Weight and balance adjustment

Questions about boat repairs with our resins and fiberglass: hull patches, transoms and stringers, foam, rot etc.
MarkOrge
Very Active Poster
Very Active Poster
Posts: 544
Joined: Tue Jun 23, 2009 4:48 pm

Weight and balance adjustment

Post by MarkOrge »

Good day all. I have a question around how much to adjust for a potentially new engine that will weigh about 100 pounds more. I am going to buy new, but unfortunately they are all heavier now, likely also because they are all 4 strokes. So, I have my boat almost right as it is now, FYI on a 10" jackplate. Good news is I can shorten the jackplate to offset the extra 100 pounds but cannot figure out by how much? In other words, would I go down to an 8, 6, or 4" to even it out?

Many thanks in advance,

Mark
My Son calls me "The Fish Whisperer" I love it !

fallguy1000
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 10198
Joined: Tue Jan 12, 2010 12:25 am

Re: Weight and balance adjustment

Post by fallguy1000 »

The jackplate shortening is a waste of effort.

To evaluate weight effects on a boat; you use lever moments.

In order to do so, we must make some assumtions about your boat. Also, because you left out a bunch of information.

Let's say you have a 17' boat, and a planing hull. A typical longitudinal center of mass would exist around say 35% of WL from aft. Or, 0.35•17= 5.95' from the aft waterline.

Now, let's say you add 100 pounds to the back of the boat on a jackplate with a new engine and the center of the mass of the new engine is 2' behind the waterline. You have added a moment 100 pounds, 7.95' from the boat's center of mass (effectively also moving the center of mass, but we'll not worry about that now). The moment is 795 foot pounds.

So, let's say you can put a battery up on the front of the boat and the battery weighs 63 pounds and due to space limitations, you can only put it 4' back from the waterline. This is 13' forward from aft, or 7.05 feet from the old center of mass. This moment is 7.05•63 = 445 foot pounds.

Is that ideal, no.

So, your jack plate question. Let's say you have a 10" jackplate and you change it to four. That is going to be a moment of 0.5' times the jackplate and engines weight. Or for the engine, a 50 foot pound moment. And for the plate itself, say a weight of 30 pounds 6" or a 15 pound moment. So, for the jackplate effort, you got a total of 65 foot pounds the other way.

In order to perfectly offset the engine, more levers forward of the center of mass (generally) are needed. It is a bit more complex than that as any weights moving forward contribute or reduce levers.
My boat build is here -------->

viewtopic.php?f=12&t=62495

User avatar
cape man
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8268
Joined: Sun Jan 20, 2008 12:02 pm
Location: Lithia, Florida

Re: Weight and balance adjustment

Post by cape man »

Without more information it is hard to recommend anything. FG is right that the easiest way is to move other stuff forward, but will adding 100 pounds aft be a problem to begin with?
The world always seems brighter when you've just made something that wasn't there before - Neil Gaiman

MarkOrge
Very Active Poster
Very Active Poster
Posts: 544
Joined: Tue Jun 23, 2009 4:48 pm

Re: Weight and balance adjustment

Post by MarkOrge »

Thanks Capeman. Unfortunately I dont have an option unless I go buy an older 2 stroke - which I am adverse to as I seek the best reliability I can. Strength is not a concern, Jaques gave me a conservitive layup schedule (foam core) and I used top quality transom foam, and even substituted one of the 12 oz layers either side with 18 oz - she is rock solid.

Back to the weight of the motor - the other factor is parts even for Evinrude not that old are getting scarce - especially in Canada. I had to buy a fuel pump last year and got one of 2 left on the planet (or so I was informed)

With my TM batteries up front already it seems the only other option left is the starting battery....
My Son calls me "The Fish Whisperer" I love it !

User avatar
cape man
* Bateau Builder - Expert *
* Bateau Builder - Expert *
Posts: 8268
Joined: Sun Jan 20, 2008 12:02 pm
Location: Lithia, Florida

Re: Weight and balance adjustment

Post by cape man »

what boat do you have? Fixed or portable fuel? Coolers/seat? Are you sure about the 100 lbs extra to repower with a 4 stroke?

My first comment was meant to suggest that maybe you will have no problem with adding 100 lbs to the transom and your looking for an issue to fix that may not need fixing.

And FG is right. You will gain very little by changing the jack plate set back.
The world always seems brighter when you've just made something that wasn't there before - Neil Gaiman

MarkOrge
Very Active Poster
Very Active Poster
Posts: 544
Joined: Tue Jun 23, 2009 4:48 pm

Re: Weight and balance adjustment

Post by MarkOrge »

Thanks Capeman. PH18, Fixed fuel tank (cant move - built in) fixed bass boat style seating

Here is the link to my build if it helps: viewtopic.php?t=21717&start=380

Three of four batteries are in the front locker where the fire extinguisher used to be. I am dead sure about the weight, it is not a light boat especially with all the gear, so 150 it is. I might get away with 60 or 70 pounds more if I can find a reliable E-Tec but still worried about parts. FYI I know about the cautions of overpowering and have read the threads that hold good caution and explain the added slamming loads - I inly go fast when it is dead calm and, a larger block will not have to work so hard pulling tubes.... Worst case I guess is just stay with a 115 and 13" pitch prop.
My Son calls me "The Fish Whisperer" I love it !

TomW1
Very Active Poster
Very Active Poster
Posts: 5844
Joined: Sun Apr 19, 2015 10:57 pm
Location: Bryson City, NC

Re: Weight and balance adjustment

Post by TomW1 »

Mark need a little more info to help you fully. I can give you the calculations and they are not hard. Mark remember you building your PH18 but I need the length of the waterline, placement of the back of the console on the waterline from the transom. Depending on the boat Jacques places the LCG along the waterline from the transom. It is from 42-45 % and on a center console 6" behind the console. From there we can determine what moving lbs. along the waterline will accomplish. We do not worry about vertical COG as most lbs. are on or under the deck and on the centerline if possible. I'll be honest the Evinrude new ETEC's were not that much lighter than some of the redesigned Yamaha's and Mercury's. Etec 115 390-405lbs., Yamaha 115 377-386lbs. Mercury 115 Pro XS 359lbs. Here are the 150HP motor lbs. for comparison, ETEC 418-436lbs, Yamaha 479-487lbs, Mercury 456lbs. So 390lbs * -.5 = -195lbs that can be used to adjust other weights.
Here is where I would start

Wt old motor vs Wt new motor = __?_ What size and brand motor are you looking for and are available to you. An example of this is using your ETEC at 390lbs and using a 4" jack plate instead of a 10" would move the motor .5' closer to the console. This is a relief of 195lbs.

Where is your starter battery now and could you move it to the console?
There will be other calcs when you get some of these numbers put together.

Tom

PS: You still entering the Bass Tournaments or did Covid kill them off?
Restored Mirror Dinghy, Bought OD18 built by CL, Westlawn School of Yacht Design courses. LT US Navy 1970-1978

MarkOrge
Very Active Poster
Very Active Poster
Posts: 544
Joined: Tue Jun 23, 2009 4:48 pm

Re: Weight and balance adjustment

Post by MarkOrge »

Thanks Tom ! Sorry for the delay, work is crazy (a good thing)

I believe my COG is slightly aft of design, I have a double console and although did my best during the build to follow the great advice on this forum, do not have a lot of options due to built in fuel tank etc. For length of waterline, forgive me but is that as simple as measuring from the transom to the tip of the bow along the little "stain" one gets after being in the water, doubling it, then adding transom width at the waterline? If it helps, at the transom, I draw about 9" and she is almost level since I move 3 our of 4 batteries forward. She is considerably heavier than the stock PH18 but she is loaded with a lot of gear, electronics, extra weight of resin and glass from the foam core approach, etc. When she came back from the paint shop, no hatch lids just hull and deck but on trailer she was exactly 2000 pounds - I still have to visit a scale and figure out how much the trailer weighs so I can get a sense of what kind of beast I built.

Tournaments - absolutely! Last year was tough, but the two years previous I finished top 10% in my club both years (Hawgtown Bassmasters) If you follow bass fishing, you'll see we have some rising stars from north of the border; Chris and Cory Johnston, Jeff Gustafson, and now young Cooper Gallant - you should see them all in the next Classic.

Back to W&B - I seem to be having trouble posting the below image - hopefully this works - if it does and you can see it it may help as it has all the dimensions (2D birds eye view)

Thanks again Tom - great to see all the veteran Bateau Gentlemen still helping out!

Image
My Son calls me "The Fish Whisperer" I love it !

pee wee
Very Active Poster
Very Active Poster
Posts: 2276
Joined: Sun May 01, 2011 8:29 am
Location: Georgia

Re: Weight and balance adjustment

Post by pee wee »

Here you go:


Image
Hank

TomW1
Very Active Poster
Very Active Poster
Posts: 5844
Joined: Sun Apr 19, 2015 10:57 pm
Location: Bryson City, NC

Re: Weight and balance adjustment

Post by TomW1 »

Hi Mark, no problem it is your time. :D OKay let's look at the weight of the new motors, your Evinrude is around 369lbs. A new Suzuki 115 is rated by the mfg. at 401lbs, not worth changing platforms over, The Yamaha 115 is listed at 377lbs almost identical. And the Mercury 115 is listed at 359. As far as gear ratio goes and pushing a larger prop the Suzuki has a 2.59, the Mercury ProXS 2.07 or in CT trim 2.32, standard Mercury 4ST is the same, Yamaha 115 has a 2.15 gear ratio. Gear ratio is important as it indicates the size of prop you can swing, the Evinrude had a 2.90 gear ratio. There is not a lot of difference it max rpm range, Mercury ProXS and Yamaha 5300-6300, the Mercury 4ST and Suzuki 5000-6000. So don't waste your money on changing your set back.

Like you I am leary of when Bombadier will stop making replacement parts for the Evinrude''s. A big company only has so much patientce when a product is outq of warrenty and no longer profitable.

So about the only change you will need to make is the prop, due to the higher ratio of the Evinrude. Let me know which motor you select and I will run it through the prop calc again to get you the right one again.

As far as boat weight is concerned weigh it after the new motor then on the trailer there should be a lable or stamped its weights, carrying capacity, etc. and its weight. Or you can go on the web for the manufacturer to get the info if know the make andd model.

I would choose the Suzuki if it was my boat and then the Mercury CS and finally the Yamaha. One caution right now hear in the States I am hearing of wait times as long as 6-7 months, don't know if the same is above the border.

Mark, take care please ask if you have any questions.

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

Post Reply

Who is online

Users browsing this forum: Google [Bot] and 6 guests