High X Dim.

[Tinkerer]

Hi - I have a 1992 34 SCARAB.
I have installed IMCO 2 inch shorties and am currently running 28 Bravo 4 blades ( not labbed )
Best speed I have seen GPS is 83 at 5600 RPM
80 at 5400. I have 509's that put out approx. 650 HP give or take a few. The prop shaft is now 4 inches below the hull. The question I have is I am thinking on raising the drives another inch maybe 2.
If I go to Maximus props at that height do you think I will have problems getting on plane?

Right now with the Bravos I just push the throttles untill about 4500 to 5000 RPM and the boat just comes on plane. The props dont blow out they just slip untill you get going and they hook up - then the boat takes off like a rocket. This takes about 3 seconds.
When the boat was stock ( 502's and 385 HP each ) the boat had 25 pitch Mirage 3 blades and if you took off too fast the props would blow out and you would have to try again slower.

I have been thinking of adding stand off boxes and going up at least 3 inches.
I haven't been able to find anyone that has added boxes to a 34.
This boat is not the same as the older SCARAB III.
I don't know the differance but it is about 10 MPH faster with the same HP.

Will I be able to get the boat on plane with the higher X ?

I have cable trim ind. - neutral trim is 3.
I have to trim to 9 for max speed.
I am hoping raising the X will keep the trim setting down to a reasonable setting.

Thanks Steve W.

 

[ThrottleUp Props]

Don't go higher. If you have to use a lot of positive trim now, you will need even more trim as you go higher. Higher is not always faster.

 

[Tinkerer]

When the boat was stock with the shafts 6 inches below the hull it had 25 pitch three blade Mirage props and they would blow out easily while getting on plane. I went to 26 pitch 4 blade Bravos and that solved the problem. I then built the 650 HP engines and went to IMCO 2 inch shorties. The 28 pitch Bravo 4 blades now slip getting on plane.
They never blow out no matter how much throttle you give them - they just slip untill you get to the break point and they just hook up and off you go.

When stock with cable ind. ( neutral trim is 3 ) I always had to trim to 8 or 9 depending on water conditions. With the added HP and shorties I still have to trim the same.

It seems with this boat I am triming to ventilate the props more than for bow lift.

If I go to 5 blade Maximus props how much higher can I go and still have the same plaining ability?

I also took some weight off of the boat ( 300 lbs.)
( radar arch, radar, third 27 series battery )

 

[jetmech]

I have found Maximus props to be 5 mph slower than the 4 blade bravo 1.You might try some hydromotive p5 x's.

 

[Tinkerer]

Jetmech - If I was going from 4 blade Bravos to 5 blade Maximus props I could see that I should see a drop in top speed since there is one more blade in the water.
But what I want to do is raise the drive so that I don't have to trim so high. With less drive and prop in the water I would think the extra blade should not be any more drag than the 4 blade is now.

 

[Griff]

Jetmech - If I was going from 4 blade Bravos to 5 blade Maximus props I could see that I should see a drop in top speed since there is one more blade in the water.
But what I want to do is raise the drive so that I don't have to trim so high. With less drive and prop in the water I would think the extra blade should not be any more drag than the 4 blade is now.

You will have to trim more to get the same bow lift. Not to mention, the more blade surface area you add, the more stern lift that is created and it will be even harder to lift the bow.

 

[Geronimo36]

Raising the X will require more trim. If you go higher you might have to go to a 5 blade to get any bite. Regardless a 5 blade might give you better cruise.

At 4" I think you're at a pretty good all around setting now. To get better trim maybe you can send the props out and have them add a little cupping to the blade tips.

 

[blackhawk]

I don't want to hijack but how can you trim more when you raise your propshaft? If you raise your propshaft 2" it will come to the surface sooner with less trim.

That is the point of raising your X or running a shorty, less trim = more forward thrust(from less trim angle) = more speed. And in turn the less trim makes boats run flatter because you lose bow lift.

Am I missing something?

 

[MOBILEMERCMAN]

I am with you Blackhawk. Plus The higher you go the more "carry " you lose. So as you lose the ability to carry one tends to try to trim more and look for more bow lift.

I also agree with Geronimo 4" below is s good all around height for 80 some MPH. You really can't raise it much more if any effectively. At your speed the bottom straightness comes into play. The bottom needs to be true and straight with NO hook or hollows to find your best speeds. At which point one may add a little rocker to carry the bow higher.

The boats that are getting there prop shafts near the bottom are going near 100 mph or more.

Best of luck. As you know those last few MPH get hard to find and sometimes a change slows you down.

 

[Geronimo36]

I don't want to hijack but how can you trim more when you raise your propshaft? If you raise your propshaft 2" it will come to the surface sooner with less trim.

That is the point of raising your X or running a shorty, less trim = more forward thrust(from less trim angle) = more speed. And in turn the less trim makes boats run flatter because you lose bow lift.

Am I missing something?

Not all boats are the same but when you raise the X you begin loosing trim levrage, as Jim said you loose the ability to "carry". so on a straigh bottom boat that needs carry, you'll more than likely need to trim more to carry the bow.

Before I did my X dimension, at cruise speed, I would have to run my drives at 2 on the indicator (slightly negative) and now that I'm two inches higher I have to run it at 3 (neutral) at the same cruise speed.

As you raise the prop up it gets closer to the surface and depending on the style prop you're running (Bravo 1) you start getting more sternlift and you need to trim a little more to get the bow up.

this has been my experience...

Now step bottoms...totally different ballgame and prop selection cause slip numbers are so high and they run fast, flat.

 

[Rik]

This is so damn funny..

Everyone says there is no bow lift with an Arneson and you guys are finally coming to the same conclusion about a Bravo being surfaced. Truth is any drive surfacing gives no bow lift as you cannot trim it against the air.

You want or need bow lift, lower your X so you can trim more, change propellers to one with more rake, or examine and adjust your bottom.

Maximus has 30 degrees of rake but 5 blades and a diffuser ring (cut it off)

Bravo 1 has 25 degrees of rake and a much smaller diffuser ring

Hering has variable blade #'s but only 22 degrees of rake I believe

 

[Tinkerer]

I raised my X dimension 2 inches with the shorties - handling improved drastically, speed went up 1.5 MPH.
The amount of trim for top speed did not change at all. This led me to believe that I am trimming to ventilate the prop not so much to lift the bow.
Removing the two anchors in the bow locker and ALL of the stuff in the cabin makes NO differance either. ( I did get a little more speed but the trim angle stayed the same )
Trim angle stays the same.
The bottom doesn't have any hook, The inside strake has been cut off 12 inches forward of the transom to cause bow lift. The transom and strakes have been sharpened. There is a slight rocker in the bottom the last 8 ft.
The boat carries the bow very easily.
I am thinking on putting boxes on the boat but can't find anyone that has tried them on this hull.
I plan on increasing HP next year to about 800 from 650.
I plan on keeping this boat for a while so why not make it what I want.

 

[pullmytrigger]

Id never hi jack but how'd you make out with your nieghbour.....??

 

[DAREDEVIL]

I raised my X dimension 2 inches with the shorties - handling improved drastically, speed went up 1.5 MPH.
The amount of trim for top speed did not change at all. This led me to believe that I am trimming to ventilate the prop not so much to lift the bow.
Removing the two anchors in the bow locker and ALL of the stuff in the cabin makes NO differance either. ( I did get a little more speed but the trim angle stayed the same )
Trim angle stays the same.
The bottom doesn't have any hook, The inside strake has been cut off 12 inches forward of the transom to cause bow lift. The transom and strakes have been sharpened. There is a slight rocker in the bottom the last 8 ft.
The boat carries the bow very easily.
I am thinking on putting boxes on the boat but can't find anyone that has tried them on this hull.
I plan on increasing HP next year to about 800 from 650.
I plan on keeping this boat for a while so why not make it what I want.

Boxes are not always better,if the boat runs the way u want then leave it.
On your boat the rocker carries the bow,,so thats why. Not all the boats are the same as u know ,even if it came from the same mold u never know what the former owner did unless u buy new and even then some are different.
Personaly, i am a friend of HIGH X dimension.
My bullet is flush with the hull and it runs best @ trimm 4-5 at the most,beeing 3 is neutral .

 

[Tinkerer]

Daredevil - What boat do you have ? what speed ? is it stepped or flat ?

This is not the first boat that I have changed the X on.
My first boat was an 18 ft Glastron Carlson CVX-18 ( bought it used in 78 it was a 77 ) I was 21.
It came stock with the 888 merc package. ( 188 HP Ford 302 )
Due to fitting the engine under the cover the drive was installed with the cav plate 1/2 inch below the hull. ( boat had a 8 inch wide ride pad.)
I installed an AlPha SS ( 2 1/2 inches shorter than stock ) I gained 10 MPH.
I had already increased the HP to about 300 before the SS. The drive mounted too low was pulling the stern down the more trim you gave it.
I continued to increase HP and blue printed the bottom.
I got to over 500 HP and was running mid to high 80's ( stock was 49 MPH )
Had to replace the transom and stringers so I thought what the heck and recut the transom 2 1/2 inches higher.
If memory serves me the prop shaft was now 3 inches below the pad.
With a 4 blade prop the boat was a rocket out of the hole.
Top speed was mid 90's
This boat was the ultimate sleeper.
I just FEEL I need a higher X.

OH and trim was about zip.
with the drive all the way down you pushed the up button for about 1/2 second.

I ended up selling the drive and hull seperate.

I could blow the drive at will - Way too much HP for an Alpha SS.

 

[Tinkerer]

Pullmy trigger - talked with my neighbor for the first time yesterday. He was still arrogant and couldn't remember what he had done and said. But I think the problem is behind us. He will never be a nice neighbor - he is a bully. He has learned that he can't bully me. He likes to lie and I caught him at it yesterday. I spent almost $3000 and had some landscaping and 4 - 17 ft tall blue spruce trees installed. I can no longer see into his backyard.
His whole beef is the noise I make maintaining my yard. And also all of the work that has been done to get it to where it is today.
I told him yesterday thay I had the right to mow my grass and that the projects were 95% done.
We shook hands and parted ways.

 

[MOBILEMERCMAN]

If the boat carries the bow well and you are trimming out for max speed I would think you can go up more. The thing is the effort it will take. If you are determined then do it. You can space down easily if need be.

When the motors are side by side somewhere around 3" below will be the highest you will likely care to go. It is no fun to run a boat when the props leave the water before the hull. Not sure what that ultimate height is on your boat. To give you an idea figure out at what height the drives can be to put the prop shafts even with the lowest part of the V. I think if you raise them so that the prop shafts are higher than the bottom you will be giving up handling in the rough.

 

[DAREDEVIL]

Daredevil - What boat do you have ? what speed ? is it stepped or flat ?

Its a 32 foot V ,single engine , no step but a notched transom.(same as box).

Speedmaster #4 ,~800 HP running 90+ with my big prop,set up for 84+ MPH in P class right now.
Remember ,surfacedrive props are a little different then Bravo style,although they work somwhat a like.
The survace drive prop is best when half in and half out of water @ full speed.
(cleaver style blades)
I would not try boxes on your boat , also i would maybe go up 2 more inches with drives on transom because like Jim ,(Mobil) said ,u could always space back down.
But again,,,is it worth it ????? I did so much to my previous boats and waisted alot of $$$,,in the end when u did it all ,its boring so u sell it anyways for what u get and find a new project,,LOL. Thats just how it is.
So be happy and actually enjoy the ride ,,,thats what i have learned.
OK,OK,,not guite,LOL ,,,i am still messing around .HAHA,,just not as much since money is getting tight and i need to stop waisting it on a boat that actually runs fine .
Think about it.:seeya:

 

[blackhawk]

Not all boats are the same but when you raise the X you begin loosing trim levrage, as Jim said you loose the ability to "carry". so on a straigh bottom boat that needs carry, you'll more than likely need to trim more to carry the bow.

Before I did my X dimension, at cruise speed, I would have to run my drives at 2 on the indicator (slightly negative) and now that I'm two inches higher I have to run it at 3 (neutral) at the same cruise speed.

As you raise the prop up it gets closer to the surface and depending on the style prop you're running (Bravo 1) you start getting more sternlift and you need to trim a little more to get the bow up.

this has been my experience...

Now step bottoms...totally different ballgame and prop selection cause slip numbers are so high and they run fast, flat.

Okay, you guys were commenting that you needed more trim for bow lift when the propshaft is higher. That I agree with.

Tinkerer, are you planning on actually raising the X by reglassing/recutting the transom?

 

[VetteLT193]

Have you thought about playing with props more? I can trim my Bravo 1's easily. Mirage +'s that I had on it before I was tucked in some. You might find that a set of M+'s totally change the feel... or just another style of prop all together.