My 1975 SeaCraft 25' Seafari rebuild thread

[hermco]

the max bending stresses are at the outer surface.

Is why you want as close to an equal laminate on both sides of the core as possible to achieve the "I" beam.

[FishStretcher]

Decks with sandwich core construction tend to have unequal skin thicknesses. This isn't contradictory, rather that the minimum thickness required for stiffness isn't enough for when you drop the anchor on the deck. So you add more laminate there. But it isn't needed for the bending stiffness.

[Blue_Heron]

Originally Posted by Bushwacker

A bracket will evenly distribute motor loads across the entire transom, so you'll actually have lower concentrated loads and stresses on your rig than you'd have on a conventional outboard or outdrive transom mount!

... (When you bend an I-beam, the compressive and tensile stresses are directly proportional to the distance from the neutral axis, which is at the center of a symmetrical part, so there is NO bending stress at the neutral axis. A cored panel is similar to an I-beam, so the max bending stresses are at the outer surface. This is why cored panels can be so stiff and strong with relatively weak and lightweight core materials like foam. As long as you use epoxy or vinylester resin for good bond strength, that transom will be stiff as a brick!

Originally Posted by hermco

Is why you want as close to an equal laminate on both sides of the core as possible to achieve the "I" beam.

Originally Posted by FishStretcher

Decks with sandwich core construction tend to have unequal skin thicknesses. This isn't contradictory, rather that the minimum thickness required for stiffness isn't enough for when you drop the anchor on the deck. So you add more laminate there. But it isn't needed for the bending stiffness.

This is all good discussion about the fundamentals of boat structure. It got me thinking that maybe we should start a thread on the basic structural concepts of cored laminates and other aspects of boat construction. If it proves useful, it could be made a sticky. I've bookmarked some good links, maybe I'll start something when I have a chance to pull it together.

Dave

[flyingfrizzle]

Originally Posted by Blue_Heron

This is all good discussion about the fundamentals of boat structure. It got me thinking that maybe we should start a thread on the basic structural concepts of cored laminates and other aspects of boat construction. If it proves useful, it could be made a sticky. I've bookmarked some good links, maybe I'll start something when I have a chance to pull it together.

Dave

I think that would be a very great Idea!

[FLexpat]

I'm sure lots of guys have developed a table like this but here is one I work with...
It took me a while to notice that mat is is units of oz/ft2 while most other glass in in units of oz/yd2 and why the biaxial with mat didn't weigh what I expected

[flyingfrizzle]

Flex, Good info on the weights and glass to resin ratio!

I got the transom cleaned up a little more this weekend while dodging the rainfall. I fitted the core in the hull and made sure it would fit plus lay flat. I ended up having to notch the top port side gunnel to get it to drop down in there but it was the only way with out cutting something else. It will be easy to glass the piece back in once the core is in place. I had already cut the top of the core section on a 45 and rounded it so the glass would roll the corner well but after dry fitting it I realized I needed to cut the back of the side edges on a 45 too and round them smooth as well. The radius on the transom skin to the hull edges were holding the core off the back of the skin a little bit. Now that it is fitted I will seal the wood core section this week after work and have it hot coated and ready to bond in to the back of the boat. Just need some good warm weather to do it. It is 75 degrees today but rainy. Then come this weekend the highs are suppose to be 30 something degress!

Few pics of the rear cleaned up:


A pic of the the core were I cut the 45 degree angles on it:


Were I had to notch the gunnel to get the core to drop in:


Pic of the core dry fitted in the boat, (still needed trimming at this point so it is not sitting all the way down and back yet):

[flyingfrizzle]

I got the core hot coated on the back side that will be bonded to the old hull skin. I used thinned VE resin with 1% hardner so that it would cure slowly and allow the wood to soak up as much as possible. I also cut out a section of CSM and laid it up on the core to help hold a little more resin and full seal the wood off from soaking up any moisture. I double coated the outer edges that wick up the water the worse to ensure a good seal. Once this dries I will scuff it up with some 120 grit to give it some tooth for the Epoxy resin to bond to when I install it into the boat on the rear hull skin. I feel like the VE resin thinned with styrene will soak in to the ply a lot better than the epoxy would of done if I waited and done in place with a raw wood ply in the boat. I can lay it flat on a table and thin the resin thin as possible with in the recommended amount. If I would of installed the raw wood into the boat the resin would of run right off/out of the cloth with it standing up vertical. To keep the epoxy resin from sagging or running it needs to be thickened and that would keep it from absorbing in to the wood as well so I think it helps to pre-coat it like this with the thin VE resin. Plus I know that the wood is sealed as well as possibly now, if done once installed into the boat there is no guarantee you will get it 100% sealed off (it may trap air pockets) plus you would get much less to penetration into the ply. I will hot coat the front side as well but not till I am ready to install it into the boat. I will do it laying flat on the table like the back with thinned resin but would like to get a few coats then once it tacks up a little install it. Then while it is still tacky in the green window time, lay up the laments on the front side to get better continuous bond. This will give you a good wet on wet primary bond that will be stronger and have better peel strength than if let fully cure and bonded latter as a secondary bond. Secondary bonds can be good (especially with epoxy) but the more wet on wet green primary bonds you can do the better off the bond is. Sometimes you cant help it just like bonding to the old rear poly hull skin from 1975, that's why I use epoxy there to help get a better secondary bond to the old glass that is original poly. The epoxy has more peel strength and should maintain a better bond to the old skin than a standard poly or VE. Both resin types have their advantages and VE will help water proof the Glass where pours poly will not as much. VE thinned will absorb better than the non styrene based epoxy and the cost less while the epoxy has superior water proofing effect with a stronger peel strength and elongation. You can use the Epoxy over the VE and poly just fine with out problem but to use VE or Poly on top of epoxy is a no no. They say it will not bond well and if I dose it is a waist due the weaker bond under it will fail first. There are may ways to install a transom, and I do it differently sometimes from situation to situation but with wood I like to do it this way to make sure I wont have water intrusion problems in the future. If using foam I would of done this differently.

I will be waiting on some better weather so I can get the transom in. Due to the boat is a few feet to long to go into the heated shop, I have to do it outside or under my shelter that is not heated. It is suppose to get into the single digits this weekend at night. Highs in the 30s during the days so I will play around with the bracket till the weather breaks and it warms up a bit so I can get the core bonded in and glassed down/tabbed to the hull.

Picture of the core with the rear sealed: The resin is almost cured and turned white in this pic. It starts out redish bay colored then turns white as it cures. It will be solid white once fully cured and you wont be able to see threw it. The VE was formulated to do this, the rep gave me a few reasons such as- easier coverage with paints and gels, it keeps the lay up from shrinking, and a few other things as well. It is amazing how white it gets after starting out so red.

[flyingfrizzle]

Well while I wait for a good warm day to bond the core in with the thickened epoxy I decided to move forward with the build and get the outboard bracket built. I got some 3/4" melamine from home depo and started to figure out how I was going to build the bracket. I decide to make the flotation tub as large as possible so I can get as much flotation at rest as possible. With the deep vee on this hull I think I should be able to get a fair amount more out of it than the one I did on my 20'secptre. I will be going 60" wide which will take it out almost to the outer edges of the second or middle steeps on the hull. I am coming up 2" from the hull bottom to allow for the water rise and following the deadrise of the second panel. This is a 20 degree panel and will give me about a 40 degree vee on the bottom of the tub. I thought about following the steeps of the hull bottom and changing it from 20 to the 26.5 degree of the first panel but think I will keep it a constant grade across both panels. This will allow me 4" at the bottom of the vee so I can get two garboard drain plugs at the back of the transom with out having clearance problems with the tub. The variable deadrise would look cool on the bottom of the tub but would not work well with the drains and it dose not add much air space so it would be a lot of extra work for little gain. The set back will be 22". I want to keep it to a absolute minimum so it dose not kill the balance of the hull too bad. At 22" I hope to be able to tilt the motors up enough to clear for safe travel and for good clearance in shallow water if needed. If I cant obtain full tilt I will limit the tilt on the motors so it will never hit the top of the rear of the transom but with twins space out wide it might actual clear tilted all the way up.

[flyingfrizzle]

I have a question for the knowledgeable up here, I have some questions about setting up twins on the 25'. I know most people set twins up on 26"-28" centers, sometimes 24" center to center minimum - to 30" center to center is common. You see more set close together than you do apart but I have read up here and told by some of you guys that you will get the cleanest water for the props if you set the motors center of the center panels (middle hull steep). This would set the motors at 50" centers apart. I looked back at some pictures, and the early 21' race boats had motors spaced far apart with the little low power (55hp) homelites but most of the latter 21'race hulls had the (mercury 110hp) motors very close together. I am wandering if they found to have better performance running them close together. I know that people say the closer the more speed you gain and less blow out when turning. Most of the go fast guys run them close together as possible for several reasons but know the vdr may behave differently. The turning may be the biggest issue with the wide spread due to the 25' rolls so much, I would think it could cause a bad blow out issue on this hull. Also I have heard that too close together may cause water to be stole from prop to prop and counters could cavitate if too close. The engine height will vary depending a pond where I place them, and far as now I figured on setting the bracket mounting tab at 31" above the hull bottom so that it would take a single 30" leg motor with the AV plate slightly above the keel. So with me running twins (25"legs) that would work out if I did the 28" centers but if I run them out apart further the mounting tab would need to rase the motors an additional 3" or so to compensate for the deadrise. You don't see many outboards today spread apart far except for cat hulls and such so I am guessing there is a reason. Also one other question, How much toe in do the motors need? I have read that at the standard 28" apart they need 1/4" of tow in for counter rotators. If spread further apart would this change? What would work best 1/8" or 1/4" or? I assume there is a lot of trial and error here but like to have a good starting point. Bushwacker, and any of you other engineers or experienced guys in performance twin set up help!

Here is a few pic's to show the spacing on the twin outboards on the 21' race hulls which as very very similar contour to the bottom of the 25's as shown by another member before up here that graphed the bottoms on a cad computer program and compared them.


These two are close together:





These two are further apart:

[Blue_Heron]

You bought your boat from George, right? Didn't it have twin I/Os? How far apart were they?

That being said, the original spacing may or may not have been ideal. I think your thought process on the pros and cons of wide versus narrow is good. As you said, the boat heels a lot in a turn, and that may cause the outer prop to blow out in a sharp turn. On the other hand, with outboards you have the flexibility to adjust engine height to compensate.

The best guy to ask would probably be bilgerat. He's got the only 25 I've seen with twins on a bracket. If he doesn't respond here, you should consider sending him a PM.

Dave