Extreme ventilation issues?

[oregonape]

Ok, first some back story. New to me 2001 North River Mariner, 20ft, 2001 Mercury XR6 150hp, 20" long shaft on a 25" transom. XR6 came with a jet pump on it but also included the factory lower unit without a prop. XR6 mounted in hole 3 of 5 so I can go up or down another 1.5".

I hated steering with the jet pump so I bought a Bay Manufacturing 5" spacer kit to go from long shaft to extra long shaft and reinstalled the factory lower unit. Prop is a quicksilver black max, 3 blade, 14.5" with 19" pitch. New flow torq II hub kit.

Initially prop to pad measured 6.25" which is a lot more than I would have thought based on a lot of people saying 3.5" is a good starting point.

Ventilation plate was about 1.75" above the bottom of the transom.

We tool it out to test it. Started out brother up front me in back and there was "slipping" issues. Regardless of trim, RPM was constantly spiking and you could feel us lose "traction" and speed. He was able to eventually get us up to 38mph (we could easily get 35mph on the jet pump).

I weigh more than my brother and when we traded places it got even worse. With him in the back I couldn't even get up to 10mph (GPS measured). Not mucher how I trimmed it it would "slip with the slightest throttle". Every now and then it would "catch" and you could feel the thrust as we started accelerating. However I couldn't even get on plane.

I don't have a tach on my console, and the little digital induction tach I put on a spark plug wire was making now sense (set to 2 stroke mode but it was displaying like 6000rpm at idle).

It shifs into forward and reverse cleanly and as far as I know the lower is brand new. Changed the gear oil after installing the lower unit and now sparkles.

I doubt it is spun prop as the flo torq hub was new and we didn't hit anything. Doubt it is the drive shaft coupler as it pissed from the tell tale with no stop.

I did go mark the prop and the hub just in case so I can check for spun prop next test trip.

So this sounds like ventilation, the prop sucking air from the surface or losing grip in exhaust. However this was absurdly bad. It's not like I'm looking for a little more speed. I couldn't get on plane. I couldn't give it throttle without it feeling like my prop went from water to air. No porpoising. Just felt like a car with too much motor and not enough traction spinning its wheels. Like I said, it did run a little better with more weight in the back.

I'm just shocked that it could be this bad. Going down the last 3 holes will give me prop to pad of 7.75" which sounds wrong.

There was a little rectangle of plastic mounted to the transom from the jet pump that closed the gap from the transom bottom to the jet pump shoe. Could that be doing this?

I tried to attach a photo but the site says something about upload failed.

Do I go up to get prop to pad to 4.75" or down to get prop to pad to 7.75"?

Anything else it could be?

 

[oregonape]

Wow, a lot of typos in that post. That's what happens when I write a short novel on my phone.

 

[jimmbo]

The first sentence:
"...2001 Mercury XR6 150hp, 20" long shaft on a 25" transom..."
Pretty much sums it up

You might get away with it at the Top End, if you use Props that are designed to run half out of the water, but they really only work well at High Speeds, and will still slip a lot when attempting to Plane...

For Speeds up to about 45-50mph, you want the AV Plate "Even" with the Bottom of the Boat. Once you get above 50mph you can start raising it higher, but each Transom Bracket Hole Up, only gives about 1 mph more when in the 50s. I don't think you are going to get much past 50 with that combo

 

[oregonape]

The first sentence:
"...2001 Mercury XR6 150hp, 20" long shaft on a 25" transom..."
Pretty much sums it up

You might get away with it at the Top End, if you use Props that are designed to run half out of the water, but they really only work well at High Speeds, and will still slip a lot when attempting to Plane...

For Speeds up to about 45-50mph, you want the AV Plate "Even" with the Bottom of the Boat. Once you get above 50mph you can start raising it higher, but each Transom Bracket Hole Up, only gives about 1 mph more when in the 50s. I don't think you are going to get much past 50 with that combo

See second paragraph, added Bay kit 5" spacer with drive shaft and shift shaft and water tube extensions to take it to a 25".

Presently the AV plate is about 1.75" above the bottom of the boat.

 

[jimmbo]

1.75" is way too high out of the water, for the speeds that combo will yield

 

[racerone]

Agreed with above.----Is the AV plate is that high out of the water why is it even there.-----Lower the motor by 2" or even a bit more !

 

[oregonape]

To clarify, the AV plate wasn't 1.75 out of the water, it was 1.75 above the bottom of the transom or the bottom of the boat. Regardless, it was definitely too high.

I lowered it the last 2 holes so 1.5". Ventilation plate about .25" above the bottom of the boat now. I was able to hit 45mph going up river without problem. No more issues turning (bit of pull to the right but that can be addressed with the trim tab). No ventilation or slippage at all as long as you trim down enough before taking off. Super nice to be able to maneuver at the ramp.

Marks I put on the prop and prop hub were still in line so no spun prop. Didn't matter who sat where.

All passengers up front and it worked great. Would have liked to hit 50mph but considering I put a cheap prop and guessed on pitch and haven't messed with any carb calibration or throttle yet I'm satisfied. Thanks all for your help.

 

[jimmbo]

To clarify, the AV plate wasn't 1.75 out of the water, it was 1.75 above the bottom of the transom or the bottom of the boat. Regardless, it was definitely too high.

When the Boat is on Plane, the AV Plate, will be out of the Water, and so will the Tips of the Prop Blades