Center of gravity location MA20?

[Chaser]

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!

[mnwnvc]

I think a drawing is available on this site. Should be able to scale it from the drawing.

[wattaway2]

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

[Chaser]

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.

[jorgeinmiami]

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

[Bushwacker]

Originally Posted by Chaser

. . . 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 . . .

Don't know about Potter, but Moesly always centered fuel tank on the CG so trim wouldn't change as you burned off gas. (With over 13,000 hrs flight time in WWII, most of it flying cargo, he understood the importance of CG location better than most!) Probably safe to assume Potter did the same thing.

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!

[Chaser]

Originally Posted by Bushwacker

Don't know about Potter, but Moesly always centered fuel tank on the CG so trim wouldn't change as you burned off gas. (With over 13,000 hrs flight time in WWII, most of it flying cargo, he understood the importance of CG location better than most!) Probably safe to assume Potter did the same thing.

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!

Thanks BW

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.

[Chaser]

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...

[flyingfrizzle]

Looking good, keep the pics coming!

[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.