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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.
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Dave |
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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 :o Attachment 9192 |
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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: Attachment 9197 A pic of the the core were I cut the 45 degree angles on it: Attachment 9195 Were I had to notch the gunnel to get the core to drop in: Attachment 9196 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): Attachment 9194 |
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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. Attachment 9198 |
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.
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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: Attachment 9229 Attachment 9230 These two are further apart: Attachment 9231 Attachment 9252 |
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 |
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Thanks for the imput, True, I didn't think about the dimensions of the twin outdrives. They appear the be as close together as the motors would allow. I think there may be some clean water to be had in different spots off the vdh but after reading for hours I have a good idea what works best for most boats so I know where to start. There is also a fellow north of me in Virginia that has 76 25' with twin v6 mercurys on a bracket and I have spent some time talking to him over the phone. I would like to reach out to bilgerat if you read this give me a shout. |
Twin Engine Set-up on VDH
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If closely spaced motors see much air from that inner step on the VDH, they may be just as likely to blow out or might have to be trimmed down lower than if they were mounted further outboard! However I only know enough theory to be dangerous, and have absolutely no experience with twins and so I respectfully defer to bilgerat, and especially Fr. Frank, who has lots of experience rigging and running twins on very deep narrow hulls. Forum member Flying Time is a naval architect who also owns a race boat hull that's similar to your 25 and may also have some good advice. I'd also suggest you send a PM to Carla (65Bowrider) to see if she can ask her dad what he'd recommend for a 25 Seafari with a bracket. I know he recommended the wide spacing for their Moesly 21 project boat which originally raced with the twin Homelites, both for "clean water" and minimum draft considerations. He also commented that the twin 250's on the bracketed 27' Seamaster "Sea Mistress" he bought were mounted too close together. Both of those boats have less transom deadrise than the 25 however, which may be a totally different animal. At the transom, GFS's race boat hull looks a lot like the 25 Seafari without the outer panels, so whatever worked best on the narrow race boat hulls might also apply to the 25. You probably have the motors mounted close together on your race boat, so that gives you a test vehicle that you could use to experiment with toe-in/out, etc. I'm sure that the Merc folks probably did lots of testing at Lake X on the race boats to determine what combinations gave the highest WOT speed. On a family cruising boat however, other factors like draft, handling characteristics, fuel efficiency, and low speed planing ability are more important than a couple of mph in WOT speed, and that might be why Carl recommended the wide spacing! |
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Well I have most of my bracket mold cut and assembled. I used 3/4" Melamine sheets to form the mold so that I could wax the surface and have few to no sticking issues. I sometimes use MDF sheeting and then line it with plain white Formica which works better for doing curved parts. I decided to make it 22" high to the top of the swim platform with a 22" set back. Not a 100% sure the motors will get a full tilt here but if they don't I will modify the rams so they will not over extend and hit. I am much more concerned with minimizing set back vs getting more tilt. If I can tilt the motors up enough to get past the hull bottom and also clear the road when on the trailer then that will be all I need. Just want to help the balance by keeping the setback short as possible with out notching the transom and loosing the factory fish boxes across the back. The flotation tank will be 60" wide to go all the way out to the start of the 3rd steps. That will leave me just enough room for trim tabs or k planes. The swim platform will be 72" wide extending 6" past each end of the floatation tub. The flotation tub will be mounted low as possible, at 2" up from the hull bottom to clear the rise of the water rolling off the back of the transom. The tub will also have a 20 degree vee on the bottom to match the second step deadrise so that it will parallel with them. This will leave just enough room under the bottom of the tub for a garboard plug set center of the transom. The vee section of the tub will be angled slightly upwards to the rear point to allow an additional 2" of clearance for the water surface. This should be enough to keep the flotation bracket from dragging the water when the boat is running with no trim.
It was fun figuring out all the angles when cutting the pieces and designing the bracket. The transom in the 25' seafari is about 12.5-13 degress just like some of the other seacraft hulls. With the transom lay back and the upwards taper of the flotation tub aft made for some complicated compound angled pieces that were a little brain racking to figure out but not so bad once it was plotted out on paper. I made the sides of the tub section of the mold taller than needed so that when I glassed it in the top edges would not roll away from the side of the mold when the fibers curl out past the top edges. I will just cut it to height with a thin cut off wheel once popped out. The front side that will bolt to the boat will have a 4.5" notched lip inwards to allow for the swim platform front lip to lap over it and in place while maintaining a flush surface. I actually cut the last 4.5" off of the front side of the mold then used a 1/4" piece of ABS plastic to space the top strip inwards towards the rear of the mold then backed it with some scrap 3/4" ply that was scrap from the transom core. I also added a 6" wide strip of 3/4" ply to the top of the front of the mold just to hold it square. In the pics below you can see the stepped section. I will contour the corners tonight with clay and round them with my radius forming tools. Then it will be ready for waxing and layup. Here is a few Pic's: Cut and ready to screw together: Attachment 9277 Attachment 9278 Attached together: Attachment 9272 Attachment 9273 Showing the notched edge for the overlap of the swim platform: Attachment 9276 Attachment 9274 Attachment 9275 The sketch and design...lol Attachment 9279 |
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On the width of swim platform, you might want to consider extending it far enough past flotation tub to be able to mount a swim ladder underneath platform! Regarding angling bottom of swim platform, you're losing some flotation doing that. Looks like you're planning to mount it a bit lower than mine to gain flotation (good), so understand desire to prevent dragging in water, but might want to send a PM to Don Herman about that. The Hermco bracket is straight on bottom and longer (30"), but it doesn't drag in water when on plane, even at 12 mph. Even if it did drag temporarily coming on plane, it would act like a trim tab and provide more stern lift to help boat get up on plane, so I don't think that's a bad thing! Might look at making bottom straight with no slope but mount about 4" up from bottom, and see which design has more flotation. |
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Now about the taper on the tub, I may be worried about something that will not be a problem. I figured it may need the extra room due to me mounting the tub so low. I didn't realize the Hermco is straight outward on the vee part of his, Every bracket that I have seen other than his has had a taper up wards on the vee and more than the 2" that I used. Most taper is like 4-5" up wards. I don't want to loose the volume and had it sketched out as a straight leg but after pulling strings and taking measurements I figured it would drag unless the boat is riding very flat, level or have the tabs buried to keep the nose down. Even with the 2" up wards angle it will only allow for a minimal attack angle before it drags. I changed this last minute after reading about another guy who made one without the taper on the vee and it caused his to porpoise back and forth he clames. He said it would hit the vee and then it would make the nose dive like a tab would but then it would lift back on it and start proposing. He said the boat did not do it before the bracket but only after installing. It was mounted low also. His may of just been a balance issue mainly but it got me worried and that is my basis for adding it. Far as what would have more flotation volume (not a math wiz) but the 2" taper vs no taper at a 2" higher mounting height would cause me to loose more volume if mounted higher even with the straight leg? The lower 2" mounting would allow the larger part of the tub to sit in the water more vs only a 1/2 of the 2" slice I am cutting off. Correct me and I may be wrong but wouldn't the lower mounting with the 2" taper yeld more volume? You got me curious now because the main thing I wanted was the most volume as possible with out any negative effects. Looking at other pics of 25' seafari's they seem to ride bow proud with the hulls planning at an angle upwards and this would greatly effect the outcome of needing the angle cut on the tub or not I am thinking? I figured to start with the water rising off the transom will rise at 1" per foot back, so that will leave little clearance at 22" back due to the water should of rise around 2" at this point letting it just scrape by with out any addition room for error and that is assuming the boat is level in the water at plane. Factor in the angle of attack the boat will ride at and that will cause the tub vee to angle down into the rise of the water leaving the transom. I might be over thinking all this but that has been on of the blind factors I have been trying to figure out. Any more incite on this would be greatly appreciated. |
I like your bracket design. I made my bracket just like the Hermco's with it straight back. It doesn't angle up. Mine is a 30" set back and mounted around 3.5-4" up and it doesn't drag. It has more then enough flotation also.
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You should be able to lay out/compare the geometry of a flotation tub extending 30" straight back from the transom mounted about 4" above keel vs. one mounted 2" above keel and only extending back 24" to see which is closer to water. The angle/rise of water coming off transom will change depending on speed; more rise at low speed. That hull should plane at about 12 mph if it's balanced right. Maybe you could make up a piece of aluminum angle/plate to simulate dimensions of tub and try it out on your race boat to see if it's clear of water! My experience is that porpoising occurs at higher speeds, 20 kts+. I think most boats will tend to porpoise after adding a bracket just due to the CG shift caused by moving the motor back, even if tub is clear of water, unless other stuff like console and gas tanks are shifted forward to offset the weight of motor moved aft. |
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Interesting project, thanks for posting.
Porpoising will an issue with the 25 hull and a bracket, because it is so sensitive to horizontal CG. You talked about a ballast tank. Particularly with bracketed OB, that is a great idea and could pretty much eliminate porpoising issues. |
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Work has kept me from finishing the bracket but I have still have been making progress. I had gotten the clay filets in the corners and rounded them off with my filet tool. I started out with some clay my coworker gave me (she makes clay art) but that was not the right stuff for the job. It dried out way too fast and shrunk. The shrinkage caused it to crack up so I had to remove it and then start over with the right stuff the second time. I got some modeling clay that was listed as shrink free and the package said that it would not harden either. It was much more playable and went in much more smoothly. It was also shaped in little sticks so it fit in the corners perfect for making radiuses. Once I got this done I waxed it a few more times. Over all I waxed the mold a total of 7 or 8 times letting it dry 15 minutes and removing it each time with a special cloth made for buffing out molds with wax (forgot what the cloth is called). I got the gel coat sprayed in with my cup gun. I sprayed 4-5 mils then let it kick slightly then did a second coat of 3-4 more. I didn't thin the gel at all so I could of sprayed it in one thick coat but I was worried about it sagging on the sides. Cold as it was it came out slow and thick. The cup gun works well for mold like this due to it gets in the corners good with out hurting the filets. After this, while still slightly tacky, I laid 2" strips of csm in the corners then a 4" wider strip of csm in the corners. I had all the cloth pre-cut so it went well laying it in at one time before it kicked off. I laid all the sides with 2 layers of csm and overlapped the sections to prevent seams. Once it was all in I rolled all the air out as well as possible and got the csm laying so that it looked like one continuous layup. I used the csm as a skim coat to help prevent print threw of the 17oz biaxel. I have several layers of 1708 and 1700 to lay in also but haven't had a chance to get that far yet. All the csm has fully cured now so I will have to scuff it up with some 80 grit to get a good bite for the new resin to bond too when I lay it in. I wanted to keep it all green on green but work would not let that happen. I hope to get the rest in tonight as well as bed the rear motor mount core in place too.
Clay filets in: Attachment 9362 Attachment 9363 The bad clay I used and had to remove: Attachment 9364 The mold after White Gel coat was sprayed: Attachment 9365 The cup gun I spray gel with: Attachment 9366 |
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Well, I can say Herman @ Hermco defiantly earns his money. If any body thinks he is high on his product they need to try to build their own cause it will change their mind. I was starting to think I should of just bought a bracket but I am on the short rows now. It has been a very labor intensive and time consuming process to get the tub laid up and the core in. I has took a bit more resin than I expected as well. Being the tub on this one is 60" wide and so big for the extra flotation I have burned threw some 1708, biax and csm. I lost count how many layers I installed but just started building it out to a desired thickness. The back of the bracket where the motor will mount ended up being 1/2" thick and 5/8"-3/4" thick in the corners. There rest of the mold is around 3/8" thick in most areas and 1/2" thick in the corners where it has the extra 1708 tape strips. After that there are two 3/4" Douglas fir marine ply cores that were pre-bonded together with two layers of csm, one on either side of the two layers of 1708, that are in the middle of the cores bonded together. The rest of the core was hot coated and soaked in thinned resin to seal the wood up. I wanted wood ply for the compression strength of the motor mounting bolts to prevent crushing of the core. The rest of the cores will be core cell. The core section that was premade was boned into the mold while the layers were installed so it is all a "green" lay up. I did smear in some thickened resin on the last layer of glass prior to laying the core in just to make sure all the voids were filled. I used a plastic card with 1/4" notches in it to spread the thick resin on top of the layers then laid the core in and clamped it in place and squeezed the air out and let it set up over night. I need to go back and add some layers over the core then it will be ready to come out of the mold. I was thinking that it would take around 10 gallons of vinyl ester but I am at 12 gals now and have to still add in the front of the core and all the stringers. That will take another 3 gallons I am guessing due to the stringers will not need much as they will be just a few layers over the core cell. I feel like I way over built this thing but I didn't want to under do it either. I will be hanging 850 lbs of motors off of this thing and will be doing 60 + mph in 3-4 seas and don't want to worry about it breaking. I just hope all the extra weight will not hurt the balance to bad. I am thinking about doing the swim platform out of epoxy and carbon fiber just to save weight at this point so I can keep the bracket under 200 lbs.
I didn't get many pics due to the intensive work laying it up and to the fact that I had resin on everything everywhere but here are a few: Attachment 9459 Attachment 9460 Attachment 9461 Attachment 9462 |
Holy 5#!+!!! What a read! Not going to question the price and value of a Hermco Bracket again!.! Frizz, does my Seafari have a removable portion of the glass inner liner of transom? I found seams just inboard of rear seats, and appears to open up rear of engine box like your beginning pics. This is about 18" of curved transom cap where engine vents and blower is located. Appears I would have to remove aluminum rub rail to access screws to remove cap. I guess I should add pics to my thread. Nice job documenting your progress. Keep at it.
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http://www.classicseacraft.com/commu...ad.php?t=25648 |
Wow! Nice finish and documentation. Then Strick hast to introduce the Gucci of all live wells! But your rebuild does show exactly how you removed the section, and it looks identical to mine. Thanks for the link.
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Good luck with yours, some people like to cut the transom from the rear but I prefer to cut the back liner skin out and do it from the inside. There are advantages both ways but way more for doing it from the inside. Just a few examples of advantages changing the core from the inside: You can make it thicker easier, you don't disturb the outer shell of the hull, you can fillet the corners then tab them in better, better access & more space to work, easier to remove the old core out, less likely to trap air when laying it up and most of all way less fairing. Did I say way less fairing?...lol |
Yes, I agree with the inside route. That's how the MA was done. And I strongly feel that paint is for cars and gel coat is for boats. I have been fortunate refurbishing both Whalers without paint. My MA I did have the cap sprayed and its holding up well, until you bump a dock. But hey, it is just a boat right?
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