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Posts: 1023
| In the spring I got some advice on this site from VMS on how to raise my 70 hp Johnson 4 stroke (thanks). It is mounted too low on my 1650 Fish hawk. The cavitation plate (the one immediately above the prop) is running 3" under water at full speed. I am planning on raising the motor 2 holes. This will put it one hole from the highest setting on my transom. Should the cav plate be at the surface or slapping the top? I think so from what I have seen on the web, but that would put the prop barely below the surface at full speed - which I am having a hard time wrapping my head around.
My boat does 32 mph with 5200 rpm max. This top end speed seems a little low for this set-up. It has a great hole shot though. The minimum recommended wide open throttle is 5200 and the max is 5800.
Feedback/opinions would be appreciated! Thanks! |
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Posts: 572
| I guess I've never paid to much attention to the cavitation plate at full speed. However, I've always thought the cavitation plate should be either be level or 1" above the bottom of the transom, at least to start. Smaller motors may want to be mounted about 1" below the transom. I'm sure someone else will be able to help you. However, you may want to read some of the comments found on the following forum...
http://forums.iboats.com/Outboard-Motors/Cavitation |
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Posts: 295
Location: Southern Ontario, Detroit River and Lake StClair | I just checked mine yesterday (carefully) while running WOT. It appears to be "on" the water but slightly be low the surface level if that makes any sense. There's no water over it at all. That's with it set about 3/4" above the bottom of the hull in a neutral trim position.
Tim
Edited by Roughneck1860 9/6/2009 9:07 AM
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Posts: 32924
Location: Rhinelander, Wisconsin | That's about right. Last thing you want is to drag the cavitation plate around under the water at full throttle; will slow you down, cost you fuel, and work your motor harder than necessary. |
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| Wait, I'm confused. I thought the cavitation plate was the (Top) plate. The bigger one. Right, or wrong??The top plate on mine is exposed while running W.O.T but the bottom one is under the water. Is this right?? I cant quite get the R.P.M's im supposed to unless i am by myself in the boat. I was thinking it was maybe a little of on the prop but maybe the motor is mounted too low? Please help! |
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Posts: 1023
| The larger plate immediately above the prop is to be above or on the top of the water then - right? I need to raise mine up 2 or 3 holes then! |
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Posts: 295
Location: Southern Ontario, Detroit River and Lake StClair | Landry - 9/7/2009 8:52 AM
The larger plate immediately above the prop is to be above or on the top of the water then - right? I need to raise mine up 2 or 3 holes then!
Yes........When the boat is fully planed out. Do you have or can you get a picture of how the motor is set in relation to the boat now? What motor/prop/boat do you have? You might just need to change prop pitch and/or style to get the performance your looking for. Raising the motor 3 holes sounds extreme. If you had to go that much right now you'd have trouble with to much bow lift, poor hole shot and proposing issues
Tim
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Posts: 1023
| I have a 1650 Crestliner Fish Hawk with a 70 hp 4 stroke Johnson/Suzuki.
Right now the cav plate is 3 inches below the water at full speed. I have a great hole shot but could likely get better top speed and rpm's I think. I had heard that setting the proper height for your motor was step one - then changing props if necessary would be step 2.
I just went out to look at it and the cav plate is level/even with the keel or slightly below it - so at this height the transom would have to be out of the water completely for my cav plate to be on top. I am assuming that since the plate is 3 inches below water that I need to move it up 3 inches but is this correct? Maybe I should move it up 2 holes. The reduced drag may help it pop up higher than the 2" I need??
Thanks for the help guys!
Landry |
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Posts: 295
Location: Southern Ontario, Detroit River and Lake StClair | That sounds about right, deffiantly move it up. I'd go two holes. I've had two Crestliners, one 16ft Angler and one 1850 Sportfish. The 16ft ran best set one hole above sitting flat on the transom and the 18ft two holes.
Tim
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Posts: 3504
Location: Elk River, Minnesota | Hiya,
As hard as it is to actually view the ongoings of the water when the boat passes over, the boat essentially makes a "hole" in the water. As soon as the boat passes over, the hole begins to fill, therefore, when you raise your motor up so the anti-ventilation plate is even or slightly above the water level back there, your prop will still be fully submerged.
If you are staying with an aluminum prop, I still would say go up all the way and try it there...each hole on the motor is about 3/4" so going up to the top hole is only 2-1/4 inches. The standard yamaha prop is a great design, and although many would say not to go to the top hole, I'll bet you can get there and not see any huge loss in handling performance. You will find a bit of down-trim will be needed as you go into a moderate turn at speed, but I don't think you will ventilate to any extreme. Either way, moving it up is going to be a nice thing...you will get more motor out of the water, and actually, more boat out of the water as well, reducing your overall drag from bow to stern.
would love to hear how the testing goes once you have it moved up.
Steve |
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Posts: 1023
| Thanks guys. I will report back soon with my results!
Landry |
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