sims super v jet drive problem or mismatch engine to impeller ?

Hi all i am new to the boating world and hope some of you may be able to shed some light on this subject. i have a 14ft sims super v jet boat witch new had a 16oo crossflow engine fitted to it like they all did and the jet drive is a copy of the pp65 made by west end marine. I have made some changes to the boat in hope to make it go a little faster than it was in back in the day. so i bought a omc 2.3l from lancing marine and fitted that thinking all would be good took the boat to the water to try her out and i am having some trouble getting this sims off the line to say the least it's almost like a clutch slip the rpm's go up but with no real thrust from the jet drive.I set my impeller clearance to about 1/2 a mm both sides and can not really go any closer at the moment as there are a few small bumps push up behind the wear ring from corrosion (i need to find a replacement wear ring if anybody knows where i could get one please) this may not be helping matters ?. or could it be more that now i have fitted a bigger engine i need to change my impeller pitch or the amount of blades on the impeller to a 4 blade impeller with less pitch ?. i did once manage to get this boat up on the plane once with a friend sat on the front but it was a struggle but once under way hit 35/40 mph at 3500 rpm and could not get any more rpm as i think the timing was way out that day due to the person before not putting the distributor in correct. any help on this matter would be great. Thanks

 

It appears that you're in "thrust breakdown" which happens in jets when you don't have enough forward speed and are pushing the impeller faster than it's supposed to go. Basically you're just overpowering the impeller. With the bigger engine you have more low speed torque. To properly match to the bigger engine you really should have a bigger jet, or at least a different impeller. If don't slam the throttle forward, and accelerate more slowly you should be able to plane off and accelerate to normal speeds. The original impeller was set up to reach full rpm with the 1600 cc engine. With that impeller on the same boat, to absorb the higher power you'll need to spin that impeller faster. Remember that the power absorbed is a cube of the speed change. With the 2.3 engine if the power is 40% higher then you'll spin the impeller about 11% faster to absorb the power. If the 1600 turned 6000 rpm you'll need to turn the 2.3 at 6660 rpm to absorb the power. If you want to absorb the power at lower speed you need a different impeller.

 

Thanks for the reply Yellowjacket i really appreciate your time and help. There are 4 different types of impeller for this jet unit as i understand it only one being a 4 blade for the v6 125 hp and the rest 3 blade with different pitch's the one i have at the moment is the one for the 1600 crossflow which was 80hp and rated to 5000 rpm and according to a graph i've seen started loading the jet unit at around 3800 rpm and ran out at 6000. i would like to find this pp65 pdf manual again and load it onto this thread to show you the different choices of impeller and the power curves to hp and rpm to see if this will ever work well enough to use with the 2.3 engine or i may have to find another 1600 and go back to that. I am hoping not ? but not really knowing much about this kind of stuff i would like your views on it please. i will try and find the pp65 pdf manual tomorrow and put it on here for you to view and see if it is something i could overcome with the right choice in impeller from the list. thanks again for your time and help.

 

If you have the 2.3 installed and running properly you really don't want to go back to a 1600. What is the hp rating on the 2.3 and what rpm does it turn at that power? Since there are several impellers available you can most likely find something that will work.

 

A 4 blade impeller ideally requires a 7 volute stator ,yours is 5.

 

yes anthony goodson mine is a 5 vane stater but i see on pp65 impeller listings there is 4 impellers 1 for the 1600 80hp one for the 2ltr 95 hp and one for the v6 125 hp 3 blade and one for the v6 125 hp 4 blade and all start working if i am reading it right at different rpm due to pitch im guessing.

 

Hi yellowjacket i think the 2.3 omc cobra i have is 120 hp from what i've read but could be up to a max of 140 hp will rev up to 6000 rpm but i have been told that its not good to go that high and stay at no more than 5200 rpm. and that is good to know that i may be able to sort this with just the impeller choice as i have put a lot of money into this sims but with no real results yet and really don't want to have to go back to the 1600 engines used. the 2.3 i have is in good condition low hours and been rebuilt all new manifolds infact all new every thing to much to list converted to electronic ignition. Thanks for your replies

 

for some reason i can not seem to copy the pp65 manual i've found on another thread and paste it on here i am not the greatest with pc stuff when i figure it out i'll try again.

 

Admin limits what new members can do.
This may be the issue you're having...

 

oh ok thanks for that Bluebell. maybe if i ask nicely one of the admin may be able to post a copy of the pp65 pdf manual on this thread for me. Thanks again

 

You can run the 2.3 at 5200, you just won't be at wide open throttle, it actually rated to 5600, the range of max power operation is 5200 to 5600. If you run to 56oo you'll be at about 112 hp and that's probably close enough. Another thing you need to realize is that if you go faster you need more power for a given rpm and this depends on the boat. Since you're going faster you need more power. If you run to something like 6000 for a few seconds it isn't going to hurt anything. Remember that boats are rated for continuous rpm. That is with that engine in a boat they expect you to lock the throttle down and hold it there at max speed. In cars that engine was rated to 6000 rpm, but that's only for short times since you don't run that kind of power continuously in a car. I'd run it on your boat with it running properly and see what it does. In reality if it ran to 6000 rpm you'd be at 138 hp and it isn't going to get there since you don't have that much power. In reality you'll run about 5750 at wide open throttle and that's going to be about it. Run it and see what you get.

 

ok so would you say i should stick with the 1600 impeller for now and just add the power very slowly allowing the boat to try and catch up or am i not being hard enough on the throttle ? normally when i try to pull away i open the throttle fairly gentally adding probably no more than a 1000 rpm at a time by the time i get to around 2000 rpm i am moving at about 7- 10 mph the front of the boat is lifting up quite high and as i add more throttle it's trying to rise more and gaining any more speed becomes a problem the rpms are then going up and up with no more speed gained and i can not get over the hump of water in the middle of the boat. once the rpms hit say 3 to 3500 rpm may be even 4000 rpm i am then backing off as its not really gaining any speed just trying to raise the front and almost feels like it is pulling the back down so seems pointless to me to keep the rpms going up and i let off and try again. i did see on this graph that with the 1600 engine and impeller that at 20 hp and 3800 rpm is where the jet starts to work its seems like below 3800 rpm the jet really is not doing anything. so am i reading this right if i have more power now that the jet is going to need even more rpm's to get it to start pressurising the water enough to push the boat up onto the plane ? so if it was 3800rpm's on the 1600 crossflow it may be more like 4/4200 with the omc 2.3 as i need to spin the impeller faster to absorb the extra bhp and torque. or should i change the impeller for one that starts working at lower rpm around 2500 and 20hp or to as near as with a different pitch as it may stand a better chance of getting up on the plane quicker? the only down side is that this type of impeller revs out at 4500 max.

 

Yellowjacket has given good advice regarding impeller/engine matching in general. But what you are describing is that your jet is cavitating when you are trying to get over the hump. If my information is correct, the 2.3 l engine is at least 60 kg heavier than the original. If so, when added to the rear of the hull the hump resistance will be increased to the extent that the original cavitation margin is lost. This is a very common problem when the jet unit is "one number too small" for the displacement at hand.

Until the cavitation issue is solved, there is no chance to use the full potential of the new engine. You may even have to sacrify max power by selecting a "coarse" impeller that may have slightly better cavitation performance, but won't let the engine spin to max power rpms. It is a very fine line between success and disaster here; has anything else been changed to the hull bottom? Additional weights? Is there any kind of hand-hole in the jet inlet duct that could introduce a leak path for air? How about the inlet grating/fingers, are they bent/damaged? Any leak paths along the attachment to the transom? Is the reverse bucket really clearing the nozzle outflow completely in the forward position?

 

Hi beackmo thanks for the reply. i would go as far to say this engine could be 80 to 100 kg more than the old. i think its around 430lbs not sure on the cross flow but somthing is telling me i did see it at around 280lbs some time ago. the weed rake on the bottom is in good shape and the hull has not been change in anyway with fresh paint as it been resprayed so not build up of anything on it all new seals on the shaft through the bearing housing 7 of with springs facing the water new bearing in the housing too. this something i did wonder about if i may need to fit trim tabs or put some weight up front to counter act in some way as when my friend sat on the front as i thought this may be affecting things it did seem to help.

 

i did wonder if i should now move the fuel tank and battery to the front as its the only moveable weight in the back of the boat ?. also i find yes that the reverse bucket may not been fully clear of the jet not by much i must say about a inch but im guessing that may have interrupted the flow from the jet the last time i took it out at swanage. there are no other ways air could get into the jet no inspection holes on this jet and all is clear and sealed up impeller is in good condition no damaged edges the wear ring to impeller clearance could be made tighter i think when i find a new wear ring at the moment is around 1/2 a mm both sides i'd like to half that.