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Author Topic:   Roostertail
rkp posted 08-29-2003 07:36 PM ET (US)   Profile for rkp   Send Email to rkp  
I have a 13 dauntless with a 40 evinrude on it. Originally I could not get it to allow any trim without porpoising. The motor was on the top hole so I raised it one hole and installed a wedge. Now I am able to get some trim and overall performance improved. My question is that I have quite a roostertail at WOT. Does this mean I could go higher. I am not getting any prop slip upon tight turns, etc.


JohnJ80 posted 08-30-2003 11:23 AM ET (US)     Profile for JohnJ80  Send Email to JohnJ80     
The rooster tail, if it is V shaped and coming off the lower unit means that you are probably too low. Also this can happen if the motor is trimmed in too far - something that can easily happen with a wedge.

What I would do, and I know this is a hassle, is to remove the wedge then adjust the motor height.

Almost always, dealers put the motor on the boat too low. If the motor is too low, the thrust from the engine is trying to lever the bow up and the thrust is not parallel to the direction of travel - you are trying to rotate the boat. This position is unstable and leads to porpoising. Getting the thrust in the right place will take a lot of the leverage away that is needed to start porpoising (I'm an engineer, but this is all quantative - there are others way more competent to give you the dynamics of this).

If you have the wedge in place, you haven't corrected this problem, you have just provided the ability to lever the bow down and you still probably won't have a lot of trim capability, you have hydrodynamic drag that is too high from a motor too low in the water. It isn't anywhere close to optimal. Wedges strike me as creating as many problems as they create.

Remove the wedge. Adjust the motor until the AV plate is 1-1.5" above the bottom of the hull. Whaler recommends 0-3/4", my own personal experience and that of others on this forum seem to indicate that better than 1" positioning up is better. Test this configuration for porpoising and your rooster tail. My guess is that this will also improve the ride while decreasing porpoising and increasing your top end speed.

What you need to watch for is to make sure that you still have the motor peeing water - otherwise you have raised it too high and you will overheat the motor. you want to have the motor as high as you can get it, still be able to cool the motor and not have the prop breakout in turns or chop on typical waters where you boat.

Next step is if this porpoises, I would look at a fin. If the motor height is properly set, the fin when the boat is on a plane, should ride at the interface to the water, not deep in it. As the boat attempts to porpoise - stern sinking, the fin will lift it back up. I, personally, would add the fin anyhow. I am a big fan of them and I think that almost any boat will benefit from them. However, you need to properly position the motor vertically to avoid drag.

You can also achieve some of the same result using a high performance prop. however, that is a more expensive solution than the fin.

If you can, I would get the new prop, a Hydro Lift fin (don't know if they make them for your motor), your tools (hydro floor jack, 2 ea 3/4" wrenches, Drill, socket set, silicon) and head to the boat launch. Remove the wedge and raise your motor up so that you have one hole remaining to go up (this should be ok, but measure to make sure you are in the 3/4-1" up range or so). Launch your boat and try it.

If you need help on how to raise your motor on the trailer, let me know. Its easy, but there is a trick to it.

Raising the motor will get rid of the rooster tail. Porpoising is usually helped by raising the motor.

If you still have the rooster tail and/or porpoising, then pull the boat and mount the fin. (The Hydro Lift does not drill the AV plate). If you need a fin that does drill the AV plate, drill it with a rechargeable drill and mount it. Retest.

If this works better, then go the last hole up. If not add the prop and retest.

I did all of this with my Dauntless 15 when I repowered. I now have a Hydro Lift fin mounted to the motor, a Stilletto 13.25x15 SS prop and my motor is mounted in the very top hole (motor raised the highest). Performance is great, steering is easier, and I can clearly hear the motor running better (not any laboring at all).

I am pretty sure you will notice substantial differences with this configuration. If you are happy with your boat now, you will be delighted with the change. It is a matter of experimentation based on your loading of the boat, the way you drive the boat and the waters you boat on.

Hope that helps.


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