Jet boat not performing

simply because they don't

A water jet is simply a big hydraulic pump on the back of the motor, therefore load on the impellor is not dependant on forward speed as does a propeller. As it doesn't rely on speed under the hull you can achieve max thrust (max rpm) at zero speed. This is one of the fundamental advantages of water jet propulsion, max available thrust at any given hull speed. It also ensures that the motor isn't overloaded once the vessel is overloaded, as the motor will always see the same load from the water jet regardless of outside factors.

 

Unfortunately there seems to be difference of opinion on this and in this forum. A water jet can also simply be a big propellor in a tube! Logically forward motion at the water intake will improve the performance and efficiency of a jet and that logically means the engine revs will be somewhat higher at speed than stationary, same as a prop.

 

Capt littleleegs, there may be differences of opinion in the forum, but among those with first hand experience on waterjet propulsion, like Anthony and Speedboats, the judgment is appallingly coherent.

Your notes here reveal your complete lack of knowledge in this area; I suggest you restrict your piece of advice to scenes, where you have at least some rudimentary expertise, instead of spreading drivel here that is of no help to Truls!

 

So why can't you or anyone else simply explain why my logic isn't logical and help us all or are we expected to just believe only you because you say so? A statement like "simply because they don't" is hardly convincing and leads me to think you don't know why! Is it a trade secret? It could be you spreading drivel so convince me otherwise!

 

The point here is that one contributor, using his boat commercially, has turned to us for advice. His problem can only be solved by the application of sound engineering science, not amateur speculations and guesswork.

As Anthony mentioned, there are other threads in this forum that can give you basic insights in the working principles of waterjet propulsion and how it differs from conventional open propellers. If you spend some time there, you may also identify some contributors that actually know the subject. Let us keep this thread here exclusively for the solution to Truls' propulsion issue and refer the novice waterjet tutoring aside!

 

AMEN!.........

Bad enough to notice that some members of our industry provided a crappy job. (and I would say Hamilton and the yard can share this disgrace 50/50)

Worst, a complete novice (at least in this field) tries to sell his opinions as knowledge.

Capt littlelegs
be nice, just leave this field to the experts as Baeckmo. That guy is trying to earn his living with the vessel, he needs help, not opinions.

Regards
Richard

 

He states that he has had advice from the manufacturer, presumably that is sound engineering science and it still didn't work. I know and he knows it's not likely to work, certainly it won't as a fishing boat but I get the feeling he'd like to give it a try anyway and why not? My advice is just as valid as anyones and I don't see any help from you!

The point here is none of you self proclaimed experts can answer my question and the way I identify contributors who actually fully know their subject is to consider their answers. Seems to me I've hit upon a blank area of your knowledge as others have!

 

Are you another jet expert?

 

These "self proclaimed" experts as you name them ARE in fact experts. And as such not willing to reduce this thread into a tutorial for a stubborn amateur!
It was already said twice: there are more than enough threads where you can improve your knowledge about propulsion and hydrodynamics, this is not one of them!

Regards
Richard

 

Truls, there may be a few tricks that could improve your situation, even if not presenting a complete solution, but that needs some additional info from you.

First on the hull; what is the waterline length? Draft loaded aft/fwd (straight part of keel before rising fwd)? Deadrise angle?

Then on the jets; what diameter is the jet nozzle (not steering nozzle)? How many rails are there in the inlet gratings?

Finally; could you check the trim angle at a few speeds (borrow an inclinometer from someone in the building industry or sailor)? Refer to keel line!

Forget about chasing "high thrust impellers", they only exist in sales brochures! Impellers are a strict match to engine power and corresponding rpm's. There are ways to improve cavitation behaviour when high thrust at low speed is asked for, but Hamilton are often good in this respect as built.

And Apex, thanks for the crossfire, nice to see you again......fed up with the Bavaria (or whatever) now?

 

Welcome as always mate!

A bit far oT but I will reply short.
We tested 6 mass production boats of the 5 best selling brands in the past 5 weeks. All cr@p. But I had fun...............look here:
http://www.boatdesign.net/forums/open-discussion/what-have-you-done-weekend-23233-61.html#post360184

 

So what qualifies them to be experts? I didn't expect or want a tutorial, just a simple answer to my specific question! How do you know if I'm an amateur or not? If I wanted to improve my knowledge about propulsion and hydrodynamics then a forum is probably not the best place, who would I believe was giving me the best advice, those that said so?

A true expert on a forum doesn't have to wave a flag and doesn't always know everything, he just gives convincing advice and answers questions to the best of his ability leaving the amateur to make up his own mind.

What he doesn't need to do is criticise others comments, at least not without giving plausable reasons, they don't have a monopoly on knowledge.

I'm sad for this forum that you and your so called arrogant expert friends feel it is beneath them to answer a simply question because you think I'm an amateur!

 

I'm sure if you were requesting knowledge then you'd be pointed in the right direction. What makes people experts? Knowledge and experience. To whom should you pay attention? Look into their backgrounds, check their experience, listen to their reasoned arguements (both here and around the forum), you will get a feel for who knows what they are talking about and who likes to drivel on.

Unfortunately, as on most forums, it is easy to hi-jack someones thread which then changes the focus and is of no help to the original author, some here try to maintain that focus. If I may, I'd suggest opening another thread and colate your questions and thoughts, no doubt you'd be quickly pointed in the correct direction.

Baeckmo, "...the judgment is appallingly coherent." lol

Would a couple of big arse strakes at the front of the vessel help lift to the plane? Boating in anything more than a light chop would require a chiropractor on-board though!...

 

First of all, I want to thank everyone that have contributed. I am sure everyone of you know more about the subject than me. It can only benefit me as a total novice, from a design point of view, to have a discussion such as this on my posting. At least I am getting opinions from obviously knowledgeable professionals that I have not been able to obtain from the supplier of the jets.

Baeckmo -

I was able to locate the performance estimate from Hamilton Jet from when I bought the vessel. Based on this estimate I gutted the boat and moved the engines aft, with some professional help, to obtain their weight and LCG figures. The scan is attached.

Below are the figures you requested based on what I know. The boat is currently located in Alaska, and I am in Seattle.

• Waterline length is 30 feet (might be slightly longer).
• Draft at the stern is 28-30 inches. I have not measured the bow, but I would say it is very even.
• Deadrise is a constant 12 degrees.
• I do not know the jet nozzle diameter, but it is the standard delivered with the HJ321. Based on drawings from the manual I see three different options - 165 mm, 170 mm, and 175 mm. The nozzles have a smaller diameter top and bottom. I am not sure which one I have. I would think a larger diameter would be better at low speeds, but the response from Hamilton Jet was that they had delivered the right one for the boat.
• I am not sure how many rails are in the inlet. However, they are cast aluminum and beafy. They cover a significant percentage of the inlet area. I would say they are spaced with about an inch gap between them.
• I am not sure about trim angles at speed. Generally I would say it stays even, though we are talking low speeds here. I believe that with some horse power behind it the stern squats rather than the bow coming up. I am always on the bridge and there is usually a lot going on around me if I am going to waste a bunch of fuel in order to gain a knot.

I hope this helps. I may be able to come up with the number of rails in the inlet if I dig through some old pictures, but there was no drawing in the manual that showed.

Thanks again,

Truls

 

Sorry, correction to above. When measuring the water mark at the stern when dry, it measures 28 - 30 inches. When operating in the shallows I always bump in the stern first (muddy bottom everywhere). Otherwise the boat visually seems level. I know it is not very scientific.