell it seams if i leave an air gap between the hull laminate and bulkheads it should fix the hard spot issue. i assume this is true because the glass used to tab them in will flex with the hull . u think that would help? U cant see in the pics but i cut out the lowest corners of the bulkheads so water will drain to the bilge.

[cdavisdb]

Just a clarification. The Seafari 25 was built with one less bulkhead than it needed, plus all mine were rotted. The 4 layers of 1708 were laid on the hull under the new bulkheads and tapered so that the bulkhead load would spread. The gap between bulkhead and hull, I filled with thickened epoxy which was also wider than the bulkhead by the time it got to the hull. 3 layers of tapered 1708 to tab the bulkheads to the hull. In retrospect, I probably should have used trapezoidal foam blocks, but so far, no problems.

[Bushwacker]

Originally Posted by afishin82

ok did some research and form what i could tell it seams if i leave an air gap between the hull laminate and bulkheads it should fix the hard spot issue. i assume this is true because the glass used to tab them in will flex with the hull . u think that would help? U cant see in the pics but i cut out the lowest corners of the bulkheads so water will drain to the bilge.

Yes, the air gap would eliminate the hard spot issue. However I think gravity + capillary action might cause resin to wick out of any cloth spanning the gap, so it would help to fill the gap with something soft to prevent that. I believe cabosil is a little too hard for that, but any type of caulk that provided a large radius fillet to support the glass would work.

The purpose of the bulkheads is to tie the stringers together to provide lateral support, which prevents them from buckling while also adding a lot of torsional stiffness to the structure. This would be particularly important if the stringers were tall thin 1x12's with a large difference between their "stiffwise" and "easywise" bending resistance. (bending about the respective horizontal and vertical axes.) However your stringers are ~2 inches wide at the top and probably 2.5-3" wide at the bottom, so they're already pretty stiff in both directions. Simply tying all 4 of them together at the top, combined with the robust tabbing of the stringers into the bottom laminate, has proven to create a very stiff and torsionally rigid structure that doesn't really need bulkheads.

After thinking about the changes you've made to that hull, I've concluded that it won't have nearly as much torsional stiffness above the deck as the original Seafari had with it's cabin top and bulkhead that tied the hull sides and gunnels together, nor will it have the bending stiffness provided by the tall cockpit coaming. Therefore any extra torsional stiffness provided by the under deck bulkheads is a good thing! More robust tabbing of them to the bottom with 2 layers of 1708 would also improve torsional stiffness; taper the tabbing so top layer is a little shorter than the first one. That horizontal rib about halfway up the hull sides will make a handy shelf, but it adds virtually none of the torsional stiffness that you'd get from adding a core to the hull sides. You could maximize the overall torsional and bending stiffness of what you have at this point by building a cap with wide gunnels, a thick core, a large front deck, and a wide full transom cap. Some vertical ribs/rod holders to tie that horizontal shelf to the deck and gunnel would also add a little more bending stiffness to the hull, although it probably won't do much for torsional stiffness.

[afishin82]

Originally Posted by Bushwacker

Yes, the air gap would eliminate the hard spot issue. However I think gravity + capillary action might cause resin to wick out of any cloth spanning the gap, so it would help to fill the gap with something soft to prevent that. I believe cabosil is a little too hard for that, but any type of caulk that provided a large radius fillet to support the glass would work.

The purpose of the bulkheads is to tie the stringers together to provide lateral support, which prevents them from buckling while also adding a lot of torsional stiffness to the structure. This would be particularly important if the stringers were tall thin 1x12's with a large difference between their "stiffwise" and "easywise" bending resistance. (bending about the respective horizontal and vertical axes.) However your stringers are ~2 inches wide at the top and probably 2.5-3" wide at the bottom, so they're already pretty stiff in both directions. Simply tying all 4 of them together at the top, combined with the robust tabbing of the stringers into the bottom laminate, has proven to create a very stiff and torsionally rigid structure that doesn't really need bulkheads.
Not entirely sure wat torsional stiffness is but I want it haha. I was gonna build boxes to go from the floor to the cap
After thinking about the changes you've made to that hull, I've concluded that it won't have nearly as much torsional stiffness above the deck as the original Seafari had with it's cabin top and bulkhead that tied the hull sides and gunnels together, nor will it have the bending stiffness provided by the tall cockpit coaming. Therefore any extra torsional stiffness provided by the under deck bulkheads is a good thing! More robust tabbing of them to the bottom with 2 layers of 1708 would also improve torsional stiffness; taper the tabbing so top layer is a little shorter than the first one. That horizontal rib about halfway up the hull sides will make a handy shelf, but it adds virtually none of the torsional stiffness that you'd get from adding a core to the hull sides. You could maximize the overall torsional and bending stiffness of what you have at this point by building a cap with wide gunnels, a thick core, a large front deck, and a wide full transom cap. Some vertical ribs/rod holders to tie that horizontal shelf to the deck and gunnel would also add a little more bending stiffness to the hull, although it probably won't do much for torsional stiffness.

Not entirely sure wat torsional stiffness is but I want it! I am going to widen and core the cap l, build three boxes to go from the floor to the cap and put them roughly where the bulkeads are! Glad u like my shelfs haha!