Hi All I've got a 225HO which has the 1.86:1 lightning gearbox. I'm dialing in the wake on my boat, trying different props etc. I find that I have to mount the motor really, really high otherwise the cav plate pushes down and makes a bump in the centre of the wake. I'm also getting spray up the front of the leg, maybe from the nosecone. Just wondering if anybody has any experience with the HO gearbox in skiing applicaitons. Even better would be a experience comparing the normal and HO on the same boat. Thanks Greg
Greg, It is fun to fine tune a boat. Here are some things to consider: There is an interaction between the prop and the boat that needs to be considered. A prop with rake will tend to push the stern down. It is called "bow lift" in fast boats because at speed the stern cannot go down so the bow goes up. In boats that are set up purely for speed the motor is high enough that the cavitation plate is no where near the water and this is not a problem, not to mention that they are not too concerned about wake shape. A double cupped prop (cup in the tip and trailing edge) increases the effective rake and will produce the down force also. This force requires you to add negative trim to balance the boat at ski speeds and will make the "hump" bigger. My experience has led me to believe that tip cup is undesirable in a ski boat as long and the prop will (grab) when you stab it at low speed. I don't run any trailing edge cup either because it seems to produce turbulence in the wake. These theories may not work as well on a V bottom as they do on a Dyne because the V bottom boats tend to want to ventilate in the turns specially if they have built in set back. jim
I forgot to add that raising the motor reduces the leverage that the prop has with respect to the hull and therefore reduces stern dig also. This reduces the negative trim needed (or less positive trim) and will reduce the hump and the spray up the front of the lower unit. This techniques has limits because of prop aeration in turns also. jim
The trailing edge of both the lightning gearcase and the Magnum case is squared off, unlike past OMC V6's and the current small block gearcase which is a V shape. Many of the newer gearcases also have a longer cav plate. Couple that with the extra negative trim of the Evinrudes vs Yam/Merc and that cav plate may be sticking further out and down and causing your bump. The lightning case is made for high engine mounting, so you may indeed be seeing some drag spray from the front of the gear case. I have run both and the Magnum case needs to be sunk low in the water - it has a huge diameter bullet and will cavitate on take off if not fully submerged. I think the Lightning case performs better. Can you bump the trim up a bit to get rid of the bump?
Bumping up the trim doesn't help until about blowout. Maybe 60% on the icommand gauge. I started with the cav plate 2.5 inches above the bottom of the boat and it was still an issue. Then I installed a jacking plate with 6 inches set back and lifted it about another 2 inches and it still does it. The hull has a step at the back of it anyway (inbuilt setback). Introducing more set back probably doesn't help. I'm looking at replacing the jack plate with a 1" aluminium block so that I can lift the motor further. I've lifted the motor to the point that traction was reduced to below acceptable levels and it did start to flatten the wake out. This isn't an issue with other motors so it must be hydrodynamic. I looked back through old posts and saw a lot about prop tuning. I'd love to play with this. I've tried the BRP 21" Cyclone, Merc 22" Bravo One and Merc Large hub Trophy Plus 21". The trophy had the best wake but it didn't have nearly as much traction. I really like the traction I get with the Cyclone or Bravo.
If you check out the deck angle under way, you can see that more set back actually lowers the engine in the water. It is not riding at the measured distance of the extended running surface to the prop shaft. It is lower than that. And, slight pitch changes have a rather dramatic effect on the depth. If the prop is right the engine should run with the drive shaft perpendicular to the water at the target speed. The prop shaft would then be parallel to the surface. Try having someone else drive the boat and hang over the transom and watch the wake around the gearcase and the wake coming off the boat. I learned a lot by doing that. jim
