[Jayhawk04]

I've got a 93 Diablo with the 4.3... yeah it's ass heavy. Doesn't matter the prop... If you don't have tabs, you need weight in the front. I have a 19 three blade SS and a 20 4 blade AL... I use the 4 blade for boarding... I have hydraulic tabs now. I started with Smart Tabs... and they worked great (I ripped them off doing 50 in moderate chop... John at Nauticus replaced them though) but we board and the Smart Tabs washed the wake out. I took them off and installed a set of Insta Trim tabs... I will tell you what, hydraulic tabs are the way to go... I can tab down and plane at ~17 even with all the weight in the rear... and that is a slow approach. I can then adjust for weight side to side at the flip of a switch.

I love my Diablo. It's a quick hull but the 4.3 is heavy for the short hull... Props wont help if you are trying to plane w/o distributing weight.

[Jayhawk04]

Originally Posted by indykoch

Smart Tabs will definitely help, but let's get these numbers before anything (especially the girlfriend's, but understood if they're missing from your response)!!

The only way to really "tune" in on a correct prop size suggestion is to know the current max speed, RPM, gear ratio, etc. as already pointed out. Another factor - what's the condition of the current prop?

21 pitch is a tad bit high for the 4.3, but definitely doable. Does your boat "growl" at all while idleing or turning? That could be the gimbal bearing which robs power when it goes bad. Also, what 4.3 is it - standard or LX (what horsepower)?

A 21 is not high for mine... I can spin one to 5+k... Hell I spin the 20 4-blade to 5.4k ( I know I should be at 4.8K )

[Rick of Austin]

Got the numbers! This weekend I took the boat out with the GF and her sister so it was lightly loaded, at wide open throttle I am seeing 41 mph and right at 4250 rpm. On the engine cover it says the allowable range is 4400 to 4800 rpm and being the gasburner that I am I would rather see 4800 rpm! I tried to slalom ski and there is no way it can pull me up. The boat mushes bow high and drags me through the water even though my ski is a fairly wide one.
So... called Nauticus and ordered a set of ST-1290-60 Smart Tabs this morning.
Now my plan is to inslatt the smart tabs then run more tests and if necessary optimize the prop size with the tabs. The prop is a 3 bladed stainless 21 pitch in very good condition.
Both girls got up on a pair of skis with no problem though. And of course her sister learned the hard way that she needs a one-piece bathing suit when skiing!

[Jayhawk04]

It's good you went with the stainless Smart Tabs.... They will make a huge improvement.

As for your prop, what make is it? Both of mine are Michigans and though the SS 19 is great for low end grunt, it over revs like crazy. I don't know my max speed with it because I can come close to 5800 rpm and still not be WOT. With my 4 blade 20, I know I'm getting some flexing on the top end because it is aluminum, but I can hit 5400 RPM and about 51 MPH on the wish-o-meter. I usually have myself and 1 to 2 other people when I get these numbers. Now when fully loaded, the numbers change.... I found that the 4 blade 20 was a good all around compromise.


Are you trimming the drive out? I've got a 1.84 outdrive (you should too).

[Paul F]

4250 is way too low. It will take more than smart tabs to get those rpms up to an acceptable range.

Wonder if your engine has issues?

[Jayhawk04]

I was thinking the same thing... When was the last time a full tune-up was performed? When I did mine this spring, I found that the timing was off a few degrees. When adjusted properly, my acceleration improved and I gained a few more RPMs on the top end.

[indykoch]

Originally Posted by Paul F

4250 is way too low. It will take more than smart tabs to get those rpms up to an acceptable range.

Wonder if your engine has issues?

That's exactly why I asked if it "growled" at low speeds especially when turning. Could be a gimbal bearing.

Originally Posted by Jayhawk04

though the SS 19 is great for low end grunt, it over revs like crazy. I don't know my max speed with it because I can come close to 5800 rpm and still not be WOT

You sure that tach is accurate??? I would think you'd have a hole in your sunpad about the size of a piston if so! There should be a rev limiter that wouldn't even allow it to come close to that.

[Jayhawk04]

Originally Posted by indykoch

That's exactly why I asked if it "growled" at low speeds especially when turning. Could be a gimbal bearing.



You sure that tach is accurate??? I would think you'd have a hole in your sunpad about the size of a piston if so! There should be a rev limiter that wouldn't even allow it to come close to that.

Yes.. the tach is accurate. It's only 600 rpm over the recommended MAX. The non FI Thunderbolt IV doesn't have a rev limiter...

Besides... it's not like I run it at that all the time... It was just to see what it did.

I spend 90% of my time doing 20mph pulling a boarder.

Edit: I see you quoted the 5800 not the 5400... yeah It's right and I didn't let it stay there. With that 19, I would not go over 3/4 throttle...

[Rick of Austin]

Went home for lunch and confirmed its a 2 bbl carb meanings its a 155 hp engine according to this table for 1993 engines.

http://www.piranha.com/motor_specs.php?id=4732

Am thinking with a Quicksilver 21 inch pitch it may be overpropped
The engine runs strong, I just changed the plugs, rebuilt the carb and changed all the fluids.