Homebuilt jetboat-project

[cwoav8r]

Agreed, that is quite a gap... one of the guys at SBT told me a quick way to check was, if you can fit one business card between the impeller and the housing you are okay... two business cards, you will begin to see cavitation.

 

[Speedling]

I just found this thread, and I don't know how I missed it!!!
1. Yes, what you describe of a clutch slipping feeling is almost ALWAYS cavitation, which everyone has addressed here.
2. The motor is plenty to push that boat, and I'm jealous, because I have always wanted to do a jet conversion like this!!!
3. I know NOTHING about that paticular pump. As such, you need to find out if they have an impeller chart for that pump, and what kind of horsepower you can absorb with it.
For example, check this out:

This is for the common Berkeley drives on most of the small go fast jet boats. If you have 400 hp, for example, you have to determine what rpm you will be making that at, and choose an impeller for that, as well as perhaps giving it a lower pitch than suggested due to having a larger displacement hull.
4. I will stay tuned here for sure as the prospect of someday taking a go fast boat and converting it to a Jet greatly interests me so this is an AWESOME project you have!

 

[Matz]

Thanx everybody for Quick input and lightningfast replies.

I Will post some pics after The new impeller is installed. I Will also report back here after a test drive, before winter sets in. To see If it has Made any diffrent.

 

[Matz]

Dont know if this will help you guys to help me... i got this chart from our friends at Castoldi Australia.

 

[Englewoodcowboy]

What pitch impeller did you order? I would guess that a 355 chevy could be anywhere in the 200 to 300 hp range easily by how it was built. That bit of info an this chart would help you tremendously. Also, is the pump direct drive (1:1) to the motor? If so, I would look at a max RPM range of 5000 but again, you need to know where it was built to run, maybe 4500 might be recommended max for it. Once you figure those two items out, then apply them to the chart and order your pitch. If I was just ball parking it, I would start with something like the 31/35E and go from there.

 

[Matz]

As far as i know , there are just one standard pitch for this jet pump unit , and there is an build in gearbox in The pump. That chart i uploaded, just above, is over The gearbox ratio, ingoing rpm/ outgoing rpm.

And you are correct about the engine power, a standard 350 is around 260 hp, this is built to a diffrent spec: diffrent camshaft, high pressaure oilpump, bigger bore, high compression pistons, bigger carb and so on.

previous owner said something about 330 hp... ( not sure, cause i never put it to a dyno...)

But still, i have never seen/ heard anything about castoldi impellers with diffrent pitches, if i understand everything correctly, i think that the pitch is fixed, and you would just adjust the gear ratio instead of fiddeling with the pitch.

There is a "direct drive" from engine -> pump unit. ( just a splined axel with dubble U-joints ) BUT there there is not a direct drive from engine -> impeller, there is a gearbox with changeable gear wheel-ratios.

Engine-> gearbox->pump/impeller.

So to answer your first question: i orderd a standard composite impeller, no variable pitch, dealership didn´t ask for what pitch aswell. ( i have a "sparepart-list" with the differnt serialnumbers for the diffrent details in this pump. And there is just 2 diffrent numbers when it comes to impeller: one number for composite/plastic and one number for the stainless steel version.)

The pump WAS connected to a 305 volvo before, so the gear ratio installed at the moment is supposed to fit/suite a V8 engine and its range of RPM/power, this is what i was told be the previously owner. ( right now it´s just connected to a bigger and more powerful V8 than before...)

I will contact castoldi and ask about diffrent pitches, and post the answer here.

//Matz

 

[Englewoodcowboy]

The only standard for a pump would be diameter. The pitch is variable as it can and will need adjusting via performance increases, elevation changes etc.

On another note, when you reassemble your pump, use some marine silicone caulk to seal the pump housing together. I do not know if there is a gasket and its condition but if the nozzle is not sealed well to the housing and you get blow by, it will cause cavitation issues there as well even though it is after the impeller.

 

[Matz]

Hi. Got an anser from castoldi last night about The pitch:

Hi Matz,

Castoldi use an axial flow impeller that has been optimized for its diamter
and pitch. There is only 1 available per diameter of waterjet.
The gears are what matches the engine/s output to the absorption curve of
the impeller.
If you know the RPM/HP of your engine, I can check to see what ratio would
be the better suited.

Hope it helps.

Regards

 

[Englewoodcowboy]

I would also get an answer from them on the wear ring tolerance. It doesn't take much of a gap to cause performance issues and loss. My sons sea foo sucked up a rope and it made a small gouge in one spot about 2mm in depth and we instantly saw his motor over revving, blowing out, loss of speed etc. and when we tore it down to look, I thought that to should have been ok but it certainly was not, night and day difference as soon as we put it back into spec.

 

[Matz]

latest update: Spoke to my Castoldi contact and the gear ratio in my jet drive is correct, when i tore it apart, i noticed that i couple of bearings was scrap.
So, i ordered some new ball bearings, a axial seal for the outgoing axel due to water in the oil, and a brand new impeller.

When i get the goods from italy i will assemble everything and get back to you.

I doubt that i will able to testdrive it anymore this year, the snow has already started to fall, just a couple of miles from here.

//Matz

 

[Bruce]

Sounds like progress.

 

[Speedling]

I'll be waiting for your report!
I'm guessing you won't be going super fast or anything, but it'll be a fun ride that can go shallow waters, and actually be fairly efficient if you can get this thing to go on plane correctly. Good luck!

 

[Matz]

Ok, so the more "boatible" temperatures has arrived here in the far north of Europe.

The bearings and impeller has been replaced, took it out for a testdrive this morning, and reached 27 knots before i realized that i had no idea what amount of fuel was left in the tank.

But, the fact that it went up in plane, and had more speed to give before i turned back, shows potential.

Now i will start with the "fun" bits, tuning of carb and timing igintion. ( still a bit rough at idle, and a bit of hesitation when opening the second throttle )

Sorry that i´ve haven´t been active for a while, but... he who waits and all that....

//Matz