How To - Cure That Porpoising Wetbike
Topic: Porpoising - What is it and how do you cure it?
Expertise Required: Beginner
Estimated Time: Well, it just depends on what you find during the inspection phase
- Rear Ski (worst case scenario)
- Couple of washers (easy fix)
- 1/2 tube of 5200 - to re-glue ski and skeg
- Creeper or piece of cardboard to lay on if your bike is on trailer
- Finely tuned CE's (calibrated Eyeballs)
- Allen Head socket to remove scoop and/or skeg
- Screwdriver and small wrench to remove skeg hardware (if needed)
- All of the tools required to remove ski/pump assembly (if you need to re-glue that ski). See the technical article for ski removal/replacement
Step-By-Step: First of all, you need to be able to identify it correctly - then you can choose a plan of action. You also need to understand that there are just some bikes out there that are more prone to this than others (fiberglass bikes more so than Metton).
What Is Your Wetbike Doing?: It is important to diagnose the behavior correctly. That is, if your bike only starts to porpoise right at full speed, you could be experiencing over-stuffing of the pump. Not very common, but it has happened. If your ski is in excellent shape, this might be you - and you'll need to modify your scoop (see tech article).
However, if your bike porpoises all of the time - it's time to dig in deeper.
Identify The Bad Yoga: If you have an older bike (50hp or fiberglass hull), the odds are that your rear ski is abused/shot. If it looks bent lika a banana, it's probably gonna porpoise. If you have a super scoop on it (big double-bladed red one) and you see daylight between the ski and the two front tips, it's definately bent. Example:
The best solution at this point is 2-pronged. First, fab up a 1/4" spacer to go between the forward brace (round brace or H-shaped one with 2 rubber donuts). This will help hold the front of the ski down and reduce flexing. NOTE: The rubber bumpers should always be in contact with the hull while at rest.
If you have a bike that has the H-Beam style of brace (Says WETCO) on the top (where you can't see it - wtf?) - then you need to inspect the ski from the bolt holes all of the way back to see if it is cracked. If you have cracks from the wear-ring to the back (on each side of the skeg) it's probably game-over for that ski.
OK, your ski looks good and you've got the front supports setup properly . . . and it porpoises a lot, the next area to look at is the Hull Seal. With bike sitting stationary (on trailer or the boat ramp), put your foot on the front of the ski and lift up on the hull. If you have a buddy - float bike out into waist-deep water and try it again. If you see your motor rocking back and forth - time for a new hull seal. They are still available - and/or the ones for a Mercury Sportjet are an excellent option - especially if your bike has been reinforced and the hull thickness is not stock. The Grommet P/N's are:
1/4" 25-820663-250 3/8" 25-820663-375 1/2" 25-820663-500
If everything looks/feels fairly firm, it's onto the next thing . . .
Doin' The Dirty: OK, it's time to pull the ski off of the pump and re-seal the ski (see tech article). Tech - Ski Removal and Replacement
The Last Resort: So, you've done all of the above and it still porpoises. More than likely, the ski is flexing (internal cracks and old age) and there's not a whole lot you can do about it. One last thing to try is to loosen that 2 rear screws holding the skeg on and slip a washer in there (one each side) and tighten it back down. What you are doing is making it like a trim tab. Some folks have had success with this - and with 2 or 3 washers too. Others, not so much.
If that didn't do it, it's definitely time for another ski !
Copyright 2011 Capt'n Obveeus