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Old 30 December 2014, 07:02   #31
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Old 30 December 2014, 07:03   #32
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Duplicate post, "Database error" grrrr!
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Old 30 December 2014, 07:10   #33
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I would give it a go, I think the shape of the hull also plays a part, with a deep V the cav plate can be higher than the bottom of the hull.
Mine is about 1 1/2 inches above but cannot go any lower without cutting.
I suffer a bit of cavitation when turning tight but can live with it as it runs very well otherwise. I will have a tinker with props when I get hold of a couple.
My setup is 5.1m, 90 ETEC with a 13 1\4 X 17 prop, had 3 adults today and managed flat out 36mph at 4600 revs.
Any views on this setup?
Thanks in advance.
If you are only getting 4600rpm at WOT, you are way over propped. You need to see another 1000rpm. That prop looks a bit steep for that rig, I would have thought a 15" tops, which should gain you another 400 rpm, even then you'd be borderline.
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Old 30 December 2014, 09:20   #34
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When I spoke to Powertech they suggested a 19" pitch SST prop. They also suggested increasing the engine height 1 or 2 notches and then doing some tight turns as fast as you dare -
You don't have to do "fast" tight turns to check for cavitation during turns. Set your trim at the usual level (trimmed out) and set a course at say 20kt's then do a starboard and port progressive hard turn. As your RIB tilts over it'll lift the prop towards the surface. If you suffer with no cavitation I would lift one hole and try again until you start to lose grip in the turn (engine revs rise and speed drops off). You might find that either Port turns or Starboard turns suffer more more from cavitation than the other. This is because your outboard is probably off-set to port or starboard depending on whether you have a CW or CCW rotating gearbox.

Once cavitation is experienced I would then try the turn again at the same speed but with the engine trimmed in. If you can turn without cavitation and loss of grip then I would say that you're pretty much at the height you want to be at. If you are still suffering with cavitation I would go down a hole and stick with that.
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Old 30 December 2014, 15:44   #35
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You don't have to do "fast" tight turns to check for cavitation during turns. Set your trim at the usual level (trimmed out) and set a course at say 20kt's then do a starboard and port progressive hard turn. As your RIB tilts over it'll lift the prop towards the surface. If you suffer with no cavitation I would lift one hole and try again until you start to lose grip in the turn (engine revs rise and speed drops off). You might find that either Port turns or Starboard turns suffer more more from cavitation than the other. This is because your outboard is probably off-set to port or starboard depending on whether you have a CW or CCW rotating gearbox.



Once cavitation is experienced I would then try the turn again at the same speed but with the engine trimmed in. If you can turn without cavitation and loss of grip then I would say that you're pretty much at the height you want to be at. If you are still suffering with cavitation I would go down a hole and stick with that.

I was thinking I should try raising one hole to see what happens thanks for the advice. I've noticed with the Vipermax that the rear end of the tubes sit quite low on the water, pretty much up to the rubbing strake even when moving at 20knts. Is this unusual or is this another indication that the engine is too low?
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Old 30 December 2014, 17:16   #36
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Originally Posted by Pikey Dave View Post
If you are only getting 4600rpm at WOT, you are way over propped. You need to see another 1000rpm. That prop looks a bit steep for that rig, I would have thought a 15" tops, which should gain you another 400 rpm, even then you'd be borderline.
Yes i did think that as the spec says 5000-5500 for this motor, I did have 3 adults onboard and 70l fuel but I will try and drop to a 15, I was just not sure to buy one or try and lend one. I plan on taking the kids out in the summer on a donut so would you think a 15 would be ok for this?
Cheers!
Gary
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Old 30 December 2014, 18:08   #37
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Originally Posted by Newto View Post
Yes i did think that as the spec says 5000-5500 for this motor, I did have 3 adults onboard and 70l fuel but I will try and drop to a 15, I was just not sure to buy one or try and lend one. I plan on taking the kids out in the summer on a donut so would you think a 15 would be ok for this?
Cheers!
Gary
Before shelling out on props, I'd make sure that the engine is setup correctly first, as it's the cheaper option. You need to ensure that when the throttle is at maximum travel, the throttle plates are fully open & the TPS is at WOT voltage. Ideally you'd use a laptop with the diagnostic software to do this, but iirc it can be checked with a multimeter. Propping is a bit of a black art, but generally speaking 1" of pitch = 200rpm or thereabouts. So dropping 2" should gain you 400rpm ish. Also switching from an ally to stainless prop will get you about another 100rpm for the same size prop.
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Old 30 December 2014, 18:15   #38
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Originally Posted by Pikey Dave View Post
Before shelling out on props, I'd make sure that the engine is setup correctly first, as it's the cheaper option. You need to ensure that when the throttle is at maximum travel, the throttle plates are fully open & the TPS is at WOT voltage. Ideally you'd use a laptop with the diagnostic software to do this, but iirc it can be checked with a multimeter. Propping is a bit of a black art, but generally speaking 1" of pitch = 200rpm or thereabouts. So dropping 2" should gain you 400rpm ish. Also switching from an ally to stainless prop will get you about another 100rpm for the same size prop.
Sounds a bit much for me, how would I setup up to use a muntimeter?
The engine is 2009 model only done 35 hours. I do have a full engine printout from the dealer just before I bought it would this info help?
Thanks for your help Dave.
Gary
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Old 30 December 2014, 18:25   #39
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Sounds a bit much for me, how would I setup up to use a muntimeter?
The engine is 2009 model only done 35 hours. I do have a full engine printout from the dealer just before I bought it would this info help?
Thanks for your help Dave.
Gary
If it was setup by a decent dealer, then all should be well, I'd assumed it was a self install. Has it been serviced since new? if not, it's due & all the checks will be done as part of the service. If you post up the engine report I can have a look. Look on the report for "TPS Calibration" the number should be 190 +/- 30 so between 160-210
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Old 30 December 2014, 18:38   #40
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If it was setup by a decent dealer, then all should be well, I'd assumed it was a self install. Has it been serviced since new? if not, it's due & all the checks will be done as part of the service. If you post up the engine report I can have a look. Look on the report for "TPS Calibration" the number should be 190 +/- 30 so between 160-210
Cheers Dave ill get that sorted for tomorrow I dont think it was serviced because the chap I had if off bought it Dec 2008 and it was due first service Dec 11 but so very little use. It was showing 32 hours when tested in November just gone.
Gary
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