motor height question

[longshot]

I have a new 1860 lowe roughneck with a 90Etec. It is running 5350-5530 with a 13 7/8X15 SST prop depending on the position of people in the boat at WOT.

My motor is all the way down on the transom. The cav. plate is right at the bottom of the boat right now. The motor wont cavitate in a turn or takeoff. Should I mess with the motor mount height??? How much should I move it??? What will I gain or lose with moving the engine?

thanks
longshot

 

[Dhadley]

Re: motor height question

By moving the motor up you'll reduce drag which should increase efficiency. It'll probably gain something like 75 - 100 rpm (top) for every hole you go up. That prop has a pretty big diameter which will allow some added height.

The obvious goal would be to be able to run a 17" prop in the correct rpm range with an average load.

 

[tashasdaddy]

Re: motor height question

up an inch. you should gain. typing at the ame time, again.

 

[longshot]

Re: motor height question

Ok....

How do I know when it is high enough??? Like I said I have NEVER had any blowout(cavitation/ventalation) of the prop. I have 3 more holes that I can go up I think.?.?.

The boat is currently running 38mph(per GPS) WOT trimmed up. THe boat is a 10degree V that is 770# dry unrigged. Rigged with normal load of people/gear @2000# total weight.

So I should just go up 1 hole at a time till???????

thanks
longshot

 

[tashasdaddy]

Re: motor height question

you got it. i'm running a 1988 88spl (90 without vro) on the same weight boat, DeepV, and getting 45mph.

 

[longshot]

Re: motor height question

tashasdaddy

The 88spl you have is it the 105cubic inch V4 90 block isnt it? That is a VERY strong 90.......my 90 Etec is only 79CI(I think). The carbed 90 Johnson is hard to compare to ANY other 90 in my opinion. It also has a different gear ration than the 90 Etec.......

I figured that I could run a 17 pitch on the Etec of mine, but the 15 ran just right.........so I will probably just run it and fine tune motor height.

thanks
longshot

 

[Dhadley]

Re: motor height question

As a very general rule of thumb -- when you get to the point (up) that you can use 80 -90% of your trim without venting the prop then you're about as high as that prop will want to be. If you can use all the trim without venting, that prop will run higher.

Now, if you're getting max performance at, say -- 60% trim, and your prop doesn't vent until 90%, then you can still go up. It's a testing thing to find the relationship between your hull design, load and style of prop.

Also watch the slip numbers. Calculate your percentage of slip now and compare it as you test.

We'll assume you have the 20" motor with the 2:1 gear ratio. At 5500 and a 15" prop you're at about 15.5% slip. Not too good. We'd like to see that around 10% - 12%. Then you'll be at about 40 mpg, give or take a shade.

 

[longshot]

Re: motor height question

Ok.....

With my buddy riding in the front seat which is what we usually do this is what we got. I can trim up till it stops trimming without venting and it is running 5530 rpm at WOT.

That is in water with a little chop. In glass water conditions the boat will start to bow hop a little with max trim.

I notice that running WOT trimmed up the boat will change behavior suddenly while trimming up. It will act like it gets "unstuck" from the water and gives you a floating sensation and change in motor noise and will turn easy to the left, but is harder to turn right. What is happening?

So raising up the motor will drop the slip rate???? I assume it will to a point correct????

thanks
longshot

 

[longshot]

Re: motor height question

Dhadley

you have a PM from me.

thanks
longshot