Cavitation Plate Location Relative to Hull Bottom

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Author Topic:   Cavitation Plate Location Relative to Hull Bottom

dlschutz posted 05-04-2010 07:46 PM ET (US)
As many of you know, Boston Whaler makes recommendations regarding the [engine shaft lenth of outboard motors], that is, 20-inch or 25-inch shafts. Many of you more experienced guys also know that 20-inch or 25-inch varies as much as a couple inches by vintage and manufacture, so this is a reasonable guide, not the answer. It would seem that there is an optimum measurement for the relationship between the location of either the propeller shaft or [anti-ventilation] plate and the bottom of the hull. Any facts or experience? Look forward as usual to "Wave Wisdom"
My application is the addition of a kicker to my [unclear, perhaps means a 22-foot boat]. The kicker is a 9.9 Yamaha 4-stroke with 25-inch shaft. It's too long; the skeg touches the water in up position. Having looked at different options to correct, will jack it up using very solid mahgony plate bolted to the transom. I'm thinking up, 4 to 5-inches.
Dave
Wiley Cat posted 05-04-2010 10:28 PM ET (US)
The [anti-ventilation] plate should ride on the surface or slightly above the surface when the boat is on a fast plane. The height can only be determined by testing, as the setback of the motor also comes into play. There is no hard and fast rule. The relation of the propshaft to the bottom of the boat is not important in a "not for racing" boat.
Dickie
jimh posted 05-04-2010 11:11 PM ET (US)
I am confused. Are you asking about the mounting height of the main engine on a Boston Whaler 22-foot boat? Or the mounting height of an auxiliary engine on a Boston Whaler 22-foot boat?
dlschutz posted 05-04-2010 11:24 PM ET (US)
Hi Jim(?),
My interest in particular, in the end, relates to how far I should jack up my 25" 9.9 kicker on the back of my v-22...
The bigger picture I believe, with more important relevance to our peers, is the general subject of [anti-ventilation] plate or propeller shaft location relative to hull bottom for ANY attached propulsion in order to achieve some kind of efficient starting point...
I'm a little unclear on the science behind it and am inclined to believe that experience is the better yardstick...
What are your thoughts jimh (do your friends call you Jim?)....
Dave
jimh posted 05-05-2010 12:26 AM ET (US)
The mounting height of an auxiliary motor is quite a separate topic from the main motor. For the auxiliary motor the shaft length and mounting height should be chosen so that the propeller is nicely immersed in the water and shows no sign of [ventilation]. The proof of proper mounting height will likely be when operating in rough seas. If the propeller of the auxiliary motor breaks the surface as the boat moves in rough seas, the engine is probably mounted too high. For a starting point, I would refer to the owner's manual for your boat. If you have a classic 22-foot Boston Whaler, you can find the information in the REFERENCE section, where the owner's manual is reproduced in hypertext form:
http://continuouswave.com/whaler/reference/manual18-25/operations. html#auxiliary
Tohsgib posted 05-05-2010 11:41 AM ET (US)
The deeper the better with an aux engine as then it won't [ventilate] in heavy seas. It does not need to be mounted high as it will never plane a boat or create enough drag.
kwik_wurk posted 05-05-2010 11:56 AM ET (US)
Like Bigshot said.
Mount that kicker as deep as you can, but still be able to tilt it clear of the water. (If the skeg is in the water, you can get an adjustable bracket.)
Last thing you want is the trolling motor popping out of the water and running up, every time you roll over a wave.
dlschutz posted 05-05-2010 01:35 PM ET (US)
Well you guys have done it again, spoiled a perfectly good, controversial, subject!
OK, you guys are right, the aux motor shaft SHOULD be as deep in the water as possible in order to avoid [ventilation] and run-up on rollers...
So, the conventional wisdom should be: Get the boat on plane and adjust the aux. down until the skeg clears the water. This would give you the maximum bite, and minimum chance of [ventilation].
Anyone disagreeing with us on this is wrong..... or stubborn...
Now, the science behind the main propulsion relationships?
I shall go back outside to mount my kicker while the discussion heats up.
Thanks to all. As usual, the solution is on "The Wave".
Dave
Tom W Clark posted 05-05-2010 01:38 PM ET (US)
There is absolutely no way on Earth that a Yamaha 9.9, Hi-Thrust or otherwise, 20 inch or 25 inch, will get a 22 foot Boston Whaler up on plane.
number9 posted 05-05-2010 02:54 PM ET (US)
Hey Dave,

quote:
So, the conventional wisdom should be: Get the boat on plane and adjust the aux. down until the skeg clears the water. This would give you the maximum bite, and minimum chance of [ventilation].

That was not recommended by any of the replies. You communicate like a trailer boater. Those who keep their boats in the H2O normally would wand to mount the aux high enough to have the lower unit out of the water at rest. If just out on plane, probably in at rest and cause some drag during getting on plane phase. Use some common sense, just mount so the A/V plate is level or lower than the hull bottom at mounting point. If still in the water when tilted invest in a heavy duty EEz-In type mount.
dlschutz posted 05-05-2010 03:23 PM ET (US)
This is better than I hoped for!
Tom: you seem like one of the most experienced guys out there.... Ooops, how did you figure I could "plane the boat" on other than the 225, while I was "adjusting" the kicker? Which, would have to be out of the water, if I was checking to see "how low before the skeg touches the water"? I'm glad you mentioned it though, some might think you could plane a big heavy boat like this with a 10 horse OB... I was hoping for 4 or 5 knots (with current and tail wind?).
Happens to the best....
And number9: I'm a commercial Striper fisherman on the south shore of Boston. I don't even know what a trailer is...
"Common sense" would seem to indicate that at rest with say 1/2" clearance to the water (aux. skeg tilted up), on a calm sea, would BURY the skeg on plane...... Maybe I should have said to lower it until it "just" clears the water....
Go back and read what I said, I think it was right...
I could be wrong, and I love you guys.
Thanks to all. I am sincere in this, I DO appreciate the help out there. I've never seen a more selfless and helpful bunch of guys, dedicated to a wonderful product and great community!
Best,
Dave

Tom W Clark posted 05-05-2010 03:36 PM ET (US)

quote:
Tom...how did you figure I could "plane the boat" on other than the 225, while I was "adjusting" the kicker?
I didn't and I don't.

quote:
"Common sense" would seem to indicate that at rest with say 1/2" clearance to the water (aux. skeg tilted up), on a calm sea, would BURY the skeg on plane
Why? I wouldn't expect that at all, it will clear by a mile while on plane.
I'm honestly not sure what you are talking about or what the problem is. Clamp the kikcer on the transom, tilt it up and away you go.
Ideally you want the kicker as high on the transom as you can get it; some use a plank or block bolted to the transom as a simple way to "jack it up" on the transom. This helps keep the powerhead from getting sprayed or swamped in sloppy conditons.
The [anti-ventilation] plate does NOT need to below the bottom of the hull at all but it should be deep enough so as to not get near the surface of the water and ventilate in big waves as had already been mentioned.
Tom W Clark posted 05-05-2010 03:41 PM ET (US)
Dave -- You may want to read this articel:
http://continuouswave.com/whaler/reference/yamahaT8Kicker.html
You would normally use a 20" shaft kicker on a 22 footer so if you have a 25" shaft kicker, your idea of jacking it up 5 inches is a good one.
dlschutz posted 05-05-2010 05:13 PM ET (US)
Thanks Tom,
I'll do that.
Dave
WhalerAce posted 05-05-2010 09:50 PM ET (US)
Y'all please change the word CAVITATE to VENTILATE. There is no way that a kicker can reduce the water pressure enough to cause the water to boil.
And please change the words CAVITATION PLATE to VENTILATION PLATE.
As an engineer, I appreciate using the proper technical ternm. Jimh is still working on my English (grammar), but I hope to get there soon.
--- WhalerAce
And yes, I'm on a job out-of-town, cross because I don't have any boats with me, and on top of that, when we brought the Tempation back hone Sunday, I found a crack in the "bullet" section of the Port lower unit.
jimh posted 05-05-2010 10:03 PM ET (US)
Ace: Good idea; done.
WhalerAce posted 05-05-2010 10:10 PM ET (US)
Jim: Thanks. Now we can work on my idea of a MOTOR being an electrical device (e.g., starter motor, trim motor), while that petrochemical-consuming device is an ENGINE (e.g., Outboard Engine, 225 HP E-TEC Engine).
But really, who has time for this when they can laugh their A## off reading the shmuck from Eastern Canada!
dlschutz posted 05-06-2010 10:05 AM ET (US)
Thanks for the input WhalerAce and this gem: "I appreciate using the proper technical TERNM"...
You gotta admit, that's pretty funny.
Your point however is well made.
Now then y'all.....
I live on the south shore of Boston down the street from where "BOSTON Whalers" WERE made.
In the interest of consistency, I would appreciate it if you guys with the newer, white, pointy boats would please correctly refer to them as EDGEWATER Whalers....
Goin back outside... fixin the boat.
Best to all,
Dave

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Author	Topic: Cavitation Plate Location Relative to Hull Bottom
dlschutz	posted 05-04-2010 07:46 PM ET (US) As many of you know, Boston Whaler makes recommendations regarding the [engine shaft lenth of outboard motors], that is, 20-inch or 25-inch shafts. Many of you more experienced guys also know that 20-inch or 25-inch varies as much as a couple inches by vintage and manufacture, so this is a reasonable guide, not the answer. It would seem that there is an optimum measurement for the relationship between the location of either the propeller shaft or [anti-ventilation] plate and the bottom of the hull. Any facts or experience? Look forward as usual to "Wave Wisdom" My application is the addition of a kicker to my [unclear, perhaps means a 22-foot boat]. The kicker is a 9.9 Yamaha 4-stroke with 25-inch shaft. It's too long; the skeg touches the water in up position. Having looked at different options to correct, will jack it up using very solid mahgony plate bolted to the transom. I'm thinking up, 4 to 5-inches. Dave
Wiley Cat	posted 05-04-2010 10:28 PM ET (US) The [anti-ventilation] plate should ride on the surface or slightly above the surface when the boat is on a fast plane. The height can only be determined by testing, as the setback of the motor also comes into play. There is no hard and fast rule. The relation of the propshaft to the bottom of the boat is not important in a "not for racing" boat. Dickie
jimh	posted 05-04-2010 11:11 PM ET (US) I am confused. Are you asking about the mounting height of the main engine on a Boston Whaler 22-foot boat? Or the mounting height of an auxiliary engine on a Boston Whaler 22-foot boat?
dlschutz	posted 05-04-2010 11:24 PM ET (US) Hi Jim(?), My interest in particular, in the end, relates to how far I should jack up my 25" 9.9 kicker on the back of my v-22... The bigger picture I believe, with more important relevance to our peers, is the general subject of [anti-ventilation] plate or propeller shaft location relative to hull bottom for ANY attached propulsion in order to achieve some kind of efficient starting point... I'm a little unclear on the science behind it and am inclined to believe that experience is the better yardstick... What are your thoughts jimh (do your friends call you Jim?).... Dave
jimh	posted 05-05-2010 12:26 AM ET (US) The mounting height of an auxiliary motor is quite a separate topic from the main motor. For the auxiliary motor the shaft length and mounting height should be chosen so that the propeller is nicely immersed in the water and shows no sign of [ventilation]. The proof of proper mounting height will likely be when operating in rough seas. If the propeller of the auxiliary motor breaks the surface as the boat moves in rough seas, the engine is probably mounted too high. For a starting point, I would refer to the owner's manual for your boat. If you have a classic 22-foot Boston Whaler, you can find the information in the REFERENCE section, where the owner's manual is reproduced in hypertext form: http://continuouswave.com/whaler/reference/manual18-25/operations. html#auxiliary
Tohsgib	posted 05-05-2010 11:41 AM ET (US) The deeper the better with an aux engine as then it won't [ventilate] in heavy seas. It does not need to be mounted high as it will never plane a boat or create enough drag.
kwik_wurk	posted 05-05-2010 11:56 AM ET (US) Like Bigshot said. Mount that kicker as deep as you can, but still be able to tilt it clear of the water. (If the skeg is in the water, you can get an adjustable bracket.) Last thing you want is the trolling motor popping out of the water and running up, every time you roll over a wave.
dlschutz	posted 05-05-2010 01:35 PM ET (US) Well you guys have done it again, spoiled a perfectly good, controversial, subject! OK, you guys are right, the aux motor shaft SHOULD be as deep in the water as possible in order to avoid [ventilation] and run-up on rollers... So, the conventional wisdom should be: Get the boat on plane and adjust the aux. down until the skeg clears the water. This would give you the maximum bite, and minimum chance of [ventilation]. Anyone disagreeing with us on this is wrong..... or stubborn... Now, the science behind the main propulsion relationships? I shall go back outside to mount my kicker while the discussion heats up. Thanks to all. As usual, the solution is on "The Wave". Dave
Tom W Clark	posted 05-05-2010 01:38 PM ET (US) There is absolutely no way on Earth that a Yamaha 9.9, Hi-Thrust or otherwise, 20 inch or 25 inch, will get a 22 foot Boston Whaler up on plane.
number9	posted 05-05-2010 02:54 PM ET (US) Hey Dave, quote: So, the conventional wisdom should be: Get the boat on plane and adjust the aux. down until the skeg clears the water. This would give you the maximum bite, and minimum chance of [ventilation]. That was not recommended by any of the replies. You communicate like a trailer boater. Those who keep their boats in the H2O normally would wand to mount the aux high enough to have the lower unit out of the water at rest. If just out on plane, probably in at rest and cause some drag during getting on plane phase. Use some common sense, just mount so the A/V plate is level or lower than the hull bottom at mounting point. If still in the water when tilted invest in a heavy duty EEz-In type mount.
dlschutz	posted 05-05-2010 03:23 PM ET (US) This is better than I hoped for! Tom: you seem like one of the most experienced guys out there.... Ooops, how did you figure I could "plane the boat" on other than the 225, while I was "adjusting" the kicker? Which, would have to be out of the water, if I was checking to see "how low before the skeg touches the water"? I'm glad you mentioned it though, some might think you could plane a big heavy boat like this with a 10 horse OB... I was hoping for 4 or 5 knots (with current and tail wind?). Happens to the best.... And number9: I'm a commercial Striper fisherman on the south shore of Boston. I don't even know what a trailer is... "Common sense" would seem to indicate that at rest with say 1/2" clearance to the water (aux. skeg tilted up), on a calm sea, would BURY the skeg on plane...... Maybe I should have said to lower it until it "just" clears the water.... Go back and read what I said, I think it was right... I could be wrong, and I love you guys. Thanks to all. I am sincere in this, I DO appreciate the help out there. I've never seen a more selfless and helpful bunch of guys, dedicated to a wonderful product and great community! Best, Dave
Tom W Clark	posted 05-05-2010 03:36 PM ET (US) quote: Tom...how did you figure I could "plane the boat" on other than the 225, while I was "adjusting" the kicker? I didn't and I don't. quote: "Common sense" would seem to indicate that at rest with say 1/2" clearance to the water (aux. skeg tilted up), on a calm sea, would BURY the skeg on plane Why? I wouldn't expect that at all, it will clear by a mile while on plane. I'm honestly not sure what you are talking about or what the problem is. Clamp the kikcer on the transom, tilt it up and away you go. Ideally you want the kicker as high on the transom as you can get it; some use a plank or block bolted to the transom as a simple way to "jack it up" on the transom. This helps keep the powerhead from getting sprayed or swamped in sloppy conditons. The [anti-ventilation] plate does NOT need to below the bottom of the hull at all but it should be deep enough so as to not get near the surface of the water and ventilate in big waves as had already been mentioned.
Tom W Clark	posted 05-05-2010 03:41 PM ET (US) Dave -- You may want to read this articel: http://continuouswave.com/whaler/reference/yamahaT8Kicker.html You would normally use a 20" shaft kicker on a 22 footer so if you have a 25" shaft kicker, your idea of jacking it up 5 inches is a good one.
dlschutz	posted 05-05-2010 05:13 PM ET (US) Thanks Tom, I'll do that. Dave
WhalerAce	posted 05-05-2010 09:50 PM ET (US) Y'all please change the word CAVITATE to VENTILATE. There is no way that a kicker can reduce the water pressure enough to cause the water to boil. And please change the words CAVITATION PLATE to VENTILATION PLATE. As an engineer, I appreciate using the proper technical ternm. Jimh is still working on my English (grammar), but I hope to get there soon. --- WhalerAce And yes, I'm on a job out-of-town, cross because I don't have any boats with me, and on top of that, when we brought the Tempation back hone Sunday, I found a crack in the "bullet" section of the Port lower unit.
jimh	posted 05-05-2010 10:03 PM ET (US) Ace: Good idea; done.
WhalerAce	posted 05-05-2010 10:10 PM ET (US) Jim: Thanks. Now we can work on my idea of a MOTOR being an electrical device (e.g., starter motor, trim motor), while that petrochemical-consuming device is an ENGINE (e.g., Outboard Engine, 225 HP E-TEC Engine). But really, who has time for this when they can laugh their A## off reading the shmuck from Eastern Canada!
dlschutz	posted 05-06-2010 10:05 AM ET (US) Thanks for the input WhalerAce and this gem: "I appreciate using the proper technical TERNM"... You gotta admit, that's pretty funny. Your point however is well made. Now then y'all..... I live on the south shore of Boston down the street from where "BOSTON Whalers" WERE made. In the interest of consistency, I would appreciate it if you guys with the newer, white, pointy boats would please correctly refer to them as EDGEWATER Whalers.... Goin back outside... fixin the boat. Best to all, Dave