Performance Issues

Hi
I have a 8m grp mfv, it has a Volvo TAMD 63P (370HP) inboard diesel driving what I believe to be a PP jet drive, I don't know the model and don't know how to find out.
The problem is it won't go very fast. At 1800 rpm it can do 7.5 kts, at 2800rpm (max RPM) it will do 8.5 kts, a lot of noise but no speed.
Is there a simple answer to this?
Many thanks to anyone who can help
Nick

 

Is it a displacement hull?

 

No its a planning hull, but doesn't seem to want to get on the plane
thanks

 

the first thing I would check out is the impeller and intake on the jet. I am not familiar with that brand. Does it appear to discharge a large volume of water. You have pleanty of power, perhaps the jet is to small.

 

It does seem to discharge a large amount of water, and yes the jet does seem rather small, but this was fitted by the manufacturer, BJR Westral

 

If it is a PP Jet and was correctly spec'd then it will probably be a PP 115 or a PP140 , VOSPOWER took over from PP several years ago and are still selling spares ,just google it for contact details There is almost nothing goes wrong with these jets if they are used intelligently ,Richard Parker was way ahead of other jet manufacturers in his time and made the best jets available ,look for a simple or obvious problem. Blocked inlet, blocked outlet ,weed growth on boat hull ,high impeller tip clearances [ajustable] damaged impeller blades or sucking air round the handhole cover Jets are the simplest of machines they suck water in one end and blow it out the other with a minimum of moving parts .

 

Hi, thanks for the replies
The inlet and outlet no not appear to be blocked, the hull is clean.
When I take the inspection hatch off and put my hand in, I can fit my fingers between the impeller tip and the tube, is this the right clearance, how do they adjust? One of the blades has a small chip and a few barnacles can be felt, but i don't think it will cause such a degradation in performance.
How would I find the handhole cover to see if its sucking in air.
Sorry to be a pain, any help is greatly appreciated
thanks
Nick

 

The handhole cover is the inspection hatch, this is on the suction side of the impeller so just check that the gasket is intact and sealing.or it will draw air rather than water The impeller runs in a stainless steel wear ring which is cone shaped .The tip clearance is adjusted by adding or removing shims around the shaft in front of the impeller,the less shims the smaller the clearance. If you dismantle the jet,remove and replace the impeller without the woodruffe key but with the tailpipe, you will be able to turn the impeller by hand through the inspection hatch and ascertain how many shims are needed ,in practice the clearance should be as small as possible WITHOUT TOUCHING the wear ring, count the shims and replace the key .Impeller blade condition is more critical than propeller blades ,barnacles will cause cavitation/ventilation These instructions apply to most models of PP jets why not post a photo so that we know which model we are dealing with and that it is a PP jet

 

Hi
Thanks again here is a picture of the boat, I will go and take some of the jet tomorrow and post them

 

Hi
Yes looks like a PP jet the PP embossed on the bucket is a bit of a clue. Think it's probably a 140, a succesful fibreglass bodied Jet .Measure the impeller diameter ,if its 14 inches [356mm] then thats what it is.
This model went through several development changes and there were several shaft, nozzle and rear bearing options depending on application If this is a 140 it is the right spec for your boat and should perform well Vospower will almost certainly send you a parts list and exploded diagram which will help you with dissassembly call them on 02392539739 or e-mail vtmpspares@vtplc.com

 

Well its been a while now but I had the boat out of the water, after cleaning the hull and putting her back in the water I managed an extra knot but no significant improvement.

Does any one see that the impeller may need replacing, its not very smooth at all, many pits and chips around the outside, are these repairable?

I cannot measure the diameter very well because of the shaft, but it appears to be just under 12 inches.

Any contact info for anyone who could help would be most aprreciated.

Thanks for looking and to anyone who can help.

regards

Nick

 

The diameter you mentioned, is that the dia of the outlet side of the impeller? And, please check the outlet dia of the nozzle as well, because if you have an impeller outlet dia of less than 12 in., it will be a PP100. If so, the nozzle would read 6.0 in (152 mm). As Anthony has already noted, it ought to be a bigger size jet; the PP100 is not a very good match for your boat/engine combination.

Your jet is probably cavitating or ventilating heavily, which means that we have to be quite precise in diagnosing, but there is room for improvements, provided we have correct info.

Any ideas on boat wheight?

Could you produce a pic from below, say midships, looking aft along the center line, so the inlet position and those "vanishing tunnels" are visible? There may be a problem with air trapped in the tunnels beeing sucked sideways into the jet when the jet is demanding increased flow to overcome the hump.

 

Hi,

thanks for the quick reply

if you can zoom in in pic 2 you will see the tape measure, i'm not completely sure of all the part names

i've added another pic, which is about the best one, the boat is back in the water now

i am not too sure of the boat weight and would be guessing at around three ton

i did locate a part number which is 1155029, which was somewhere on the bucket - do i assume that the first part 115**** is probably the model number?

thanks again for your help

regards

nick

 

Good morning Baeckmo ,could also be a PP115 which wouldn't be quite so bad. measure the inlet side of the impeller,this is where the measurement is taken also the standard nozzle on a 115 was 7.5 inches but there were other options.

 

Your last posting crossed with mine ,if it is a 115 then it was designed to give a planing performance with boats from 2 to 4 tons with engines up to about 300hp, not as good as the 140 I originally thought it might be ,but never the less,should perform ok the two jets look very similar from the bucket end ,just different sizes ,both were GRP.As Baeckmo says more information is now needed