Fish-Hawk.Net

Porpoising is caused by the prop losing his grip, failing to keep the bow higher. First start with playing with the engine height. How's your AV plate in relation to the bottom of the keel (both being measured level)? A good starting point is to have the AV plate 1.5 inch higher than the bottom of the keel...

CN I originally took the boat in to have them raise it higher on the transom but what I was told is that the mechanic said it would just cavitate if they raised it any higher than it was at that point. We dropped it down to see if that would make any difference and it didn't and started side spraying like crazy.

Brian

There's something wrong with the initial setup, don't buy anymore props or a hydrofoil yet..

Let's start with the basics, what type of boat do you have and what kind of engine/hp.
Next you'll need to measure the prop to pad.
From top to bottom.
1. Leveling the boat using a level on the pad.
2. Measuring the distance from the pad to a level concrete floor.
3. Leveling the engine. (Use the trim button and move the engine, not the boat)
4. Measuring the distance from the center of the prop shaft to the floor. Now just subtract the prop shaft measurement from the pad measurement. For my set-up, the prop is 2.75" below the pad.
5. This picture shows that on my boat, I can still raise the engine .75" on the jackplate by removing the mounting bolts and raising the engine on the plate.

Follow this link if you need pictures to help you.
http://www.bbcboards.net/showthread.php?t=157208


Once you have all that we can move on.

So I took the measurements from the pad to the ground at level was 12 3/4 inches and from the center of the prop shaft the the ground was only 5 1/4 which gives us 7 1/2 inches which is ridiculously low even I know that. The boat is a 1990 bayliner cobra 1804 with a 2001 Mercury 135 optimax DFI. After seeing how low it was I brought back to the shop to have it raised 2 holes on the transom.

Brian

Even raising it up two holes isn't going to be enough.

Can you confirm the shaft length is the same between your old motor and the new one?

at 7.5", I am thinking you may have bought a 25" shaft opti and the black max was a 20". Just a thought. And If I am correct, it doesn't matter where you mount the motor on the transom, it will be too far down.

Lape it is definitely a 20 we ran the serial numbers on it and came out as a 20 i think that its a bigger gear case than the old motor that it just has a different set up than the old one. I just got it back they raised it 2 holes and it is working a lot better getting about 5300 rpm and 46 MPH wot. I get a little trim on it now but not a whole lot. So for now I'm happy I can use it now. A dealer is going to let me try a different prop tomorrow so I'll give that a shot and see if we can only get better. Appreciate all the help from everyone I'll try it tomorrow with the new prop and see what happens and will post a follow up.

Brian

Brian, I know a fella that has a jackplate for sale, 150$, would solve your issue.
All you'd need to make sure is your transom is solid and you've got enough length in your throttle cable that you can move the outboard 6" back.

With a Jackplate you'll be able to move it up and down wherever you need it. set it and forget it.

Let me know I can check with Pete if he still has it.

ShawnD

ShawnD wrote:Brian, I know a fella that has a jackplate for sale, 150$, would solve your issue.
All you'd need to make sure is your transom is solid and you've got enough length in your throttle cable that you can move the outboard 6" back.

With a Jackplate you'll be able to move it up and down wherever you need it. set it and forget it.

Let me know I can check with Pete if he still has it.

ShawnD

Attaboy!

I would second that.

It would save all of these trips to the marina too!

Your max RPMs are supposed to be 5600 so you've got a little room to play around. a jackplate may make all of the difference in the world and if you still have room, a prop can make up the rest.

Shawn could you see if he still has it or not. is the pete you are talking about Pete Mcormack (definitely spelt that wrong). I'm currently on vacation and not in ottawa until the first week of august.

Brian

ShawnD wrote:Brian, I know a fella that has a jackplate for sale, 150$, would solve your issue.
All you'd need to make sure is your transom is solid and you've got enough length in your throttle cable that you can move the outboard 6" back.

With a Jackplate you'll be able to move it up and down wherever you need it. set it and forget it.

Let me know I can check with Pete if he still has it.

ShawnD

Jack plate could help but depending on the size/setback could also hurt. what is the size? 4" 6" 8" 10" 12". the bigger the number the more stress on the transom.

CN wrote:

ShawnD wrote:Brian, I know a fella that has a jackplate for sale, 150$, would solve your issue.
All you'd need to make sure is your transom is solid and you've got enough length in your throttle cable that you can move the outboard 6" back.

With a Jackplate you'll be able to move it up and down wherever you need it. set it and forget it.

Let me know I can check with Pete if he still has it.

ShawnD

Jack plate could help but depending on the size/setback could also hurt. what is the size? 4" 6" 8" 10" 12". the bigger the number the more stress on the transom.

Clearly wrote that the transom must be in good condition, it's a 6" Jackplate.
We've identified the issue, the jackplate will let him fine tune the engine height and won't hurt anything..


Brian
Yes I am talking about Pete Maccormack, he purchased it from me, but turns out the throttle cable was too short and decided not to use it.
He had it for sale on Kijiji for 150$

Send him a message on facebook or call him at Pirate Cove Marina, he works there now.

Good luck and keep us up to date on what you do.

Thanks

ShawnD

If you have a forward livewell, fill it with water so you can at least enjoy your time away. Other wise throw a couple jugs of water up front to drop your front end down. It is cheaper and quicker, and when your holiday is over, sort out your cavitation issues.

Best of luck

Any update on this issue? There's a lot of good info in this thread but it would be nice to have a "final result" post.....

So we brought it up another hole on the transom and are currently using a 4 blade aluminum prop. Both have helped but I'm sure that the motor definitely needs a 3 blade as there is signs of cavitation on the aluminum prop (Paint is peeled in spots) As of now we are sticking with the 4 blade prop and looking for a SS 3 blade prop. As Shawn said a manual jack plate would be the best solution but it just isn't in the budget right now so we are currently sticking with what has been basically working.

Brian