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boat porpoising

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earlbuddy

Regular Contributor
I have a 1960 Starcraft Jupiter with an '66 80hp Evinrude. When I hit approx. 3/4 throttle the front end starts to go side to side and I have to throttle down to maintain control. Picture shows how it's sitting now. Prop say 14p and is approx. 9" in diameter. I appreciate any help. Thanks



Boat.jpg
 
Usually that type problem shows its ugly head right at the point when the boat is becoming airborne.

Is the keel on that boat straight and true... no hooks or bulges?

The engine/cavitation plate looks to be about perfect... you might try placing the angle setting rod in more hole just to see what effect that has... BUT... that will probably cause the bow to plow which of course is something you want to avoid.

How fast is that rig moving at the 3/4 throttle setting you mention?

Does the problem change any with a heavy load... 4 or 5 people?
 
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If it's going side to side, it's not porpoising... that's an up and down motion. Check and see if the steering cables are tight, and not letting the motor turn a bit by itself.
 
If I'm right, and we are talking about the same boat, it's rated for an 85, so it's not overpowered. Looking at the pic again, I think the motor may be trimmed in too much.. I'd try raising it one hole and see what happens.
 
Not sunk and not fishing either. LOL. I checked the gear oil and it's milky so got water in it. What seals should I replace? 1966 Evinrude 80hp Starflite.
 
I think your boat may be chine walking. First thing is make sure there is no slop in the steering. Then google chine walking and read up on what it is and how to handle it. What I know about it is that it will scare the #@*& out of you. Besides that, it is dangerous.
 
thats not a whale tail on that motor but there is some type of tail bolted on it...i would remove it and try it...
 
A tail won't affect the boat if it's chine walking, which I really doubt. You say the bow goes side to side. Can you give a better description than that? Lets start with a couple of definitions.. Porpoising is the bow going up and down, in a pretty steady motion. Plowing, the bow is trying to go underwater. Chine walking, the boat is rocking side to side. That hull won't chine walk, you need to have a V hull to get that, and the Jupiter has a flat profile at the stern.
 
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bill_w said:
You say the bow goes side to side. Can you give a better description than that? Lets start with a couple of definitions.. Porpoising is the bow going up and down, in a pretty steady motion. Plowing, the bow is trying to go underwater.
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Splendid suggestion.
 
one other question then...is the boat pulling left or right or running straight without torque on the wheel when you are up on a plane and about 500 rpms before the problem occurs?is the boat new to you?...if not did the previous owner have the problem?...probably a waste of time to ask him though as he will say no....is it the same motor that has been on the boat for a while?
 
papyson said:
one other question then...is the boat pulling left or right or running straight without torque on the wheel when you are up on a plane and about 500 rpms before the problem occurs?is the boat new to you?...if not did the previous owner have the problem?...probably a waste of time to ask him though as he will say no....is it the same motor that has been on the boat for a while?
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If he answers your questions papy, perhaps he'll take the trouble to answer mine also (post reply #2).
 
saw that joe but re asked anyway...i dont think we know whats actually happening with the boat...i have never saw a whale tail on the market built like that either...if that boat is trying to come out of the water and fly there is no telling what will happen...
 
I edited that lower unit and see nothing attached to it (below). I wonder if the lower motor mounts could be broken and if so, that engine would move quite a bit within the housing... in turn changing the angle of attack.
V4a.jpg
 
earlbuddy stopped replying here and went to private messaging me...he now has the motor off the boat to replace the transom...i am outa here on this problem...
 
I didn't reply because with all your responses I was able to trim it out and it rides fine now. I appreciate your help papyson
 
earlbuddy said:
Why are you guys being this way? I'm just asking questions because I'm new to this.
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No offense intended on my part and I've still been interested in this problem and its cure... however none of my questions were answered which leads me to feel ignored (See My Tag Line), then you left the board to go the PM route, essentially choosing one member over all others. Like I said... no offense on my part, just telling you how that came across to us. Hopefully, in the future, your engine problems are solved in a smoother manner.
 
Ok, I understand. I guess I thought some of my questions made me sound stupid so I thought using the PM route was better. I guess I should have remembered there is no such thing as a stupid question and that others maybe having the same issues. I'll post all my questions on the open forum from now on. I'll post what I did and my results following this post. Thanks
 
Sorry I forgot to response with my results. I appreciate all your help guys.
The boat was chine walking like was described above, so I trimmed it out one hole(middle hole of 5) and tightened the steering cable. Now she cuts through the water smoothly at full throttle. I removed the lower end and pressure tested it and found it was leaking around the shift cable assembly. Someone before me must have removed it and it wasn't pulled through enough to make it seal. I don't think these are made to take out and put back in but with some effort I got it back in. After having it out on the lake the other day I then drained the gear oil(Type C) and no signs of water. It's a blast to be on the water especially with a classic boat. I'm getting a lot of looks and compliments Thanks

Boat-1.jpgBoat-2.jpgBoat-3.jpgBoat-4.jpg
 
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Boat-5.jpgBoat-6.jpg Here's how I have the 1960 80hp Starflite trimmed on my 1960 Starcraft Juptier.
 

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