1990 Trophy twin Force 120s prop question

[The Force power]

Ok I found 2 13x19 3 blade props on Ebay that should fit the 4in. The Vortx 992105 from Michigan Wheel. Got reallt good deals on used ones so it's all good. Let's hope they do a better job on RPM than tye 4 blade of the same specs and also the 13.5x15 that over revved.

I just swapped out my (3blade) 17P with a 21P and got my over revving lowered but can still do 6200RPM. I have also a 23P that I want to try however I do a lot of tubing wakeboarding so it may not work for that? The 21P maybe for me the sweet spot for both.

 

[gica]

I just swapped out my (3blade) 17P with a 21P and got my over revving lowered but can still do 6200RPM. I have also a 23P that I want to try however I do a lot of tubing wakeboarding so it may not work for that? The 21P maybe for me the sweet spot for both.

What size boat and outboard do you have?

 

[gica]

So the boat is a 24 footer trophy 2302. The port engine was fully rebuilt as was the lower drive. So I got the boat with two 4 blades 19p props. The props were doing 4500 at WOT so I first got 2 15p and the port engine and sometimes the other starboard were over-revving or cavitation/ventilation with RPM going to 6k.
So I got two 3 blade props at 19p took it out to the islands 15miles out, fished behind the islands came back in choppy seas, pretty heavy closer to shore as is always the case at the Channel Islands at 5PM. Again I did get the over-revving on the port side. When I use the port as a complementary engine along the starboard one it is fine. The over revving is at below 3k RPM mostly. So if I push the port engine by itself it goes to 6k right away if I max it for hole shot. If I do it progressively it goes ok. I have trim-tabs on both sides. I also have the speed sending unit next to a transducer TM-165HW(HW) which is a short one. The starboard doesn't do that at all. I am thinking of putting the 4 blade back on. Have no clue if that will fix that. What the heck could it be?

 

[dingbat]

So the boat is a 24 footer trophy 2302. The port engine was fully rebuilt as was the lower drive. So I got the boat with two 4 blades 19p props. The props were doing 4500 at WOT so I first got 2 15p and the port engine and sometimes the other starboard were over-revving or cavitation/ventilation with RPM going to 6k.
So I got two 3 blade props at 19p took it out to the islands 15miles out, fished behind the islands came back in choppy seas, pretty heavy closer to shore as is always the case at the Channel Islands at 5PM. Again I did get the over-revving on the port side. When I use the port as a complementary engine along the starboard one it is fine. The over revving is at below 3k RPM mostly. So if I push the port engine by itself it goes to 6k right away if I max it for hole shot. If I do it progressively it goes ok. I have trim-tabs on both sides. I also have the speed sending unit next to a transducer TM-165HW(HW) which is a short one. The starboard doesn't do that at all. I am thinking of putting the 4 blade back on. Have no clue if that will fix that. What the heck could it be?

The transom is pretty cluttered close to the prop. Cavitation/venting is the most likely scenario

Anything mounted in the vicinity of the prop from the bow to the stern is suspect. A misplaced thru hull will do it.

Sensors should be mounted 4”+ outside the radius of the prop. Some transducer mfg recommend 12” from radius of prop to prevent cavitation

The fact that it only happens on port side is a good hint.

4 blades are more tolerant of “venting”, but I would find and remedy the problem, not band-aide it with a prop change

Counter rotating props?

 

[gica]

The transom is pretty cluttered close to the prop. Cavitation/venting is the most likely scenario

Anything mounted in the vicinity of the prop from the bow to the stern is suspect. A misplaced thru hull will do it.

Sensors should be mounted 4”+ outside the radius of the prop. Some transducer mfg recommend 12” from radius of prop to prevent cavitation

The fact that it only happens on port side is a good hint.

4 blades are more tolerant of “venting”, but I would find and remedy the problem, not band-aide it with a prop change

Counter rotating props?

Thanks for the answer. Both props are clockwise props. But let's say I only start one engine the port side and I take off it over revs by it self. The other one does not. I know there is clutter there.
The only thing extra from the other side is the speedometer sending unit. The transducer in the middle stays up most of the time.

 

[dingbat]

Thanks for the answer. Both props are clockwise props. But let's say I only start one engine the port side and I take off it over revs by it self. The other one does not. I know there is clutter there.
The only thing extra from the other side is the speedometer sending unit. The transducer in the middle stays up most of the time.

Is this a new to you boat?
Assume problem stays with motor if props are switched?
What props are you running?

 

[gica]

Is this a new to you boat?
Assume problem stays with motor if props are switched?
What props are you running?

It is fairly new to me I already fully rebuilt the motor and drive on this particular motor. It is not the prop itself I used a 15p 3 blade with the same results. When I got it, the boat had 4 blades 19p. Originals were 21p 3 blades.
The reason I changed to the lower pitch is the RPM at WOT it was 4500 with the 19P 4 blade 5K with the 15P. Since I do have twin engines I am thinking that they supplement each other and the stress on the engine is not as bad so 4500 should be fine. I will put the 4 blade back and see. If it continues I will switch the drives. That should narrow it down.

 

[dingbat]

Since I do have twin engines I am thinking that they supplement each other and the stress on the engine is not as bad so 4500 should be fine.

Unfortunately, that’s not how it works.

Motors are limited in rpm when demand exceeds supply. The 4500 rpm limitation tells us the motors are max’ed

With rpm the determining factor in both HP and torque, the 700-750 reduction (15%) in RPM is costing you roughly 30 HP in available HP

I’m pushing a 22’ Grady W/A at 40 mph with a single 225 at 5850 rpm.
What is your speed and rpm at WOT?

Thinking you either have different gear ratios in the lower units or you have a hull problem on the port side

 

[gica]

Unfortunately, that’s not how it works.

Motors are limited in rpm when demand exceeds supply. The 4500 rpm limitation tells us the motors are max’ed

With rpm the determining factor in both HP and torque, the 700-750 reduction (15%) in RPM is costing you roughly 30 HP in available HP

I’m pushing a 22’ Grady W/A at 40 mph with a single 225 at 5850 rpm.
What is your speed and rpm at WOT?

Thinking you either have different gear ratios in the lower units or you have a hull problem on the port side

I don't remember exact speed at wot I think it was around 35. Yesterday I did remove the speedo sending unit(using GPS for speed anyways) and moved the TM165HW transducer in its place to the right farther away from the engine. I also removed a supply L-shaped hose that was installed for the live bait tank right behind the starboard motor to get raw water while running without using the pump. All these I think were creating interference and triggering the over revving issue. Too many things there causing the cavitation. I have to test it. On the port side there is only one transducer and no over- revving.
So now instead of 3 things getting in the way there is only one transducer.

 

[jimmbo]

First of all what is the WOT Rpm Range for that Engine? In the 70s Merc ran them at a lower Speed than later engines, My guess is around the 3800- 4200 rpm

 

[gica]

Ok good news, the over revving was caused by the clutter behind it. So, removing the big hose and the speedo sending unit and moving the transducer to the side did the trick. And it is a good thing because the other engine decided to quit on me, so I had to use this one to get back to port. It actually got me to 20mph. I replaced and clamped all the hoses on the other one, removed and cleaned the fuel pump that was all gunked up, replaced the membrane also and it started again. Maintenance is key that's what they say.

 

[gica]

So wanted to update on the over-revving issue. I had lots of clutter behind the affected engine, and I removed the supply hose for the fish tank (see pic), the speedometer sending unit since I use GPS for speed and also moved the transducer further to the right and installed the 4 blade 15p back on, that was successful as it no longer over revs. So, the clutter is a no no.
It's a good thing too because the port side engine decided to quit on me when I was out. It has been doing the sneezing at start up symptom for a while now, it also did it that day, but I managed to start it and it got me out there and then it quit starting. So, the engine that was over revving before ran great got me close to 20mph. Very smooth running motor given all the rebuilding and replacing of parts. I still have to do some adjusting as it still wants to die at idle in gear, but not all the time.
So, the SNEEZING at start up is that a sign of the fuel supply being restricted?
I did open up the fuel pump and it had a lot of sand behind the screen (see pic). I changed the diaphragm, cleaned all the crud, replaced all the hoses and clamped them, filter was new, it now starts again.
Let me know what you think. Thanks in advance.

 

[jimmbo]

You need the WOT Rpm to determine if the Prop is Suitable...

 

[jerryjerry05]

Sneezing at startup?? That could be the motor is sucking air and needs repaired.
Do the Starting Fluid Test: start the motor, spray SF around the intake side of the motor and see if the spray makes it run different. Spray under the coils, packs and below the carbs. The port covers have a slim gasket and it gets pushed out and starts sucking air.

 

[dingbat]

I also removed a supply L-shaped hose that was installed for the live bait tank right behind the starboard motor to get raw water while running without using the pump.

Didn’t see that in the original pictures.
99.8% probability that was your problem.

Have bronze, scoop intakes on the bottom of my hull for my intakes

 

[gica]

Sneezing at startup?? That could be the motor is sucking air and needs repaired.
Do the Starting Fluid Test: start the motor, spray SF around the intake side of the motor and see if the spray makes it run different. Spray under the coils, packs and below the carbs. The port covers have a slim gasket and it gets pushed out and starts sucking air.

So, I put back the 13 3/8 by 15 4 blades as they are the only ones that give me 5k RPM. On the last outing the while in port the port side motor (the one that was not yet rebuilt) decided to suddenly stop. This is the one that has the sneezing when I press the key for choke assisted start up. It only sneezes on choke. I looked down and there was a plastic bag in the water right by it. I took it on the boat and the motor restarted. I drove the boat to the fishing spots close to shore about 1 mile and it's there where it didn't start anymore. I used the starboard motor that now doesn't over rev anymore since I removed all the clutter. When I got home, I replaced all the fuel hose clamped them and replaced the membrane and cleaned all the built-up sand crud from behind the round scree, and there was plenty. I started the motor in the tank but there was no water coming from the nipple. So either something went inside, or the water pump failed. So now it starts but not right away holds idle, but something is wrong. It could be a coincidence with the bag but there was a lot of stuff floating around the launch bay. I probably have to open up the water pump and look.
I will also do the spraying of stating fluid as it is a great way to find leaks. I do have to resolve the water issue first to have it run and not overheat.

 

[gica]

Didn’t see that in the original pictures.
99.8% probability that was your problem.

Have bronze, scoop intakes on the bottom of my hull for my intakes

There is a picture of the hose up above. It was placed there to scoop up water while going.