Extension box?
 

I have a question for the extension box experts. I'm running ITS boxes on my Formula. The ITS box has a 7" offset vs 12" on an IMCO. I believe that the general consensus is that on the imco the water will rise 2" by the time it hits the drive. By this I mean when the water leaves the hull, it will come up 2" by the time it gets to the prop. If this is true, I'm assuming an ITS box would have a 1.2" rise? Would everyone agree with this?

I'm going with IMCO -3's this summer, last year I was 5" below the hull with a -2 set up and I was still too deep. -3 will put me at 4" below which I would think would be optimal on my boat.

Thoughts?

I have tested many stand off boxes. 12" box is closer to 1" lower. There are so many variables, speed, angle of boat when running, spacing of drives,ect.. Most boxes will give you the option to bolt gimble at different heights. I found the major benefits to a box was control when you trim up or down the boat responds better, like a larger breaker bar more leverage.

Are you referring to the actual measurement of the box and the propshaft to the hull? I'm talking about the water coming back up due to displacement of the boat. I agree that a lot has to do with a variety of factors, but on average, I thought it was 2" for 12" box. If it's only 1" for a 12" box, that is better than I thought.


I started at 7.5" below the hull with no box, with box and -2 I'm 5", with -3 I will be at 4" below.

I set my boat up,bravos,a 2" spacer added,then put my boxes so w/ the spacer prop shaft was 2" below the bottom of the hull.the control was awesome,I could put the nose of the boat,up or down quickly.It was easy to plane this way but not the fastest set up.
I took the spacers out and yes the boat is faster,,but and a big but,,the boat likes to fly more,,which means,,in rough water,its not as fast as when the drives are deeper.of course the videos now are better,,more hang time.
this summer the spacers are going back in.
I also have 380 kplanes.

A up 3 box will net you 2". water rises 1" at 60+ mph, more at slower speeds

I think it all depends on the hull and speed.
I had this conversation with Dave Wesseldyke and his answer was at speed the water doesn't come up much at all.
I was at 7/8 inch above the lowest part of the hull bottom without the box ( with the -3 drive ) and then added the neutral box ( but installed it 1 1/8 inch higher )
Running the same prop the boat is harder to get on plane but the speed and handling is much better. My slip numbers are low ( < 5 % ) at top speed but my cruise speed is slower at the same RPM. I also can't stay on plane at the same speed I used to unless I trim the drive way down. ( I have a lot more negative trim than stock)
I have been told by BBlades that my boat is one of few 26's that can run my combo and have it work.
Right now I am running an OLD Bravo 28 that is stock.
I bought a labbed 30 from BBlades but haven't run it yet.
I have a 28 Maximus that the boat DIDN'T like that is going to BBlades to make it friendly to my boat.

My info is on V-Hulls

Class6
I agree with what You said. I have heard the 1 inch from many people. BUT the faster you go the less the rise. Also the faster you go the less the boat sinks into the water.

and another factor is how rounded or sharp were the bottom of the hull meets the transom. A rounded edge the water will curl up against the transom. A very sharp edge will release the water at that point and IMO will probably be a 2 inch rise. Other factors play in the mix that was already mentioned.

Gopro off back of boat to see where drives are in relation to water while running top speed? If hard to measure, maybe some bright paint marks that are measured out on frt or side of drive ?

^^^^^ totally agree and we have done that plus Go Pro comparing different drive showers as well. I forgot to mention that. Good post SB

Thanks for all the info guys.

A Go Pro is absolutely going to be mounted on my trim tab this summer so I can see exactly what is going on with the drives. Formulas don't exactly have the sharpest transom. At some point this spring I'm going to add some material and get the transom and hull to meet at a point. Time to spend some more money!

Just get a 25' line and intertube for a birds eye view. I'll drive and take it real easy on ya. Ha. Trav has a GoPro if you need to borrow. I have a feeling I might be using it a few times myself for same reason and also to strap on my starboard side so you can see what your boat looks like getting passed. Hah fken witcha!!!

We'll see about you passing me! That's a lot of boat you need to push through the water. Maybe you can draft me so you can get your speed up, cyclists do it, cars do it, why not your boat.

How'd you make out with exhaust options?

I got the Gil's off. That was a lot of fun with the engines in the boat.

Mike have you cked in the forrmula section to see what guys are getting for speeds with various set ups but near same power?

Yeah, I've been digging for years on this subject in that forum. Nobody has installed extension boxes, nobody has run shorties, few guys have had medium power in 311's, which speed wise are the same as a 292. For the most part I think the fastest SR-1 in my range was 82 mph with a pair of 540's in a 272, but it was stock drive height and stock lower. As a whole, I'm the experimenter when it comes to SR-1's, very very few people have done anything other than install bigger engines. I've had a few riggers e-mail me and say to get the props at 4" below, that they had good luck with 311's doing that, but they didn't remember speeds. I know I'm too deep at 5". I could get on plane with out issues with Mirage props, the only issue I had was rough water in lake Michigan. So with that said, I know I can go higher, and you drove my boat, at WOT there were zero handling issues. If it turns out to be to high, I can add a spacer until things improve. I sold the Sportmasters today, so -3 have to happen now.

I was just popped over to the Formula section and there is a 311 with 525 hp 496's in Traverse City, It runs 72 with TRS drives. There was another 311 with "strong" 540's that did 78 mph (GPS). So at 80-81 I'm one of the faster SR-1's, I know I have another 5-6 mph on the table getting everything dialed in. The exhaust is certainly going to help, and drive height should be good for at least 2 mph. We will find out when spring rolls around- shooting for 85 this summer. That is going to be about all my old boat is good for. I might have to start saving for that Active Thunder 33 if I want to go any faster!

Endeavour - make sure you have very near 19.5" length from flange of KE's tail to end of exh tip sticking out the transom. Seriously. 18"-20" is good, but closer to 19.5" will be the scwheet spot. :)

Thanks SB! The next time I go up to the boat I'm going to work on mocking up the exhaust so I can get new tails made. That was going to be my question for you, do I measure from inside the for total length or outside. There is 4" of collector inside the header, I know you are familiar with the KE's so it sounds like measure from the outside of the KE. Thanks again.

Tap Tap ........ Progress?

29 Magic Wizard
 

Looking to put and Mercury racing ITS extension on a 98 29 foot Magic Wizard. Has anybody done this with this particular boat?