Single vs Twins and Transom Weight

[json]

I feel like I have seen this discussed here, but I wanted to talk a bit about transom weight. As it stands I am thinking I want to have a single high power yamaha (f250 or f300), which weighs about 560 lbs. I am intrigued though by running twins for the helm master digital controls, which allows the engines to maneuver the boat with a joystick as well as a lot of other benefits, namely redundancy. Since I will likely throw a kicker on the back as well for redundancy if I run a single, that ups my transom weight into the 700lbs range, throw a bracket on there and it's in the 800lb range. Digital yamaha outboards start at the inline 4 cylinder f150, which weighs a whopping 490 lbs. So, is a thousand pounds hanging off the transom unrealistic? Or can I balance out the weight and not worry about my transom getting snapped off like a lever? It's expensive and I am not sure I can justify the cost, but I would like to at least know what's a realistic option before I make the investment.

[Brian.Dixon]

From a strength perspective, the transom can handle the weight - the big issue is the movement of the center of gravity too far aft (gonna be a porpoising issue).  Since I hate ballast, and there's no good way to add weight forward, e.g. fuel burns off, water runs out, it's best to try to work within the weight limits.  Note that the manual gives transom limits for a 26-footer (725# plus weight of brackets etc ... derated more as the bracket gets longer).

BUT if you are building longer, say 28' or so, the transom weight limit can probably go an extra 100# without issue, likely even more if you are building a 'water camping special' boat with plenty of accommodations and appliances, or even if you will likely have 2 or 3 extra people in the boat on most trips.

I allow 1000-1100# for a sterndrive, but that's with a big V6 or V8 (etc) inside the transom.

Since boats, like most people as they age, tend to gain weight in the stern ... more gear, bigger ice boxes, more friends, more stuff on the transom etc.  It's best to start out a little on the conservative side.

Hope this helps... 

Brian

[json]

That does help, thanks Brian. So, just to understand right in my feeble brain, when I make adjustments they are done on a lever basically right? IE if I put an extra 100 lbs on the back edge of the boat, and want to balance that with weight half way between the center and front of the boat I need to add double the weight in that location? So to realistically offset 300 extra pounds on the back of the boat in any manner other than hanging weight off of the pulpit it's going to be in the ballpark of probably 500 to a thousand extra pounds somewhere forward of c/g? This sounds like it's probably not really very realistic given the other amenities I want to add like a bait tank in the middle of the cockpit that can hold a hundred gallons of water, etc. Perhaps when the time comes I can go put this in the water and play around with some ballast before I pull the trigger, although I don't think that that's really going to surface any porpoising issues. 

[json]

...and I talked to a yamaha rep who says you can't buy their helm master system unless it's on a manufactured boat, so duals isn't really in the cards. Feedback about C/G and effects of too much weight in the stern is great though, definitely helps to keep that in perspective.

[Todd j]

That’s a bummer.  Helm master still has a lot of great features even for a single main.  I hadnt list all hope until now

[Farmboy]

I truly have my heart set on twin 115hp tohatsu’s. On a very short manual jack plate.

Plan on building a 28’ GA

[Brian.Dixon]

That does help, thanks Brian. So, just to understand right in my feeble brain, when I make adjustments they are done on a lever basically right? IE if I put an extra 100 lbs on the back edge of the boat, and want to balance that with weight half way between the center and front of the boat I need to add double the weight in that location? So to realistically offset 300 extra pounds on the back of the boat in any manner other than hanging weight off of the pulpit it's going to be in the ballpark of probably 500 to a thousand extra pounds somewhere forward of c/g? This sounds like it's probably not really very realistic given the other amenities I want to add like a bait tank in the middle of the cockpit that can hold a hundred gallons of water, etc. Perhaps when the time comes I can go put this in the water and play around with some ballast before I pull the trigger, although I don't think that that's really going to surface any porpoising issues.

Yes, like a teeter totter around the center of gravity (noting that the center of buoyancy is just forward of the CG, hence the 1-1/2" bow-up trim if in a perfect world).  The CG on a 26-footer is about 9' forward of where the main deck strikes the transom ... add a few inches for longer boats.  Close 'nuf.  If you figure that's the fulcrum and the motor/bracket combo has a CG about a foot behind the transom .... it's more or less 10 feet from the motor weight to the CG (fulcrum).  To balance extra weight in the motor, say 100# in our example, you'd want ... 200# at 5' forward of the CG ... or 100# at 10' forward of the CG ... or 67# at 15' forward of the CG.  In other words, the forward and aft 'moment arms' are kept equal.  The aft moment arm is the extra 100# times 10', or 1000 lb-ft.  The forward moment arm of the 200# example is 200# times the 5', or ... tadaaa ... 1000 lb-ft.  It's just a big floaty teeter-totter... If you know where the typical CG is, then the rest falls out naturally ...

Brian

[json]

That’s a bummer.  Helm master still has a lot of great features even for a single main.  I hadnt list all hope until now

I am thinking now that I am going to buy a single with digital electronic control in hopes that they open helm master up in the future. I don't think I can justify twins due to the weight but like you say, they still have a ton of killer features now for even a single.

[json]

After talking to another local shop around here, it appears I was misinformed about Yamaha's Helm Master (at least the ex version). This dealer said they can get me the components for helm master ex and I can install them myself, so the dealer in Medford I was talking to earlier must have been misinformed. The dealer I just spoke with is quoting me for all components in a complete helm master ex system, including digital control, digital steering, autopilot, and joystick, plus any other components required to make that work.

[Todd j]

Is this for.a single?

[json]

Is this for.a single?

Yep, single f300 (or any of their digital control f outboards).