Duoprop and dpx drive

[Cgoddard1]

I need a little help... I have a 28' commander cat with a dpx 420, dpx duo prop drive, 1.78 gearing, and e3 props. It's slow out of the hole, a top speed of 52, and max rpms of 4500. I think the boat originally had 1.59 gearing. Should I switch the gearing to 1.59, or the props to e2s? Or any other ideas?

 

[Scott Danforth]

do a compression check first

1.59:1 gearing would be slower out of the hole than the 1.78:1

that is a big boat, it is going to be somewhat slow out of the hole. your max RPM's of 4500 are about where you want to be prop-wise assuming the motor is healthy

 

[Cgoddard1]

The compression is good. I'm just wondering if the gearing and and the props are going together well. It takes me at a minimum of 12 seconds to plane out with just me and my wife. I was told going to the 1.78 I would be quicker out of the hole. But that's ridiculous slow.

 

[Scott Danforth]

You do have the drive all the way in for the hole-shot, correct?

 

[Cgoddard1]

Unfortunately, yes. I wish that was all it was

 

[Scott Danforth]

what is your top speed and RPM?

a quick google search has a 500hp or more motor in these things.

I found one with a DPX500, one with a 700hp 540 cubic inch and one with a 555 cubic inch blown motor putting out 950hp.

your 8.1 DPX420 may not be enough motor for the hull.

 

[jakedaawg]

Doesn't this have twins?

 

[Scott Danforth]

The commander 2800 cats have singles

 

[Cgoddard1]

It only has one engine, and it's the original engine. Per the last owner, prior to putting in the 1.78 drive, with the last drive it went about 80. With the 1.78, it's max is 52 @4500 rpms. I'm not sure what the last drive was, but the last owner thinks it was a 1.59

 

[Maclin]

I am having trouble reconciling the 80, is that mph or kmh. At a 1.6 ratio and approx 24 pitch the engine has to be above 6100 rpm to hit 80mph. Not sure that the E series prop numbers relate to regarding pitch. A B/C/F series number of 3 is closer to 17p. For example B5 is about a 19p, B7 is closer to 23.

 

[Cgoddard1]

I was just told 80. I don't need that kind of speed. I'd be fine with 70 really. I'm just not sure how to fix this. I was hoping to find advice, since I'm not having much luck with VP dealers

 

[Maclin]

Well, 80kmh is about 55mph, maybe the "guy" is used to kilometers per hour where they are from.

I do not see a way for you to get to even 60mph. With your engine not being able to get past 4500 with the 1.78 and E2 propset it just seems it does not have the power you would need to get much more mph than you achieving now.

 

[Maclin]

I should have looked at the VP prop guide first. There are engines listed with the 1.78 that are in the 70 mph top speed, and 1.59 ratio engines that show 85mph top speed. I apologize as I had never looked at the E series DPX props/drives in the prop guide.

I do not see a DPX420 listed, there is a DPX415 listed with 1.59 ratio with E2/3/4 ranges from 70-85mpg. So, if the engine is in tiptop shape and is at this power level as the 415 then 80 is possible with E2 or E3.

See if this link gets you to a post with the VP prop guide attached, look in E series prop section...

http://forums.iboats.com/forum/gene...39405-what-size-duoprop?p=5531833#post5531833

 

[Scott Danforth]

Gear ratio change from 1.59 to 1.78 should have you ventilating the prop on the holeshot or jumping on plane vs making it worse.

Have you had the engine scanned to make sure its delivering full power?

 

[Cgoddard1]

I just found out the boat has bad compression in the motor. I've been told our options are to rebuild for 7k (they didn't recommend), a new long block for 10k, or a merc 420 with all components, exhaust and electronics, etc. for 20k (they recommend). What would anyone here recommend? I'm sticking with the duoprop 1.78 with e3s

 

[Cgoddard1]

After reading some sites, I'm not sure the compression is bad.
Cylinder 1- 145
2-150
3-130
4-150
5-140
6-150
7-145
8-150

The mechanic said it should be 170-175, but I can't find that info anywhere else.

 

[bruceb58]

was the engine warm when you did the test?

 

[Cgoddard1]

Yes