Outboard Mounting Height/Low Water Pressure

[Hiteck22]

Just mounted an Optimax 115 to my '97 17.5' procraft fish and ski. The motor is reman'ed so I brought it out yesterday to start the break in procedure, which calls for fully trimmed in and 3000-3500RPM. At that RPM totally trimmed in I was getting rooster tails as tall as the engine and the water pressure was too low.y smartcraft system monitor was squeaking about it and it was in the 7PSI range so I shut the boat down and called it a day after about 10 minutes. 

 

I know rooster tails can be a product of running fully trimmed, but I'm supposed to stay trimmed in during break-in, and combined with the low water pressure I figured I was just mounted too high on the Transom. I have the cav plate about 1" below the bottom of the boat. I also only have one hole left to go down, it was in rhe second to lowest hole, so I'm a little nervous that won't be enough to resolve this. Any thoughts? Am I way off in my assessment? 

[redmexican5081]

You shouldn’t be getting any type of rooster tail with the motor fully trimmed down.  The anti cavitation plate sounds like it is set correctly. Your motor should be straight up and down during your break in running.

[Hiteck22]

27 minutes ago, redmexican5081 said:

You shouldn’t be getting any type of rooster tail with the motor fully trimmed down.  The anti cavitation plate sounds like it is set correctly. Your motor should be straight up and down during your break in running.

Do you think the low water pressure is a result of being fully trimmed in also? I had the smartcraft gauge set for RPM at the time because I was trying see see how accurate the analog tach was. Then the low water pressure alarm came up and I switched over to that and it was around 7 psi at which point I shut it down. 

[Hiteck22]

The guy I bought the motor from said to check my impeller. I ran it on a set of muffs for a couple minutes and he is wondering if I toasted the impeller. I'll be checking tonight. 

[redmexican5081]

6 hours ago, Hiteck22 said:

The guy I bought the motor from said to check my impeller. I ran it on a set of muffs for a couple minutes and he is wondering if I toasted the impeller. I'll be checking tonight. 

running on muffs wouldn't ruin the impeller but if you are running it above idle on muffs then you can risk  motor damage from lack of water flow.  Not changing the impeller as part of you regular maintenance can cause it to fail.  If the previous owner is suggesting that you cooked an impeller running it on muffs in your driveway then there may be more to the story as to why it was re-manufactured.

[WRB]

Trimmed down! 

Can't check the water pump with hose pressure.

Idling the water pump pressure should be 10 psi minimum.

Wouldn't be concerned about the water pump, the power head overheated if a alarm went off.

Outboard engines come with 3 lower unit lengths; 15", 20" and 25" for example. 20" is standard for most bass boats. The propeller centerline depth below the plane surface is 3 1/2" paralell to the plane pad surface. That is the right height. Trim the engine up too high and you rise the propeller shaft and water intake out of the water. Trim the engine down you lower the propeller and water intake deep into the water. Trim is met to optimize bow lift, trim down the bow is pushed down, up the bow is pushed up to gain speed.

Why would anyone think gaining speed is needed when breaking in a new powerhead at 2500-3000 rpm's?

Tom

[Hiteck22]

On 5/26/2020 at 6:50 PM, WRB said:

Trimmed down! 

Can't check the water pump with hose pressure.

Idling the water pump pressure should be 10 psi minimum.

Wouldn't be concerned about the water pump, the power head overheated if a alarm went off.

Outboard engines come with 3 lower unit lengths; 15", 20" and 25" for example. 20" is standard for most bass boats. The propeller centerline depth below the plane surface is 3 1/2" paralell to the plane pad surface. That is the right height. Trim the engine up too high and you rise the propeller shaft and water intake out of the water. Trim the engine down you lower the propeller and water intake deep into the water. Trim is met to optimize bow lift, trim down the bow is pushed down, up the bow is pushed up to gain speed.

Why would anyone think gaining speed is needed when breaking in a new powerhead at 2500-3000 rpm's?

Tom

Yes I meant trimmed down. Everything that I read about the break in process said to do the break in at full load. Full gas tank, gear loaded, trimmed all the way down, etc. I bought the motor from a re-man shop. And he said the same this as you, running on the muffs isn't an issue unless you are running above idle. Just to check it off the list of potential suspects I pulled the leg to check the impeller. It was fine but I put a new impeller and gaskets. The passage from the water inlet to the bottom of the water pump was clear/clean. It's a 20" leg btw. I will double-check the prop center to the planing surface tonight.

 

So after doing the water pump I took the boat out and had the exact same performance. At idle I was getting 3-4psi, and on the slow crawl up from idle to getting on plane the water pressure did increase. When I was right at the verve of popping up onto plan (the moment when really plowing water) the psi got up to 10. As soon as it climbed up on plane the water pressure went down to 7-7.5psi. I ran through this cycle 7-8 times and got the same behavior the entire time. I kept swapping over the the engine temp throughout the process and the temp seemed fine. between 90 and 120 degrees depending on RPM/speed. When I was on plane I was seeing the 7 psi in the 3000-4000RPM range. The manual for my engine says pressure at WOT is 12-14. I wouldn't expect to see 10 psi at idle. I know some mercury models get PSI up into the 30's so maybe those get idle pressure of 10psi?

 

I am getting the water pressure from a system monitor gauge. I don't have a tube run to an analog water pressure gauge. I am planning to check the tubing to the water pressure sensor for obstructions. the line comes out of a port on the block. It comes from the block into a tee, one line going up the to water pressure sensor, and the other is currently capped off and is there to connect an analog water pressure gauge. I'm considering buying one to hook up just to confirm the water pressure I am getting off the sensor is valid.

 

 

[WRB]

2 important things with cooling the engine is the water pump is working and the thermostate is opening when the engine has warmed up. The pee hole indicates the water pump is circulating water, the exhaust indicated the thermostate is open and exhausting water though the powerhead. 

Your PSI water pressure isn't bouncing around and stead pressure it probably OK just reading low.

I have owned several Merc's they all had 5-7 psi at 800 rpm idle, 3 psi is within the normal idle range.

Tom

[Hiteck22]

On 6/1/2020 at 10:46 PM, WRB said:

2 important things with cooling the engine is the water pump is working and the thermostate is opening when the engine has warmed up. The pee hole indicates the water pump is circulating water, the exhaust indicated the thermostate is open and exhausting water though the powerhead. 

Your PSI water pressure isn't bouncing around and stead pressure it probably OK just reading low.

I have owned several Merc's they all had 5-7 psi at 800 rpm idle, 3 psi is within the normal idle range.

Tom

Understood. The water pressure at idle isnt the concern for me, it is the 7-8psi at 3500-4000 RPM when on plane. That is too low and I am trying to sort out why.

[Jig Man]

I had an Optimax for 14 years it always showed low water pressure.

[WRB]

Did you sort it out?

If the impeller gets too hot it takes a set and does't work right. The gauge could also be off. Over 1500 rpm's it should be 10 psi increasing with higher rpm's. 

Tom

 

[Skunked again]

Did you lower the engine, to the next set of holes? Too see if that would help?

[Hiteck22]

I did lower the engine and it didn't really change anything. When the boat is fully in the water and I am ramping up the RPM's to get up on plane I do see up to 10psi. But as soon as the boat comes up onto plane that goes back down to 7.5-8 and stays very consistent at that pressure. I have solicited as much feedback as I can locally and the general consensus was if it is cooling right then the lower than expected water pressure isn't a bad thing. the 12-15 I am seeking is technically the maximum pressure. I want to take that advice and and go but I fear I will always be suspect of this until a cause presents itself or I figure something out and I would love some peace of mind.