Jet boat tops out engine revs without planing

Can i just ask that if my problems related to the speed/gearing of the engine how come it dont really give me any sort of performance change when im throttling up or down?

 

Like most "problems" more information assists in the diagnosis.

For example:
what speed do you get at a set rpm?...a range speed v RPM would be helpful
What is the size of your jet?
what is the rpm of your engine
what is the power output of the engine
what displacement is the boat when you conducted the above trials...

The list can go on, so a few suggestions or ideas may help, is about all that anyone can do from a distance and without much data to provide a better understanding of the problem. All "we" can do is hon in on the problem with each snippet of info provided...there is no "press here for answer" button. But most common problems are about mismatched engines and jets. So to eliminate this first, more data is required which will also assist in answering the question 'why'?

 

From what can be seen in your photos, the inlet arrangement is one of the lousiest I've seen (and i have seen many.....). In a jet, the shape of inlet ducting is extremely important to the performance. At hump speed, the hull opening is a "hydraulic sink", and water is entering the opening both from its sides and forward (at low speed even from aft).

In your boat, the actual jet intake is somehow recessed to accommodate the jet units in a horizontal position, leading to a massive disturbance of the inflow, and consequently a serious reduction of total static pressure in the impeller inlet. You are actually throttling the pumps on their suction sides; the worst NONO in the pumping world as it leads to instant vaporizaton ("cavitation") of the water at the impeller inlet tips.

Normally, the tangential inlet tip velocity may be up to abt 45 m/s in a well designed arrangement, without cavitation. With the losses in your case, I would be surprised if cavitation-free operation could be achieved beyond 28-30 m/s! Any throttle/rpm increase above the limiting rpm's will only result in a greater volume of vapour in the inlet, without any increase in flow. As the flow times velocity increase from inlet to outlet is what produces thrust, the speed will be limited to the speed where produced thrust equals boat thrust requirement.

To make this arrangement work, the inlets have to be reshaped to meet the hull without sharp edges and with controlled curvature and flow area. With this done, the gearing must be selected so that impeller speed (=swallowed power) is matching engine performance AND so that cavitation limits at hump speed are not violated. What is maximum engine power, impeller inlet diameter and jet outlet nozzle dia?

Question is, is it really worth wile???? If I were you, I would scrap the double arrangement; it just extra ballast, and find a single used PP/Vospower 90 or equal. Remember that the impeller inlet area is the limiting factor for thrust!

 

Many thanks for your reply, do you think they have been bodged then? They look like they are standard to the boats fitment so surely the boat couldnt have always been slow.

 

A reasonable guess is that it has seen some kind of metamorphosis. Dual jets are normally fitted with inlet flush to bottom, i.e. vertically inclined. One possibility is that there is some kind of "flow straightening device" missing in the inlet. It would be interesting if you could produce more picture(s) showing the inlet recess better.

If scrapping is not an immediate issue, then we might turn to a plan B:

Check the following:

Control that there is no obstruction to the flow in the inlet (plastic bags et c.)
Check inlet grating (clean, no barnacles, no fingers bent et c.)
Are there handholes for rinsing/cleaning; check for possible air leak?
Radial shaft loads from belts may displace shaft sealing surface under power.
Exterior of jet units slightly "rough"; check that pump interior surfaces are smooth and clean.
Check impeller radial clearance, adjust if possible
Check impeller vane leading edges for dents
Hump resistance is very sensitive to weight and running trim; throw out everything reminding of surplus, idle kg's.
Are there exchangeable nozzles in the outlet (if yes, are both there.....?)

With ok on all points, but still no planing, then you have to do some exercising and measuring in order to produce input data for approximate calculations of flow and thrust. Figures, figures, figures.... Inlet dia, outlet dia, shaft dia, boat weight, boat dimensions, engine power curve.

 

ade/baeckmo

only just noticed those last 2 pic's...wow!
Yes those ducts need to be flush with the bottom, as baecmo noted above, but it also looks like they are not flush in both planes, vertically and transversely. Shocking!

 

Hi Ade,

I came across this post while doing a google search for "Almarine Jet Boats" and registered with the forum. I also have an Almarine, it's a twin seater, single waterjet layout, 10.5ft long. I have never seen your boat and imagine it is very rare, possibly one of the last made by the company, maybe even a one off/prototype. Almarine made lots of small speedboats like the Seafire and Spitfire that sold quite well in Holland and the UK.

I'm very surprised by some of the suggestions like scrapping the tunnel arrangement. As you say, this is how the boat was originally configured judging by the photos, but admittedly the tunnel design with the waterjets recessed is far from ideal, however I expect the problems you are getting are related to the condition of the waterjets and the re-power to the single motor rather than the hull design. As baeckmo correctly suggested, it would be well worth measuring the ID of the wear rings and the OD of the impellers by dropping the rear section off the waterjets. The difference should be very little (about 1mm or preferably less). If the gap is large you will get slip around the impeller tips and poor performance from the jet.
I know it sounds obvious but presumably the reverse buckets were raised when you throttled the boat up?

Regarding the re-power, is there any way of checking for slippage on the drive belts? Are they chain/toothed?

The UA jets were fairly common in their day, but apparently they were limited to about 40hp! My boat runs a very torquey 50hp Rotax 2 stroke and accordingly the shaft was bent when i bought the boat. The front wear ring was also badly worn, but I had no idea of how this affected the boat because the engine was also knackered. The unit had also swallowed up a large stone while in shallow water which had removed most of the stator vanes. Again as baeckmo suggests this is something you definately want to check. Your boat has no inlet grating so its more than likely something big has been sucked up at some point.

I hope you stick with it as it would be an awesome boat when up and running. And please ensure you have checked all the drivetrain before making any alterations to the hull design!!!

Cheers

Dom

 

Have you got any more pics of the inlets to the waterjets?

 

Many thanks for your help, i have attached more photos, i think you are right, my plan was to remove the jet casing this weekend and check the tolerances of the lining to impeller. The engine runs lovely and i feel is more than capable of delivering the power needed for the jets, it 105bhp 1.5 Alfa which drives toothed belts on almost a 1:1 ratio. I think it revs up to about 6000-7000rpm, do you feel this is over reving the impellers?

 

Well, surprise surprise, as Pist'nbroke noticed, it seems to be an original installation even to the lousy inlet. Meaning that it once possibly worked slightly better than it does now. So if you have a nostalgic reason to tolerate less than top performance we have to dig further into the pump. Blade leading edges are not too badly dented, so let's look further down the water passage for the solution.

The lower part of the inlet, the lip area, is badly corroded. If the stator or the nozzle has suffered correspondingly, it could result in too high flow. This in turn will cause increased cavitation. In pumping, a parameter called suction specific speed is used as a "quality indicator" for cavitating performance. It has the form:

nss=rps*(Q)^0,5/NPSH^0,75, where Q is flow (m3/s) and NPSH is the margin of static pressure over vapor pressure in the impeller inlet.

When the combination of flow and inlet pressure at constant speed reaches a critical level, the pumping breaks down, no matter how the speed is increased. All jets have a throttling area in the outlet, after the stator section. Some units have exchangeable nozzles to allow close engine matching. You may have to take away the paraphernalia around steering nozzle and reversing bucket in order to see anything there (more photos??). Check for any signs of a missing component or eroded material. Some of these "metering nozzles" can simply blow out when eroded.

 

6000-7000 rpm should be ok. My engine revs to about 6000rpm.
The inlet plate design is really bad, my boat did not have a plate so I made one up from fibreglass and have achieved an improved design. It seems that the plate is quite deep at the front bottom leading edge, and then presumably this thins out as it approaches the bottom part of the front wear ring? Is this the case? If so, its not good but still dont think thats the problem.

Looking at the photos, the tolerances on the front impeller look really good, no problems there. The impellers also look good. We really need to see the stators. Just out of interest how freely do the impellers turn? Can you decouple the driveshaft from the motor and turn them freely by hand?

Going back to the reverse buckets, does the reverse control stay in the same position when the boat is running? Check the reverse buckets are not dropping when the boat is running (or just take them off initially!)

 

What seems to be missing here is a large scoop with an intake grate. Is was or should be attached to the corroded vertical plane at the end of the intake chamber. Is almost certainly is an alloy casting with a more or less hydrodynamic shape to guide the approaching water to the impeller. Without it, the pump draws in water from all sides, pulling the transom down.
The corrosion in your pictures looks much like on my 2 years old Berkeley jets. They used an aluminum-zinc mixture that dissolves in water almost like a sugar lump.

 

Many thanks for all your replies, from what i can see the intake plates seem original, i can see what you mean though, also it does seem to suck the back end down when you rev it high, could this be that the intake needs reducing to create a venturie. The impellers turn fine when they are dettached from the belts, i am going to remove the rear casings tommorrow and take some more pics. Can someone please explain the Stator, i have done some research and cannot find out what it means.

 

The stators are the fixed blades/vanes between the impellers. Yours is a twin stage jet so the first stator is between the 2 impellers and the second stator is behind the rear stator. The stators straighten out the flow of water passing through the jet to improve its efficiency.

Some more pics of the underside of the hull especially the section before the intakes would be also useful.

As for intake grates, yes most jet boats have them, but i dont believe they were fitted to Almarine's. Probably due to the fact the inlet section of the jet was moulded with the hull, as opposed to most jets which have the inlet as a cast section bolted to the hull making it easier to fit a grate.

 

confusing name

Hi this is the original pistnbroke here I see you have chosen the same name though spelt it slightly different ....this will be confusing for many members and you dont want my bad reputation ...why not re register as something else as I see you only have 4 posts to date.

the onle and only genuine PISTNBROKE