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Center of gravity location MA20?
Does anyone know the as designed location of the center of gravity on a Potter built MA20? Doing a refit and changing the fuel tank. The new tank is quite a bit smaller/shorter and I want to install it in the right location, directly over the CG...
A distance from the bow or stern would be helpful. Doesn't have to be perfectly accurate. I can always guess or maybe even use a hoist to figure it out but thought I would give you guys a shot first. Thanks in advance! |
I think a drawing is available on this site. Should be able to scale it from the drawing.
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Don't think you could be to far off with the new tank being smaller if you placed it against the original forward bulk head
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I did some digging back through some other threads and think I got the info I need. Looks like the center of the old tank should get me close, around 63%/12' aft of the bow.
I'm repowering with a 135 or 150 4 stroke (I know it's heavy I'm going to deal with it). Originally I thought moving the fuel tank forward was a good way to balance the weight of the heavier outboard. But then I though that would change the trim depending on the amount of fuel in the boat... So I think I am going to put the new tank right over the CG and shift the console a little bit forward. The new cockpit sole should be about half the weight of the old, same with the new console. That will eliminate around 150-200 lbs (those old consoles are heeeeaaavy!) and that should all help a lot. I am also going to leave a space to add ballast forward under the sole to adjust the trim once the boat is all back together and I see how she sits. Pulled the old sole out yesterday, as well as about 50 lbs in wet foam, mostly from the aft section. So I'm thinking the boat will sit simillarly to how she sat before, even with the extra 100 lbs of outboard back there. May not need much ballast if any at all. But, since the boat will be significantly lighter than it was, adding a little ballast to make her float even shouldn't be the end of the world. Will post pics as the project progresses. |
This post got me thinking also on my MA rebuild.
I recently dug out the old wood after I took of the skin of the deck and also the old wood from the hatches covering the gas tank. I think that just in the hatch it is at least 40-50 lbs lighter and that does not include the wood that was soaked under the deck. I am replacing all of this with Nida Core and it is going to be much lighter. Also the transom which was also water soaked will go back in with a pourable transom which will also lighten the load. If I replace the original 70 gallon tank with a 50 gallon tank and move it forward, place the batteries under the console along with the weight savings on the deck and transom, I should move the CG up some and then be able to go with a 4 stroke. The gas saving will also compensate for the smaller gas tank and get the "same" range as a older 2 stroke with a larger tank. You get so many ideas here ...it great |
Careful in thinking that moving the tank forward will change CG. This is only true when there is fuel in it! Without fuel it doesn't change CG, and you don't want the CG to change with more/less fuel.
I removed the entire floor, not just the outer skin. Going to replace it with coosa, laminated both sides. Check out Peter Allaires rebuild in the photos section of this site, very helpful. I think the job will actually be easier this way, instead of re-coring the existing deck. Plus, what I found underneath the sole makes me even more glad I'm doing it this way, another 50 lbs of useless putty and foam gone! Going with a 40 gal Moeller poly tank (ft4037). Will fit right between the stringers. Build and install small bulkheads forward and aft of tank to secure it, just like in Peters pics. Paint all with Bilgekote and cut access hatches in the new sole to have access to the entire bilge, which will be really nice. Should be able to keep the boat bone dry, no more inaccessible areas where water collects. With 40 gal of fuel, getting around 2 gph cruising @ 25 kts, should give me a range of 20 hrs/500 miles. Plenty! Just guessing on the fuel burn from some research on other boats... Maybe won't be quite that good but I expect no more than 3 gph @ 25 kts. Likely going with a 135 Honda. Going to go look at it today. My mechanic says they're the best for reliability and lifespan. Did the 2 stroke thing for 3 years, but now the old Mariner needs a lower unit, and compression on #2 cylinder is low. Plus, it's hard for the wife to start/run and she hates the smoke/smell. So, 4 stroke it is! Happy wife happy life! I have to say I am looking forward to a sewing machine quite engine... |
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So to begin with, assume the original CG was at center of fuel tank, with a 300 LB MOTOR that the 20' hull was originally designed for! Now you're talking about changing lots of stuff, so I'd look at each change independently and calculate the CG change of each. Going to a 500 lb motor will move CG AFT, as will installing a lighter deck, console and removing wet foam! Even if the total package is lighter, it still won't float and handle right unless CG is right. I would NEVER accept the performance penalty of ballast when you can move batteries, gas tank, and console forward to get it right. Since people, coolers and gear tend to congregate in the back where ride is softest, and ALL of your planned changes are moving CG AFT, I'd put that gas tank as far forward as possible! My gut feel is that you're gonna have to move console fwd a fair amount, but before you bolt it down, maybe do a flotation test after you hang motor with pictures of transom/bootstripe to document how level boat sits in water. Then you can move console to see what effect it has. Use trash cans filled with sea water to simulate crew weight. And if you DO still have to add ballast, consider using 20' of chain on your anchor rode . . . makes a HUGE difference in anchor holding power! |
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My original thought was to move the tank all the way forward. But then I started thinking that with it far fwd of CG the trim of the boat would change as fuel was used up. Do you think this would be a problem and how noticeable would it be? Would I just figure on running with the tank mostly full all the time if it was far forward? I think the sole replacement will be neutral for CG. Just as much weight fwd of CG (if we figure CG is center of the tank) as aft of it. There may actually be more weight aft of CG in the sole, so that would be a positive. I may do something like use plywood for the forward section of sole and Coosa board aft, which would make the new fwd section heavier than the aft. Plywood is twice the weight of Coosa sq/ft. The battery was in the console before and will be there again. Agree with you the lighter console will shift CG aft. Maybe I can make use of the old one just so it is heavier... So much work to fix it though... I plan to movie the whole console, leaning post, and T top about 6" forward. This will also put 2 people (mostly run with 2-3) 6" further fwd. I always have chain on my anchor rode :) You can't anchor here without chain. |
Some thoughts.
Batteries forward. Foam core in fuel tank hatch. Stern lifting prop. Hydroshield. ( the last two help running) A bench seat tank forward. A smaller tank forward. A tank that doesn't let 30 odd gallons slosh aft as you try to get on plane. No t-top. The t-top moment is like a heavier motor when running. I have dry decks with 420 lb of motor(s) on the transom of a 20 MA. Tank and batteries are well forward. Will put a whaler style under seat tank forward of the console this year. |
I should also mention that the boat sat stern down before I started this process, with the 400# Mariner on there. Despite anything I've read on this site about poor handling characteristics that may be attributable to stern down trim, I still think the boat rides amazing. And I have had it out in some pretty rough water, always been happy with it. So I'm not hoping to get to factory trim here, just hoping to keep the trim I had.
Oh, and the trolling motor bracket is coming off the transom too. |
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The fuel sitting all the way aft in the old tank was always a problem. The 72 gallon tank ended about 10" ahead of the motor well, and sitting stern down already it all ended up back there. The boat always rode better with more fuel, I think mostly because it pushed the CG forward. But I had to have >50 gallons of fuel in the boat for that! So the tank I am thinking about is half the length of the old one. I am thinking to put the back edge of the tank at the center of the old tank (CG). This will put the fwd end of the new tank about where the old tank was. Don't have any confines with the tank trough, it is not going back in the boat. The new tank will sit on a platform between the stringers. I kind of like the multiple tank idea. My wife suggested that too. I just don't want the hassle of another set of plumbing etc... An easy under seat square tank isn't a bad idea though. |
I have what might be an unconventional layout for my 20MA. No tank under the deck- that is full of flotation foam. I have huge console that would house a head. I put a 20 gallon tank in there. That is fine for Buzzards Bay cruising. I have no T-Top. A T top might help static CG a little when stopped but will hurt underway. I may put one on the boat at some point.
I am about to put a 22-24 gallon Whaler style tank ahead of the console. They are like $250 at West Marine. I plan to tuck a fuel line under the cap so no under floor plumbing required- just a little step over hat channel on the deck for the 1 foot run to the bench seat. That gives me 42-44 gallons. I get 5MPG, so that's a lot of range for me. (I don't think any 20 footers do a lot better than 5NMPG. So I think a full to empty range of more than 300 miles is really unusual for a 20MA.) If I took the kicker (58 lb) and stowed it forward, that would help with static CG. Because it is behind the transom, that's about like one guy standing in an aft corner. I still need to pull the fuel tank hatch and take the plywood out and foam core it. I really think you won't be happy with that Honda. I think it is 200 lb over ideal (under 300 lb), 100 lb over the sort of normal compromise (375ish), and 30-50 lb more than most "performance" or "plus kicker" compromise setups (like mine with the 58lb kicker). And it isn't insanely powerful, so planing out might still be a bit of a slog. My 20 MA came with a 175 Yamaha and it was 415-420lb, I think. 60 lb lighter and I am pretty sure it planed out well. I never used it. I ditched it for the lightest 4 strokes I could find- 2 stokes make me seasick, it wasn't an option. I also wouldn't run an old motor without a kicker in Buzzards Bay, so the main engine had to get lighter to support that change. All that outboard weight is, well, outboard. Behind the transom. You aren't floating it 1:1. It is on the other side of the seesaw. You float it, then you float the lever arm it makes again by sinking the transom further. You can offset it with anchor chain to a degree so you pay the least amount of stern sink by having "ballast" all the way forward. But not when you are anchored. Then that ballast is on the sea floor. Which is when it might be when you need freeboard the most. This isn't to say that anchor chain is a bad thing- quite the contrary. But it isn't ballast at anchor. At any rate, I think batteries and tanks forward are the way to go. Fore aft fuel sloshing is bad. I think the comment about CG from fuel burn is a bit overthought, in so far as I don't think tanks can be too far forward on a 20 footer. There isn't the volume in the forefoot to really mess it up. Plus: If your tanks are forward of the console, and you are too light from fuel burn, a 5 gallon pail or two of seawater on the casting deck will fix things quickly. Along with coolers and other gear. That's how I run my 20MA and do it without tabs. Maybe I don't know any better, but it seems pretty good. |
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For reference- here is the "marine survey edition" transom scumline with a 20" shaft F100, a 30" shaft kicker and four, yes FOUR transducers. Two of which I was just testing before the big boat was ready. You can see the scumline versus the notch, and what the kicker does to "tip" the scumline. On paper, the F100 is 367 lb. Plus maybe 8 lb of oil. The 6hp Tohatsu SailPro is 58 lb.
Batteries are forward. The fuel tank is in the console- centerline maybe 3 feet forward of stock. The thing under the skeg is this hydroshield. Plus that's a stern lifting prop. And no. The dog isn't mine.:D |
Carl Moesly would place the tanks at the cg so that it would not cause a shift as fuel burned off and that works well if possible. With a heavy four stroke you will need that weight forward. The two tank deal will work well to keep the weight forward. You can burn from the rear tank and save the front for reserve. The small amount of gas burned in the second tank will not change the the cg much due to a honda 130 will not burn much unless u are running long distances. If you can gain perfect balance with a cg centered tank go for it but I personally think you will need the extra weight to make it balance. You could ballast weight but why use extra weight when you can utilize the fuel weight u have. If you do this with dual tanks make sure you can tie them together and as u re fill equalize the tanks and top of each one so the fuel stays fresh in both tanks.
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Thanks for all the responses guys. I do have a good understanding of lever arms and how weight affects trim, having sailed and worked on boats my whole life. So I am not coming into this cold...
Like I said in my previous post, the boat was already stern heavy and I'm not necessarily trying to improve it, just make sure it doesn't get worse. I raised the transom to 25" four years ago. The pic of the MA with Yammi on it is about where my scum line sat, just below the motor well scuppers. Mine was "tipped" as well from the kicker. Kicker and bracket are both going away with this refit. I did some math last night on what kind of difference the fuel tank will make. With 20 gal in a new tank (aft end of tank at CG, 4' long tank, so fuel ~2' fwd of CG) vs 20 gal in the old tank (8' long tank and fuel all sat ~3' aft of CG) that equates to ~75# on the transom. Tanks full (new tank 40 gal, old tank 70 gal) it equals about the same change in weight on the transom (Weight of fuel in the old tank moves fwd as it is filled, to centered on CG). The motor is a done deal, brought it home yesterday. I think I'm pretty well settled on placing the tank just about as far fwd as I can, which will be centered 2' fwd of CG. I'm going to push the console as far fwd as is reasonable. I'm also going to utilize my "fwd plywood, aft Coosa" sole construction technique to make the new sole heavier fwd and lighter aft. My feeling is from all this that I will at least maintain the trim I had if not improve it just a little. Again I appreciate all the comments and it is all great information! |
Looking good, keep the pics coming!
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http://www.classicseacraft.com/commu...ad.php?t=22189
Everything about this setup was perfect. Fished the boat a lot offshore w bait tank full, loaded ice, 3-4 guys. Sat good in the water, ran good, and self bailed. |
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Noticed your stringer was cut out for the fuel fill/vent also, maybe this was standard... Mine is solid stringer now :) I am raising the sole only 1-1.5". It self bailed before and should be fine only 1" higher. The method I am using is to build an "H" frame of laminated plywood overtop the stringers on which the sole will sit. I am going to use 1/2" divinicell for the aft 6' of the cockpit sole and the fwd sides, and 1/2" ply for the center section between the stringers on the fwd 6' of deck. I'm leaving the casting platform/anchor locker. Will glue and tab the new sole to the hull sides and casting platform, as well as the stern molding. 35 gal plastic tank under the sole, with provision for a 20 gal seat tank in the console if I end up needing more capacity. Valve, filter, and flowscan will all be under the sole accessible via access hatch. I am also eliminating all the original molded hatches (CC hatch, bow hatch, casting platform hatch). Glassing them over and replacing with plastic drop in units. I know the boat will be about 150-200# lighter after all this. Materials for sole and new H frame support is only 2 sheets 1/2" ply, 2 sheets (+ a scrap) 1/2" divinicell, 20-25 yds 1208 glass, 5 gal vinylester resin, plus misc reinforcements, resins, bog, and such. New tank is less than half the weight of the old tank. Made great progress yesterday getting the H frame laminated and glued and tank supports glued in. Will get some pics and update the Google brain. Cheers! Go Hawks! |
Thxs buddy.
Sounds like you have a good plan. Cant wait to see some pics. |
Couple new photos in here of the cockpit sole sub frame.
https://drive.google.com/folderview?...Hc&usp=sharing |
Nice
I did the same thing to my console. Worked out good. You may have already checked and yours looks good...but make sure the binnacle will go wide open w/o hitting |
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Great progress so far this week. My 8 hours of "regular" work being augmented by 3-4 hours of SeaCraft work every day is paying off!
Got the bilge painted underneath the tank. I realized I had to do this now if I wanted it painted as the tank can't be removed easily once the sub frame is glued in. Got the tank in and bolted down. Then Wednesday got the sub frame glued in place. Last night I ground down the tops of the two short sections of sub frame that the T-Top will be screwed to. They stood a little proud of the rest of the frame since they were on the other section of stringer, and the stringers aren't exactly even or square. Now the whole frame is glued in, even, and I marked the hull side where my cleats will need to go to support the outer edge of the new cockpit sole. Got all the old silicone sealant pulled out of the edge of the casting platform, and the edge ground down so it is now ready to be glued, filleted, and tabbed to the hull. I expect I'll get the cleats installed this weekend, as well as the final shims on the fwd and aft flange (old sole) that the new sole will sit on. Then its on to patterning floorboards! We also scored a SWEET pair of Springfield Marine pedestal seatposts on Craigslist! They have gas springs to adjust the height and also they provide active suspension, about 3" of travel! These will be great on those rough days! The boat had a Birdsell Ind leaning post, but I noticed the original MA's had pedestals... I want to make the boat as original as possible, and I like the idea of pedestal seats instead of the leaning post anyway, we have a lot of situations where it would be nice to turn the seats sideways or backwards. So, I'm stoked that we found these things! https://drive.google.com/folderview?...Hc&usp=sharing |
Got a pic of the seats?
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Seats suck. I'm thinking about some Todd marine seats that look just like the originals in the 1979 brochure on this site.
Got a ton done tonight will post some more pics in the morning. Ready to start building the center floorboards and patterning the outside sections. |
Lots of progress made these last couple of weeks.
Casting platform and anchor locker tabbed to the hull, faired and fitted for the drop in hatches. Still plenty of detail work to do but the major grinding and fitting is done. The fuel tank vent was always a problem being aft next to the fill, with the hose running from the forward end of the tank back. It always got fuel in it and I had to mind it while filling the first 1/3 of the tank to catch any fuel sputtering out of the vent. So, I built a fairing (got the idea from this site, just did it a little different) to cover the vent line up the hull side so I could take it directly from the tank up the hull side, with no low spots. So now I should never have a problem with the vent line filling with fuel again! First bits of wiring and hoses are in. Stuff that would be difficult after the sole is complete and glued in. Got the side sections of cockpit sole modeled, cut and bottom side laminated. This will be the next really big step to get these glued and tabbed in. Then I will have a complete floor to work on! I'm doing the side sections in Divinicell full length. I had thought doing them in two sections, with plywood forward and foam aft, to push weight forward, but I had enough Divinicell to do the whole thing, and I thought, why not just make the whole boat lighter? Especially towards the outside of the hull where more weight will only increase rolling moment. So, the cockpit sole is about 3/4 foam and 1/4 plywood, with plywood only in the center section under and forward of the console. This comes back to my original CG issue... And I know you all have said you think it is stupid, but, if I have a problem with fore aft trim, I am going to ballast the bow down with some chain. I left a compartment forward of the tank that is accessible via the 8" inspection port just aft of the casting platform just for ballast, if I need to add it. I finally came back to the original thought that if the boat is significantly lighter overall than it was (which it will be, by a lot) and I still have a problem with fore/aft trim, I can put ballast forward to help trim the bow down, and still be under the original weight of the whole boat. We'll see how this plays out once everything is back together... I laminated the bottom skin of the side floor sections, and will trim them and do a final dry fit tonight. Then I should be able to glue them in and laminate the top skin in place this weekend. I plan to tab them into the hull sides and sub frame at the same time as the top skin is laminated. The console got a few sweeps of fairing and is looking really good. One more sweep and then it will be time for primer. So, after this weekend I should be at the point of fairing, priming and painting, and putting things back together! Getting very excited to see it coming together, and seeing the end. Our winter has been so mild we've had three weekends in a row that it would have been awesome to have the boat ready to go so we could get out into the islands. That must mean by the time I am done the weather will turn permanently shitty till July 4, per usual! https://drive.google.com/folderview?...Hc&usp=sharing |
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I should say where I foresee needing to put the weight :)
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What year is your MA? BTW, nice work!
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Its a '78.
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Lots of progress this afternoon and evenng. It was raining so I brought the console inside the shop. Got all the instruments, radio, wheel, and throttle cut in and dry fit. Pretty happy with how it turned out. Built a new glove box lid and a shim for the throttle lever so it doesn't hit the face of the console. Did these with 3/4" starboard and they came out great. Now the throttle lever is just shy of the console face at full open.
Now a bit more fairing and some primer and she'll be ready for paint. I am tired. But looking forward to fitting the floorboards tomorrow. |
Well the past few weeks have been crazy busy. Besides one of the busiest starts to the season we have ever had at our business, I have been laboring every night and weekend on this silly SeaCraft. It will be worth it in the end though!
I haven't gotten as many pics as I'd like, mostly because I'm just pushing through the projects as quickly as I can... New floors glued and tabbed in. Primed the waterways and gloss areas of the aft cockpit and casting platform. I was going to just paint the inside of the hull, and leave the roving texture, but I didn't plan well enough and protect it when I tabbed in the floor and casting platform, so there was fairing going onto the roving and when I primed a section I decided it would look like crap to just paint it! So, yesterday I layed lightweight fairing over the whole interior, sanded it back, and painted it with BilgeKote. Its not perfect, but it looks better than roving would have, and I'm pretty sure once I put everything back together I won't even notice the bad spots. I'm thinking I will get the gloss sprayed on the waterways and console this weekend. I hope. Then its on to bolting in parts, mounting the motor and rigging things up! The end is in sight! https://drive.google.com/folderview?...Hc&usp=sharing |
Gloss is painted! Console, aft floorboard waterways, instrument panels. All Cloud White.
Hope to get some pics tonight when it is safe to go back in there. |
sounds great ,, need picts, please ..
that's what its all about!! |
Well. Got the console filled with instruments. And it's lifted into the boat. Still have the rest of the wiring to do but that will go pretty quick.
Still got some work to do on the floorboards and to finish the nonskid. I want to get everything dry fit before I mark out the waterways for the nonskid. Hatches are all bolted in. Hydraulic hoses and cables are ordered. Outboard is ready to bolt on. My wife is a pretty good seamstress and is making new coaming bolsters. We have made them before for our clients powerboat. There is a pic of them coming together in the Google gallery. We painted doorskin to seal it, then ran bolts through and hot glued them in place. Then Joy glued on the nice draining foam. She is in the process of cutting the material to be stapled on. They are going to turn out really nice. And, it goes with the theme of getting the boat back to as close to "classic" as I can (or want..) Besides, when I thought about filling all the holes in the coaming and making it look nice, I figured there was about the same amount of work to make new bolsters! Still haven't made a final decision on seats. This might wait till we can find a good deal on something. It is going to kill me to put the ratty old green seats we got with the posts back in there... At least just about everything else will be final. The hope is to have the boat ready for next Wednesday night. First race of the year for the YC and I am supposed to run race committee. We can do it in the YC boat but I would much prefer to show off our restored SeaCraft :) https://drive.google.com/folderview?...Hc&usp=sharing |
Just ordered seats. Todd "Key West" style. They are on closeout from Todd but the shipping was ridiculous. So, I had my wholsesale supplier order them on their next Todd shipment so I don't have to pay shipping. They don't come with cushions but Joy will make cushions (better than factory...) with matching material to the bolsters and the rest of the cushions, so they should look very nice.
Pics coming soon of the bolsters installed. Getting close... |
Oh, one nice part I did that I forgot to mention is the aft motor well barrier, the panel that further seals off the motor well from the cockpit. Previously this was a fiberglass panel that was screwed into place. It was heavy and being screwed into place made accessing the motor well and the bilge hatch a pain in the ass. So, I bolted down a piece of plastic U channel on the front of the motor well for the panel to sit in. I built a new panel of 1/4" balsa core with fiberglass skins; it is much lighter than the previous solid fiberglass panel. The panel will sit in the U channel and then I'll use barrel bolts on the upper section to secure it in the motor well. That way, to remove it, simply undo the barrel bolts and the panel will pull right out. I'm also going to build a storage space for when the panel is not needed and out of the motor well. It fits perfectly under the deck all the way aft. This will be one of the final items that gets finished up, but there is a pic of the U channel in the motor well in the Google gallery. Here is the pic:
https://lh6.googleusercontent.com/rD...gUg=w1497-h695 |
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