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Old 04 January 2005, 19:04   #11
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I did a fair bit of testing using the Piranha prop a few years ago. Number one, the first one I got had an incorrect distance from the bronze insert containing the thrust taper to the front of the hub. It appeared ok after fitting but it was so close that once the propshaft endfloat was taken up by the thrust, it rubbed against the retaining ring inside the gear casing. Using the lathe to remove a few though off the front edge fixed it. Also, the machining of the taper in the bronze was not circular... back to the lathe. The guys at Piranha were very helpful and pleased to know about it and a later hub was ok. But, nevertheless, it's an indication of the quality control at that time. I can't comment on it's impact strength. I don't hit things with my gear casing.

The performance was pretty good on a 200hp merc. It had good acceleration and it gripped pretty well. When it finally broke loose, it easily recovered and regained its grip. It reduced the top end speed by about 3-4mph and, despite trying various pitch blades, I could not regain the speed. This is in comparison with a merc ally prop with teddy bear ears for blades.

I changed to a torqueshift which was initially disappointing but when I really learned how to set it up properly and how to drive it, I never looked back. I tried other stainless props but nothing came close to the handling and grip in all conditions and the softening of the ride in bumpy conditions.
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Old 04 January 2005, 19:26   #12
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JW - what setup is required for the Torqueshift? And have you still got yours for sale?
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Old 04 January 2005, 19:39   #13
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I had two, sold one and I'm keeping the other for when I feel the need for speed again and buy a Phantom.

Setting up is done to match the boat and the way you wish to drive it. There is a range of cams which control the rate of pitch change. There are also springs which modify this rate to fine tune the setting. There are maximum pitch adjustment screws too but, when you get it right, these can be set to maximum and the setup controls the maximum pitch. When it's right, it's cool. However, you can do a reasonable job by setting the maximum pitch using the screws and benefitting from the progressive pitch change only. Provided the cam selection is in the right ball park.
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Old 06 January 2005, 02:24   #14
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Jwalker,

This torqueshift sounds like a good idea, never heard of it before. Do you know how much pitch change it will give, what is the variation in pitch?

Thanks,
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Old 06 January 2005, 04:00   #15
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Plastic props

For about 7 years we used plastic props on our commercial operations on twin instalations with up to pairs of 175s powering 10m boats with 12 pax.

The pyrana prop is probably the better lived though not as well engineered as the propulse. The propulse has the advantage of being adjustable but the blades do tend to sheer off after a while when simply under load ( to be fair our loads can be extreme). With both you are limited on sizes if running a counter rotation as well. We also set our cavitaion plates quite low (about 20mm below intersect) and open up CL of engines which no doubt helped a twin instal. using plastic props.

There was probably about 10% loss in general efficiency. Cavitation is slightly higher with a full load as would be expected.

We still carry them as spares but do now fit all of our boats with 3 or 4 blade s/s props as for marketing reasons it is "required" of us. There is a potential issue on warranty of gearbox if not using a dedicated prop.

No doubt the better the prop the better the performance but at the same time the plastic probs do seem to come up with good results and overall cost is about 30% of s/s whilst on damage and assuming just one blade broken then cost of replacement is about 10% of s/s.

As an aside it is possible that the plastic prop is more ecco when considering impact with cetaceans and pinipeds at low speed. Whilst still causing lacerations they will not be a severe as a ss prop.

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Old 06 January 2005, 07:24   #16
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Quote:
Originally Posted by jwalker
I had two, sold one and I'm keeping the other for when I feel the need for speed again and buy a Phantom.

Setting up is done to match the boat and the way you wish to drive it. There is a range of cams which control the rate of pitch change. There are also springs which modify this rate to fine tune the setting. There are maximum pitch adjustment screws too but, when you get it right, these can be set to maximum and the setup controls the maximum pitch. When it's right, it's cool. However, you can do a reasonable job by setting the maximum pitch using the screws and benefitting from the progressive pitch change only. Provided the cam selection is in the right ball park.
Having seen the video on their website the acceleration looks awesome - so much so it worries me a little bit - if you are going fast and leap out of the water does the change in pitch happen so fast that you might get such a surge of acceleration that you flip the boat on landing???
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Old 06 January 2005, 08:36   #17
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We ran a Pirahna prop on our lead dive boat a while back, the logic was that we had a number of new boathandlers and if they ran into something the prop would break and save the gearbox.

The first time this happened the fact that it couldnt limp home on the what would have been bent remains of the ally prop spelt its death knell, this despite the fact that the same mob had to borrow my socket set to get the old one off, not having anything onboard to change the prop with. As a method of quick change and repair they are good, we didnt notice any loss in efficiency but youd have to be super sensitive to tell on a dive RIB full to the gunnels.

Obviously the doomsayers did not reflect upon its gearcase saving potential. I can post some pictures of the snapped blade. We carry one now as spare and run ally as a main prop.

New blade set = 25-00
Steel dev repair = 36-00 (ish)

If you are in the habit of having only the one prop and wont or cant change the prop at sea then stay with ally. If there is the chance of gearcase damage you'll get a lot of blade sets for one gearcase.
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Old 06 January 2005, 09:07   #18
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Quote:
Originally Posted by roohairy
This torqueshift sounds like a good idea, never heard of it before. Do you know how much pitch change it will give, what is the variation in pitch?
Thanks,
Yep. There are two alternative blade sets available, one is 11" - 26" and the other is 14" - 32".

Quote:
Originally Posted by codprawn
Having seen the video on their website the acceleration looks awesome - so much so it worries me a little bit - if you are going fast and leap out of the water does the change in pitch happen so fast that you might get such a surge of acceleration that you flip the boat on landing???
Nah. If you are running fast then the prop will already be pitched up. It will pitch up on a jump but it just comes back down as you land. You will be unable to tell it is happening. Your notion is correct though. On landing you have good power as the prop re-enters the water. Just what you need.

Don't let the vid fool you. If you watch the porpoising and look at the engine you can see that there is more trim than you would want for a good start. It's probably done for dramatic effect... it's worked on you...


Back to the plastic props.... The Piranha, compared to a S/S prop, is VERY light. This must be good for the dogclutch in the gearbox.
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Old 06 January 2005, 14:59   #19
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Here are the broken blade shots from an encounter with an orange fishing float (hard GRP? type) you can just see a smudge of GRP and some melted blue polyprop rope on the edge of one of them.

They are quite strong given the are a composite the float was drawn into the prop and managed to bend the AV plate into a gentle curve before the blades failed. Again not sure how ally or SS would have faired
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Old 06 January 2005, 20:01   #20
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Quote:
Originally Posted by jwalker
Back to the plastic props.... The Piranha, compared to a S/S prop, is VERY light. This must be good for the dogclutch in the gearbox.
Nope ... JW you seem to have a good idea of engineering but I must disagree on this one. Forgive the words but a positive dog if what you want rather than a slack one.

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