# Tunnel hull Engine Height.



## ryan_riggle09 (Jun 14, 2018)

I know that this has been brought up before in the past but I am looking to get some help. Just re powered my Pathfinder 15t with a new Suzuki and trying to get motor height issues resolved. From what I have gathered I think the motor may be to low, when i get on plane i am getting alot of spray on the back of the boat and cowling. The spray is shooting out of the sides coming from the front of the lower unit. If I trim high enough for the spray to go away, the motor sounds like it is starting to cavitate. Also going along with that i have gotten some porpoising and used trim tabs to correct that. From what i have seen researching the topic, the symptoms seem to say the motor is to low. But I am also making a big change I went from a lighter 2 stroke to a much heavier 4 stroke. Just for info i am running a 14p 4 blade that is 11.5 diameter, and not running jack plate. If anyone could give some advice on proper height for tunnel hull, maybe even some pictures of a similar setup so i could have a reference point to where my motor should be i would really appreciate it. 

thank you in advance.


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## Guest (Apr 18, 2019)

Keep raising it until she blows out then drop it down a hole.


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## Guest (Apr 18, 2019)

Starting point should be cavitation plate at or slightly above the top of tunnel.


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## ryan_riggle09 (Jun 14, 2018)

Just making sure I am doing this right, when checking height boat should be close to level, and get the motor where cavitation plate is level? Not trimmed all the way down like I would before I would i was trying to get on plane.


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## devrep (Feb 22, 2009)

stupid question but did you put a long shaft motor on a short shaft transom?


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## ryan_riggle09 (Jun 14, 2018)

The Suzuki 50 only comes in 20 inch shaft. This is about propeller height with boat level and cavitation plate level as well


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## ryan_riggle09 (Jun 14, 2018)

View attachment 70186
Different angle


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## ryan_riggle09 (Jun 14, 2018)

Level on center of tunnel, cavitation plate is just under it


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## Guest (Apr 18, 2019)

Looks close, raise it a hole at a time until it cavitates then drop it a hole.


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## ryan_riggle09 (Jun 14, 2018)

View attachment 70190


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## ryan_riggle09 (Jun 14, 2018)

10 4, y’all had me scared for a min with the short shaft question. Haha


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## Mike C (Feb 27, 2017)

I have a Tailfisher with a 60TLR 4 stroke Yamaha.
No jack plate
My cavitation plate is above the tunnel. 
Also, mine planes much quicker if I start with the motor upright. If I tuck it, it takes three times longer to get on plane.
I leave my trim tabs up almost all the time. 

Disclaimer, I do have a 24v trolling motor and two PC 1500 batteries up front.


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## zthomas (Jan 14, 2014)

I think you can probably get away with raising it two holes. My 17T with a 4-blade runs best in the second-to-highest position.

Also, which prop is that specifically? I'm running a PT SCD, which isn't an especially grippy prop. If that's a PT SWC, you might be able to go even higher.


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## ryan_riggle09 (Jun 14, 2018)

It’s actually a solas prop, I tried powertech swc3 and due to diameter I had a lot of slippage and was getting way to high of rpm. The solas is what my mechanic recommended and it was also the largest diameter shelf prop I could find. It’s a 11.5 diameter 14 pitch. Box describes prop as great stern lift, handling, and speed/efficiency in the engines mid range.


Next time I get a chance to run it I will try with motor upright thanks for the advice. I have 18 gallon gas tank up front only thing in the back is cranking battery. I don’t have trolling motor at the moment.


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## zthomas (Jan 14, 2014)

I'd raise it two holes and give it a shot. On takeoff, I find quickest to plane is tabs down about halfway and propshaft horizontal. That transom is angled so much that with the motor tucked all the way in, it's almost like you're trying to lift the stern vertically at the expense of forward thrust.


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## ryan_riggle09 (Jun 14, 2018)

Okay thanks for the info, I appreciate it I’ll let y’all know results when I get it dialed in.


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## ryan_riggle09 (Jun 14, 2018)

One last question when taking off with motor level, I would assume once your on plane there isn’t trim adjustment because motor is already where it needs to be?


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## Mike C (Feb 27, 2017)

Mine is usually about two bumps on the trim from plane to running


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## devrep (Feb 22, 2009)

usually the tunnel puts







up a bulge of water. I would think you would want the prop shaft near the top of the tunnel. Mine is a different design but I can run with the prop almost halfway out of the water, which is where the SWC4 really shines. I have my motor all the way up on the jack plate and drilled new mounting holes in the jack plate to get it mounted higher. I'm not even using the standard bolt pattern. The Hatsu 50 has drain holes all over the lower unit that take in water also, don't know if the Suzuki will cool at that height.


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## devrep (Feb 22, 2009)

here's a vid of a 15T running on plane, he's got the motor pretty high up. its all about the cooling intake and prop cup though.


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## Mike C (Feb 27, 2017)

I have a SCD4 on mine that I had Admiral add cupping to.
I've looked back there while on plane and the cavitation plate is at least 2-3 inches above the water. 
You can hear the prop chopping the surface


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## Capnredfish (Feb 1, 2012)

Not so sure you need a stern lifting prop either.


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## Smackdaddy53 (Dec 31, 2012)

The higher the better


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## zthomas (Jan 14, 2014)

devrep said:


> here's a vid of a 15T running on plane, he's got the motor pretty high up. its all about the cooling intake and prop cup though.


Save the baby seagrasses!

Seriously, it's a little bit different animal without a jackplate. Yes, you _can_ run that high, but do you really want to be stuck there all the time? Without a plate, it's more a matter of finding the sweet spot between minimal running draft and optimal handling and efficiency.


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## Vertigo (Jun 3, 2012)

A tunnel hull without a jack plate gives up a lot of performance. Tunnels have little or possibly negative effect from a still start and at idle speeds. The motor will need to be lower at these times. Tunnels come into their own at speed on plane, at which time the flow of water thru the tunnel can allow the motor to be raised as much as 6". It might cost a thousand or so, but lots of tunnel hull trim problems can be solved with a jack plate.


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## noeettica (Sep 23, 2007)

Great thread ! Anyone running a "Compression Plate" ? if so What is your set back and height in relation to the "Tunnel" ?


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## devrep (Feb 22, 2009)

I sold my shaw wing compression plate. I ran with and without it several times and couldn't tell any difference.


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## ryan_riggle09 (Jun 14, 2018)

Ran boat with mechanic today and looking back when you go to plane it’s creating a air bubble and cavitating bad. Once you get it to plane cavitation plate is about 1 inch above water line, but prop is coming out of water. If you turn at all it blows out. Mechanic suggest jackplate to have more motor set back to hopefully get cleaner water and then the ability to finely tune and adjust motor. I would love to get hydraulic jack plate but just spend 7k on motor and not looking to drop another grand on that, so looking like I will get manual jack plate for now at least.


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## Smackdaddy53 (Dec 31, 2012)

You need a good prop and that cavitation will be nonexistent.


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## ryan_riggle09 (Jun 14, 2018)

This whole thing is just blowing my mind because the mercury I had didn’t have any issue and it had a standard powertech that wasn’t heavily cupped or anything. And now I’m having issues


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## Smackdaddy53 (Dec 31, 2012)

ryan_riggle09 said:


> This whole thing is just blowing my mind because the mercury I had didn’t have any issue and it had a standard powertech that wasn’t heavily cupped or anything. And now I’m having issues


Is your motor too low and causing the water to go over the top and pulling the stern down? 
Mechanics don’t always know everything but they like to tell you things and take your money.


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## Smackdaddy53 (Dec 31, 2012)

Judging from the photos you sent me your motor is mounted too low and the apron of water coming out of the tunnel is actually going over the top of your lower unit cavitation plate causing an air bubble behind it and the prop isn’t really a surface piercing prop so it’s cavitating and blowing out. I bet if you raise that motor to where the bullet is about even with the top of the tunnel when trimmed flat you will lose a lot of that cavitation.


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## Finsleft258 (Oct 7, 2018)

I'm assuming that's a Saturn 4 you are running. That prop has zero cup. As others have mentioned, the engine mounting needs to be high on the tunnel. You're also not propped correctly to bite at high elevation when the prop enters a ventilating condition.


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## ryan_riggle09 (Jun 14, 2018)

its a solas rubex c4, i also have a powertech swc3 that has plenty of cup but it lacks diameter where is is only 10 3/4, the solas is 11 1/2 inches diameter. Besides going to a custom built prop all i could do is get cup added to the solas.


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## Finsleft258 (Oct 7, 2018)

ryan_riggle09 said:


> its a solas rubex c4, i also have a powertech swc3 that has plenty of cup but it lacks diameter where is is only 10 3/4, the solas is 11 1/2 inches diameter. Besides going to a custom built prop all i could do is get cup added to the solas.


I would do that first. The Solas props also tend to be very thin (hence why the price is less), which means less drag/bite.


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## ryan_riggle09 (Jun 14, 2018)

Finsleft258 said:


> I would do that first. The Solas props also tend to be very thin (hence why the price is less), which means less drag/bite.


being that its thin, is that going to make it harder for prop guy to add cup?


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## Finsleft258 (Oct 7, 2018)

ryan_riggle09 said:


> being that its thin, is that going to make it harder for prop guy to add cup?


No.


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## ryan_riggle09 (Jun 14, 2018)

Well learning the hard way I am doing what I should have done in the first place. I mailed my check out today to jack foreman to get me a prop, can’t wait to get it and see the results.


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## Smackdaddy53 (Dec 31, 2012)

Finsleft258 said:


> No.


Bwahaha any real cup won’t last long on a thin blade like that. Glad he got a Foreman, they are thick and built to last.


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## No Bait / Lures Only (Apr 10, 2011)

ryan_riggle09 said:


> Ran boat with mechanic today and looking back when you go to plane it’s creating a air bubble and cavitating bad. Once you get it to plane cavitation plate is about 1 inch above water line, but prop is coming out of water. If you turn at all it blows out. Mechanic suggest jackplate to have more motor set back to hopefully get cleaner water and then the ability to finely tune and adjust motor. I would love to get hydraulic jack plate but just spend 7k on motor and not looking to drop another grand on that, so looking like I will get manual jack plate for now at least.


View Vance in Tennessee for Manual jack


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## crboggs (Mar 30, 2015)

ryan_riggle09 said:


> I mailed my check out today to jack foreman to get me a prop, can’t wait to get it and see the results.


Smart man.


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## Finsleft258 (Oct 7, 2018)

Smackdaddy53 said:


> Bwahaha any real cup won’t last long on a thin blade like that. Glad he got a Foreman, they are thick and built to last.


Agreed. The Solas is damn near disposable. It is half the price, however.


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## Smackdaddy53 (Dec 31, 2012)

Finsleft258 said:


> Agreed. The Solas is damn near disposable. It is half the price, however.


Because you’ll wear three of them out before you even begin to wear out a heavy bladed prop like Foreman builds. I have had two and the first one is still running in Louisiana over 6 years after I bought it. My current Foreman has at least 200 hours on it and looks new. It might be from how high I run my motor but still...


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## ryan_riggle09 (Jun 14, 2018)

Big girl came in today, can’t Wait to get it on and see how she does.


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## ryan_riggle09 (Jun 14, 2018)

Got her put on gotta get some washers so that castle nut lines up, picked it up from new platform too.


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## ryan_riggle09 (Jun 14, 2018)

Took it out today, Great hole shot, turning and ability to run elevated everything you would expect form foreman prop. Hit pitch and diameter perfect as well wot I was running right at 6200 to 6300 which is on the upper range of my max. Max wot for my motor is 5300 to 6300 rpms. Lost about 2 mph but I am okay with that wot I’m running 30 with one person and cruise about 24.


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## ryan_riggle09 (Jun 14, 2018)

Got motor height dialed in with prop, boat runs great. But on to the next thing ha, always something else. I am considering cavitation plate, when i trim up on plane i have to use tabs to keep from porpoising. Also the idea of even more hole shot and better turning would be great as well. I was looking at bobs true tracker, as i can get one already color matched but I am open to suggestions. My real question is with the prop, I am already so close to the factory caviation plate will i be able to clear adding a larger plate.


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## Smackdaddy53 (Dec 31, 2012)

Tran compression plate


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## ryan_riggle09 (Jun 14, 2018)

do they have a website?


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## Smackdaddy53 (Dec 31, 2012)

ryan_riggle09 said:


> do they have a website?


They don’t have one for their compression plates that I know of but quite a few on here have their plates. I’ll text the shop number.


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## Zika (Aug 6, 2015)

Respect Mack's recommendation on the Tran. I've had two of the Bob's True Tracker plates and they work well and the powder-coating option is nice. Reasonably priced. It was close when I had the Power-Pole mounted afterwards, but engine clears. Better hole shot, lower rpms at cruise, prop stays hooked up. Glad I added mine. 

Current mount is on a BT Mosquito, by the way, not a tunnel hull.


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