Jet Cavitation Issues on Prototype Hull

Hi guys,

I'm new to the forum but I have been reading a bunch of the posts to try and help me as much as I could before reaching out. I'm part of a team that has been developing a unique boat that is powered by a Hamilton 361 and a Cat C8.7L 650HP. The project is a prototype so unfortunately I can't post pictures directly on here but I could send them privately. I understand that Baeckmo is probably the guy I should talk to, unfortunately this site won't let me send him a private message on here so hopefully he finds this post.

The issue we are having is the boat is cavitating at around 5 knots. It is a deep V hull with a 30 deadrise and weighs approx 6 ton. We don't know whether the issue is in the jet or the hull design, so we are planning some more testing and hopefully some underwater video to try and see where the air may be coming from if it is a hull design issue or if the issue maybe be in the jet itself. The impeller looks good and the gap is well within tolerance and it is the correct pitch according to Hamilton for the engine we are using.

As I've read at these low speeds the air can even come from behind the hull, which may very well be the issue. I would like to know what type of tests we should perform when we put it back in the water. I have read about the test of tieing it up and slowly increase RPM and see if it cavitates and then turn the steering nozzle and see if it stops or not.

Any other help you guys can provide is greatly appreciated!

Cheers!

 

Indeed he is...

Have you manged to match the engine power curve as you increase rpms... does it race (slip).. does it die ....???
Is there any obstruction in way of the inlet?...is the inlet supplied by Hamilton, or your own mod's to it to make it fit etc?
What is the deadrise angle at the inlet...if the inlet on the CL of the hull?

Many questions really... which I understand you can't post images of for clarification.

 

I would check loading of jet impeller, there is a graph which I published at SURV-8 conference, see attachment.
Usually cavitation is caused by a) appendages, seacocks, strakes or b) jet diameter is too small in relation to resistance/thrust - i.e. jet loading too high.

 

It seems to pull good until about 1500 RPM and then is just start slipping and cavitating. There are no obstructions in way of the inlet and it is the stock aluminum weld in intake from Hamilton, no mods were done to it. The deadrise at the intake is 30 degrees. Yes the jet is mounted on the CL of the hull. If you can please send me a Private message I will give you my email address and I can send you some pictures of the 3D Cad model. Thank you!

 

What is the Lwl and displacement at testing, of the boat too?

You need to have "start a conversation" - some how checked on your profile...as it currently does not show. Thus I cannot send you a PM. Noted here:



It is not shown.

 

Are you sure about the 5 knots ? That wouldn't even be "hull speed", one might expect the onset of problems would wait till "on the hump", and with 30 degrees of deadrise, I imagine that hump is going to be considerable. Is it 30 degrees, with a pad ?

 

It has to be around 5 knots, it is not very fast at all. I agree there shouldn't be many issues at that speed. It is 30 degrees at the inlet, with a flat delta pad approx 24" wide at the inlet and tapers to a point, roughly 8 feet long.

 

I can't seem to find the setting anywhere in my profile, I have the accept conversations from members checked but that's all I can find. I also can't find on any other members profiles where I could click send a PM.

The LWL is approx 22 feet and the boat weighs roughly 12,000 lbs at testing. If I can't figure out the PM issue I will just post my spare email address.

 

Waaoohh.... that is an LD ratio of just 1.34. Incredibly low.
There is half your issues.... jets are not known for their ability to perform well at slow speeds on low LD ratio hulls.

But... if mitigation has been taken... it may not be the culprit.

 

The hull shape is quite unique so it may not be the issue. Can you please email me at ride_hyperlite@hotmail.com and I will send you some pictures

 

I can visualize a delta pad that has way less angle of attack than the hull bottom either side of it, if you have 2' of width, and yet it is only 8 feet long ?

 

Can you please send me an email to the address above and I'll send you some pictures of the bottom of the hull. Thanks!

 

OK, looked at the graphics, which are a little unclear, but my first thought was, is there any chance that aerated water in wash off the forward parts, is being channelled into the upstream flow to your jet inlet ? It does seem a possibility.

 

It is definitely a possibility as there is a bit of a bow wave, we are hoping to do some more testing in a couple days in which we will try to film the boat from underwater and see if we can see where the aerated water may be coming from if that is the issue.

Are there any other "tests" you can think of that we could do to try and find out what the culprit may be of the cavitation?

There is also a possibility that it is sucking aerated water in from the back of the boat.

Thanks for your time!

 

More likely from forward of the inlet, you would think. I wonder what might happen if you can apply restraint to the boat to stop it moving forward, and use the throttle, that would eliminate aerated wash, if it does the same thing.