OK
I've been hemming and hawing trying to determine whether I should run extension boxes on my boat. I've been reading WAY more than I really should on the subject. I've turned up allot of info. I was looking thru the APBA rule book and found some interesting thing on page 76 in the F2 section.
I'm not looking to compete, I've just been reading everything I can find!

1.APBA F2 extension boxes: only allows extension boxes on conventional V bottoms with absolutely no hull steps , hull pads transom notches or other modifications that would effect the conventional V bottom design. Standoff boxes are limited to 15 inches. This includes the Merc ITS, can't run that unless it's a conventional V.

2. APBA F2 on X dimension: The maximum drive height for all boats eligible to run standoff boxes shall be limited to the centerline of the propshaft being a minimum of 2-1/4 inches below the bottom of the boat as measured with a straightedge off the aft running surface of the bottom. All other boats shall be limited to the centerline of the propshaft being a minimum of 3-1/4 inches below the bottom of the boat as measured with a straightedge off the aft running surface of the bottom. That is a 1 inch difference.

So what are they saying? Are they saying the one inch compensates for the drive being back (up to 15 inches) or are they saying that having steps, a pad and a notch makes you clearly advantaged so much that you must drag more of your drive thru the water?

DS

The Boxes Do Work On Some Application's, How Ever They Do Have A Tendency To Make A Non-step Porpoise. Either Way Make Sure It' Worth The Investment, As Far As Speed Verse Dollar Spent.

I agree on both counts. For the same amount of money, you could just add small blowers. My boat came with boxes and it porpoised a bit. Getting weight off the back of the boat (ditched a radar arch and got aluminum heads) got rid of the porpoise so it may have something to do with c of g.

I 'am running ext. boxes on a 35 fountain, the boat runs great but would like to know how the boat would run with them off. The fountain has blowers and does not porpus at all , and it takes off like a jet ski.

i'm in the process of re-rigging a 38'. it has no boxes, and plains in like 3 seconds. we've run every prop you can think of, and the best all around was the 5 bld maximus. as for the 35, i've never run one with boxes and no step, but the stepped version does'nt appreciate them. especially in the moderate to heavy chop.

My 386 Sonic has them and it wants to porpoise in real flat water around 50-60. Not real bad but noticeable. Top speed was 83 with 575 SCi's. I don't know how this would compare to a 386 without them. I would like find out.

Good points.
I certainly don't want a boat that porpoises. I spend a good amount of time on the chain of lakes (relatively calm water for the most part). Seeing as my boat has a notched transom the extension boxes will really be putting the drives back there behind the running surface.

One thing I noticed while reading thru threads on the subject, is nobody mentions their transom angle!
IMCO boxes are designed for a 13-1/2 degree transom angle. My transom is 14 degrees, so my drives will end up around 1/16 of an inch lower (deeper in the water). If I didn't take that into consideration it could skew my results.
I didn't check the Stellings.

I have already have acquired small blowers and may be able to get myself into the IMCO boxes without killing my budget.
More info???
Thanks,
Dan

Do you have the room under your hatches to raise your engines. Older boats have very deep X dimensions. Switching to Bravo's will raise that a couple inches but you may need more. Going to boxes will require going higher than that. You won't go any faster with the boxes if you don't set the drives higher. I would check a couple of 10 meters with boxes and see what they use for an X.

Changing from the TRS to the stock bravos and leaving my crank centerline at the same height (" X " dimension at 18.5") will put my prop centerline about 3.5 inches below the running surface.
I think I could move the engine up maybe an inch more.


BTW Ian, I'm still kicking myself for not buying those heads from you last year.

That seems close. Next you need some big dollar ported alum. heads.