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Old 05-03-2009, 09:26 PM   #1
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Country: Norway
Boat make: Phantom 25
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Phantom 25 - propheight

Hi!

I hope you are all well.

I have a 1985 Phantom 25 with twin Merc 2.5 EFI SS engines with PSU's from Brucato. I run the engines 19 inches senter senter, and current propheight is approx. 1,25 inches below the bottom. The best speed so far has been with a labbed set of Mirage Plus (!) running 87/88 mph at 7,400/7,500 rpms. I use CLE lowers with 4 water inlets. I have a water pressure of 25 psi on each engine.

I have tried various props and also a 2 inch setback spacer. Then the boat was very tailheavy when standstill. The water was far in in the splashwell. Since my boat is located where lots of waves can occur, I removed the spacers to be on the safe side. Now the engines are mounted directly to the transom.

Since the waterpressure so good, I plan to buy a pair of labbed Chopper II 26 inch props and rise the engines one hole (approx. 0,75 inch) making the propshaft approx. 0,5 below the bottom. I presume the increased height will offset the taller prop, and giving me a couple of mph on the top end. What do you think? Does the propheight seem ok to what you have experienced? Since the techs are very expensive in Norway (approx. GBP 75 per hrs work) and the props cost approx. GBP 1,500, I appreciate very much your thoughts before I move on.

Thanks!

All the best, ToffenG
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Old 06-03-2009, 06:55 AM   #2
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http://boatmad.com/forum/showthread....hlight=phantom

not selling it then?
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Old 06-03-2009, 07:46 AM   #3
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phantom25

is your boat the former WIRRAL TILES,owned by bill symington?i heard it went to norway,i have several pictures of it racing on the mersey,also,i am currently rigging a phantom25 myself with twin 2.5 mercury engines,i will be in touch with performance figures after tests,it would be nice to have another identical rig to compare with.cheers,den.
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Old 06-03-2009, 06:17 PM   #4
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Prop Height

19 1/2 centre 6" setback, 3/4" up with 2 x 28 big ear chopper (Loverly)
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Old 06-03-2009, 08:22 PM   #5
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setback

got a pic of the setback?i am at that stage next,cheers,den. p.s. i got the first agfa.
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Old 07-03-2009, 06:12 PM   #6
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Set back & height

A couple of pics of my set up. It works very well.
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Old 07-03-2009, 08:22 PM   #7
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Country: netherlands
Location: Mijdrecht/holland
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Boat name: bernico cat
Boat make: bernico 26 cat 2x 2,5,s
Engines: merc,s 2,5 EFI,s
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Post a pic of the boat running at max speed ,so we can see the angle the boat is running,or a short video will be better,is the boat porposing (bow up and down)at speed ?
In my findings ,most boats like yours are having the propshafts too high ,so the propellors don,t have enough bite to lift the bow .
If your boat is running at a good angle ,and no bow bouncing ,and the amount of wetted surface is not bigger than 3-4 feet ,i guess you have already found its sweet spot at the speeds you are seeing.
You didn,t say what pitch size you are using with the mirages,but by looking at your numbers it has to be around 27 pitch .
Around 12 years ago ,I had a 21 Bernico with 2 x 2,5 ,s and placed the engines as close as possible ,when steering full left or right ,the cowlings were making contact.
Did you measure the propshaft height direct in front of the gearcase ?
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Old 08-03-2009, 08:50 PM   #8
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set back

cheers for that,you must go thro some polish on those cowls!
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Old 09-03-2009, 09:03 PM   #9
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Hi Toffen;
You seem to like those twin OBs.
If you calculate your percentage slip, that will tell you if you can go up in hight as far as RPM goes. As menstioned, are you getting the bow lift you need. (my stuff depends on bowlift) We never got anywhere with the Mirage. Depending on your boats needs as flight attitude goes, you might try Trophys, they are cheap and work well in stock form. The Bravo will get you bow lift, but you must run it high or you wont turn it. The Choopers should work after you get it on plane.
Also, what rotation are you running? That will change your lift alot.
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Old 10-03-2009, 02:09 AM   #10
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Smile Phantom 25

Hi!

Thanks for help!

Late night here, so I have to be short this time. More tomorrow!

The Mirage Plus is labbed 25 pitch from BBlades in US. The gear ratio is 1:1,87. So the slip is good. The propheight is measured in front of the torpedo. I spin the props outward. I have tried them inward. It was 1 mph faster 88/89 mph at the current height, but a handful to drive in high seas (and at wot too). Spinning outward makes the boat a dream to drive in all knds of conditions. As on rails.

I tried a pair of second hand Chopper 26 pitch, and I got on plane very quickly. Just as fast as with the Mirage props. A little slower than with 4 blades. The Choppers I borrowed gave good bowlift, but was not perfect balanced. So therefore I think a pair 0,75 inch higher will be the trick compared to the Mirage Plus 25, which can not be run higher due to their construction. They loose bite.

The reason why I have water into my splashwell if using setback is the fact that my transom is a cut down 25 inch version. The original owner had a OMC V8 3,6 litre 300 hp mounted on it.

Jonnyboy; congratulations with your boat! How fast did it run?

All the best, ToffenG
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Old 10-03-2009, 07:31 AM   #11
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Country: netherlands
Location: Mijdrecht/holland
Occupation: truck repair
Interests: fast boats/porches
Boat name: bernico cat
Boat make: bernico 26 cat 2x 2,5,s
Engines: merc,s 2,5 EFI,s
Cruising area: ijselmeer

Join Date: Dec 2005
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Posts: 382
Is the fuel tank mounted on the floor ,and +-1 feet from the transom and +-5-6 ft long aluminium ?
By looking at the last pics ,it shows the boat is having most of its weight in the back.
Tuffboat is right ,I found also that miraged are not the best for this these application ,a set of choppers will most of the time a lot better when bow lift is needed,if no bowlift is needed due to al the weight mounted in the back,you might give a set 14 1/2 x 26 or 14 1/2 x 28 in 3 or 4 blade,these will make your boat run ore flatter,and when the weight is good and the CG at the rear it will probably run faster ,I had the centerline propshaft +- 6 cm (2 3/8 inch) below the bottem,and I used total of 9 inch settback (bernico 21)
I used a set 14 1/2 x 30 and turned 7100 rpm and around 95 mph.
Overall,I think your doing ok with a 25 ft boat at speeds around 88-close to 90 .
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