Jet jon cavitation issues

[willholl79]

I just recently purchased a 1436 jet jon with a 500cc Yamaha Waverunner engine. With just me sitting in the back driving, it will jump on plane fairly quick but still has cavitation issues. With another person in the boat it won't get on plane. The boat was built by the previous owner using the cut out hull section of the wave runner grafted to the jon boat. It's not the neatest rig in the world, but I plan on reworking some areas, if I can get cavitation under control.

This is my first experience with a "jet boat". I have had jet skis in the past. I am trying to figure out where to start with this issue. Is it the pump itself, weight, or angle/draft of the jet, not enough water in the intake, all of the above?

Any help is appreciated. This is a great site!

 

[PSG-1]

It could be several factors. Flat bottom boats don't exactly make ideal jetboats. Most jet skis and boats put the pump a couple of inches lower than the hull by forming a "spoon". Angle of pump and type of impeller could have an effect, as well.

I run a Yamaha MR-1 FXHO engine in a Duracraft 1648SV, using a Yamaha XL1200 pump (155 mm) and it runs without cavitation issues on relatively calm water. If I get in chop about 1 ft in height, then it starts cavitating like crazy. Or, if I try to go too fast into a corner, it will also catch air. But hey, so will my Merc 50 prop drive on my Triton johnboat. ANY boat will cavitate under the right conditions.

One thing you could try is to install a top-loader intake grate, this will feed more water to your pump to keep it from starving. Also, you could try a wet wolf adjust-a-thrust nozzle cone. The AAT cone has a spring-loaded shuttle that allows more water to flow through the pump at low speeds, and closes up a little at higher speeds, for more thrust pressure, and as such, it can significantly reduce cavitation.

When you put another person in the boat and it won't take off, is that person in the front, or the rear of the boat? If there's not enough weight on the stern to put the pump low enough in the water, it will cavitate like crazy.

Try shifting some weight around. On my jetboat, the battery and both fuel tanks are in the stern, and if you look at most jetboats, they try to put as much weight to the rear as possible. If you start to porpoise, you can always install a trim tab, which you can make from a 1/4" X 2" wide strip of aluminum plate, welded to the back end of the ride plate, at a slight downward pitch. That little tab will be enough to take out the porpoising, it's what I did with my boat.

One other thing.....take a look at the hull in front of the intake scoop. Is there a strake located there? If so, this can channel an air pocket along the keel of the boat and into the pump, causing cavitation. If there's a strake, the best thing would be to remove it for at least 2-3 feet in front of the pump (even better, remove the entire length of the waterline and be done with it), then, use a piece of 3/16" X 3" wide aluminum plate, welded, riveted, or bolted flush to the hull of the boat.

Hope this info helps. Keep us posted!

 

[willholl79]

Thanks for the comments. The fuel tank, battery, etc is in the back. With a 2nd person in the boat, I tried sitting both people in the back and it still wouldn't plane. I am going to try a different grate like you suggested and see if it helps. At first I thought it was "blowing out" because the pump nozzle sits too low in the water, but the more I think about it, I believe the pump is sucking air.

I'll try to post some pictures later today.

 

[willholl79]

Here are a couple of pictures I already have. I will get some better ones later.



[URL=https://s247.photobucket.com/u...MG_20130603_175231_6791_zps60cf61ae.jpg[/img][/url][URL=https://s247.photobucket.com/u...MG_20130603_180009_1321_zps6c4b7970.jpg[/img][/url]

 

[willholl79]

And, yes it does have 2 strakes in front of the grate. I will get a picture of the underside of the boat later.

 

[PSG-1]

willholl79 » 1 minute ago[/url]"]And, yes it does have 2 strakes in front of the grate. I will get a picture of the underside of the boat later.

Bingo! This is probably a large part of the issue with cavitation.


BTW, here's a picture of a top-loader grate:



Notice how it has the scoop toward the rear edge like a conventional grate? But it also has that blade farther toward the front. This blade is pitched at a slightly different angle, and it feeds water to the "top" of the pump (the upper radius) which keeps it from sucking air and cavitating. Also, it only has 2 longitudinal tines, which eliminates some of the air pockets that would be created with a multi-tine grate.

Only drawback of the top-loader is that it becomes a vacuum cleaner, and that's not a good thing if you plan on operating in a lot of grass and other debris. Due to the areas I operate in, I had to decide whether I wanted to eliminate cavitation, or be able to keep weeds out of my pump, and I decided to go back to a factory grate, then modified it to be a swing-down "stomp grate" for easy cleaning of weeds.

In your case, I still think the strake may be the culprit. Especially if there's 2 of them!

 

[willholl79]

The intake is centered side to side in the hull, and the strakes are just to either side, cut off about a foot or so in front of the intake.

The jet ski used was a 1991 Yamaha WR500. I wonder if i can find a direct fit top loader intake grate for that model?

 

[willholl79]

The strakes actually end a little over 2' in front of the intake.

 

[PSG-1]

willholl79 » Today, 10:16[/url]"]The intake is centered side to side in the hull, and the strakes are just to either side, cut off about a foot or so in front of the intake.

The jet ski used was a 1991 Yamaha WR500. I wonder if i can find a direct fit top loader intake grate for that model?

Check Riva Yamaha performance parts, you should be able to find a top loader grate for that model. These grates are model specific, and will bolt right on with no modification required.

With the strakes being on either side of the flow path, they may not be the culprit as I initially thought. Especially if they've already been removed a foot or two in front of the intake area. If the strakes were directly in line with the intake, they would definitely cause issues.

That said, you should first try a top loader grate, that's the cheapest, quickest, and simplest variable to experiment with, and I think you'll see some improvement.

Next variable would be the AAT nozzle cone, and experiment with different shuttle lengths (be careful, though, too long of a shuttle will over-lug a 2 stroke engine, so, follow the instructions carefully)

Another variable to experiment with: the impeller. My guess is that you're probably using a conventional type blade, as that's what most factory impellers are. You could try a Concord (swirl) type impeller, they tend to run better with less cavitation. In my boat, I'm running a 13/19 pitch Solas Concord impeller.

 

[willholl79]

Here is a picture of the intake:

[URL=https://s247.photobucket.com/u...IMG_20130609_115906_862_zps335879f1.jpg[/img][/url]

 

[willholl79]

Thanks for all your help PSG-1. I'll order an intake grate as soon as I can find one.

 

[PSG-1]

It looks like they've got enough of a flat area ahead of the intake that it shouldn't cause cavitation.

I do notice a livewell plug in the middle, this small protrusion could be enough to cause an issue. Not saying that it is, but it could. Maybe try running it with that plug removed and capped off with something flat, see if it makes a difference.

If that doesn't do anything, next step would be the top loader grate.

 

[willholl79]

PSG-1 » 09 Jun 2013, 14:44[/url]"]It looks like they've got enough of a flat area ahead of the intake that it shouldn't cause cavitation.

I do notice a livewell plug in the middle, this small protrusion could be enough to cause an issue. Not saying that it is, but it could. Maybe try running it with that plug removed and capped off with something flat, see if it makes a difference.

If that doesn't do anything, next step would be the top loader grate.

Good call on the live well plug.

 

[PSG-1]

I'm thinking it could be that plug. You'd be surprised what a difference something as simple as that can make. Keep us posted.

 

[willholl79]

I am having a hard time find a top loader intake grate for a 1991 Yamaha WR500. Maybe I can find the part number for the original and see if it fits another ski?

 

[willholl79]

Well, In calling around I found that the only company that made a top loader grate for the WR500 is no longer in business. I did find pictures of one:
[URL=https://s247.photobucket.com/u...ums/gg138/willholl79/G6_zpsbc0ad789.jpg[/img][/url]

[URL=https://s247.photobucket.com/u...ums/gg138/willholl79/G2_zpsfec3b4e8.jpg[/img][/url]

 

[PSG-1]

Try to cross reference some similar Yamaha jet skis and see if the part number is the same for the intake grate. If you can find a similar model that uses the same part, you may be able to find a top loader grate for that.

Typically, a lot of parts are similar for pumps that are similar sized, for instance the GP1200 and XL1200 have parts that are interchangeable, because both pumps are 155mm.

 

[willholl79]

PSG-1 » 10 Jun 2013, 19:04[/url]"]Try to cross reference some similar Yamaha jet skis and see if the part number is the same for the intake grate. If you can find a similar model that uses the same part, you may be able to find a top loader grate for that.

Typically, a lot of parts are similar for pumps that are similar sized, for instance the GP1200 and XL1200 have parts that are interchangeable, because both pumps are 155mm.

I haven't gone as far as cross referencing part numbers yet, but I called RIVA and Pro Tec and they both told me the best chance is finding a used one, as no one makes them anymore.

I may have to build one based on the pictures above.

What's the best way to eliminate a worn impeller/wear ring?

 

[PSG-1]

I haven't gone as far as cross referencing part numbers yet, but I called RIVA and Pro Tec and they both told me the best chance is finding a used one, as no one makes them anymore.

I may have to build one based on the pictures above.

What's the best way to eliminate a worn impeller/wear ring?

Excessive impeller to wear ring clearance will cause loss of top end, overall performance, and guess what else? Cavitation! Generally, acceptable range is .005" to .020" To check, remove the intake grate, and use a feeler guage to take a reading.

If it's excessive, this could be your problem.

On a typical Yamaha jet pump you have 4 parts. The wear ring, the stator, the thrust nozzle and then the steering nozzle. Some models like mine also have a reverse gate.

This is a breakdown for how to disassemble the XL1200 pump. Yours is probably similar as it's also a Yamaha.

First, remove the ride plate. This will allow access to the pump. Remove the steering nozzle. The remove 2 support bolts for the thrust nozzle. Should also be 2 more bolts that hold the wear ring assembly in place. Remove those. Then, remove the 4 long bolts that go through the pump assembly, located on the 4 corners of the thrust nozzle flange. Tap on the thrust nozzle with a rubber mallet, and wiggle it, then remove it. Now, disconnect the cooling line from the stator assembly by loosening the hose clamp.

For the stator, you'll need to use a pry bar, and pry at the designated points (small tabs on either side of the components where they mate together) The stator assembly should slide back when you pry. Once you get it to move back, then, you'll need to remove the engine coupler from the driveshaft, so you can slide the shaft out through the hull with the stator unit.

After removing the stator, then you can remove the wear ring assembly. Same procedure, use a small pry bar on the tabs, and carefully alternate from left side to right side, so as not to break the tabs.

Once you have the old wear ring assembly removed, install the new one, and re-assemble pump in reverse order. PAY PARTICULAR ATTENTION TO THE ALIGNMENT PINS THAT GO BETWEEN COMPONENTS, they are easily lost! Be sure pins are in place before assembling the components.


As for your impeller, you will need 2 BFW's (large wrenches) :mrgreen: Place one BFW on the flats of the shaft. Place the other on the nut of the impeller. (If impeller has no visible nut, but a rubber bushing, pry out the bushing. You will need a specialized impeller removal tool to remove a splined-face impeller) To remove the impeller, remember it is a LEFT-HAND THREAD, so, turn right to loosen.

You can have a worn impeller re-built by a company called Impros. They can also re-pitch it to whatever pitch you want.

 

[willholl79]

Thanks again for the help PSG-1.