Cavitation and Porpoising 14' Runabout

[DDAADDYY]

Greetings out there. I've recently tuned up an old 14' K&C Thermoglass boat that used to have a short 15" leg 1961 Merc 350 40hp motor and it ran smooth as buttah. Feeling the need for speed I followed a topic at another forum to use aluminum angle iron as a transom jack. I did this and installed a 20" 65hp 1975 merc 650. I was told to mount the motor so the cav. plate was 3/4 to 1 inch below the tip of the keel which I did. Now I've got enormous backsplash at all trim positions and severe porpoising in any but the first trim setting from the transom. I wish I had the time to experiment but I have to get to work for around a month. Any ideas? would those nautilus auto trim tabs do any good?<br /> Or a wider less pitch prop? I have a set of trim tabs that bolt on to the cavitation plate and hope that'll help but this is terrible splashing up between the clamps. On the other hand it planes pretty fast and clips along real fast but this splashing is taking all the fun out of it. Gotta go thanks ahead as I won't be home for a long time

 

[LubeDude]

Re: Cavitation and Porpoising 14' Runabout

Welcome DDAADDYY:<br /><br />First off, whoever gave you the measurements had no clue as to what he was talking about. You need to have the engine about 1" above the bottom of the boat to start with, and because you set the engine back, you are going to have to raise it even more. I would say you will be 2-3" above before you are done. Your splash is coming from having the engine way too far down.

 

[Mercury140-I6]

Re: Cavitation and Porpoising 14' Runabout

DDAADDYY:<br /><br />I don't know whered you got the idea or information that a Merc 350 was a 40hp motor, but it isn't. It's a 35hp motor. Just thought you should know for future reference.<br /><br /> http://www.myfishingpictures.com/showphoto.php?photo=13002&sort=7&thecat=500&password= <br /><br />Craig

 

[DDAADDYY]

Re: Cavitation and Porpoising 14' Runabout

http://www.oldmercs.com/models.html#60-63 <br />Hey Mercury 140-16 copy and paste this will you<br />thank you. It also says "40hp Four" on the cowl.Mine's the 10th one down on the '60 to 63 list according to my serial number. I'm guessing that picture is yours, very nice. And Lube Dude yeah I'm getting that impression. The jacks are 1/4" angle iron so the motor is only 1/4" set back from the transom. you're one of the many who say that. Is there a reference you could link me to as there are just as many who say the opposite. I'ts vey confusing as I want to be sure the elevation is right next time as I'm sure you'll appreciate this isn't just a simple job raising this whole thing. Thanks for your interest folks.

 

[LubeDude]

Re: Cavitation and Porpoising 14' Runabout

I would rasise it 2" to start if it were me and go from there, but that should be about right.<br /><br />There are many that are willing to give advice, but havnt been there done that. Id be VERY surprised if any of the old timers on here would give you the advice to have your engine where it is.<br /><br />What Prop and RPM #s can you give us?

 

[DDAADDYY]

Re: Cavitation and Porpoising 14' Runabout

Thanks LubeDude it's a 14 pitch prop at around 5500rpm. at WOT although I've only had it there once for 5 seconds as this is a freshly rebuilt motor only 6 hours on it with all new bearings rings etc. Also using quicksilver oil but am getting a squeak like a fan belt noise now and then in the last 1/2 hour testing yesterday at the powerhead. Any ideas thanks gotta work now