here is the start to my 21V ballast install

[IDBoating]

if you have an air bubble trapped at a high spot you do not want that high spot to be the pump.

Actually, that's exactly what you DO want. As long as the pump's inlet is below the waterline, you DO want air to escape upward toward the pump.

Just being below the waterline does not guarantee no air bubbles. Don't believe me? Imagine this: A piece of hose, or assembled pipe and fittings, that is U-shaped. Hold the U upside down. The assembly will be full of air. Now lower it into the water. What happens to the air in the pipe? It goes to the highest point - where there is no outlet.

Now, imagine tilting that assembly to one side. Some air will escape but until you rotate it such that one outlet is the highest point in the assembly you will always have some trapped air.

This situation does not change just because there's a pump on one end. As long as there is a "high spot" in your inlet manifold you run the risk of a trapped bubble preventing air from reaching the pump inlet. And since these are aerator pumps, they cannot draw a vacuum and cannot force-purge the inlet plumbing. You are entirely dependent upon passive purging, which works due to buoyancy, which is sensitive to the highest point in the system.

I'm glad you haven't had any problems. I hope you never do. But someday....

[dom w. forte]

All I can say is we have had a lot of problems with MC and CC and 2007 and 2008 tiges and it was always due to the pumps being lower then the feed and even when the feeds were correct the hose comeing off of the pumps once used several times would keep water in the line and cause the hose to droop , once it droops the air gets trapped in the pump and wont prime, and once the vent gets water trapped in the hose it wont vent causeing the pump to over work.
Next time you run the system check and see fill time, and see if it meets pump specs per tank size, I would be currious.
Im not saying your system wont work I just believe its going to work harder, and periodically not prime.

[Pro-Fab]

the pumps are lower than the hoses can droop for each pump. I guess there is always a possibility of air in any system. but like I previously stated if any air is trapped it will be above the pumps and water will flow past and iff this water so happens to push the air to the pumps it will flow through the pumps and past them. Fill time is ~2-3 minutes for each rear sack which I believe are actually 750 lb sacks I need to actually measure them.

[dom w. forte]

We never had any problem with the 03 to 06 tiges that had a simular set up to yours, except the pumps were straight up on the manifolds and worked perfect, only problem they had was you couldnt be moving because the pickup was on the transom and would starve the pump if you went to fast.

[IDBoating]

there is always a possibility of air in any system. but like I previously stated if any air is trapped it will be above the pumps and water will flow past

That's the exact situation I've been warning about. That would be a bubble trapped in the inlet plumbing above the pump inlet. Think about that upside-down U plumbing example again. When the boat is initially lowered into the water, water will be forced UP into the manifold - but even if the outside water level goes above the top of the manifold the air bubble won't be magically forced DOWN and through the pumps.

I get the impression we're not going to convince you of what's really happening. So just keep this in mind and if you have a priming problem, you'll know why. If you ever need to reinstall your manifold and pump system, give serious thought to rearranging things so that the flow is continuously upward from the thruhull to the pumps.

Please remember we're trying to help, not criticize you.

[IDBoating]

One more way to visualize it:

If you lowered a drinking straw vertically into the water, the air in the straw would naturally be pushed up and out through the top.

What would you do if you intentionally wanted to trap some air in that straw, while leaving it open on both ends? You could add a bend or loop in the middle such that the straw went up, back down a bit, then back up. Now, when you lower the straw in the water, an air bubble will be trapped at the top of that loop. Indeed, a bubble would remain there even if you completely submerged the straw!

That's the situation you create when you have a manifold above the pump intake.

[Pro-Fab]

Please dont get me wrong, I understand what you guys are saying, and that is the exact reason I did testing before intall, because I had the same worries. I then realized that if I mounted the pumps straight up and down I would have an issue of the pumps being above the water line periodically, specifically while under way or in choppy water, and when angled upwrads seemed to trap air in the pumps fairly frequently, while angling the pumps downward seemed to eliminate priming issues even if there was some air in the manifold because the water would fill the pumps rather than the air. one other thing is that the manifold angles down towards the rear which keeps any bubbles towards the front of the manifold and away from the pumps.

I think another contributing factor is some water is force fed through the intake underway.

I really dont mean to sound like im arguing but I am just stating how I have tested this system and set t up. If I have priming issue, the manifold will be the first place I look. I really do appreciate everyones input.

BUt because I diligently stated my point I am sure the first trip out this year we will have priming issues. if that is the case I will have to configure a new set up that places the pumps lower but I am unaware of how that might be.

[IDBoating]

if that is the case I will have to configure a new set up that places the pumps lower but I am unaware of how that might be.

Not lower. Higher. The problem is that your manifold is above your pumps. I understand you are concerned about the pumps being above the waterline but you solve that by lowering your manifold. Making the pumps even LOWER with respect to the high point in the intake system increases the likelihood of problems.

The pumps need to be as low as possible (to keep their intakes below the waterline) but still at the top of the intake system (so air naturally gets purged out of the intake system, up through the pumps). Trapped air isn't going to flow back down out of the thruhull, so the only place it can get out is through the pump itself. And to insure that, the pump must be the highest point.

one other thing is that the manifold angles down towards the rear which keeps any bubbles towards the front of the manifold and away from the pumps.

But that's precisely what you DON'T want. This arrangement traps air in the manifold system by keeping it "away from the pumps". You want air in the system to rise up to the pumps, because if that happens the reason is because there's 100% water behind it. In your arrangement, any air in the manifold is stuck there and cannot escape. I think you'll agree that in a system intended to pump water, it's best to let any trapped air escape?

[IDBoating]

I just thought of something: Are you using a scoop thruhull and not a mushroom? If so, the force of the water being pushed into the manifold while underway might drive air out of the system through the pumps. However, you'll always have that pressure and the aerator pumps will pass it right through to the bags, constantly trying to fill them. You'd mask your purging problem but suffer from no way to stop inflowing water other than shutting off that ball valve.

So... are you using a scoop?

[Pro-Fab]

any through hull on the bottom of the boat will either create a vacuume or force feed water through the system. In our case we are using a mushroom through hull and it does actually autofill very slightly (we really dont mind) this could be why we havent had the problems you are discribing.

The way I interpreted the information deducted from our tests is that consideringthe entire intake/ pump system is under water the amount of water in a 1.25" i.d. pipe the little bit of air that remains will be insignificant and stay at the highest point. And the amount of water that will feed the pumps will keep them running. If the air were to enter the pumps it would escape the pump rather quickly, because of its insignificant quantity.

So far my theory has proven right for our system. The forced induction was uncalculated for but I am certain that it will not hurt our situation.

Im not an engineer by any means but I am a man with more common sense than you could ever put in any homedepot shopping cart. I design things and build them for a living I do measure and calculate everything I build but I like trial and error as my main source of testing, because it allows for real world situations, including miscalculations and unnatural occurences. If this system should ever fail I will fix it in a way that it will last longer than the previous time, because then I could recalculate my miscalculations, and plan for the unnatural occurences.