power cat project

[aitkenkingfish]

I have just finished re.habing a 2001 custom cat. I rigged the boat with twin 65 evinrudes & am disapointed with the performance. I have to keep the motors above 4200 rpm to stay on plane. running into a heavy wind will lift the boat & I notice performance gains. This leads me to belive I am not reaching the corect speed to compress enough air to reach the sweet spot of air klift in the tunnel. I will be repowering with a single 225 evinrude 20" on a 6" setback jackplate with 6" of vertical travel. Im planing on building a swim platform/outboard bracket out of 3/4" ply 3 layers thick with a 31" set back. My question is motor higth the outboard is coming off of anouther cat a skatter cat the hulls are close to the same design however his tunnel is 8" deep mine is 16" deep. the motor is set up where the center of the driveshaft is even with the botom of his sponsoons. I am conserned with a 31" set back this will be too low, is there a way to figure out the mounting highth for the motor? the yellow haul is the one im working on the white haul is donor.
I can take mesurments & pics if needed

[Jimboat]

aitkenkingfish - all performance hulls, and in particular, tunnel hulls, are very sensitive to power and weight. You will find with your hull, i expect, that 2X65hp engines will not give sufficient power to overcome the additional weight of the two motors, and still give the kind of performance that you would normally expect of a well setup tunnel boat. The dynamic CofG of the hull is also somewhat altered in such a setup.

When you go to a single engine, you'll see better performance. Motor height (and also setback) are setup issues that are best fine-tuned with some testing, unless you are familiar with the performance and design of your hull. You should be able to set the 'torpedo' of the lower unit close to the height of sponson bottoms for good performance, but you'll have to make sure that you are getting sufficient water pressure to ensure proper engine cooling.

[Tague]

as a basic rule of thumb, you can go up 1 inch for every 12 inches of setback

Tague

[aitkenkingfish]

the main question since I'm setting it up with a swim platform motor mount I have no clue on how high to set the motor same as the originals or higher since the motor is running in the tunnel & the water height is higher in the tunnel. I will have 8" of adjustment with the jack plate but should I start the motor height off figuring the center of the drive shaft even with the bottom of the hulls?

[Tague]

You will never need to be 8 inches above the running surface of the bottom. There are two key components to your question. First is your water pickup. If you are using a low water pickup then you can use the prop shaft as a reference. If not you will use the cavitation plate as your guide. You can also run remote pickups to get the motor even higher like this




I think my motors were like 4 inches above the bottom, but I had the remote pickups. And this boat ran in the mid 120's

My guess on your application I would start 1.5 inches above the running surface and go up from there.

[aitkenkingfish]

thanks thats very helpfull info & pics. Im running a bobs nose cone. Do you have a side pic?

[aitkenkingfish]

oh I forgot wow what a sight. thats an awsome looking monster.

[Tague]

these are not great photos of the height but may be of some help.

[aitkenkingfish]

these do help some thanks. ill set it up with the jack plate in the middle with the prop shaft even with the bottom ill have 3" each direction with the hydraulics & anouthe 3 " up with mounting holes.

[Tague]

that sounds like the right idea to me. Good luck with it.

Tague

[u4ea32]

Tague, it looks like you did not have an adjustable jack plate, that these engines were simply bolted to exactly that location. Its that correct, or was there something all black not clear in the photos that was the adjustable jack plate.

I'm simply amazed that it would work without a hydraulic jack plate, but maybe its just that 600 HP on a fly weight hull allows speed to become SPEED!

Looks like fun!

I bet it took several times to get that setback just right?

[Tague]

these were fixed, and the setup was more in the props than the motor height,what I found on this boat was taking the height up to the limit and then working backward to find a prop that fit the setup. Most people do it completely backward. This boat was no lightweight it was designed as a twin from day one. The three knee's on the transom were like 3 1/5 inches think. I knew the cg was right, so I knew that i wouldn't need trim to carry the bow. I set the height so just the skeg and 1 blade was in the water at any one time. Then set the prop shaft in parallel with the running surface. Never touched the trim (never look for rpm in the trim button) and looked at the performance. It took 5 sets of props before I found the ticket.

that boat was an blast. I know it went to a good home in AZ.

Tague