Cavitation Plate Location

[blackshear]

The transom on my 2001 War Eagle 648 is 20" and I am running the new Yamaha F25 with long shaft. With the motor mounted directly on top of the transom the cavitaion plate is approximately 2" below the bottom of the boat. I want to raise it by building an aluminum bracket on top of the transom but I don't know what the best height is for an aluminum all welded hull. On the newer War Eagle boats they now have a 21" transom. Should I raise it 1" to match the new War Eagles or could I go more than 1" and get better performance? Right now I am running approximately 5700 WOT by myself and would like to get to the 6000/6100 range so that when I add another person it will not bog the motor down too much. Any ideas or experience with cavitation plate location on all welded aluminum jon boats would be appreciated.

 

[lovedr79]

my boat had a merc. long shaft, they are longer than the standard long shaft. this was on a tracker 1448 all welded grizzly. i bought a transom riser bracket that raised the motor up 3" or so, from a guy off here, it raised the cavitation plate to about 1" above the bottom of the boat. solved my porposing issue, picked up some speed and time to get on plane was reduced.

 

[Crazyboat]

That plate should skim the water surface while running on plane.

 

[nccatfisher]

You would be pushing the limit trying to peak 6K RPMs out of that motor.

 

[turbotodd]

You would be pushing the limit trying to peak 6K RPMs out of that motor.

Explain.

 

[Pappy]

What I can tell you about what you want to do is to raise the engine until you get prop ventilation and then lower it to a suitable height. There is no perfect height. Hull loading, load, load placement, propellers, all make significant differences. You are on the right track with your thinking about initial height though.
Secondly I can tell you that boat manufacturers are generally very conservative with engine heights because the engine manufacturers shaft lengths are all different. Keeps them from having to take boats back or pay to have them modified.
Remember also that if you do get it a little high you can always have the prop cupped some to minimize or eliminate blow out or ventilation.

 

[blackshear]

it is difficult for me to play with the height because the motor clamps are currently only about an 1.25” from the top edge of the transom. I don’t feel comfortable placing the motor that high without a decent riser bracket that is bolted to the transom. I may try and build something that is adjustable but I haven’t figured out how to achieve that yet. I plan on using two bolt holes from a previous motor that was mounted to the boat to secure the riser. The previous motor was much wider than the one I have now so I can pick up the old holes without interfering with current motor bracket. Once I get it all figured out I will post pictures. I’m thinking I can get by with a 1.5” riser that will place my cavitation plate about .50” below the bottom of my boat.

 

[Pappy]

That height is a no-brainer and will work just fine.

 

[blackshear]

I found this 1” riser that I built a few years ago for another boat but never used. I think it will do for a test run or two but I plan to build more robust bracket with 1.5” lift if this one works out.

 

[nccatfisher]

You would be pushing the limit trying to peak 6K RPMs out of that motor.

Explain.

If he is trying to set the motor right at the top of it's operating range for two people what is he going to do when he is running it solo on a choppy day etc? I know he could let off the throttle but it has been my experience that most don't. He will wind up bumping against the rev limiter.

 

[Pappy]

You may want to read that again.
The way I understand it is that he is currently running at around 5700 and would like to get closer to 6000 so that when he adds a second person the engine will "not bog". Of course the engine will not bog but his reasoning is sound.
Feel free to correct me if I am wrong in my understanding.

 

[blackshear]

I took my boat out for a test run today with the 1” riser. I gained about 100 RPM to approximately 5850 and gained about 0.5 mph to 27.8. I was hoping for a little higher RPM but maybe I will gain a few more by raising it another .50”. The boat didn’t come close to ventilating so I’m wondering if I can go up another .75” instead of the .50”. It is a real pain to get the parts made so I want to get it right the first time.

 

[blackshear]

You may want to read that again.
The way I understand it is that he is currently running at around 5700 and would like to get closer to 6000 so that when he adds a second person the engine will "not bog". Of course the engine will not bog but his reasoning is sound.
Feel free to correct me if I am wrong in my understanding.

You are correct. I’m trying to achieve 6000/6100 RPM With just me in the boat. That way when I add another person the motor will be under less strain. I’m getting closer after my test run today but still looking to gain few more more RPMS. I’m hoping that another .50” in transom height will get me where I want to be. My guess is the closer I get to the surface with my prop the more impact raising the motor will have on increasing my RPMS. Once I get the most out of the factory 11.25 pitch prop I plan to order an equivalent stainless prop, 10 pitch.

 

[nccatfisher]

I guess I just look at it wrong. I have one of those motors and my manuals says 6K RPMs is max. I am not going to set it where it is bumping against the max loaded heavy knowing that with a lighter load it is going to over rev if you twist it's tail. But that is just me.

 

[blackshear]

I guess I just look at it wrong. I have one of those motors and my manuals says 6K RPMs is max. I am not going to set it where it is bumping against the max loaded heavy knowing that with a lighter load it is going to over rev if you twist it's tail. But that is just me.

I completely understand your concern and appreciate you responding. I certainly don’t want to turn more than the recommend RPMs either and I am doing a lot of testing to make sure that my setup is correct. What boat are you running with your Yamaha and what kind of performance are you getting out of it?

 

[blackshear]

This is a view from my computer of my next transom riser bracket. It will be 1.6" tall which will place my cavitation plate around .40" below the bottom of the boat. This new bracket will also pick up the existing bolt holes for a more secure attachment. Hopefully this will get me closer to 6000 RPMs.

 

[Brian121804]

Perhaps make it a bit longer & put slots where you're existing holes are.

Then you can insert wood shims between the top of the transom
& the bracket until you're happy with the height.

Then bolt it up through the motor bracket holes.

 

[blackshear]

That is a nice looking 3D model and interesting concept as well. What software are you using?

 

[Brian121804]

Yeah, ghetto jack plate. I may do something
similar on mine, I have 1 1/2" of shim on it now. Think I could go up another inch, but my clamps
are at the top of my transom. I'm even with my
hull, and happy with performance, so it's not high
on my priority list.

I came across models and drawings for the F25
a while back, IIRC on a Yamaha corporate website.
Downloaded them for hell of it. I imagine it's what
they supply builders for layout. No internal components, just an external surface model.

I use Siemens NX (aka Unigraphics), doing CNC programming & injection mold design.

Edit: PM sent.

 

[blackshear]

That is so cool. I am running SolidWorks and mainly use it for sheet metal design of HVAC equipment.