Battery placement

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Barry_CF
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Battery placement

Post by Barry_CF »

I am going to move my batteries forward to counter a weight distribution change when I go from a stern drive to a bracket and outboard. The batteries are currently located alongside the engine. I have room under the sole to move them forward, but I also have a cuddy cabin with room under the cushions to mount two batteries. Obviously they would have to be completely sealed in that location, no out gassing possible. What I'm concerned about is the pounding they take in the very front of the bow. I can build a suitable mount to secure them. And I understand the cable is going to be long and heavy, but that is something I have. Anyone have experience with the batteries mounted this far forward?

For reference, viewtopic.php?t=66184

fallguy1000
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Re: Battery placement

Post by fallguy1000 »

Battery lockers require venting.

If you move them to the cuddy, add a class T fuse about 200-300A near the battery to make sure they do not kill someone in the house. Add smoke detection if not there now.

All batteries require a drip tray.

Personally, I would avoid long runs; especially for start batt. And, keep them closer to discharge sources for house. I do not know if the boat has anything to port, but opposite the helm area is also good. I would probably try to be less worried about offsetting moments and balance. Batteries close to the center of the hull do not offset a lot of weight, but also keep the longitudinal center closer which is also desirable. To translate, lotsa weight on each end of the boat is also not good design. It results in uncomfortable sea movements and even pitching.
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viewtopic.php?f=12&t=62495

Barry_CF
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Re: Battery placement

Post by Barry_CF »

Good points FG.

I misstated the sealing. When I meant completely sealed, I meant an enclosure withing the cuddy that seals off the cuddy and vents externally.

Opposite the helm is where they will go if I do not move them further forward. At the moment, my guesstimate on the weight and balance (a guess because the exact longitudinal COG of the mercruiser is unknown to me) has that position providing an almost exact re-balance. I'm just thinking ahead should I need a bit more. I'm very familiar with polar moments of inertia, and the perils of having too much weight at one end or another, just never applied it to boating. The nice thing is, I can at least keep the batteries low and on the centerline there.

I'm less concerned about the length of the cable as I have in stock diesel locomotive cable in gauges more than sufficient for the length and amperage. It's not light by any means, but I'm going to lose about 200-300 pounds in the process, so a bit of extra cable weight will not matter. We have all the equipment for proper crimping, terminal, fuses, etc.

The boat is fairly symmetrical side to side, with the exception of the batteries and the helm. Placing the batteries to the port side opposite the helm is probably the ideal place to improve the side to side balance, plus yields the needed centerline balance, so that is my plan. I'm just planning for a case worse than my numbers currently show.

I appreciate the advice!

TomW1
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Re: Battery placement

Post by TomW1 »

Barry, you do not need to know the LCG of the MerCruiser you need to know its weight versus what you are replacing it with, then we can get into the calculations that are needed to balance the boat fore and aft. As far as the batteries as they have the right size cable and fuse you can place them anywhere. Though I like the starting battery close to the motor. Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

Barry_CF
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Re: Battery placement

Post by Barry_CF »

Tom, I do know pretty closely the weight of the mercruiser engine and sterndrive. What I don't know is where the center of that mass is located. I'm guessing at this point understanding the symmetry of the V8 engine would place it's weight in the center, and the heavy manifolds and risers pushing that aft a bit. Then the stern drive itself has fairly defined position. So I have taken the point in the front of the engine as my center, and I'm working off that with the judgement that the actual center of the engine and drive weight is located at a distance 20 inches behind that center. I calculate that as 1070 pounds including the pair of batteries. The current balance then has to be 20 inches forward of the center and equal at 1070.

I remove the 1070 and place 675 for the OB and bracket at a location that is 50 inches aft of the original center. And I move the batteries forward 100 inches with the batteries allotted 125 pounds total. That comes out roughly equal if my 3rd grade math is correct :)

The 100 inches is not in the cuddy, but under the sole pretty even with the helm. The cuddy would allow me to move them another 40 inches or so.

My concern with the batteries all the way forward was the pounding they get in that location. Perhaps it's not an issue. I agree that the closer a starting battery is to the starter the better, and maybe I'll end up with the batteries remaining in their current position, but I doubt it. I do not want to have to use excessive trim tab to get the boat trimmed out properly. I'm sticking with a 2 cycle to keep the weight down Vs a 4 cycle. And I'm probably going with a minimal bracket as those can get heavy when you get the flotation type (that add nothing on plane) and swim platforms.

Let me know if you think my calculation is off. Those numbers, weight and position are just guesstimates at the moment, although I think the weight is pretty close. Thanks!

TomW1
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Re: Battery placement

Post by TomW1 »

Barry_CF wrote: Fri Sep 16, 2022 12:44 am Tom, I do know pretty closely the weight of the mercruiser engine and sterndrive. What I don't know is where the center of that mass is located. I'm guessing at this point understanding the symmetry of the V8 engine would place it's weight in the center, and the heavy manifolds and risers pushing that aft a bit. Then the stern drive itself has fairly defined position. So I have taken the point in the front of the engine as my center, and I'm working off that with the judgement that the actual center of the engine and drive weight is located at a distance 20 inches behind that center. I calculate that as 1070 pounds including the pair of batteries. The current balance then has to be 20 inches forward of the center and equal at 1070.

I remove the 1070 and place 675 for the OB and bracket at a location that is 50 inches aft of the original center. And I move the batteries forward 100 inches with the batteries allotted 125 pounds total. That comes out roughly equal if my 3rd grade math is correct :)

The 100 inches is not in the cuddy, but under the sole pretty even with the helm. The cuddy would allow me to move them another 40 inches or so.

My concern with the batteries all the way forward was the pounding they get in that location. Perhaps it's not an issue. I agree that the closer a starting battery is to the starter the better, and maybe I'll end up with the batteries remaining in their current position, but I doubt it. I do not want to have to use excessive trim tab to get the boat trimmed out properly. I'm sticking with a 2 cycle to keep the weight down Vs a 4 cycle. And I'm probably going with a minimal bracket as those can get heavy when you get the flotation type (that add nothing on plane) and swim platforms.

Let me know if you think my calculation is off. Those numbers, weight and position are just guesstimates at the moment, although I think the weight is pretty close. Thanks!
Barry, I am on vacation on the banks of the Mississippi Rivier watching it flow by in our cabin, I will get back to you tomorrow on the COG calculations. If you know the weight of the Mercruiser and stern drive let me know. Also, what motor are you replacing them with, how many HP.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

Barry_CF
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Re: Battery placement

Post by Barry_CF »

Hi Tom,

I hope you are having a fine vacation. I certainly appreciate having you take a look at my data and calculations. I've done similar calculations for aircraft and my racecar, but I'm a bit rusty :)

I found that the 5.7 liter Mercruiser Stern drive assembly had a total weight of 938 pounds from this website:
https://www.sterndrives.com/merc/sternspecs.html

I took the engine weight of a 350 Chevy from somewhere and added 100 for the water cooled manifolds to get 675 pounds for the engine, with the remainder going to the outdrive assembly at 263 pounds (938 - 675). Just looking and measuring I decided the center of that was basically at the back of the engine, roughly 10 inches from the transom. The center of the engine to the transom is approximately 24 inches, and the outdrive center is approximately 7 inches from the transom.

The sterndrive will be removed obviously and replaced by a bracket to be determined and a 2 cycle outboard of 250-300hp. I'd prefer the 300, but 250 will do nicely I think. The outboard will be either a Yamaha or Evinrude, year 2005 to 2015 or so. Those typically weigh just under 550 pounds. The bracket makers seem to recommend a 30" setback with a 30" shaft. I'm figuring on that for now as I do not yet know how much I need for full tilt on the motor, and if it will end up being a 25 or 30 inch shaft. I'd prefer the 30, but buying used means I may have to settle for what I find when I'm ready to purchase. The bracket weight varies depending on what I choose. At the moment the lowest weight is listed as 85 pounds for a single, but I think it's closer to 125 with hardware and 30" setback. So that is my current target. The other bracket I'm considering is 200 pounds (approximately) and includes a swim platform. I have been using the motor and bracket weight of 675 for my calculation (550 + 125).

The two batteries are currently located just about in the center of the engine, to the port side. I can easily move those forward 100 inches, outting them under the sole opposite the helm. I could locate them another 40 inches forward in the Cuddy if required.

Also, I have so far ignored any potential weight increase of the transom I will rebuild. I suspect it will increase 40 - 60 pounds or so.

I'm mainly concerned with trim while running and less with trim at rest. I can go with a flotation bracket for the at rest trim, but then pay a weight penalty for the running trim since the flotation is lost on plane.

Thanks for taking time to check my calculation and see if I'm in the ball park. I appreciate it.

fallguy1000
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Re: Battery placement

Post by fallguy1000 »

The COG is probably forward of the engine now, but not much.

Using the leading edge of the engine.

Engine moment
675 lbs • 14"/12" = 787.5 ftlbs
OD moment
273 • (28+10+7)"/12" = 1023.75 ftlbs
Those are going away.
So 1811.25 ftlbs exit

New motor moment
550 • (38+30)/12 = 3116.67 ftlbs

Bracket moment
125 • (38+15)/12 = 552 ftlbs

Transom amendments
25• 38/12 = 80 ftlbs

New moments
3748.67 ftlbs

Net changes moment
1937.42 ftlbs.

Moving say two batteries...

Let's say they are 2' behind the engine front edge now and weigh 100#.

Batteries moment 200ftlbs aft

Moving them forward 20'

2000ftlbs

It is a nice offset, but if it were me, I'd still move the batteries to the port helm offset. Then you can more easily put an inert weight up under the cuddy. Epoxy resin and lead shot will catch fire if poured thicker than about 3/8" at a time. Hell, you could put a 50 pound bag of sand up there which would shape to some odd shape really easily versus batteries.

The nice thing about using an inert item is once the boat is what you want it to be; you walk in the boat with a GOOD level and add weight until you like the bubble. Shot bags are 25 pounds and would also conform to an odd shape up in the cuddy.
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TomW1
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Re: Battery placement

Post by TomW1 »

Barry, Tom I am going to go through each item one step at a time to get the new COG and what steps you need to take.

Motor 675lbs now 0lbs We will get back to this later.

Stern drive 263lbs Replaced by platform and OB. 125+550=675-267lbs=408lbs. COG shifts 18" back. Assuming 30" bracket.

So, we need to offset that 408lbs someway. By moving the batteries to the Cuddy, you will have most of the weight accounted for, if needed a small bag of sand as fg mentioned works well. Make sure the batteries are in a compartment of their own that is vented.

Now, that engine just sell it. It can help pay for some of your costs. :D You have put enough weight aft of the stern to compensate for it being gone. Leave the top open and see how she sits on her lines. If needed add 400-600lbs of sandbags where the engine was to bring the lines down.

Now, on to outboards. The new Yamaha 4st 250-300HP weights are in the 550-581lb range depending on shaft length. No need to go for an old 2st anymore due to weight. The 4st have proven their reliability and strength over time. I would stay away from Evinrude they stepped away from there outboard section and you may not be able to get parts if you need any. They have closed all their dealers.

Brackets unless you and your family do a lot of swimming or diving off the boat the full width bracket is not worth the extra weight or expense, IMHO.

Tom
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

Barry_CF
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Re: Battery placement

Post by Barry_CF »

Tom, FG, you guys are great. Thanks for the help. I wish I could return the favor someway.

I had not considered adding weight to trim the boat out. In all of my experience we're always spending big $$$ to get rid of weight :) But I should have a fair amount to work with and move forward enough to get it correct. The idea of adding it until I get it correct makes sense. There is quite a bit of room under the sole and in the cuddy under the cushions. I've already figured how to add mounts in the cuddy for batteries and sealing/venting that off, but I think I'll just put them opposite the helm as suggested. Much easier and then I can add some conforming medium to the cuddy as needed, as long as it doesn't require too much. Once I take the sole out I may be able to get a little bit by moving the tank forward, hard to tell right now how much room is available.

Tom, I haven't kept up obviously, didn't realize Evinrude was discontinued. I guess that narrows it to a Yamaha, 2 or 4 cycle. I actually like the 2 cycle motors. I don't know how good the Mercury Verado is, but they seem a bit heavy to me.

I'll sale the sterndrive eventually, but I'll keep it around for now, just in case. Plus, I may decide on a 2nd boat someday, something more suitable to spending a few days on, and that might just have a pair of these...

So, thanks greatly, you have confirmed I'm not way out of the ball park and I can proceed with the modifications. I could always put the stern drive back in, but I really want the outboard on this boat. I always had a problem because I would use it in the winter so I would have to leave it in the water with heat in the engine compartment. I was out of town a couple times when the temps dipped into the teens and that was pushing it. If I was home it was easy, just go out and spend the night on the boat. With an OB I can get rid of that problem with a minimum of effort. Plus, I think this boat will be better with the extra space I get removing the stern drive and transom fish box. I can put the fish box in the floor, along with a cooler or livewell or both in the old engine area.

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