Porpoising on re-powered boat. Need Advice

[surfman]

Can you take some pictures of the motor on the transom from the side view?

 

[Stumpalump]

Overpowered 16 footer is also what I run. Get a 1/8 plate of aluminum and cut out your own oversized cavitation plate. Nothing beats a flat thin plate. Make it smaller than store bought hydro foils. I picked up 2 mph and that plate reduced the bows up and down movement at all speeds. There are also some hulls that like the engine higher and some lower. Mine likes lower. Cavitation plate even with the bottom is the best starting position and often the fastest. Not true on my Valco 16' but your results will vary. Setup with trim, height, prop and weight distribution is very important on these light boats. Mine ran fine when I bought it at 26 mph. Tweaking setup resulted in 37 mph. Over 35 and it's getting unstable. Drop it to 33 and it's a dream. The hull will just not take any more speed unless I'm driving on what feels like the ragged edge. You might be at the hulls max before it wants to fight you like mine does above 35.

 

[Stumpalump]

This is the flat plate I copied. https://www.fastcompanymarine.com/fast_company_technical.php

 

[srr5008]

I took the boat out on the river today to look at that cavitation plate. It looked to me like it was sitting a little too low. I've attached a couple pictures so hopefully someone can give me an opinion. First one is the motor on the boat, second one is looking down on the cavitation plate while running the boat on plane, and the third picture is a close up of the lower unit in relation to the transom. Let me know if you think raising a bolt hole is a good next step. Thanks.

 

[onthewater102]

You could probably raise it a bolt hole but no more. Looks like it's running great. Is the profile picture (1st pic) with it totally trimmed down?

 

[surfman]

The plate above the prop is the anti-ventilation plate, you can raise the motor until that plate is even with the bottom of the boat, it also looks like the transom on that boat has very little angle to it. That could just be the picture, also it looks to me like there is a trim pin in the lower hole, if this motor has hydraulic trim and tilt you don’t need that trim pin and I would remove it. When the motor is trimmed all the way down it should be at close to the same angle as the transom.

 

[surfman]

This is what I am calling the trim pin, is there a rod or pin in that hole?

 

[srr5008]

onthewater102 » Yesterday, 22:36[/url]"]Is the profile picture (1st pic) with it totally trimmed down?

Yes.

onthewater102 » Yesterday, 22:36[/url]"]it also looks like the transom on that boat has very little angle to it. That could just be the picture, also it looks to me like there is a trim pin in the lower hole

You are correct, the transom has very little angle to it. And it does appear that there is a pin there - I will have to investigate tonight.

 

[srr5008]

Also for what it is worth, I played around with weight distribution while on the water. I had to have 2 people sit on the front deck to get it to stop porpoising when I trimmed up (about 300 pounds). So I don't think that is a viable fix. I'm hoping that raising the motor a bolt hole will help stop the cavitation plate from digging it's nose in when I trim. I also gained 3-4mph when I was able to run trimmed up.

 

[juggernot]

It looks like raising 2 holes would be about right.

 

[srr5008]

juggernot » 6 minutes ago[/url]"]It looks like raising 2 holes would be about right.

That was my thought as well, but I figured it couldn't hurt to raise it one at a time and see what happens.

 

[richg99]

Great advice above. All of those holes are there for a reason. Keep playing with them.

For what it is worth.. on one prior boat, transom wedges did the trick. On a few others, Smart Tabs worked. richg99