Dual down shafts performance
 

I switched from Imco SC lowers (because I was eating XR gears) to same length dual down shafts. Have about 15 hours on them and all is going well but I lost solid 5mph over the SC lowers. I am not complaining because I bought them to be strong but was surprised in speed lose compared to SC. I have run with same props same loads and boat is 5mph slower every day. Just thought I would post my experience for guys looking to switch because no one could answer this question when I asked few months ago. Imco was straight up and just said your going to loose a little. If your swapping from stock XRs to full SCXs and dual down shafts figure 6 to 7mph total lose but you will be boating not breaking XR's.....

How deep are your drives? I've talked to a few guys that went from Sportmasters to DDL and losing 2 mph was the average. I was thinking about switching my standard SCX lowers to DDL, but not if I'm only going to gain back a few mph.

The way I see it, yes the bullet is smaller on the DDL, but you’re still spinning more gears and mass through the heavy torco RTF. Props to you Jesse for posting your real life results for others to read. Most are too proud to give their real numbers after dropping a lot of coin on new complete drives or lowers.

What are your slip numbers like sc lower to DDL lower?

Rv, what kind of power are you running to damage the SC lower

I am running prop shaft height 3/4 inch below bottom of the boat no boxes. Motors started out as 525s and Todd Goodwin designed new 525 intake manifold with cnc heads and 540 ci running 100 percent mercury injection and ecms dynoed 712hp and 747tq with wet exhaust and all accessory's in good air. More like 650 to 675 in summer air. I pitted both sets of XR gear sets in under 30 hours. I do run harder then most guys. The boat is a heavy 42 and I ran it last year at loto Shootout 94mph in 3/4 mile with SCX uppers and SC lowers. The boat would run 96 mph in longer stretch. This year with DD lowers I have all I can do to run 90 MPH. It was a big change.

Thanks for the detailed info. Just for comparisons sake, I'll post my initial results from my single down shaft SCX lowers.

I have a 39 TG with 600's that last year had a 700 Stage 2 tune, yes not the optimal set up. Last year max boost with the 600 cams was around 8 psi. With XR uppers and -2 Sportmasters and 34 factory labbed Maximus props my best speed was 96 mph, normal around 94. I was on the limiters at 5400 rpms.

This year, I have the correct 700 cams, but so far only 6 psi boost. With the full SCX drives and 16"x 35 5 blade cleavers my best was 92 mph at 5000 rpms. Immediately after that run, my oil pump pick up fell out of the engine, and that was that. So now I wait until the engine is rebuilt. It's hard for me to compare, as I did change the cams. My drives are -2 below the hull. Nothing was dialed in yet, so time will tell where I end up.

Thanks for data. I would think with 16 inch diameter props you could raise drives to get the bullet out of the water and free the boat up a lot. I have a good friend that runs a 39 TG with 700s stage 2s or 3s with NXTs and 17x35 herrings and boat runs 105ish every day and best ever was 112mph. I could measure his drive heights if you wanted to know? The boat really runs good. He is planning on racing it this year at Shoot out. I think it will do good in 700 class.

Yes, that would be great if you could measure his drive height. Ha, I've had a post in the Cigarette section for a year trying to get that info.

When I bought the SCX I talked with Tres Martin on whether to go SCX or SCX-4. He told me to go SCX with sportmaster lower. Not wanting to deal with broken drives anymore I just went full SCX. He said the issue with the SCX-4 is due to the large bullet, it will want to surf on the wake and in turn lift the stern. His opinion was to keep the bullet buried or get it high. He said it would take a lot of work to get the -4 set up right on a 39 TG. With all that said, the NXT bullet has to be every bit as large. It's not a slim drive. So I am curious where the NXT prop shaft is located.

Thanks in advance!

the scx-4 is a true surface drive, the sc is not and probably shouldn't be ran like it

02 T/S w 785 HP over 800 TQ. I went w full SCXs 6-7 yrs ago now and will never look back.
Best the boat saw w XRs was 96. That was w old factory blowers, no chiller and not optimum tune.
Yes I lost 3-5 probably now. Still playing w props. And water pressure issues :hothead:
BUT w XRs I was eating drives 1or 2 Per Year. One Shootout weekend Both. I got to where I had a spare set of XRs that I always carried on trips out of town.
So how fast does your chit run when you are floating w one drive puking oil out the top cap that is no longer there ?? And the Wife is biching in your ear?? Not fast enough.
Sorry but top speed is no longer a priority in my life. Trouble Free time on the water is what matters. Maybe one day I will be there :lolhit:

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Did I mention its for sale??:poopoo:

For comparison sake, my 06 38ZR imco SCX-4s are about 2” above hull bottom. Haven’t gotten a precise measurement on height or how far back they are. They are on ITS boxes. 17.25” 5 blades 18 degree rake. Planes out good with help of 280 k planes but isn’t challenging by any means. In a video that was taken with a go pro mounted in the notch you can clearly see the tip of the lower above the water. My slip isn’t the greatest however, 15-18 percent at cruise. From my experience and shops I’ve talked to the big imcos run the beat when up high out of the water and have a 16”+ cleaver to get around the big bullet and reach the water. There’s also the handling advantage with the cleavers. My boat was set up high to begin with with long sportmasters. 100+ 4 people coolers, full of fuel. 105 light, best so far of 108


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i lost about 7mph but may need prop adjustments, but i have a low water pressure issue with mine, igetwet i see you have transom pickups, do you still send water through the drive to cool it?

Yes water goes through the drive only from the lower pickup. The water exits out the two holes in the front of the drive top cap.

so the 1 1/4 hose inside is capped off?

Correct.

I’ve been trying to get an answer out of Imco about this and haven’t been able to. I figured it would be ok but just trying to make sure

Tori, I think you are spot on with your set up and bullets just surfacing. Most guys I know are running 15 percent slip when surfacing the propeller's. I will bet your water pickup location is causing you a little more slip also. Not sure on Donzi hull but on Fountains if you move water pickup to outside hull you will help with slip because props are getting cleaner water. Hope to see ya soon. Are running Top Gun poker run this year?

Yes we are looking forward to top gun! You probably right about the pickups down in the V, but they do scoop up enough water for the two stage pumps so I’m happy with that! I’ve asked a couple guys with NXT’s what their slip is but no one seems to know that or how high their drives are. From what they’ve told me about planing I’m betting they’re way up there though.


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thats where im putting my pickups at id rather get good clean water all the time deep in the v to feed my twin stage pumps

Carter plumbed my drive intake water to exit out the transom. We were going to do like a drive shower placement to spray on top of drives but swim platform brackets were in the way so it comes out the center more.

Imco just said let it spray out inside the helmet and cap the transom plate

Holy smokes Torey 108 ! That's awesome, i didn't know you made such a big step up in boats and equipment.

Thanks Dan. Yea life comes at you quick! The Baja is still in WI and we know the new owners well, we were just out on it last weekend actually.

Torey,
What engines are in your Donzi? Are they Whippled 525's?

Yes whippled 525s. Valve train obviously redone but other than that they’re still 525 guts. Boos did the tuning, run a 4.5” pulley, never on the dyno but should be around 825hp. Imo the whippled 525 is a really good package. A guy can’t help but think what it’d do with NXTs or #6s. The imcos are power/speed suckers but like others say you’ll be out boating not on the trailer!


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38 Donzi with NXTs and stage 2 700s would be comparable HP to your charged 525s and the ones I know of run 105 to 107. So your right in the ball park with them.

Endeavour, I measured my friends drive height. 2007 39 TG 700s with NXTs 17x35s and his drives are 1 inch above bottom straight ahead of drive. Hopefully this can help you?

Thanks that does help!