Clearing up a misconception about reserve capacity in my boat, probably others.

[H€PHÆ$TU$]

I've heard it repeated everywhere, this forum and others, that on the fuel pickup in our boats, the RES port straw dips down lower in the tank than the ON port straw does so as to give the operator one engine when the tank runs low instead of running both dry and getting stranded. This is not true, at least on my boat, and here is why.

Here is the upper section of the sending unit from my boat. It is obvious that the RES straw is longer than the ON straw, which would result in the RES being able to suck from a lower point of the tank. Different boats use this different ways, but ultimately the result is to have a reserve capacity to get you to a fuel source if you run the tank low. But this is not how it works on my boat and probably not on other boats as well. This is important to know if you own a boat and think you have a reserve capacity.



They obviously use the same top section in many applications. But what differs is the bottom section of the sending unit:



Notice how the ON barb is longer than the RES? This takes up the entire void length of the reserve straw in the top section and effectively eliminates any reserve capacity. I've confirmed this by pulling the sending unit and putting it in a glass of water. I can suck water like a straw from both ports at the same water level. There is no point at which the RES port siphons before the ON port.

Look at the factory parts manual. They obviously use a generic pic of the sending unit because it even shows one without the larger cup that my boat has:



And now look at this eBay listing for sending unit repairs:

https://i.ebayimg.com/images/g/rw4AAOSwIgNXsNar/s-l1600.jpg

The tops of most of the units are essentially the same but the bottoms differ greatly.

So long story short, there's no reserve in my boat, probably not others as well. Both the ON and RES port do the same thing. They're only named differently for economy of manufacturing scale.

 

[mikidymac]

Have you broken open the extension yet?
My guess is the pickup tubes are staggered in the extension also just like the top one. If they are not there would be no reason to have 2 pickup tubes.

 

[H€PHÆ$TU$]

Have you broken open the extension yet?
My guess is the pickup tubes are staggered in the extension also just like the top one. If they are not there would be no reason to have 2 pickup tubes.

Yes. They both go all the way down. Like I said, they both sucked water from a cup at the same depth. There is no reserve capacity. If the screen was as shown in the parts guide there would be though. I think they use the same top portion in multiple models and in some there is a reserve capacity so that's why it's there. It just happens that they rig it in my model to not be used in that function.

 

[H€PHÆ$TU$]

Have you broken open the extension yet?
My guess is the pickup tubes are staggered in the extension also just like the top one. If they are not there would be no reason to have 2 pickup tubes.

Here's pics to prove it. You can see how the bottom of the top section is made the same as the extension so that, if there was a different, shorter circuit board inside, the screen would fit the top section directly without the bottom extension at all, as shown in the parts diagram. In that case there would be a reserve capacity.

I understand that some boats have a selector switch. Mine doesn't. My bet is that those boats do have a different extension on them where the ON straw does go deeper but I don't know for sure since I don't own any of those boats.

 

[jts32]

The Sportster 1800 does not have a fuel selector like the skis or some boats. It just has a low fuel level alarm that is silenced by the rocker switch on the helm controls. So basically the same concept, if you have to silence the alarm with the switch, you are using the reserve

 

[mikidymac]

There’s the reason. If there’s no reserve selector there’s no reason to have the staggered pickup hoses.

 

[H€PHÆ$TU$]

Yes. In my recent experience, everyone just seems to be under the impression that all the boats have a reserve capacity just because they have a fuel nipple labeled RES, which isn't true. I had posted another thread somewhere else about the fuel system and literally everyone in that thread told me that the reason one engine feeds off the RES nipple is so that you still have one engine to get home on. That's 100% false. If I went out on that assumption, I would end up with 0 engines to get home on.

 

[chipskittles]

That's good stuff to know, thank you for sharing this information. I have had my baffle out on my 97 but I did not pay attention to the inlets but I know it did not have that plug on extension like yours. My boat has the switch between regular and reserve so I guess I would have to pull my baffle again to know for sure but it sounds like it's just that extension that has the change.

 

[H€PHÆ$TU$]

That's good stuff to know, thank you for sharing this information. I have had my baffle out on my 97 but I did not pay attention to the inlets but I know it did not have that plug on extension like yours. My boat has the switch between regular and reserve so I guess I would have to pull my baffle again to know for sure but it sounds like it's just that extension that has the change.

Yes I would say that you have a reserve capacity, especially since you have the switch. So basically when you throw that switch it switches from the ON nipple to the RES nipple.

 

[JexxElectron]

the 1992 XP has a selector switch and the pickup tubes in the tank are 165mm and 195mm. So the reserve goes to the bottom of the tank and the main does not. There is no fancy float sensor. Just pickup lines in this boat. The fuel sensor is a separate hole.

 

[mikidymac]

I think it's just a few of the boats that are different.

The easy thing is if you have a fuel selector you have a reserve capacity. If you only have a low fuel alarm then you have no reserve and once you are out you really are out.