I have a 1988 38 topgun, old, flat, heavy, its got 540ci blower motors, just over 1,000hp on dyno. The drives are ssm#5s on boxes prop center is 1 inch below the bottom of the hull Merc props 17" by 28 pitch 4 blade. When taking off it blows out and revs to about 4500rpm after a few seconds it will grab and get on plane. Im almost sure the trannys are not sliping. Drives turn out, im redoing the boat and drives are off so im going to swap them around to turn in. Should I change props, space down, or is this a good set up for speed? Havent changed anything from previous owner, looking for opinions?

Thanks for any ideas/help! Mike

Hi,

This sounds typical for #5's with 17" and the drives set that high from what I have heard. I'm in the process of setting up a 35 Mistress with #5's and boxes, also with 1000 SC EFI motors (being built now). I have 3" spacers in the drives, and I set the boxes 1" down. This puts my prop shafts 2" above where they would have been with 3A's, (about 4" below hull blottom) and this is where I am going to start off. I have 17" x 32" 4-blades and 1.48:1 gears. I may take 1 or 2" of spacers out of it if I don't have too much trouble getting on plane.

Depending on where your boxes are set now, you could drop your boxes 1" or 2" without having to open up the drives. That might be just enough to drop your rpms to 3500 or 4000 getting on plane, which might be acceptable. That's where I would start. If you don't know where they are set, reach your hand into the extension box, and feel where the unused bolt holes are in relation to the transom bolts. If you have one set above and one set below, you are in the middle, and can go either way. Oherwise, you can't see what's going on with the boxes on the boat. I just put mine on today...

If you need drive spacers, I know RMPRam on this forum has some he might sell.

Have you stretched your rig out yet? Curious to hear what your setup will do. I know a 84 TG with 1100's and #5's on boxes was reported to go around 103. I'm hoping to push near 100 with my setup eventually.

Was the boat originally from Florida....Purple and white?

Yes, this is the boat your thinking about. Motors are out and being refreshed and ALOT of stuff done diff. I was also told that a set of 5 blades would help out..? I like the drives high but want them set where I get the best speed.

The props should be 1+ inches above the bottom !!!!!!!!!!!!!!! especially with boxes,,,don't forget " THESE DRIVES ARE CALLED SURFACE DRIVES FOR A REASON " !!!!!!

I have a 87 39 stinger 4s on boxes 540 blower motores took all of 4500 rpms to get it on plain with 17 28 4 blades went with 17 1/2 28 5 blades it plains at 2500

My 93 Gun with Vs on boxes does this to....You get used to it
A friend of mine with a Big Gellner Poweedr 87 has them to. He could be alot of help...He is on Boatfreaks as "Trickey"...He has tried hundreds of combos

Same here, 4000rpm then it starts to catch. #4's on boxes on a 33 powerplay. I would like to keep this thread going now that some of you guys with this set-up have chimed in. I actually just swapped my drives last night. Now they are spinning in. Dont really care about docking and I heard I will gain some mph. Also heard spinning in will give me a better roost I have never heard that you can adjust the boxes up or down, is there any truth to that?? Does it matter what kind of box it is???

I'm talking about the Mercury extension boxes. The service manual states that the lowest position is actually the "normal" position, and then they give you a +1" and a +2" position. I set mine up on the lowest position. This is contrary to what I said in my previous post (before I looked at the manual I thought it was -1" normal +1").

So, if you have Mercury boxes, odds are that they were set up already on the lowest setting, so the only way you would have to go is up.

Stellings also had a SSM IV-V extension box, but I do not know anything about them. I think they have access covers on the sides, so that might be a giveaway to tell what you have. The Mercs have no access ports.

i have the mercs and they are in the lowest position. i talked with gary (rmpram) this morning and he said with a short drive shaft i shouldnt go to the highest hole because my u joints would catch hell. Also going up to the last hole would put my drive shafts about even with the bottom of the hull with the props being about 48 inches from the transom. Do you think I could go that high???