Center of gravity location MA20? - Page 2

Chaser · #11 01-17-2015, 11:56 PM

Quote:

Originally Posted by FishStretcher

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.

Gotta have the t-top. Wrapped 3 sides it's the only way to stay anywhere near warm enough at 25 kts.

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.

FishStretcher · #12 01-18-2015, 10:56 AM

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.

FishStretcher · #13 01-18-2015, 11:21 AM

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.

flyingfrizzle · #14 01-18-2015, 11:51 AM

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.

Chaser · #15 01-18-2015, 12:45 PM

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!

Chaser · #16 01-30-2015, 01:12 AM

The last couple weeks have been pretty busy. Removed the deck, cleaned, and now I am drying everything out and prepping to begin building the sole and support. Going to raise the whole thing about 1/2-1" from where it was.

I found that the factory had sawed out a section of the port stringer to route the fuel fill/vent to the tank. Crazy since there is a channel molded in for that... Maybe they didn't have quite enough hose to reach the tank

flyingfrizzle · #17 01-30-2015, 07:59 AM

Looking good, keep the pics coming!

Seacraft84 · #18 02-01-2015, 02:28 AM

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.

Seacraft84 · #19 02-01-2015, 02:33 AM

http://www.thehulltruth.com/boats-sa...dded-sold.html

Chaser · #20 02-01-2015, 12:40 PM

Quote:

Originally Posted by Seacraft84

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.

Very nice job! I love the FLY hull color.

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!