[Kujo4321]

Hi people
I've owned a 2001 mariah diablo for about 3 years now. The previous owner overheated the motor and it sat outside for 10 years. I put a new motor in it and have been using it almost every weekend.
We love the boat, this is my 3rd boat, 1st mariah, and it has been beyond reliable. My issue is speed/rpm.

I don't know what it SHOULD top out at with the tiny little 3.0 but I honestly expected more. When you run the original 14.25x19 what is your max speed.

More than anything it's my rpm that I know is low. I propped down to a 17 pitch and my rpm maxes out at 4200 rpm.

I've been over damn near everything on this boat and it's so reliable and smooth I'm at the point where I think my rpm restriction is due to being overweight. So I took it to the scales yesterday. Weighed in a whopping 3300 pounds with half a tank of gas. I've read that dry weight is 2200. So how much of a difference is that extra 1000 pounds going to make.

I was planning on redoing the floor and foam this winter to try to lighten it up but I was honestly expecting the scale to say it was much heavier

[mikeyt]

High 30's and maybe touching 40 is about what to expect for a well-tuned 180 with the 3.0 and that 19 pitch prop is just about ideal for that boat / motor combo. Going to a 17 pitch should result in your rpm's being higher but your top speed will likely be a touch lower. Another option would be to go to a 4 blade prop (likely an 18 or 19 pitch) which wont help your top speed but it will make the boat more responsive at low & mid range.

Nauticus Smart Tabs will help your boat get on plane faster and hold it on plane at slower speeds. DONT use one of those damned hydrofoil fins on your outdrive.

Your 3.0 has a WOT rpm range of 4400 - 4800 where it operates efficiently. I'm thinking it may be your timing is off a bit if you can only get 4200 rpms with a 17 pitch prop.

That 3300 lbs includes your trailer right? The 2001 brochure shows your dry weight at 2600 lbs and your fuel capacity at 38 gals. Half a tank would then weigh about 114 lbs (19 x 6 lbs per gal). IF you are saying that 3300 lbs is just your boat weight then something is wrong. My 180 (with the 4.3) with my trailer comes in at 3550 lbs.

Propeller info

[Kujo4321]

No the 3300 is not including the trailer. I do have trim tabs. And the 4200 rpm is with the 17 pitch. I think I was 3900 with the 19 pitch

[Dalone55]

Originally Posted by mikeyt

High 30's and maybe touching 40 is about what to expect for a well-tuned 180 with the 3.0 and that 19 pitch prop is just about ideal for that boat / motor combo. Going to a 17 pitch should result in your rpm's being higher but your top speed will likely be a touch lower. Another option would be to go to a 4 blade prop (likely an 18 or 19 pitch) which wont help your top speed but it will make the boat more responsive at low & mid range.

Nauticus Smart Tabs will help your boat get on plane faster and hold it on plane at slower speeds. DONT use one of those damned hydrofoil fins on your outdrive.

Your 3.0 has a WOT rpm range of 4400 - 4800 where it operates efficiently. I'm thinking it may be your timing is off a bit if you can only get 4200 rpms with a 17 pitch prop.

That 3300 lbs includes your trailer right? The 2001 brochure shows your dry weight at 2600 lbs and your fuel capacity at 38 gals. Half a tank would then weigh about 114 lbs (19 x 6 lbs per gal). IF you are saying that 3300 lbs is just your boat weight then something is wrong. My 180 (with the 4.3) with my trailer comes in at 3550 lbs.

Propeller info

I am working on my brother's '01 Diablo 180. The engine cover says 4.3L. Who knows? The boat has no manuals, literature, nothing. It does have the tabs. 17" pitch on the prop. Oh, it now has a working bilge pump.

I think an important factor we miss is the altitude of the water where you are boating. Navajo Lake, NM is at 6800 feet and that makes a huge difference even coming from Lake Powell, AZ/UT.

I now have a GPS speedometer on my phone and will check the speed and WOT when we go out tomorrow (Monday).

[mikeyt]

Originally Posted by Dalone55

I am working on my brother's '01 Diablo 180. The engine cover says 4.3L. Who knows? The boat has no manuals, literature, nothing. It does have the tabs. 17" pitch on the prop. Oh, it now has a working bilge pump.

I think an important factor we miss is the altitude of the water where you are boating. Navajo Lake, NM is at 6800 feet and that makes a huge difference even coming from Lake Powell, AZ/UT.

I now have a GPS speedometer on my phone and will check the speed and WOT when we go out tomorrow (Monday).

So spend the $20 and gain access here to all of the available documentation & manuals....

I agree that altitude plays a significant role in performance. Perhaps kujo can tell us where he boats.

[Kujo4321]

Sorry my elevation at raystown says roughly 800 ft above sea-level

[mikeyt]

Originally Posted by Kujo4321

Sorry my elevation at raystown says roughly 800 ft above sea-level

So then elevation is not a factor here. It comes down to a motor that isn't working quite as well as it should. Timing ? compression down a bit? Maybe someone else here has another thought?

As for props, if the above is okay then a 4 blade 17" will increase your rpms a bit and improve the responsiveness of your boat. That 3.0 (despite how reliable & economical it is) was never a top speed motor so checking everything is usually necessary to have it work at peak efficiency.

[Kujo4321]

Would the 900 extra pounds of weight lower my rpm that much

[mikeyt]

Originally Posted by Kujo4321

Would the 900 extra pounds of weight lower my rpm that much

Of course it would but I just dont understand how your boat can weigh so much more than other 180's.

[Kujo4321]

Ah, that comes back to the sitting outside for 10 years. I assume there is closed cell foam under the floor on these boats. I've read posts in the past from different boats saying that once the water gets into it, it'll never dry and the wet foam/water can weigh upwards of 2000 pounds