Transmission ratio prop pitch/diameter on Vdrive

[450rdawg]

I have a 95 Sundancer that I purchased as a project. It came with the Hurth 1.56:1 tranny and they both needed replacing. I couldn’t find 2 of the same ratio that were in my budget so I had to go with two 2.0:1 transmissions. The props on the boat are 17x16. I never drove the boat before so I don’t know what the performance was with the existing props. I also put two remanufactured 7.4 engines in it. I’ve been through about 12 hours of break in at lower RPM’s. I took it out a couple of weeks ago to stretch her legs a bit. I can’t get it to come up on plane and the engines won’t rev above 2400. I’m completely lost when it comes to transmission ratios and their relation to prop size/pitch. That being said, I think my props are either too big and/or need to be below a 16 pitch. I’m just looking for some good advice on what to do to remedy my situation. As always, thanks in advance for your suggestions!

 

[Strecker25]

I have a 95 Sundancer that I purchased as a project. It came with the Hurth 1.56:1 tranny and they both needed replacing. I couldn’t find 2 of the same ratio that were in my budget so I had to go with two 2.0:1 transmissions. The props on the boat are 17x16. I never drove the boat before so I don’t know what the performance was with the existing props. I also put two remanufactured 7.4 engines in it. I’ve been through about 12 hours of break in at lower RPM’s. I took it out a couple of weeks ago to stretch her legs a bit. I can’t get it to come up on plane and the engines won’t rev above 2400. I’m completely lost when it comes to transmission ratios and their relation to prop size/pitch. That being said, I think my props are either too big and/or need to be below a 16 pitch. I’m just looking for some good advice on what to do to remedy my situation. As always, thanks in advance for your suggestions!

I'm surprised the higher ratio transmission is having that effect, I would expect it to be the opposite all things equal. If anything I would think the 2.0:1 would get the boat on plane but have slower speed and higher revs, unless I have my ratios mixed up?

 

[km1125]

If it has the same props as it did when it had the 1.5:1 gears, then it should have popped up on plane but would have had a lower top end. You'd max engine RPMs before the boat really got to what could be top speed.

I wouldn't go with lower pitched props. Those seem on the small end anyways. There must be something else wrong.

As a comparison, I had two 7.4L (350HP Crusaders) with 2.56 gears and I was running 22x22 props. That was on an 18,000 lb boat.

 

[Strecker25]

If it has the same props as it did when it had the 1.5:1 gears, then it should have popped up on plane but would have had a lower top end. You'd max engine RPMs before the boat really got to what could be top speed.

I wouldn't go with lower pitched props. Those seem on the small end anyways. There must be something else wrong.

As a comparison, I had two 7.4L (350HP Crusaders) with 2.56 gears and I was running 22x22 props. That was on an 18,000 lb boat.

That's what I thought, so my math is right. At 2400 RPM and not on plane (I plugged in 10MPH) that is a 45% propeller slip. Typical of not getting over the hump, but likely means there's something else going on.

Is the bottom dirty from sitting for a while?

 

[450rdawg]

That's what I thought, so my math is right. At 2400 RPM and not on plane (I plugged in 10MPH) that is a 45% propeller slip. Typical of not getting over the hump, but likely means there's something else going on.

Is the bottom dirty from sitting for a while?

I just put fresh bottom paint on it. At that RPM, I’m running around 13MPH, based on the GPS.

 

[450rdawg]

If it has the same props as it did when it had the 1.5:1 gears, then it should have popped up on plane but would have had a lower top end. You'd max engine RPMs before the boat really got to what could be top speed.

I wouldn't go with lower pitched props. Those seem on the small end anyways. There must be something else wrong.

As a comparison, I had two 7.4L (350HP Crusaders) with 2.56 gears and I was running 22x22 props. That was on an 18,000 lb boat.

I honestly don’t know what the performance was with the old ratio because I never got to drive it with them. It’s carbureted and both carbs are new. I really don’t know what else would be holding it back but it’s definitely struggling very badly.

 

[450rdawg]

I'm surprised the higher ratio transmission is having that effect, I would expect it to be the opposite all things equal. If anything I would think the 2.0:1 would get the boat on plane but have slower speed and higher revs, unless I have my ratios mixed up?

That’s what I was thinking too but I was told the opposite from the local prop shop. He’s pretty reputable too, which is why I took his suggestions seriously.

 

[km1125]

If you replaced both engines and setup both the same (and both have new carbs), I'd be double-checking everything there. Is the timing correct? Are the advance mechanisms working correctly? Are the secondaries on the carbs opening? Can you get a vacuum reading off the engines while they're under load? Are the chokes opening fully (and STAYING open?) ?? Clean flame arrestors?

Is it possible there's any slipping going on in the transmissions? Have you checked trans temp while under load? Actually... rethinking this last one... That would probably result in engine speeds going up but not boat speeds - so, maybe not THIS issue but with everything new you might want to check trans temps when you DO get it up to speed for a while.