Classic SeaCraft Community  

Go Back   Classic SeaCraft Community > Recovered Threads
FAQ Community Calendar Today's Posts Search

Notices

Reply
 
Thread Tools Search this Thread Display Modes
  #11  
Old 07-28-2014, 12:30 PM
htillman htillman is offline
Senior Member
 
Join Date: Oct 2007
Location: houston, tx
Posts: 517
Default

I measured the motor and it seems too low. There is no marking on the prop except Rebel. I am going to take it off where I can get a better look at it. I confirmed the motor only turns 4800 rpm.
Reply With Quote
  #12  
Old 07-28-2014, 02:38 PM
Fr. Frank Fr. Frank is offline
Senior Member
 
Join Date: Oct 2003
Location: Shalimar, Florida
Posts: 2,265
Default

John C and Gillie are on the right track. Porpoising is caused by direction and placement of thrust and amount/type of thrust not being able to overcome PLANING rocker pivot angle. Direction and placement of thrust plus amount and type of thrust must overcome displacement CG to thrust hull past pivot angle. If the angle is too great or thrust too little once the hull pivots, the hull pivots stern-ward again off the planing rocker pivot, at which angle the thrust is once again able to force hull back into pivot, ad nauseum.

The are three possible ways to overcome this:
1. Change planing rocker pivot angle by changing hull CG.
2. Change direction and placement of thrust (Height and angle)
3. Change amount and/or type of thrust at speed. This is more about type and shape of propeller than amount of horsepower. (Yes it is possible to simply overpower porpoising by greatly increasing thrust, but it's really stupid as your actual control becomes minimal)

First, check motor mounting height by measuring distance from bottom of ventilation plate to keel when the two are as parallel as possible.
Start with ventilation plate no less than 1/2" above keel, and no more than 1 1/2" above. (This assumes a transom-mounted motor)
If your motor is at the right height, when trimming up/out you should ventilate the prop before porpoising under 3/4 or higher power. You still may porpoise slightly under acceleration, and perhaps more when slowly decelerating.

For a SeaCraft, make sure you have a stern-lifting prop. A bow-lifting prop (particularly when mounted at the wrong height) can seriously exacerbate a slight tendency to porpoise.

Monitor your trim angle. Does your hull still porpoise with the motor trimmed all the way down? Does adjusting the trim minimize or eliminate porpoising at various speeds? If trim does not ameliorate the porpoising at all, this can indicate an issue with static CG as well.

Whatever your power, you should be able to eliminate porpoising by adjusting thrust placement and direction, adjusting center of gravity, and adjusting type and amount of thrust.
__________________
Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.

Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!

Currently without a SeaCraft
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
Reply With Quote
  #13  
Old 08-30-2014, 09:41 AM
Beaver Beaver is offline
Senior Member
 
Join Date: Nov 2011
Posts: 260
Default

My 18 was porpoising so I moved the motor up three notches. Then it was cavitating so I moved it down 2 and now it rides great. I am still surprised that one notch up made all the difference.

Good luck.
Reply With Quote
  #14  
Old 12-21-2014, 12:57 AM
htillman htillman is offline
Senior Member
 
Join Date: Oct 2007
Location: houston, tx
Posts: 517
Default

Finally changed the prop, went with a 17 Viper. This made a big difference. Now turn up 5200 rpm and speed increased to about 42 mph. The motor may need to be raised a notch but the performance is dramatically improved. The porpoising is gone and the boat seems to hve a noticeable bump in acceleration.
Picture of completed boat is attached.
Attached Images
 
Reply With Quote
  #15  
Old 12-21-2014, 10:20 AM
Capt Chuck's Avatar
Capt Chuck Capt Chuck is offline
gucci
 
Join Date: Aug 2003
Location: Sailfish Capital, fla
Posts: 2,804
Default

The boat looks great Eddie!

Did you end up using Signature Finish and are you happy?

BTW: No prop in this picture = I bet the RPM's are higher than my Glucose Count now
__________________


1978 23' Superfish/Potter Bracket 250HP --------



as "Americans" you have the right to ......
"LIFE, LIBERTY and the PURSUIT of a Classic SeaCraft" -capt_chuck
Reply With Quote
  #16  
Old 12-21-2014, 01:04 PM
htillman htillman is offline
Senior Member
 
Join Date: Oct 2007
Location: houston, tx
Posts: 517
Default

Thanks, this little boat was fun putting together.
The hull sides were sprayed with Imron. I am going to paint the bottom with Signature Finish dark blue. I removed the prop and had to re-rig the engine; the cables (controls, fuel and oil) were kinked when the motor was originally installed. During this process I bulk head mounted the hydraulic steering cables, replaced the water pump and tuned the engine. She really runs much better now with the prop and engine work. Found out this engine only had 85 hours.
There is a big difference (ride, storage, etc) between the 20 and 23. I really miss the 23 and have been looking for another one. Just not sure what model?
Reply With Quote
  #17  
Old 12-28-2014, 07:18 AM
bgreene bgreene is offline
Senior Member
 
Join Date: Oct 2007
Posts: 404
Default

Most unlikely, but finally, if hull has " hook " meaning running bottom is no longer flat, but has developed a curve, it could cause porpoising.
Reply With Quote
Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT -4. The time now is 06:46 AM.


Powered by vBulletin® Version 3.8.7
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
All original content © 2003-2013 ClassicSeacraft