Porpoising. What should I do?

[RageWelding]

The wrong prop can also cause porposing.

 

[Thunder21]

the curiosity in me wishes you would've raised the motor first, tried it to see the results and then added the fin if necessary.

We ran out of room to raise it any further so I think it would have still porpoised . In the future we may build up the transom a bit and raise it even more. It’s still low but the fin cured the porpoising.

 

[Stosh]

Start by installing a hydrofoil.

 

[beemer]

I can absolutely guarantee you that the motor is too low... if it’s too low with the trim half way up why are we still having the discussion? The video clearly shows the cavitation plate buried under the water. Also take into consideration that keel shape and deadrise at the stern can play a part in the trajectory of the water when it exits to the aft of the stern. Look at the first pic, there shouldn’t be any water rushing over the top of the cavitation plate, do we need a group buy on glasses for everyone? I’ll take a photo of my boat running this afternoon if we get out.

You are 100% right Buckethead and Pineapple Express ! Sorry I did not see the link for the video like many other of us who need glasses. haha

 

[Eco Geeko]

I can absolutely guarantee you that the motor is too low... if it’s too low with the trim half way up why are we still having the discussion? The video clearly shows the cavitation plate buried under the water. Also take into consideration that keel shape and deadrise at the stern can play a part in the trajectory of the water when it exits to the aft of the stern. Look at the first pic, there shouldn’t be any water rushing over the top of the cavitation plate, do we need a group buy on glasses for everyone? I’ll take a photo of my boat running this afternoon if we get out.

That was certainly my belief before posting here, but there were so many responses covering a range of possible causes that I began to doubt. Anyway, I've got a fin on there now and it'll be interesting to see what it does, but the next step is to raise the motor and test. Unfortunately summer weekends are getting filled up with non-fishing activities (helping my folks move next weekend). I'll try and get out for a mid-week evening test soon.

 

[Aquaholic]

That was certainly my belief before posting here, but there were so many responses covering a range of possible causes that I began to doubt. Anyway, I've got a fin on there now and it'll be interesting to see what it does, but the next step is to raise the motor and test. Unfortunately summer weekends are getting filled up with non-fishing activities (helping my folks move next weekend). I'll try and get out for a mid-week evening test soon.

Good luck, I hope you resolve it and learn to love your boat again. ;-)

 

[profisher]

Not sure if this link will produce the page of illustrations I want but here it is: https://www.boats.com/how-to/the-ou...peed-with-outboard-engine-height-adjustments/ Scroll down to the 4 pics of various height setups...

 

[Pineapple Express]

Not sure if this link will produce the page of illustrations I want but here it is: https://www.boats.com/how-to/the-ou...peed-with-outboard-engine-height-adjustments/ Scroll down to the 4 pics of various height setups...

Yeah, these pics work. Lots of good info here.

I think the OP still needs to address the dent in the hull. This is a major issue.

Also, the OP needs to figure out if the prop is even in the realm of reasonable by testing the wide-open-throttle (WOT) RPM as I mentioned earlier. If this is the 4-stroke Tohatsu 90 it needs to rev...5300-6300 WOT range. The OP should be aiming for around 5800RPM but that may change based on engine mounting height.

 

[tinboatrobb]

After reading this thread I bought a set of Dole Fins for my 14ft Lund with a 20hp Merc 4 stroke, what a difference in getting on plane and keeping the nose down. Can't believe it took this long to do it.

 

[Eco Geeko]

Yeah, these pics work. Lots of good info here.

I think the OP still needs to address the dent in the hull. This is a major issue.

Also, the OP needs to figure out if the prop is even in the realm of reasonable by testing the wide-open-throttle (WOT) RPM as I mentioned earlier. If this is the 4-stroke Tohatsu 90 it needs to rev...5300-6300 WOT range. The OP should be aiming for around 5800RPM but that may change based on engine mounting height.

Yes, I do need to look into that. Haven't been able to get in the water since your comment, but typically I can't open the throttle more than about 4000 due to the porpoising issue as it picks up speed. This motor is the MD90c2, not the BFT90 you linked to, but the WOT range isn't that different (5150 – 5850).

 

[Pineapple Express]

Yes, I do need to look into that. Haven't been able to get in the water since your comment, but typically I can't open the throttle more than about 4000 due to the porpoising issue as it picks up speed. This motor is the MD90c2, not the BFT90 you linked to, but the WOT range isn't that different (5150 – 5850).

Gotcha. If the motor is running properly then the holeshot issue is due to wrong prop or crazy weight distribution of the boat (stern heavy)...but the video of the holeshot doesn't really show a bow-high orientation.

Did you sea trial the boat before purchase? How was it performing during the sea trial?

 

[Eco Geeko]

Update here that I installed a hydrofoil and put a couple cannonballs in the bow and the porpoising is much better. However, there's now quite a bit more splash coming from the lower leg area. Not surprising, given the likelihood that the motor is too low and that I added hardware to that area, so now I'm working on getting it moved up. Thanks again for the help everyone!