Motor Height Versus Transom... again :)

[Timberwolf17044]

Ok guys and gals here we go. Attached are a few pictures to provide a visual of what I'm explaining. I have a very old 12' 8" fiberglass boat with a 1980 Johnson 9.9 long shaft recently converted over to a short shaft. When I was using the motor on this boat prior to conversion I had made a jack plate to lift the motor. After conversion to the short shaft I did the research and everything I read told me that the cavitation plate should be even to 1" or so above the bottom of the boat. As you can see in the images the cav plate now sits about 2 inches above the bottom of the boat, a miscalculation on my part when I cut the jack plate down after converting it. I took the boat out on a local river on Saturday and was very disappointed in the boats performance. I had about a 75 lb bag of concrete mix up front to help hold the front end down and at full throttle could not get on plane. Even worse was downstream. My question is, will lowering the motor another two inches have much impact on the boats performance? Thanks in advance for all your help.

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[Daviet]

Re: Motor Height Versus Transom... again

You are going to need to add some back onto the transom. With it set up like that the prop will surely cavitate on a gentle turn. On plane the prop is not able to stay submerged.

 

[Timberwolf17044]

Re: Motor Height Versus Transom... again

You are going to need to add some back onto the transom. With it set up like that the prop will surely cavitate on a gentle turn. On plane the prop is not able to stay submerged.

I'm confused, adding to the transom raises the prop higher???

 

[Daviet]

Re: Motor Height Versus Transom... again

Sorry, sometimes I don't see what I am looking at. The cavation plate needs to be even with the bottom of the boat.

 

[Timberwolf17044]

Re: Motor Height Versus Transom... again

Sorry, sometimes I don't see what I am looking at. The cavation plate needs to be even with the bottom of the boat.

Ok so I need to lower the jack plate more. Thats what I was thinking. Would having the motor too high also impact the power output of the prop as well?

 

[Daviet]

Re: Motor Height Versus Transom... again

It could.

 

[boobie]

Re: Motor Height Versus Transom... again

I always have dumb questions. Is the mtr running right? Is it hitting on both cyls? "Old boat", could the boat be water logged?

 

[reddogg]

Re: Motor Height Versus Transom... again

Tribal knoweldge says the plate should be evan with to 1/2" above the bottom of the boat. Thats what everyone keeps telling me anyhow

red

 

[Timberwolf17044]

Re: Motor Height Versus Transom... again

As far as how the motor is running it appears to run well. I do not have a tach to attach so I'm just going by ear but yes it is running on both cylinders.

 

[bassin'frog]

Re: Motor Height Versus Transom... again

Tribal knoweldge says the plate should be evan with to 1/2" above the bottom of the boat. Thats what everyone keeps telling me anyhow

red

I have an Evinrude service manual that says (and shows in a diagram) the cavitation plate should be 3/4" 'below' the bottom of the boat. I have mine setup that way and it runs great. Hmmmm, wonder which is correct.

 

[1946Zephyr]

Re: Motor Height Versus Transom... again

Yea, I'm thinking that it should be below the keel myself. Was it blelow the keel before? Did it run better before the conversion?

 

[Mark42]

Re: Motor Height Versus Transom... again

Check this info out:

http://www.marinepartsman.com/installing-an-outboard-on-your-boat.html

http://www.outboard-boat.com/index....t&catid=37:boat-systems-and-set-ups&Itemid=53

Conventional methods put the outboard cav plate 1/2 to 1" above the keel. The bigger the motor, the faster you can go, the higher the motor can be mounted. The farther back the offset, the higher the motor can be mounted.

I just installed a 90hp on my 16' boat and it was riding very nice at 3/4" above keel.

BTW, when setting the height, be sure the cav plate is parallel to the keel. Use a long straight edge to be sure.

The only real disadvantage to having the cav plate below the keel is the extra drag created by having so much of the gear case in the water. Plus you will never get prop blow out on a sharp turn! But expect to spend a little more in gas.

 

[Timberwolf17044]

Re: Motor Height Versus Transom... again

Ok guys and gals here's an update.
#1 problem discovered relating to the beginning of this thread was a throttle issue which prevented motor from opening up fully. With that issue resolved I also removed the jack plate and took a few test runs. Without the jackplate the cav plate hung about 2 inches below the boat and the boat did run fairly well probably around 20 mph. With the motor now working correctly I reinstalled the jackplate, bringing the cav plate to 2" above the bottom of the boat and took a test run. #1 the motor was louder because at full throttle the exhaust wasn't submerged and it just didn't seem to have as much speed. I then removed about an 1.5" off the jackplate and tried again. Except for some additional water spray defecting off the cav plate the boat now seem to handle very well. With just myself it will get up on plane quite quickly and I'm assuming about 20 - 25 mph. All in all, very pleased with the end result. Thanks again to all those who offered assistance and advice it was greatly appreciated. I'm guessing the next time this boat sees water will be when I drop it into Halliday Lake in Northern Ontario for a week of fishing and camping.