High X Dim.

[MOBILEMERCMAN]

Looks like the bunks may be in the way for the straight edge.

[imco offshore]

hay tinker i,v been reading your thread and sat on the side ,,, but BOXES are good and will have more benifical advatages than not... if boxes were not , then why do ALL the best and world class riggers in the WORLD put them on ??? take some time and call some real offshore riggers and ask them,, jim is a great rigger,,,, everything else is a guess ? good luck

[MOBILEMERCMAN]

Boxes have there places. Not all boats appreciate them. In some cases they are used to make up for the boats poor balance.

Up sides include they allow the boats to run freer. They move the leverage point back along with the Cg of the boat. If that is needed they may help.

Down side may include a boat that acts too light. Fast in calm but, uncontrollable in any water other than ripples. When you see them on race boats most often they are connected to engines that are moved forward with drivelines. You usually need to relocate the motor even if you don't move it forward. Then spend some time to find the new perfect balance.

Yes, there are exceptions.

[Rik]

RIK - What I mean by ventilate the props is what happens when you surface the props to get air in front of the blades to free them up. It is not what I call slipping when getting on plane.
My boat doesn't seem to need a lot of trim to raise the bow but it feels like I am trimming only to free up the props.

CORRECTION - I checked the depth of the drives tonight.
With the drives tucked in the point of the IMCO drive is 4 inches from the bottom of the hull. With the drives set at 3 ( neutral trim ) they are 5 inches down.

Can I ask, why you feel you should be "Ventilating" the propellers? Your trimming purely to vent the propellers? Sounds like you are over trimming then.

Also, your photos show your up trim, but please clarify if this is where the boat runs its FASTEST or if this is where it starts to ventilate the propellers.:bigear:

[MOBILEMERCMAN]

He may be dragging the fat part of the gearcase thru the water.

[Geronimo36]

took a look at your pics. since you installed shorties instead of raising the X, you may be dragging the upper cavitation plate in the water at speed...

[Rik]

Yea but he said he used to be able to ventilate the propellers in the past and now it will not do this. I am unaware as to why he would want the propellers to break loose unless he needs the R's to get the boat on plane for his power band..

[Tinkerer]

The props do not blow out. When the boat was stock with Mirage 3 blade props if you gave it too much throttle they would blow out. You had to ease it up onto plane.

The term ventilate to me is triming to get the top blade of the prop out of the water to free it up.

That second picture is the trim angle needed for max speed.
looks like a lot to me.

[Tinkerer]

Just went out and looked - the cav plate is 1 1/2 inches above the bottom of the boat on the outside of the drive.
No way is it diging into the water coming off the hull.
Boy I love having the boat in the garage 200 ft away.

Yes I do think I am draging the fat part of the drive through the water.

I think these drives need to go up.

HOW do I find out how much.

I would think that a 38 SCARAB would have a bigger need for bow lift than my 34.
I have seen boxes on 38's.

[MOBILEMERCMAN]

Take the time to parallel the propshaft exactly and measure its height exactly.

I would guess 3" below would work well. Given the work to raise compared to the ease of spacing maybe try a 1/2 higher. No way to know what will be best 2 1/2 , 3, 3/12 until you try .

[Tinkerer]

Thanks

My first boat had the prop shaft 3 inches below the pad bottom and it worked well.
That boat was the ultimate sleeper.
Who would think that a Glastron CVX-18 would do mid 90's.
That was back before GPS - Radar was it in those days.

[Tinkerer]

I will let you know what I do and how it works.

Tinkering is what I love to do.

[Tinkerer]

here are pictures of the drive trimmed to 8 and the level is flush with the bottom of the inside strake. Strake is paralell to the chine.
Using a protractor the drive is at 10* from paralell to the chine.
If the water coming off the back of the boat rises 1 inch as it goes back 12 inches behind the boat. The tip of the propeller is 30 inches behind the transom. Does this aquate to 2 1/2 inches rise. If it does then the upper tip of the propeller blades are 1/2 inch out of the water when trimmed.

[BGIII]

Any updates Steve?

[OneBadInjun]

boxes have there places. Not all boats appreciate them. In some cases they are used to make up for the boats poor balance.

Up sides include they allow the boats to run freer. They move the leverage point back along with the cg of the boat. If that is needed they may help.

Down side may include a boat that acts too light. Fast in calm but, uncontrollable in any water other than ripples. When you see them on race boats most often they are connected to engines that are moved forward with drivelines. You usually need to relocate the motor even if you don't move it forward. Then spend some time to find the new perfect balance.

Yes, there are exceptions.

x 2.

[BenPerfected]

Tinkerer,
Our experience is with a 30' Scarab so it may vary significantly with your 34'. In the mid-90's we converted from TRS & Mirage props to Stellings Ext boxes, Bravo Drives and Mirage Props. The engines were side by side and set up so the prop shafts were raised to about 5.5" below the bottom. Somewhere along 1998-2000, we figured out that the new boat speeds were not all about the ventilated bottoms but mostly about the new Bravo 1 props and the raised X. After a lot of testing, we ended up at 1.0 -1.5" below the bottom and 10 + MPH at the same HP.
The efficiency of the IMCO lower was an unexpected home run as it was worth + 4 MPH....a surprise as we bought the IMCO's get the + 2" in prop shaft height. In our first round of testing, we raised the drives 3" and added a 3" spacer and the IMCO lower to get a base line. This is how we were able to measure the advantage of the IMCO case vs. the Bravo. Each 1" of spacer we removed was worth + 2 MPH.
I am with Dare Devil, I would go up the 2-3" and space down if needed. My thought is plan on spacers and budget the cost. They will be about $1000 and this will be be cheap compared to what this re-rig will cost

If your experience is similar to ours, you are looking for maybe + 4 MPH going + 2" in your X. I would also budget for a fair amount of prop testing to get the max out of your new X...maybe another + 2 MPH.
Sprague

[AIR TIME]

a few boats I knew that were straight bottom hulls twin and single the magic numbers were 4 to 4 1/4'' prop shaft height no box no notch no steps bravo drives. with boxes 3 to 3 1/2''. stepped and notch 1'' to 2 1/2''. a friend had a 34 like yours on the cape and cut off the arch and swim plateform ran blower motors 250 725hp did high 80s. not sure if boxes were used it was 98 or 99 the year.

[Bad Idea Baja]

Totally different animal but all the knowledge that can be passed on is worth something.
My 29' Baja runs a 4.25" prop shaft depth. Imco neutral boxes on -3" drives. I run 9% slip @ 92 mph on the rev limiters @ 5200 rpm. My boat rides very flat for a conventional v bottom.
I think your drives are too deep. I also think boxes will help, but they need to be a raised box. IMCO made (as I understand they discontinued that box) a 3" raised box. That is the box I would look for. That along with the -2" lower would be a good place to start.
However, I really think you are over trimming. If you are collecting performance data from a liquid driven speedometer you are not collecting accurate/consistent data. Get a GPS speedometer or rent a data-tac from Livorsi & get accurate information. Give that information to Bblades or Throttle Up. They will recommend the best propeller for you. You can then test that propeller to determine what your next move should be ie: raising the x dimension alone or adding a box or a raised box.

[Tinkerer]

The #s I posted are with a GPS.

The one time that I saw the regular spedo with the needle buried against the 0 stop the GPS was in metric and I didn't remember what it showed to convert it.

Work has been crazy so I havn't done anything yet.
Still looking for a good set of boxes for the right price.

[BGIII]

Steve,

I was asking a buddy about IMCO shorty lowers on a straight bottom boat and his advice was to raise the x-dimension and use a nose cone (I don't remember which brand he said they have had the best results with) on a regular bravo lower. They believe that the sharp pointed nose on the IMCO lowers are best suited for boats that run flat and/or with little trim, rather than straight bottom boats that typically want/need bow lift and positive trim (sometimes lots of positive trim). The theory is that if you are not getting equal amounts of bow lift for the amount of trim you are using, the sharp nose on the IMCO lower begins dragging through the water rather than pushing straight through the water.

Maybe some prop changes aimed at bow lift and freeing up the props a bit would allow less trim and get you the gains your looking for for little work and short money.