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Old 20-05-2016, 12:02 PM   #1
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Performance with 5" Shaft extension?

Hi all,

So currently my Phantom 21 has a Mercury Opti 225hp engine on it with a long shaft. This is specc'd for racing as the transom is 25" in length and should have had an XL shaft engine on it. The issue I have is that the engine cavitates quite a lot, and due to wanting to ski behind the boat as well as go ballistic racing it, I wanted to fit a shaft extension kit to drop the prop by 5".

Bay Manufacturing supply these kits and they have really good reviews, but how much of a performance loss will I get dropping the prop by 5"? The boat does just under 70mph on a flat calm day presently flat out!
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Old 20-05-2016, 03:35 PM   #2
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It will kill the performance of your boat. What propeller are you running? There should be no problem skiing with a 20" motor on a 25" transom if you are propped accordingly
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Old 20-05-2016, 05:21 PM   #3
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Quote:
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It will kill the performance of your boat. What propeller are you running? There should be no problem skiing with a 20" motor on a 25" transom if you are propped accordingly
It's mainly the issue with cavitation that I'm more worried about. Even with the engine dropped right down on the transom plate it still cavitates like a b*tch on hard acceleration :/

The fact it's dropped right down on the plate means we can play about with raising it up slightly if performance is wrecked. I think the prop is a laser one but canMt remember the size.... Possibly a 23? I also have a few other props to test once I get the boat back
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Old 20-05-2016, 06:08 PM   #4
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P21 prop for overall uses

For overall purposes on your P21/225opti, as Paul Suggested, don't go to the 5'' extension, not a good idea.
You want to tell ventilation I guess instead of cavitation...
if you are looking for a good prop for your rig able to make a decent top, and also towing a skier, try to a Bravo one XS propeller, or a REVOLUTION 4.
If your wallet is big enough, a BBLADES OB/F show also pretty good results, the diam is 14.75.
I've also the PROET in mind, but the external diam is a bit less and focused more on top speed, still aving a good holeshot, they use this prop on ski racing boat bernico F2 extreme.
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Old 21-05-2016, 07:13 AM   #5
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A trophy 24 small hub works well on the 21
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Old 21-05-2016, 01:54 PM   #6
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The Laser is intended for submerged set up. As sinkunit0 suggested try a Trophy Plus. The Trophy Plus works well as a surface piercing prop and holds a 100 times better than a Basic 3-blade. Trophy is a very good overall prop. It's not the fastest on any boat but it's one of the fastest on most boats and more over one of the best to drive.
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Old 21-05-2016, 06:03 PM   #7
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Sorry was mistaken on the prop. It's a tempest not a laser
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Old 21-05-2016, 11:11 PM   #8
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Will a 4 bladed prop really make that much of a difference? The engine is dropped as low as it can go on the jacking plate and it still cavitates when we attempt to accelerate hard, turn sharply or trim the engine up to bring the bow up. The prop on there currently has one of the three bungs removed on it, the previous owner said if he tried to remove either of the other two he would get constant cavitation.

Our main concern was the issues with cavitation and also that we wouldn't be able to get up on the plane while towing a skiier/wakeboarder. The service centre I use advised that the best way to stop cavitation would be to add a shaft extension which would resolve both issues, unless someone can confirm a 4 bladed prop would make a significant enough difference to warrant buying one instead?
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Old 22-05-2016, 06:52 AM   #9
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You will knacker a good boat fitting a shaft extension. It's a performance boat not a fishing boat.
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Old 22-05-2016, 08:24 AM   #10
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Quote:
Originally Posted by will_p21 View Post
Will a 4 bladed prop really make that much of a difference? The engine is dropped as low as it can go on the jacking plate and it still cavitates when we attempt to accelerate hard, turn sharply or trim the engine up to bring the bow up. The prop on there currently has one of the three bungs removed on it, the previous owner said if he tried to remove either of the other two he would get constant cavitation.

Our main concern was the issues with cavitation and also that we wouldn't be able to get up on the plane while towing a skiier/wakeboarder. The service centre I use advised that the best way to stop cavitation would be to add a shaft extension which would resolve both issues, unless someone can confirm a 4 bladed prop would make a significant enough difference to warrant buying one instead?
By all means, do not even think about a shaft extension. Better yet, change your service center

The fact you actually have a Tempest is a lot better than having a Laser as a surface piercing prop. However the 23" Tempest cavitates like hell. I had a 23" Tempest on my late Cobra P21 with a Racing Opti 2.5 XS 225hp when I got it from the UK. It was a complete joke. Hole shot took ages and top end was only 67 mph. First thing I did was I put on a jack plate and lifted the engine 2"+ and put a 26" 4-blade Hydromotive on it. The Hydro was a bit too heavy for the rig as the Cobra was so damn heavy. Anyway, hole shot was like a sunday walk in the park compared to the Tempest and top end rose to 75 mph. Now this was a labbed Hydro that costs a small fortune. The next best thing for me was a 27" Trophy Plus.

But a short answer to your question is yes, you should be amazed by the difference a 4-blade prop makes. It's quite a difference having two blades in the water vs 1 blade in the water. And yes a lot of guys will tell you a 3-blade is faster. It most probably is, if you are an experienced offshore racer. But if you are not a champ and don't need the last mile or two, a 4-blade will be a hundred times easier and a better allround prop.

Try to get a used Trophy Plus to test. Correct pitch depends on a lot of factors. My best guess is somewhere around 24-26". (Note the Trophy Plus is only 13 3/ in diameter whereas the Tempest is 14 5/8. If you want to carry a heavy load and ski go with the low end and if you want to brake recors go at the top end
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Old 23-05-2016, 09:53 AM   #11
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would this be a suitable prop?

http://www.boatmad.com/forum/f9/trop...ale-27684.html

The current tempest pitch is also 23" but it's a 3 blade while the trophy is a 4 blade. Can someone confirm if this would perform better than my current?
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