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Old 04-03-2007, 02:57 PM   #1
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Spinning in vs spinning out

Hi!

It seems to that I have spunned my props outward last summer om my Phantom 25 with twin merc 2.5 EFI's.

I felt the boat was running very flat and high, and it needed lots of positive trim. However, I only managed to test two different kind of props (Mirage Plus 27 and Bravo One 28).

Spinning in should create better bowlift and free up the hull. The downside can be a more unstable ride and problems docking.

The Phantom 25 is very weight sensitive and bowheavy. So I wonder how much gain I will get spinning in. On bigger V hulls 1-2 mph I have read.

I will use ET 28 pitch props this summer.

Any experiences out there?

Cheeeers! Toffen G
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Old 04-03-2007, 03:49 PM   #2
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Not really sure, but why not try shifting weight? or adding setback to get more bowlift?
whats top end like now with twin 2.5's? what was it with the 1 300 ?
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Old 04-03-2007, 04:00 PM   #3
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Spinning in vs spinning out

Hi!

The 15 inch midsections have 2 inch setback built in. Adding powerlifts will (in my opinion) make the boat too sternheavy (as the black P25 with twin 2.5 EFI SS). I may add 2 inch spacers for a total of 4 inch setback.

Top speed with stock Mirage 27 was 87 mph with 2 persons and full tank. However, the engines were "detuned" (PCU set 10% rich and ignition timing conservative) last summer since I will rebuild them this spring. This will be adjusted professionally this spring by engine builder Michael Werner.

I will buy a set of Merc ET props for the summer. They are made for high propshaft heights.

The boat ran 76 mph with a single PM 300 engine and 7,5 inch setback (CMC powerlift).

Cheeeers! Toffen G
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Old 04-03-2007, 05:14 PM   #4
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Toffen

You have to put a few things straith.

You said your boat is bow heavy ,why ? a p25 is not that heavy ,with 2x 2,5,s each 180 kg,s on her back? where is your fuel tank? if it is mounted in the bow with 200L gas ,it is bow heavy ,if not and the tank is mounted behind the seats in the floor,than its fine,is your ballast tank empty?(if you have 1 )
whats more in the front under the deck?
If there is no extra weight under the front deck ,and you still have a bow heavy boat set up than add at least 5 inch setback ,your boat will have no problem at all with the added leverage.
Let someone made a video from the shore ,while you fly by in full flight,at max speed.
Then watch your video and see how your boat is sitting at the water,it should run at the last 3 ft,not much more .
Now watch the engines,or lower units ,they should be level with the water surface,or trimmed slightly up to have best performance.
If the above is not the case you have to change set up ,I believe yours should run 94 mph if everything is all right.
I feel you have to run 28 choppers ,and 5 inch setback,and outward turning ,and with the 5 inch setback you start with the centerline propshaft 2 inch under the bottem.

If your boat has no hook in the last 4 ft I will be sure it has to run dry and clean at 7500-7600 rpm at 94 mph (1,78:1)and 28 choppers.

good luck sterling
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Old 04-03-2007, 05:33 PM   #5
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Spinning in vs spinning out

Hi!

Thanks!

My feeling is that P25 is running flat. It has alsways done that, despite various setbacks. I have tested 12 inches at the most with a single PM 300. The gastank is below the rear seat. I used to have two gastanks, but the mid mounted one is now removed. It is nothing below the deck in front, exept the ballast tank (which is empty running fast).

I got an email from Steve Baker a few years back, and he told me that my patrticular hull was made extra strong (and heavy). The transom is for instance 3,5 inches thick (!). I blueprinted the hull 5 years back, and the bottom is straight as a ruler.

I will test out chopper 28 props, and add a 2 inch spacer for a total of 4 inch setback. Then test if the speed increases. Now the propshafts are 1,25 inch below. Adding 2 inch more setback will put the propshafts 1,75 below the bottom + the water hitting higher.

Cheeers! Toffen G
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Old 04-03-2007, 05:39 PM   #6
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Oh I forgot ,by looking at your picture I saw you have a set trimtabs,you didn,t mention if you used them or not ,but for all out speed you will pull them all the way in so they can,t touch the water.
Are you sure they are mounte right?
They should be mounte at least a half inch ,but 3/4 is the best height ,measured from the bottem,so fully pulled in they have to sit at least 1,5 inch higher measured with a straith edge under the bottem.

By the looks from the pic ,and I could be totally wrong about this ,it looks like the engines are 20 inch ,cause they are so high from the water.
The 2 inch extra setback is build in to your bracket,and could be ignored on such a long boat.
What I,m trying to say is you can easily add 5 inch or more .

sterling
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Old 04-03-2007, 05:46 PM   #7
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After running a rig that is so weight sensitive IMHO you guys would be better off spending your time balancing with what you've got than spending more money on trying to get that extra 1-2 mph,
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Old 04-03-2007, 06:03 PM   #8
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I didn,t get that exactly ,but I think i know your point.
But you can do what you want if the base set up is wrong from the begin,than it will never be a good runner,Toffy is trying to figure out how to get it right.

He already tried a few things,but they all need setback all the time.
Not for the rough stuff ,but for all out speed setback is the key to lift that long hull free from the water.

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Old 04-03-2007, 06:25 PM   #9
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If 'all out speed' is the priority, he has the wrong boat to start with.

Get a cat.

or,... keep your rough water phantom and forget about setback, otherwise you'll just be building yourself a compromise.
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Old 04-03-2007, 06:59 PM   #10
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The 15 inch midsections have 2 inch setback built in.
Thats not entirely true. For the same prop shaft height 15" mids are mouted 5" lower on the transom (which is angled) and so further forward than 20" mids would be . The extra setback you get from 15's compared with 20's probably works out at less than 1".
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Old 05-03-2007, 12:28 AM   #11
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Spinning in vs spinning out

Hi!

Sterling; I do not use the trimflaps at wot. Sometimes in high seas. I will check how they are mounted. The mid sections are 15 inch offshore versions with build in 2 inch setback. The engines are mounted all way down, and propshaft height is then 1,25 inches below.

Johnny Boat Dude: Pls explain. Its middle of the night here, so thats maybe the reason I do not completly understand. A short leg will maybe give less trim than a 20 inch leg, but the point is to not use positive trim. So....

As set up now the boat require much positive trim running Mirage Plus. I expect Choppers to give better bowlift and hold the water better than the Mirage at this propheight. I have seen other P25 with twins running the propshaft above the bottom with 5-6 inch setback....

In an ideal world I would like to have the engines mounted directly on to the transom as is. Steve Baker reccommended this for overall use. If so, then the questions are balance of the boat, props, propheight and rotation..

Cheeers and thanks, Toffen G
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Old 05-03-2007, 06:23 AM   #12
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Toffy ,you will never be able to run the center propshaft above the bottem ,unless you have 18-20 inch setback.
Thats how you mount engine,s on a catamaran ,or a hydrostream ,etc.
Yours is too long and heavy ,and doesn,t have a true flat pad in the bottem.
You have maybe seen others running their shafts above the bottem ,but that is no garrantee for good speed or handling.
You have to make a decision for what you going to use it most time for,more in the rough and good balance ,put them at the transom,smoother water and max speed mount them 5-6 inch back ,and try if its what you are looking for.

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Old 05-03-2007, 09:38 PM   #13
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Johnny Boat Dude: Pls explain. Its middle of the night here, so thats maybe the reason I do not completly understand. A short leg will maybe give less trim than a 20 inch leg, but the point is to not use positive trim. So....
The transom mounting brackets on the 15" mid are at the same height with respect to the powerhead as on a 20" mid (or a 25" for that matter). The difference in length between the different mid sections is between the transom brakets and the gearcase. In short its the gearcase that has been raised up by 5" when comparing a 15" to a 20". So if you bolt your 15 inch mid to your boat through the same holes as you old 20" your propshaft will be 5 inches higher than it was before. Therefore in order to run the propshaft at the same height as before you have to bolt the 15" mid section to the transom 5" lower than the old 20" mid was bolted. The transom is at an angle with respect to the bottom of the boat. The bottom of the transom is nearer the front of the boat (i.e further forward) than the top. So if your motor is bolted lower on the transom it will be nearer the front of the boat, and when trimmed level the gearcase will be nearer the transom (all the above is discounting the 2" of extra setback mercury gave the 15" mid). I don't know the transom angle on a P25 but I suspect if you did the trigonometry for a 5" drop the motor would have moved forward more than 1". So take that 1"+ that you've lost from the extra 2" mercury gave you and the reality is your motor is actually less than 1" further back than it was with a 20" mid.
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Old 06-03-2007, 12:19 AM   #14
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Spinning in vs spinning out

Hi!

Thanks for your help!

I will the following since its very easy and costeffective; order spacers from CMC of 2 inches and add these. Then the setback will be 3/4 inches. Check out ET and Chopper props. And test the rotation and propheigt. Then I will see if added setback helps and if the inward rotation makes sense.



Cheeers, Toffen G
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Old 06-03-2007, 06:23 AM   #15
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I forgot to tell ,but turning inward could be dangerous(could be I,m not saying it is)
What happens is :if your boat is out of balance ,and tends over to 1 side at higher speeds ,the torque from one of the props wil help to turn the boat upside down.
With outward turning you have your propellor help you to come back in a level position .
This may not been the best explanation but I,m sure others can help understand better.

I still think E.T ,s will not provide enough bowlift ,as it is a cut back chopper that has half cleaver behaviour.
and was made for boats that had a natural bow lift ,and need not much help from the prop to lift the boat.

sterling
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Old 06-03-2007, 07:33 PM   #16
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This may not been the best explanation but I,m sure others can help understand better.
I understood
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Old 06-03-2007, 07:43 PM   #17
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even i did
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Old 07-03-2007, 05:47 PM   #18
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Spinning in vs spinning out

Hi!

Thanks for all help.

Yes, spinning in will make the boat sensitive for chine walk. But the other prop will anyway counteract it to some extent.

However, many V hulls run with the props spinning inward to create bowlift and higher speed. Turning out is more common for cats, since many of them lean outwards in turns, and the outward turning props will counteract this.

Turning outwards create transomlift, and should also be driven by caution since the boat can loose grip in turns and spin out.

Cheeeers, Toffen G




Quote:
Originally Posted by sterling View Post
I forgot to tell ,but turning inward could be dangerous(could be I,m not saying it is)
What happens is :if your boat is out of balance ,and tends over to 1 side at higher speeds ,the torque from one of the props wil help to turn the boat upside down.
With outward turning you have your propellor help you to come back in a level position .
This may not been the best explanation but I,m sure others can help understand better.

I still think E.T ,s will not provide enough bowlift ,as it is a cut back chopper that has half cleaver behaviour.
and was made for boats that had a natural bow lift ,and need not much help from the prop to lift the boat.

sterling
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Old 07-03-2007, 08:17 PM   #19
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Quote:
Originally Posted by Toffen View Post
Hi!

Thanks for all help.

Yes, spinning in will make the boat sensitive for chine walk. But the other prop will anyway counteract it to some extent.

However, many V hulls run with the props spinning inward to create bowlift and higher speed. Turning out is more common for cats, since many of them lean outwards in turns, and the outward turning props will counteract this.

Turning outwards create transomlift, and should also be driven by caution since the boat can loose grip in turns and spin out.

Cheeeers, Toffen G
As you seem very sure of these statements, I have to ask why you asked the questions about turning in, or out, in the first place? You seem to be pretty well decided already.

My experience is that turning in, on a Cat OR a Mono, isn't something that should be taken lightly, and is usually the the remit of 'records week', not an offshore setup, but it's your boat!
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Old 07-03-2007, 10:55 PM   #20
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Spinning in vs spinning out

Hi!

I thought this website was a forum for Phantom enthusiasts. Thats the reason for asking. I assumed some had first hand experience on these boats. Spinning in will in most cases give more speed, but sometimes not. Some boats gets unstable, some not. Thats the reason for asking. At this point the boat is very very stable at wot, but running very flat.

Sorry for asking. But thanks to everyone giving me advise.

Cheeers, Toffen G
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