Drag created by aftermarket tilt/trim.

Hi,

I have a 1999 Alumacraft V16LTD Mag with a '73 Merc 500 outboard. I recently added an Atlas tilt/trim unit made by TH Marine. The unit is essentially the same as the CMC PT-130. The unit causes my motor to have about 8" of setback from its original position. The height of the outboard in relation to the boat is the same.

The problem that I am having is that the 8" of setback is allowing the water coming out from below the hull to come upwards and hit the center area of the shaft housing and the tilt shocks/struts and then sprays up into the air. This is causing a lot of drag. My top speed went from 26 mph to 13 mph, fuel consumption is excessive.

Has anyone else experienced this or know of a fix? Are there any commercial products available which will help reduce the drag?

I am a competent machinist/welder/fabricator so building a custom solution is an option. As of now I am thinking of some sort of deflector to mount to the outboard's shaft housing to keep the water down, or some type of extension mounted bottom of the tilt/trim unit's frame to create a type of false hull to reach out towards the outboard to keep the water flow down.

Thanks

Re: Drag created by aftermarket tilt/trim.

I have a CMC on mine and I do get a bit of spray on the motor but nothing like you are talking about.

Your loss of speed sounds pretty drastic and I wonder if there isn't another source, generally you gain speed as you can run the engine a bit higher with the set back.

Have you tried moving the motor to it's highest mounting position, that may help both your speed and spray problem.

I would try everything else before modifing anything, you are certain to void your warranty as soon as touch the TT and I would be REALLY shy about modifing the hull, you could get into all kinds of unintended problems there.

My other though is maybe you shook something loose while changing the engine and you have a plugged carb, your speed loss seems way to big to be just from adding a TT, as I said, that normally makes you go faster, something isn't right here.

Re: Drag created by aftermarket tilt/trim.

you motor is set too low. with set back you have to raise the motor 1/2 to 1 inch.

Re: Drag created by aftermarket tilt/trim.

Thanks for the info so far. For the record I never had ideas of modifying the hull itself, i was thinking of something similar to a hole-shot plate for the TT. The carbs are not blocked and are adjusted properly

tashasdaddy, I think you might have identified my problem. CMC's clamp-on adapter makes the motor mount several inches higher than the adapter I fabricated is. Some data on the current prop location is as follows.

Centerline of propeller shaft: 8" below hull bottom
Prop Diameter: 8"
Cavitation plate 2" Below hull

The last one is telling. A quick search show I should be 0" to 1" above the bottom of the hull. I am going to remount the outboard 2" higher an see how that goes

Here are some pictures of the current setup.





Thanks for the help!

Re: Drag created by aftermarket tilt/trim.

Plate 2" below the hull Is telling.Also keep in mind the water flow behind the boat could put the plate much farther below the actual water surface.
You may want to make a test run and evaluate if a 2" increase is enough.
My hunch is not enough.Is there a provision on the TnT or motor for installed height adjustment?

Re: Drag created by aftermarket tilt/trim.

Thanks for the reply. Unfortunately I live over an hour away from the closest lake that I can test on. I ended up mounting it 3" higher because as I got working I thought 2" might be too low.

I had to modify my custom adapter to raise the outboard up that high. If I am off a little I will simply need to mill off a little material or weld on a spacer piece depending if I need to go up or down.

After the adjustment the plate is approximately 1.5" above the bottom of the hull if a straight edge is used to project the plane of the bottom of the hull. The outboard has 9" of setback from the transom.