Motor need to be raised?

[mjr5253]

I'm trying to convert my 13.5' deep V Sea Nymph boat to a stick steering setup. I just got a 25 hp mercury tracker, and the boats rated for 25 hp. Before I drilled holes through my transom to mount the motor, I wanted to test the motor with the boat and make sure it worked ok. So I put the motor on the back and just tightened the top screws and left the bottom hang and took it to the river. Unfortunately from about 3/4 to full throttle, the boat almost seems to drag, and it shoots water out around both sides of the boat pretty bad, starting about a quarter of the way back from the front of the boat and shooting clear past the transom. Its almost like its shooting around the ribbing (you can see what Im talking about in pictures 45 and 48, they run the length of the boat. ) The boat seemed to be getting on plane (or **** close). As you can see from the pictures, my transom dips down in the middle. Also, I'm aware that the motor is about 6" to long, but it's all I have to work with.

So my question is, has anyone had this problem and is there a way to fix it? Any Ideas would be appreciated..

Would rebuilding my transom so it's flat (therefore raising the motor) help? If I did this the cavitation plate would only be about 2" below the bottom of the boat.

Also I forgot to play with the trim, even though it was all the way down when i tested it. Would trimming up do anything?

Could it be a weight distribution problem? When I tested it it was me and a battery in the back, my 200 lb brother in the middle and a 5 gallon gas tank in the middle.

If the pictures dont show up use these links...
https://www.tinboats.net/forum/images/albums/userpics/24787/IMG_0048.JPG
https://www.tinboats.net/forum/images/albums/userpics/24787/IMG_0047.JPG
https://www.tinboats.net/forum/images/albums/userpics/24787/IMG_0046.JPG
https://www.tinboats.net/forum/images/albums/userpics/24787/IMG_0045.JPG

 

[earl60446]

That motor is too long for the boat. A jack plate would probably help, same goes for building the transom up some. I think a 25hp on a 13.5 ft boat should get up on plane even though the motor seems 6 inches too long. I guess better too long than too short though.
Tim

 

[TNtroller]

IMO, the 25 should make that hull fly, but it's apparently a heavy hull. Going to stick steer will be a major change in weight distribution but the cav plate is causing a lot of drag being a long shaft motor on a short shaft hull.

You could build a "riser" from some aluminum plate with plastic cutting boards as filler material and bolt it all on the transom, you will just need to figure out the height of the riser to get the cav plate even with the drain hole on the transom or above it.

the tilt/trim will affect how the boat runs, so you need to consider that into the mix as well.

 

[srr5008]

Just to add to the original post (I'm the slightly-heavier-than-200-pound brother that was along for the ride). The water was splashing up from the bottom of the hull and hitting the stringers running the length of the boat. This was throwing water out 8-10 feet from the boat, and about 5 feet above the river surface. I moved further up the boat and it got worse. The nose of the boat felt like it dropped like it was on plane, but it definitely felt like it was plowing water.

My thought was that if we re-did the transom, making it flat across the top, that would raise the motor up at least 4 inches, and help the problem. But that's pure speculation on my part, so we figured we'd get some opinions.

We also noticed that as soon as we started to plane, the resistance we got on the "tiller" got pretty strong. We were really fighting with it to keep the boat going in a straight line (wanted to pull right). As soon as you backed down to 3/4 throttle, it steered normally. Could this be related to the motor being mounted too low as well?

 

[lckstckn2smknbrls]

About the steering issue, on the underside of the anti-cavitation plate is a small fin. This fin might need to be adjusted a little, loosen the bold holding it on and rotate the fin a little.

 

[Djknyork]

Raise the transom/motor like you plan and it will be faster, drier and easier to steer. How much of each will vary but your plan will be a definite improvement. If it were my host I wouldn't hesitate to do it.
When I had a prop I jacked mine up to where the anti cavitation plate was about a 1/2 inch above the bottom and picked up a lot of speed and hole shot (when trimmed right).
Good luck & let us know how you make out if you decide to go that route

 

[TNtroller]

Might need to try adjusting the trim position as well, what hole is the pin in when the test runs were done??

 

[huntinfool]

As stated the motor needs to be raised until the cavitation plate is flush to 1" above the bottom of the hull. Then you may need to change the tilt position also, its just a trial and error thing. But raising the motor is a must. There are riser plates made that will do the job. Or you can rebuild the transom. The good thing about the riser plates is that if you raise the motor and it still needs to be raised a bit more, you just unbolt and raise it. A permanently built rise on a transom only gets one chance. JMHO.

 

[PATRIOT]

Not to be picky but, what people mean to say is, the cavitation plate should be 30-50mm BELOW the bottom of the boat, seeing that its sole purpose is to prevent air infiltration around the prop.
I'm running a somewhat newer version of the same hull.

https://www.nissanmarine.com/tech_talk/pdf_files/Nis9-9B_12C_15B_18C.pdf

 

[SumDumGuy]

it's an anti-ventilation plate. It is designed to try and help eliminate air getting to the propeller.
It is optimally set where it skims just at to just below the surface of the water. (Of course if too high you may ventilate in sharp turns).

cavitation is something else entirely.


Sad times when the fellas at Nissan can't even get it right in their manual.

 

[turbotodd]

You want the motor as high as it can be without blow out and without losing water pressure. Some motors can run REALLY high. Dependent on the hull, prop, and a bunch of other things. Most normal props won't surface very well; however on a high performance setup like in the video, it can (and will) run about 50% out of the water. Skip to about 2:00 into the vid and watch closely. You can easily see the prop spinning and about half or more of the bullet up out of the water. Keep in mind that most of us with a tin boat won't be able to run quite that high but you get the idea. That rig in the video is running about 96 mph at full tilt.

The higher it'll run without blowing out and/or losing water pressure, the less drag there is and the faster/more efficient the rig will be at speed.

https://www.youtube.com/watch?v=PZm11iP0EUU