Where to put fuel and battery (weight distribution)

[wild.coast]

18ft custom alu RIB with 50hp Yamaha. Current have the fuel tank and battery jammed right in the back by the transom. Looking to clean up my wiring and add a proper fuse box and stuff, so wanted to move the battery somewhere else with more room - curious if anyone has any advice!

I've heard that shifting weight forward helps a lot with handling and weight distribution. The boat has two bench compartments: one is the bench the driver sits on and another bench near the bow, pic:




(I realized I don't take any pics of just the boat so here's some sea cliff rock climbing action haha)


The obvious/convenient place is the rear 'driver seat' bench, putting the battery beside where the driver would be sitting. I'm just thinking maybe it doesn't shift weight forward enough to make any differnce? The front bench is a bit more awkward, smaller access opening and would mean I would have to make super long battery wires to reach the outboard, not sure if that's a problem. The other option is to put the fuel tank at the very front, but again, would need super long fuel lines, not sure if any issues/problems with that?

Someone suggested I just move the fuel tank forward temporarily and see how it feels. I guess I just don't know what to even look for when you reference 'better handling'! The boat seems to sit flat in the water, there isn't any tilt -> but I'm assuming that's because the tubes have tons of flotation so it would take a lot more uneven weight to get the tube on one side to actually dip further into the water.

The only thing I know I don't like is the fact that boat tends to 'slap' when going into wind waves instead of cutting through them. But I always assumed that was due to the flat nature of the hull (very shallow v-groove).

What should I expect/be looking for in terms of 'improved handling' ?


In short, easiest and most convenient is the rear bench for battery and leave fuel tank where it is now by transom, but if it's worth doing all the extra work to shift it all to the front then I want to know!

[Roflhat]

The improved handling is usually in reference to big seas, keeping the bow down.

Weight at the back makes the boat more efficient, wasting less power lifting the bow up so more power pushing forward (less trim). If possible experiment with moving weight around, even if you've got another person get them to move around instead of shifting fuel and battery.

Most likely I'd say leave them at the stern

[Peter_C]

When you go to accelerate how much bow rise do you get? While cruising along does the bow porpoise, meaning kinda goes up and down?

In floor is the best location for a fuel tank, keeps a low center or gravity, with weight forward. Does it already have one there? Could you add one? How many liters/gallons is your current tank? Is it big enough for your needs?

Batteries are pretty easy to move around, and just requires new cables. Batteries are also heavy and can make a good change in weight distribution. My theory is fuse panels should be high in the center console so even if the boat gets filled with water it will stay dry. Easy access is important too.

My boat came with a replacement tank under the back seat and battery also mounted under the seat. It was seriously butt heavy with a 4 stroke Honda 75HP (Same block as Honda 90HP), so I repaired the in floor tank and moved the battery into the console. Doing so made a huge difference in bow rise.

Originally Posted by Roflhat

Weight at the back makes the boat more efficient, wasting less power lifting the bow up so more power pushing forward (less trim).

Hmmm...as someone who filled their wakeboarding boat with a lot of water to create big wakes, the more bow rise it had the more horsepower was needed, and sometimes we were so loaded we had to shift people weight forward until the boat was up to speed. I do agree using people to move around weight for testing is an excellent way to see the changes in weight distribution.

[wild.coast]

Thanks for the replies! @Peter_C:


Fuel tank is 35L, just a portable boat jerry. It's enough for short outing, which we do a lot of, but I usually bring a couple of 25L jerry cans for multi-day remote trips, these fit well in the bow. Here is the current fuel tank:





Here is an example of the boat on a multi-day trip carrying 25x3, so extra 75L of fuel using jerry cans:





An in-floor fuel tank would be sweet, but I'm assuming that would be a major fabrication project which I don't have the skill nor funding for!

When you go to accelerate how much bow rise do you get? While cruising along does the bow porpoise, meaning kinda goes up and down?

With two people sitting on the bench the bow definitely goes way up and then drops once boat goes on plane. Having the passenger step forward towards the bow temporarily definitely makes it way easier/faster to get on plane.


One thing to note is that there are also these steel plates extending off the back of the transom in line with the hull which previous owner added, I'm assuming this is to artificially extend the length of the hull backwards and make it easier to get on plane.


When cruising I don't think the bow porpoises too much, no.

My theory is fuse panels should be high in the center console so even if the boat gets filled with water it will stay dry. Easy access is important too.

Yeah, center console seems obvious as a choice. But the center console is pretty tiny unfortunately and the stiff steering cable is right in the middle and eats up a lot of room:




One battery just fits if you unhook the steering cable and slide the battery in. I have a Blue Sea Systems switch panel with integrated fuses, so I guess a lot of the wiring would have to be in there anyway. The other things I would want to add is a few T's for the NMEA2000 network (engine to Garmin communications) as well as circuit-wide kill switch.

Another consideration is whether I should be planning for enough space for a second battery. Honestly one of the main reasons I discounted the console placement (because console doesn't fit 2 batteries).



Currently I only have one battery; it's a 'combination' type, so deep cycle with high enough cranking amps to start the 50hp. Not sure if I should be wiring a second 'house' battery. Is that overkill for this type/size of boat?



We have a prawn puller which is the main reason I considered a second battery. Also do tons of very remote trips with zero boats around for days sometimes. Prawn puller only gets used for short local food-harvesting outings though. I've been using the prawn puller on the current battery and it doesn't seem to drain it much. I'd rather save the weight/space and keep one battery I think.

[willk]

Looking at your loading, I'd suggest that you potentially have enough "weight forward". If your boat is getting on the plane easily and running flattish, I'd worry about something else for a while.

We have the same deck matting BTW, neat...