bad bow steer after motor upgrade

[robertbullen11]

my 2004 300 marlin with twin 225 yamaha was a great handling boat. ran 40 mph wide open at 6100 rpm, and cruised 30 mph at 5000 rpm. upgrade. replaced 15 year old twin 225 yamahas due to alarm issues and potential exhaust corrosion but they still ran and boat handled great. now new 2019 twin 300 suzuki’s and the boat handles terrible after about 25 mph at 4500 rpm and boat will max out 45 mph at 6000 rpm. but is very scary and steering all over the place. the props are 16 dia/17 pitch 3 blade. can anyone help

 

[SeanC]

I just sounds like that combination of motors and props is giving more stern lift. Did trimming out the motors help.

Reminds me when I put a foil on the motor of one of my centre consoles. Terrible bow steer. Trimmed the motor out a couple on notches. ( no hydraulic trim) All good.

 

[Halfhitch]

I just sounds like that combination of motors and props is giving more stern lift. Did trimming out the motors help.

Reminds me when I put a foil on the motor of one of my centre consoles. Terrible bow steer. Trimmed the motor out a couple on notches. ( no hydraulic trim) All good.

I agree 100% with SeanC. Once you get on plane , trim out till you get blow-out, then trim in just till it hooks up. In that configuration your bow should be high enough to avoid serious bow steer. The Suzukis usually run a larger diameter prop due to a higher numerical gear ratio in the leg so the lifting effect may be greater than the Yamaha props. All this, you probably already know but this is just to get you to mull this over some more and gives me a reason to have more coffee and avoid getting to work.

 

[ocnslr]

Comments above nailed it. Bow steer because you have too much boat in the water.

 

[robertbullen11]

trim is only helping below 25 mph after that its almost like it losing traction or blowing out , old motors had 4 blade props , my salesman isn’t trying to give me 4 blade, also its almost like there not in water far enough

 

[PrinceofThieves]

I'm going to agree and add that you can most certainly run a larger wheel - give the 16x18.5 WaterGrip series prop a try, I'm surprised you're only getting 6000 with those wheels - with the 16x17's on my 330 (I have 300 Zukes as well) i was getting 6250 trimmed all the way out with moderate load.

 

[Doc Stressor]

Could your engines be mounted to high? If they are, you not going to get enough bow lift.

Take a look at your anti-ventilation plates with the boat on plane. The wake should be right at the level of the plates. On lighter hulls without forward cabins, you can get away with the water level an inch or so below the plate. But not on a big Grady.

 

[robertbullen11]

thanks everyone for your help, im going to check that ventillation plate

 

[robertbullen11]

Could your engines be mounted to high? If they are, you not going to get enough bow lift.

Take a look at your anti-ventilation plates with the boat on plane. The wake should be right at the level of the plates. On lighter hulls without forward cabins, you can get away with the water level an inch or so below the plate. But not on a big Grady.

Could your engines be mounted to high? If they are, you not going to get enough bow lift.

Take a look at your anti-ventilation plates with the boat on plane. The wake should be right at the level of the plates. On lighter hulls without forward cabins, you can get away with the water level an inch or so below the plate. But not on a big Grady.

Hi doc, if my motors cant be lowered , would moving them apart farther work to get them in water more. they are about a half inch up off the transom now. I have not checked the antiventilation plate yet, going to do that tomorrow. I been feeling that they are too high, they were mounted in the same hole as the yamahas.

 

[Doc Stressor]

I don't know if moving the motors away from the centerline will help or not. The SV2 hull is a strange bird. Sometimes just a little tweak will make all the difference in the world.

Just a wild *ssed guess, but is it possible that one of your new engines is not counter rotation?

 

[SkunkBoat]

Another thought...
are you sure its not a steering problem?
I assume you got 2 new steering cylinders on the 300s and they bled out all of the old fluid. Maybe you have some air bubbles.


Do you have power assist? Autopilot?
Does autopilot have trouble going straight?

 

[robertbullen11]

Another thought...
are you sure its not a steering problem?
I assume you got 2 new steering cylinders on the 300s and they bled out all of the old fluid. Maybe you have some air bubbles.


Do you have power assist? Autopilot?
Does autopilot have trouble going straight?

they only gave me one steering cylinder and told that is all i needed, i had 2 on the yamahas. My autopilot now only works at low speeds and always worked great before new motors

 

[SkunkBoat]

they only gave me one steering cylinder and told that is all i needed, i had 2 on the yamahas. My autopilot now only works at low speeds and always worked great before new motors

I'm no expert but I think 600 HP requires 2 cylinders
I'm no expert but autopilot pumps are sized based on volume.

My ox66 225s share one cylinder. It was really squirrely when I bought it. Purging/replacing hydraulic fluid improved it.
Then I bought autopilot. I got a Garmin "Smartpump" which is an oversized, self adjusting universal pump for any cylinder volume.
Did that in case I go with 2 cylinders when I re-power.

When installing the autopilot I ended up replacing the Helm too and purging again. Got more ugly fluid out. Steering is 100% better and autopilot is awesome.

My point is...maybe you're assuming its the motors when its maybe the steering/helm/fluid/air bubbles...maybe in combination with more HP feeling new to you and its on one steering cylinder.

 

[Ozz043]

some extremely valid points raised here that will certainly give you something to look at and get the cogs turning

“Bow steer” is the key phrase for me...this entails stern lift and so the motors may be too high... I will assume you’ve played around with the trim purely because you’ve gone WOT ...even just a tad should effect the way she runs...

props won’t effect the way she steers at speed
steering issues would be quite evident at all speeds with a heavy wheel as well, I would think there should be two cylinders though definitely

 

[DennisG01]

I'm wondering if the term "bow steer" might be being used incorrectly? Bow steer happens when too much of the hull is in the water at planing speeds. Often, we can purposefully cause this to happen by trimming down fully. When you are having the issue, where is the water spray leaving the hull... if it's at the midpoint of the boat (or further towards the stern), then I don't think you're having bow steer problems.

It might be best, at this point, to take it your installing dealer and tell them something is wrong and the handling of the boat is compromised. It might one of those things that's hard to diagnose without being there - whereas an experienced person can probably figure it out pretty quick in person. Go out for a ride with your dealer - if they're a decent organization, I'm sure they'll work with you to figure things out.

 

[Kizuna]

my 2004 300 marlin with twin 225 yamaha was a great handling boat. ran 40 mph wide open at 6100 rpm, and cruised 30 mph at 5000 rpm. upgrade. replaced 15 year old twin 225 yamahas due to alarm issues and potential exhaust corrosion but they still ran and boat handled great. now new 2019 twin 300 suzuki’s and the boat handles terrible after about 25 mph at 4500 rpm and boat will max out 45 mph at 6000 rpm. but is very scary and steering all over the place. the props are 16 dia/17 pitch 3 blade. can anyone help

I'm having what seems to be the same senario. Here's my post about it:
https://www.greatgrady.com/threads/300-marlin-repower-with-suzuki-300s.28018/

Anyhow, since my original post, I've move the engines up one bolt during the 20 hour service. They said the anti-cavitation was about an 1/8" below the hull at the second hole, and is now about 1/8" above on the third hole. Boat is getting better number, topping out at 5750 at 44 mph. I also had the steering lines bleed and that did get rid of some slop in the steering.

I'm running the boat with higher trim now, much closer to 40%. Also I didn't lose any trim coming up to the third hole, and can run at 50% easily. The handling is a bit better, but mostly It feels like the boat is jiggly in the rear. I'm almost certain its the toe alignment. According to the dealer, they set it to 0. Once I get the proper toe-in and the engines are aligned with the water flow I suspect the jiggling will go away.

 

[Exocet]

Your engines are too low. Move them up another hole.

 

[Mr.crab]

I was told anything over 400 hp needed two cylinders,