Motor too low?

[78 GT-150]

A Newb question;

78 Glastron GT-150, 86 Merc I-6 90hp, Ballistic SS 19" prop. Boat has no "Pad" i have a Sting Ray hydrofoil on it.
Boat is 15 feet, dry weight of 625 lbs, O/B weighs 330-350 lbs. 2 6 gallon steel tanks.

The boat is rated for a 90hp (max) with a 20" shaft. How do I know what the shaft length is?

The boat pulls pretty decent out of the hole and planes in about 20 feet. It porpoises all the time (helps when it's trimmed down, but I loose a lot of speed) When trimmed for max speed I can get about 53mph then it starts to "Death wobble" violently! I still have about 1/4 throttle left. I am taching at about 4900 rpm.

I just bought the boat and this is my first experience with an outboard. The previous owner told me he thought the motor needed to be raised, but he never did it.

I would like to be able to flat out fly, but it gets very scary running above 50. My dad had the same boat with a 150 Johnson on his and was able to run in the 70's (supposedly) I was too young to remember.

I read on another thread that the prop C/L should be less than 6 inches below the bottom of the boat. Mine has got to be closer to 12"-15" (See pic)

I have 2 mounting holes above and below where it is mounted now, will that be enough?

Sorry about the lousy photos, I didn't take them and the boat is docked 20 miles away so I can't snap any new pics right now.

Sorry for the long post, but I need some help!

Thanks everyone!

 

[Frank Acampora]

Re: Motor too low?

The "Death Wobble" you are describing is Chine-walking. When a deep vee hull has more and more power put to it, as it speeds up on plane, it rises higher in the water. Eventually if there is enough power, it rises to the point where there is not enough bottom in the water to support it. It falls off to one side (chine) then rises and falls to the other side. This CAN be a feedback situation and can intensify to the point where you lose control. Most people get too scared before that point is reached though.

Porpoising occurs when the hull weight is biased too far back.

It may be counterintuitive, but adding more weight judiciously may help both situations. Remember, your dad's 150 probably weighed around 100 pounds more than your 90.

Move your gas tanks to under the seats if you can and then try the boat carrying a couple of passengers. Move them around to see best weight placement. You actually may be able to attain more speed with the extra weight.

Glastrons were always a light, fast hull, BUT: I suspect that with the 90 engine, 53 is about max for that hull. The extra weight will just make it feel safer.

And by the way: The James Bond GT 150 had an OMC 135 on the back and did 65 top speed. It was rigged specially for the jump.

 

[Dhadley]

Re: Motor too low?

Everything you describe indicates the motor is too low, especially if you're going to leave the fin on. That wouldn't be my first choice for a prop but I'd work the set up with what you have before spending $$ on another prop at this point.

That's a decent hull as is but if the bottom gets blueprinted it'll make a world of difference. Nice, fun ride!

 

[78 GT-150]

Re: Motor too low?

I thought it was chine walking at first, but it is fast and violent! I thought chine walking was more of a lean in either direction, then would become violent?

In this boat, it comes on w/o warning, it comes on so fast, I can barely get to the throttle to slow it down! There is no "working" up to it, everything is fine and then, it's like, holy crap hang on.

The steering rack has been replaced by the previous owner and it feels really tight (not warn out).

The porpoising is controllable and it isn't that big of a deal, just a couple clicks down on the trim and all is well. It's just the violent slam from side to side that frightens the crap out of me! Can I power out of that? I ask because it does this at about 2/3 to 3/4 throttle.

Should I raise the motor a few holes, or would I be better off with a jackplate? I read that Glastrons should have the motor mounted on the transom not a jackplate? I don't know..

Please understand that I have controlled boats (20'-24' that were set up correctly) well into the 90's, so I am not afraid of going fast, but the boats that I have been in at that speed, could do it all day driving with one hand!

Is it the nature of the beast (15' Glastron) or do I have the setup all wrong?

Thanks everyone! All replies welcome.

 

[78 GT-150]

Re: Motor too low?

Everything you describe indicates the motor is too low, especially if you're going to leave the fin on. That wouldn't be my first choice for a prop but I'd work the set up with what you have before spending $$ on another prop at this point.

That's a decent hull as is but if the bottom gets blueprinted it'll make a world of difference. Nice, fun ride!

Should I remove the 'foil? I like the fact that it jumps up on plane at 10mph..

I did get 2 other (aluminum) props with the sale of the boat, I have no idea what they are, should I change them out? P/O told me they were usable but "Sucked" lol

You lost me on the "Blueprinting" part... Help??

Thanks!

 

[Frank Acampora]

Re: Motor too low?

I missed the fact that you had a hydrofoil on the engine.

I seem to remember reading somewhere recently that Mercury had posted an advisory about hydrofoils. Apparently on some hulls they can cause a vicious snap roll. Mercury disclaims any warranty repair (like a broken anti-Ventilation plate) related to hydrofoil installation. YES! Try removing it!

Want to read the actual article? Try sending a PM to Karla45 She is the one who sent it to me.

 

[78 GT-150]

Re: Motor too low?

Thanks so much for your time and replies guys! I need all the help I can get!


I will send her a PM.

Thanks again everyone!

 

[Dhadley]

Re: Motor too low?

You lost me on the "Blueprinting" part... Help??


By that I mean to straighten the bottom - take the hook out. I did a GT150 years ago and it made a world of difference. Lots of work though.......

 

[78 GT-150]

Re: Motor too low?

Oh OK Thanks! If I were to go that far, I think I would just buy a newer boat!

 

[daveswaves]

Re: Motor too low?

I think your motor is too low and the hydrofoil is lifting the engine too high, hence the chine walk. The hydrofoil will magnify the chine walk as the foil alternately dips in and out of the water. Get rid of the hydrofoil. Raise the motor if you can, a lower rake prop will give you less lift, a high rake one will make the problem worse.

 

[hwsiii]

Re: Motor too low?

You seem to have 3 problems GT.

1. Chine walking, that is absolutely exacerbated by the hydrafoil as it picks the stern up in the middle of the boat and has a tendency to let the boat fall side to side as circumstances change with the boat. Think of a seesaw and how it is the center of a pivot point, that is exactly what a hydrofoil does and how it reacts, and that is why the reaction starts so quick and is so violent and it wasn't that way on the other boats you drove. On some motors you can remove the diffuser ring and it will help stop the chine walking, depending on the circumstances.

2. Porpoising is actually caused by the center of lift on the boat being moved back and forth from the longitudinal center of gravity of the boat, a lot like chine walking except it is longitudinal instead of lateral. On some boats if you throttle up slower when you come on plane it will slow the effect down a lot, but on others it doesn't matter if you do it slow or fast it still happens. It is a basic instability in the boat that can be caused by many reasons, including hull features as Dhadley had with his hull, but usually it is caused by weight placement in the boat as Frank says. Moving some weight forward or backwards some of the time can totally cure the problem, but not always. This problem can be exacerbated at times by using a prop that has some rake and blade surface area but not enough to lift the bow and hold it, and at other times it is best to use a zero rake prop that lifts the stern only and thus eliminates the tendency of the boat to porpoise because it keeps the center of lift on the boat in the stern and it never passes the center of gravity farther forward in the boat. On some boats you can give it more speed and it will actually stop porpoising, because as the RPM increase the prop is able to provide more lift because it is turning faster, that usually means that you can use a higher rake prop to cure the problem. Brett Anderson and I have had this discussion before and I still contend that on some hull forms you need a stern lifter and not a higher rake bow lifting prop.

Cures

1. Get rid of the hydrafoil, it just causes problems and normally eats a little top speed, and you don't need it. Buy a set of Smart Tabs SX cheap, they are placed as close as possible to the starboard and port chines, which is the exact opposite of the hydrafoil. As the boat tries to fall to the port or starboard side they are there to support the hull to stop the chine walking. Think of the same seesaw, except this time it is supported on the very ends of the board instead of the middle, now it supports the ends and therefeore it increases stability instead of increasing instability.

2. Tabs also will almost always (95%) cure porpoising as they help hold the stern up and will not let a higher raked prop pick the bow up anymore than it has the power to do, instead of picking it up and then losing lift and letting it fall back down. And when you have a prop that has enough rake in it the tabs will not stop the bow from lifting and you have the best of all worlds. The stern is lifted by the tabs and the bow is lifted by the prop, so in effect the whole boat is lifted more theoretically.

3. Your motor needs to be as high on the transom as it can be without ventilating, as this creates the least drag possible from your lower unit and thus gives you an increase in RPM and speed. You need to move the motor up the last two notches and then ask Dhadley which prop to get for maximum performance.


H

 

[78 GT-150]

Re: Motor too low?

WOW! Lots of info in this post, thanks HWSIII!

I have been looking into the smart tabs, they just sounded too good to be true, but after reading your post, it sounds like a good place for me to start.

The first thing I am going to do is yank the whale tail...

Thanks for all the help guys!

Feel free to comment!

 

[indy440]

Re: Motor too low?

Im going to watch this post, im interested to see what you figure out...

 

[78 GT-150]

Re: Motor too low?

LOL, me too!