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Old 08-20-2004, 07:02 AM
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Question question for jet drive experts

I need advice from people who know about jet drives and the way they operate. I am having a problem with a big 62' cruiser which is something like 55 000 lbs. It is powered by twin MAN V12 diesels @ 1200hp. and waterjets. The boat should run 46 knots or 53 mph at 2450 RPM. The trouble is, I am not getting anywhere near that. Infact, I can't get more than 25 knots or 29 mph out of it so there's obviously a problem. The engines are running fine, everything was checked and is OK. The jets were checked and cleaned. Everything's working and nothing obstructs the water flow. Now, I am very new to jets and I was told that I must learn how to use them. I was told that the boat was "sucking herself down" and that I must use trim tabs to bring her on plane and then accelerate by raising tabs up once the engines are over 2100 RPM.

So my question is: does this make any sense? Is there a special way to operate and handle a jet-powered boat and what is meant by "sucking-herself down"? Is this bona-fide jetspeak or just plain BS.

Also, any suggestions to cure the problem are greatly appreciated...

Thanks
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Old 08-20-2004, 07:41 AM
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Default Re: question for jet drive experts

I'm no expert put I'll toss in my .02.

On my jet ski's, when I first hit the throttle the stern does squat for a second, especially is shallow waters. I know that anything that isn't nailed down when I pass over it will get sucked up into the pump. In some circles it's called "The Hoover Effect". Like the vacum cleaner? So, I'd say it's possible for the pumps to suck the stern down. Those big pumps have to suck a lot of water before it make thrust.
I have read in magazines that a hull has to be specially designed for water jet application. I understand that you can't just install a jet pump in place of shafts and props. Something about the hull design and how water flow is directed into the pump intake.

Like I sad, my thoughts and observations.
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Old 08-20-2004, 08:29 AM
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Default Re: question for jet drive experts

Thanks Iggy. The boat has been specifically designed for jets by Don Shead who probably knows what he's doing. It isn't a one-off either and there is a number of same boats around, which function well and get speeds b/w 42 and 46 knots. This is why I think that everything may well be OK with the boat, I just have to learn how to use it. Your experience tells me that this is entirely plausible. I'll try using tabs to get some transom lift and then retract them progressively to see whether I pick up any speed. IMHO, picking up 20+ knots is unlikely but you never know...
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Old 08-20-2004, 09:20 AM
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Default Re: question for jet drive experts

When i was into jetski's years back i had sucked up a rock and damaged the wall of the jetpump......scored it bigtime. The jet passed the rock but the damage was done and it couldnt build the correct pressure.....yes it still worked as it was designed to but the top speed was lost. So what i'm saying is you may want to have the pumps inspected by someone who knows about that model.
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Old 08-20-2004, 11:21 AM
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Default Re: question for jet drive experts

A 63' yatch that does 50mph ?!? NICE !

I am no expert but, WHo checked the jets ? Do they really know what they are looking for ? The walls and the seal around the impeller must be exact tollerences down to a thousanth of an inch all the way around . It a jet that big , pushing that much weight , you would HAVE to know your chit to check it right ! Any little scratch or ware mark would throw it way out of wack ! Also the impeller is like a prop, and you know how touchy they are .Have the impeller checked out too by an expert . check for alignment fro the impeller and it's seal . if it sits too far away ,it won't make pressure ..

Have you lifted it and checked the bottom for problems in the intake end ?

I really doubt you will gain 20 mph by trimming the tabs .... How about trim on the outlet nozzle . Can you trim that ?

Has this boat EVER run right , or is it a new boat ?

Sounds like a realy cool project !! Keep us posted ! Joe
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Old 08-20-2004, 11:33 AM
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Default Re: question for jet drive experts

By the way , since your into big ,fast cruisers ,I thought you would like this ...

A few years back, A guy my brother knows in NY (I think) . Built a 92' Lydia sport fish ! It was the longest and tallest on the east coast at the time . I think the tuna tower was around 75' tall !

It ran 55 mph !!!

It had 2 huge multi thousand hp 16 cyl. diesels with variable pich (hydraulic) props ,and I think it was a single 3600 or 4600 hp turbine with a jet pump for back up !! The diesels would get it up to around 25mph and then you fired up the turbine and hang on !!

MY brother was coming back from "the canyon" (way off shore fishing spot ) ,he was driving around a 50'er doing about 28 knots when he said the Lydia past them like they were standing still ! He said it was like being past by the Empire State building !

HAve you ever seen that one ? It was featured in one of the boat mags a long time ago ...

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Old 08-21-2004, 12:53 AM
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Default Re: question for jet drive experts

I would contact this "Don Shead"...If you have a boat like that it should'nt be too much of a problem to have the designer or manufactuerer be there to help...I'm sure Reggie (if you bought a 65') or Mike Fiore (if you bought any outerlimits), a Sterling rep, the lake x guys (if you are with merc) would be there to help...go to the source and have them figure it out.
A 65' with twin MAN 1200hp on a jet drive? WOW!
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Old 08-21-2004, 10:14 AM
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Default Re: question for jet drive experts

A jet pump builds pressure. The pressure is converted into thrust by squeezing it thru nozzle and increasing the velocity of the stream.

I have to assume that the big boat uses an axial flow pump. These pumps need controlled clearance on the OD plus (and even more importantly) on the bowl clearance on the backside of the impeller. (jet skis don't have this kind of pump).

Dings in the prop will affect the thrust, but on someting this big, I almost think it can stand a nick or two..

You say the motors turn up proper rpm. You also need to check to see if the motors are developing proper boost pressure at that rpm - as that will tell you whether they are putting out the correct grunt. If the pumps are somehow unloaded due to excessive clearances, etc, the motors will still spin up but won't be properly loaded. This is significant.

A 20 knot variation? This is massive.
Has anybody swam under the boat to make sure there's not a buoy stuck in the intake areas?

keep us informed..
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Old 08-23-2004, 03:44 AM
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Default Re: question for jet drive experts

Thanks guys. I am starting to think that something in the tunnel is slightly out of whack, like you said. Because everything else was checked. The hull has been cleaned, the tunnel was meticulously cleaned as well... MAN mechanics were aboard and monitored the engines for everything inc. RPMs, boost pressure, oil pressure, water temperature, exhaust temperature... and all parameters were normal. Thus, the motors are tip-top. At this point, I have contacted the manufacturer and they are sending a technician to check the jets. What I am experiencing is consistent with what you guys are saying. I actually managed to get it past 40 knots by playing with tabs but it's a long and painful process. And when I hit a big wave head on or I take a sharp turn, it's back to 25 knots and I have to start the whole process again. Which probably means that jets are having trouble achieving and keeping the intended pressure.

Boot, not familiar with that turbine-powered Lydia but this one is moored in a port very close to my home. The tower is indeed massive and even though this one will not do 55 mph, it will do about 40. Which is still impressive when you're smashing 6 footers and standing on the upper helm. Powered by twin 1800hp. V16 Detroits. The old 149-series which are massive motors...
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