Motor mounting Height - Huge Difference -

[Skoot]

I was finally able to raise my motor, and let me tell you what a difference.  Here's the story; My motor was mounted on the top holes, It had a Dol-fin and a 13.25(dia) x 17"(p) wheel on her.  I was only able to get 4200-4300 RPMs (30miles per hour) at WOT with a light load and trimmed just right.  I had a terrible hole shot and mid range acceleration wasn't much better.   I should be getting 5000-5500 RPMs at WOT

I then bought a new prop with a 15" pitch.  Took that out for a spin and maybe gain 100 RPMs and the same speed.  Thats when we started talking about the motor height.  When i was running I couldn't see the cavitation plate.  So it was decided I should raise the motor by 2 holes and "Take off that stupid Dol-fin".

So I finally raised the motor 2 holes , took the stupid dol-fin off and took her for a test run with the 15"p prop.  Heres my results; at WOT I was able to reach 5000 RPMs,(gained 800rpms)  I also increase my speed by 5mph, now im at 35mph.  My hole shot is much better and overall performance is much more responsive.  Im also pretty sure I can raise the motor 1 more hole as the cav plate still has water coming over it. I might even try a 14" prop, well see.

All in all Im stoked! I cant believe what a huge difference those changes made and theres more to come.

[Aswaff400]

i saw 43 mph with my 112 johnson, when i bought the boat it was mounted on the lowest height, raised it to the 2nd highest, shaved a few seconds off time to plane, wot before w/ 17p 3 blade was 35-37 at 5000. after 39-43 mph @ 57-5900rpm, cav plate was a 1/2 inch out of the water...

with the 200 i have the cav plate 1 to 1-1/2 inches out of the water running a 13-1/2 x 23 4-blade with a good amount of cup and rake, i have yet found out how fast it goes, it gets up to 40 quick though

[slvrlng]

This is that thread I was telling you about over on Classic Mako. On the last pae he has some new data about performance with his twins and he also has pics of his cav plate and relationship to the water. Pretty good read.
http://classicmako.com/forum/topic.asp? ... hichpage=5

[Skoot]

Thanks lewis!   That pic where he lowered it by one hole is exactly how my anti cav-plate looks now.  If I take a straight 2x4 and place it on the keel it will line up just under(.125") the cav-plate.  Once again I couldn't get any motor cavitation no matter how high I trimmed the motor.

[Aswaff400]

took the boat out for round two, got up to 52 mph with myself and full 50 gallons of gas... :twisted:

motor itself didnt perform as great as i thought, still got some kinks to knock out...

[wingtime]

took the boat out for round two, got up to 52 mph with myself and full 50 gallons of gas... :twisted:

Wow!  I can't wait to see how the 200 Merc pushes my XF.  I received the prop yesterday and hooked up the steering and throttle cables.  All I have to do now is hook up the wire harness and fuel lines.  Oh and put in the new poppet valve and stainless thermostats when they arrive.

[GoneFission]

 So it was decided I should raise the motor by 2 holes and "Take off that stupid Dol-fin".

So I finally raised the motor 2 holes , took the stupid dol-fin off and took her for a test run with the 15"p prop.  Heres my results; at WOT I was able to reach 5000 RPMs,(gained 800rpms)  I also increase my speed by 5mph, now im at 35mph.  My hole shot is much better and overall performance is much more responsive.  Im also pretty sure I can raise the motor 1 more hole as the cav plate still has water coming over it. I might even try a 14" prop, well see.

All in all Im stoked! I cant believe what a huge difference those changes made and theres more to come.

Doel-fins don't work on outboard-powered Aquasports when the motor is at the correct height.  Generally, these fins don't work on most boats except inboard/outboard setups where the lower unit is so low in the water that the fin can "bite" at all speeds.  Put the motor at the right height, get the right prop, and trim it out until it almost cavitates and you will be amazed at the difference in speed and fuel consumption.

[bumpster]

Trim Tabs will be the best money you ever spend. When I bought mine it changed everything about the ride on my 200 Osprey

[Capt. Bob]

This is what happens when you stay up way too late.

While I grew up my whole life with the word cavitation, I believe we really mean ventilation when we are talking about trimming the motor too high or taking too tight a turn.

That is, allowing air to enter the prop stream and causing the motor to over rev.

Cavitation is a completely different animal.

[seabob4]

Yamaha calls the cavitation plate an "anti-ventilation plate"...for what it's worth.

[GoneFission]

Bob - yes, maybe you are staying up too late, but you are correct.  I used the term cavitation because many folks don't know the term ventilation as it relates to boat propellers.  That's why some use the term cavitation plate where ventilation plate would be technically correct.  

Ventilation occurs when surface air or exhaust gases are drawn into the propeller blades. The load on the propeller is reduced by the mixing of air or exhaust into the water steam causing over revving.

Anti-ventilation Plate: Large plate cast into gearcase housing directly above propeller. Helps reduce surface air from being pulled into blades.

Cavitation: The aeration (bubbling) and boiling effect of water caused by creation of a low pressure area. Generally caused by a solid shape (propeller blade) passing through the water, in such a position and speed, that a low pressure area is formed due to the inability to move through the water in nonresistant manner. An example is, a propeller blade that has a rough edge would not cut efficiently through the water, thus creating a low pressure area. If the pressure drops below the vapor pressure, a cavitation bubble will form in that region. These bubbles will collapse when they reach the higher pressure region of the blade. This causes a rapid change in pressure and can result in physical erosion. You may notice burns (erosion) at some area on the face of the blade.

Common rules of thumb:
Ventilation can lead to excessive slippage.
Cavitation can lead to ventilation and/or slipping.

[gran398]

Thanks GF, very good.

Now,  next time my wife gets mad that I'm looking at "that Aquasport crap" I'll be able to determine if she's cavitating or ventilating.

[GoneFission]

G3 - the term for what you are experiencing might be better defined as fuming:

1. Vapor, gas, or smoke, especially if irritating, harmful, or strong.
2. A strong or acrid odor.
3. A state of resentment or vexation.

             :tongue:

[gran398]

Fuming.....You've hit the nail on the head!  I knew that was it...but really couldn't explain it.....

[wingtime]

I took out the XF yesterday for a test run with the "new" 200 Merc.   WOW!     Much better than the 125!  I think I need to raise the motor two holes as well.  Right now it is mounted in the lowest position.   I was getting a large amount of spray shooting up the back of the transom from the front of the lower unit/mid section.  Even so I was able to get 38 mph and 4900 WOT with a 14.5 x 19 prop.  I'd like to see 5300 or so.  Here's a pic of the freshly rigged motor: