New to tinboats.net. I come with axial flow jet questions

[Monteria]

Hello all. I have been boating and rigging hulls for some 20 years, but have zero axial flow jet (and little outboard jet) experience.

None the less, I have been bitten by the jet bug. I have a motor, pump, design, fabrication and aluminum welding ability, so I am hoping that Y'all can help me out with some technical advice to insure success. Or at least avoid catastrophic failure 8)

While designing the hull, it occurs to me that decisions concerning dedrise should include consideration for not only calculated draft, but also for pump location in relation to draftline.

Is there a general rule for pump location in relation to water line while at rest? IE: when at idle, do you want your pump to be say half under water with the intake box half full? 1/4 full? 3/4 full?

As soon as I feel confident in pump placement, Ill finalize my drafts and start a 1:5 model. Hopefully The model proves out before my aluminum order arrives...

Thanks in advance for your assistance,

Steve

 

[RivrLivn]

I assume your are going with an inboard motor with a pump. If so the bottom design is important, will need a delta pad or a radius hull to feed the pump the best.
As far as install, you really need to refer to the installation instruction for the pump. American Turbine and Hamilton both have this on their web sites.

Looking forward to seeing pics of the build.

 

[Monteria]

Thanks for the input. Unfortunately, this one aint going to have instructions of any kind. I am using what I have on hand, which is a '99 Yamaha 1200GP jet ski, for donor parts.

I have what I think you are referring to as a delta pad incorporated into the design already. I need to figure out my pump position though. If all else fails, I will approximate the original pump depth from the ski.

Steve

 

[PSG-1]

Your pump is very similar to mine, the XL1200W.

Indeed, a delta pad would feed water more efficiently to the pump, but if you look at my boat, you'll see it can be done with an unmodified flat bottom.

 

[RivrLivn]

I understand now.
PSG-1 definitely knows his stuff when it comes to the wave runner to boat conversion. Look at his thread, he did a great job.

The pump should be fully submerged at rest, the intake housing needs to be full of water, otherwise it would cavitate at take-off.

I assume the pump angle should be defined by the pump housing mounted to the bottom of the boat on the pad. PSG-1 can answer that better than I.

Give us lots of pics.

 

[PSG-1]

Thanks for the compliments! :mrgreen:

You are correct. Ideally, the pump should be fully submerged at static flow (idle) Look at any jet ski or jet boat, and this is how they are set up. Now, IN THEORY, I would guess that as long as the intake grate is in the water, it would feed water to the impeller. Because, when you think about it, when a jetboat is on plane, the majority of the pump is mounted inside the hull, and is above the waterline, the only part in contact with the water is the hull and the intake.

However, without some sort of 'spoon' or delta pad incorporated into the hull, it will run fine on relatively smooth water, but will tend to cavitate in light to medium chop. My boat does OK in chop up to 1/2 ft high, beyond that, I have to go slow to avoid cavitation. Also, if I turn too sharp with too much throttle, it will cavitate. But then again, so does my Merc 50 with a prop drive. That said, I'm willing to sacrifice a little cavitation to have that minimal draft with a flat bottom and no delta pad or spoon.

As for the angle of the pump mounting to the hull, if you look closely at the pictures of my pump installation, you can see that the intake housing has an angle to it, begins about 1/8" thick at the forward end, but ends up being about 3/4" at the rear end.

This slope will need to be duplicated when constructing the tunnel, and to tell you the truth, this, and rolling the horseshoe ring that meets the topside of the intake housing's radius, are the most difficult parts of that task.

Feel free to browse through the pictures on my Aluma-Jet thread, hopefully they will be useful for reference.

If you have any questions, PM, or e-mail me at: [email protected]

 

[Monteria]

Well, that means that I need 6 or 6.5" of draft. More than I wanted, but still less than a prop runner with 3" draft, a prop and skeg sticking out under it.

Incidentally, I do not have the cast intake, I will have to fab my own.

Time for a little math...

Thanks,

Steve

 

[PSG-1]

My boat draws about 8 inches of water at static flow. Which is about the same as my Triton 1650SC rigged with a 50 Merc outboard.

By the time you add an engine, batteries, fuel tanks, inner decks, passengers, etc, your boat will draw at least this much, which should put the entire pump in the water.

If you have to fab your own intake, you might want to check out Ranchero50's build, he fabricated the intake duct for his boat.

 

[Monteria]

I have checked out your build and Ranchero50's. Each lent inspiration and great information which will be incorporated into my build. And, because I am starting with a pile of aluminum sheet, I will have the advantage of picking and choosing which features to incorporate and how.

 

[PSG-1]

So, you're building your own hull, as well? Wow. =D> Definitely keep us posted on this with lots of pictures!

If I were going to do it from scratch, instead of using an existing hull, I'd probably go ahead and incorporate a spoon or a delta pad. The way I'd do it, is to start flush with the hull at the bow, and end up with a 2 inch drop on center at the stern, with the pad wide enough for the pump to sit flush (if I remember right, it's about 10 inches wide)

Looking at the stern, the profile of the bottom would look something like this:

----------\________/----------

Your pump would sit in the flat area in the center. Putting it a couple of inches lower, on that gradual tapering spoon, you'd eliminate a good bit of cavitation.

However, you have to take into account the alignment of the engine to the pump. With my MR-1 engine and the XL1200 pump, the engine is practically bottomed out, almost resting on the floor of the bilge, there might be 1/8" of clearance, and that's with the pump mounted flush with the hull, not dropped 2 inches.

So, depending on what engine you use, you may have to form the delta pad wide enough for both the engine, and the pump to sit in, or you'll end up with the engine being much higher than the pump.

 

[Monteria]

Yes, I am going to fab the hull. It will be a modified V hull, so at the end of the day the hull itself IS a "delta-pad". Current calculations predict that I will need ~3" of deadrise to the delta-pad, to achieve my desired draft of 6" at rest or static flow.

Steve

 

[PSG-1]

With the modified V hull, it should definitely work, and take out a lot of cavitation. Like I said, keep us posted, and be sure to get some photos of the progress as it moves along!

 

[Monteria]

I will, it will be a little while before I generate any tangible progress though. Heck, with my current workload and hunting weather still creeping into the Texas summer, it will likely take me a month to complete a 1:5 scale model. Then I will order the raw materials...

Steve

 

[Ranchero50]

My ears were burning, had to chime in.

Per deadrise etc, I don't think you have a lot to worry about in your design. Mine seems to load fine at idle and once it's moving at idle I can do a 1 second push to WOT and the boat will take off fine. The first couple trips with the boat naked it didn't load and the engine idled at 1500rpm, but once I stepped on the rear deck it loaded and stayed loaded once it was moving.