custom 18' power cat

[aitkenkingfish]

I am working on repowering an 18' power cat. origional builder had twin 50 johnsons befor stripping the boat. I repowered with twin 65 evinrudes, turned out it was still not enough power. I am repowering with single 175 johnson mouted on a jack plate. Im trying to figure out at what highth I should mount the motor. I have been told to mount the center of the prop shaft with the botom of the sponsoons. I will be installing low water pick up if needed. ive include a few pics.

highth of the tunnel is 15 1/4"
width is 22 3/4"
motor is a 20"
thankyou for the help

[aitkenkingfish]

heres some pics of the finished transom

[anthony goodson]

Nice to see pictures of a finished project ,let us know how it works for you

[aitkenkingfish]

here she is in the water. im going to have to add a diverter to the center. Im getting alot of drag & water coming up the jack plate.
any ideas for a fix?

[jonr]

I'm curious about the advantages of a small tunnel like that - seems that wetted surface area would be the same or less if the bottom were flat.

[Jimboat]

Originally Posted by aitkenkingfish

Im getting alot of drag & water coming up the jack plate.

Your back-wash may be coming from low motor mount. Your engine is mounted very low in the water for a tunnel design. You could move it up considerably without hurting control, while reducing draft and improving performance - and back-wash will be reduced.

[aitkenkingfish]

"

Originally Posted by Jimboat

Your back-wash may be coming from low motor mount. Your engine is mounted very low in the water for a tunnel design. You could move it up considerably without hurting control, while reducing draft and improving performance - and back-wash will be reduced.

ive been told once on plane the tunnel will run dry? so the motor driveshaft is set centered with the bottom. what do you recomend? If you need more photos let me know the jackplate has a 10" set back but I can reduce it to 5.5" set back.

[aitkenkingfish]

well you were right I raised it 2 holes all the difference in the world. Im going to raise it 2 more holes 1 on plate 1 on motor. I havnt had to add the bobs low water pick up yet still running 15lbs of presure on the gauge.

props ive run so far
14 1/2 x 24 3000 rpm 28kt raker
15 1/2 x 14 4500 rpm 28mph oem s.s.
15 x 17 4200 28mph best for trimming boat out oem s.s

[aitkenkingfish]

heres the pics

[Jimboat]

Originally Posted by aitkenkingfish

well you were right I raised it 2 holes all the difference in the world.

aitkenkingfish - i'm glad you're seeing the improvements i expected. (amazing...engineering really works!)

suggest that you raise engine 1 hole at a time, or better yet, use a jack plate and raise in 1/4" increments, keeping record of RPM, max mph, visual spray results, water pressure, handling characteristics. this will help your setup and propeller selection choices.

watch your water pressure as you raise engine....if performance is still improving as engine is raised but water pressure drops, then LWP is required for continued performance improvements.

[aitkenkingfish]

should I be looking at 4 blade props to help with cavitation & trim?

[Jimboat]

Originally Posted by aitkenkingfish

should I be looking at 4 blade props to help with cavitation & trim?

not necessarily....best to test your props for best performance and keep records of your slip %.