OUTRAGE 18, Twin 70-HP: Engine Height - Moderated Discussion Areas

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Author Topic:   OUTRAGE 18, Twin 70-HP: Engine Height

gmoulder posted 12-06-2009 10:05 AM ET (US)
All: Sure this is an old subject. Pardon me for getting in on this late. The boat is an 1988 Outrage 18, the engines are twin 1996 Johnson 70-HP two-cycle engines. In reading recent discussions, I realized that there is adjustability in the motor height. I just thought that the dealer knew what to do and did it. The boat is in the shop right now, some work on the console wiring, and while it is there, I might as well correct any obvious mistakes--if any--in the mounting height and start with a good baseline.
What is the intended physical relationship between the propeller, the cavitation plate, and the bottom of the boat? I read the Mercury mounting height reference but did not get the intent. The engines bolt directly to the transom, there is no Whaler Drive or other bracket. Hopefully, someone has wrung this one out, and knows where the bolts go for best performance. I would appreciate your advice on this.
jimh posted 12-06-2009 11:00 AM ET (US)
For twin engines on the transom of an OUTRAGE 18 use engines with 20-inch shaft length. Assuming the transom is drilled with mounting holes in the standard BIA hole pattern, and assuming the engine position is located as recommended by Boston Whaler at the specified horizontal separation, the typical installation would begin with mounting the engines "one-hole up" from the lowest position.
Further adjustment is possible and will be influenced by the choice of propellers. Sea state and normal boat operating load will also affect engine mounting height.
REFERENCE ARTICLES:
FAQ: Q9: What is the Standard Transom Hole Layout?
http://continuouswave.com/whaler/reference/FAQ/#Q9

Dimensions and Production History
http://continuouswave.com/whaler/reference/specifications. html#hulls15-19
Twin Outboard Engine Installation on Moderate V-hull Boats
http://continuouswave.com/whaler/reference/twinEngineMounting.html
The relationship between the propeller shaft and the anti-ventilation plate is fixed in the design of the engine.
The relationship between the transom bottom and the anti-ventilation plate is dependent on:
--the transom design
--the engine shaft length
--the engine mounting position
The position of the anti-ventilation plate relative to the maximum transom depth is generally not as significant as the relationship to the surface of the water when running on plane. Engine mounting height is generally adjusted to permit the anti-ventilation plate to ride just above the surface of the water stream flowing around the gear case when the boat is on plane and the engine trim has been adjusted to set the A-V plate parallel to the surface of the water stream. See the many pictures in the article linked below.
A-V Plate Pictures
http://continuouswave.com/ubb/Forum4/HTML/006491.html
The height to which the engine can be raised depends on several factors:
--the propeller used, and its ability to maintain a good grip or bite
--the location of the cooling water inlets and their ability to provide good water flow to the cooling system
--the overall boat handling that results
The typical approach to engine height is to raise the engine until a negative outcome is reached, that is, the propeller loses grip, or the cooling water flow declines, or the boat handling becomes awkward. At that point, your engine is too high and should be lowered one hole.

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Author	Topic: OUTRAGE 18, Twin 70-HP: Engine Height
gmoulder	posted 12-06-2009 10:05 AM ET (US) All: Sure this is an old subject. Pardon me for getting in on this late. The boat is an 1988 Outrage 18, the engines are twin 1996 Johnson 70-HP two-cycle engines. In reading recent discussions, I realized that there is adjustability in the motor height. I just thought that the dealer knew what to do and did it. The boat is in the shop right now, some work on the console wiring, and while it is there, I might as well correct any obvious mistakes--if any--in the mounting height and start with a good baseline. What is the intended physical relationship between the propeller, the cavitation plate, and the bottom of the boat? I read the Mercury mounting height reference but did not get the intent. The engines bolt directly to the transom, there is no Whaler Drive or other bracket. Hopefully, someone has wrung this one out, and knows where the bolts go for best performance. I would appreciate your advice on this.
jimh	posted 12-06-2009 11:00 AM ET (US) For twin engines on the transom of an OUTRAGE 18 use engines with 20-inch shaft length. Assuming the transom is drilled with mounting holes in the standard BIA hole pattern, and assuming the engine position is located as recommended by Boston Whaler at the specified horizontal separation, the typical installation would begin with mounting the engines "one-hole up" from the lowest position. Further adjustment is possible and will be influenced by the choice of propellers. Sea state and normal boat operating load will also affect engine mounting height. REFERENCE ARTICLES: FAQ: Q9: What is the Standard Transom Hole Layout? http://continuouswave.com/whaler/reference/FAQ/#Q9 Dimensions and Production History http://continuouswave.com/whaler/reference/specifications. html#hulls15-19 Twin Outboard Engine Installation on Moderate V-hull Boats http://continuouswave.com/whaler/reference/twinEngineMounting.html The relationship between the propeller shaft and the anti-ventilation plate is fixed in the design of the engine. The relationship between the transom bottom and the anti-ventilation plate is dependent on: --the transom design --the engine shaft length --the engine mounting position The position of the anti-ventilation plate relative to the maximum transom depth is generally not as significant as the relationship to the surface of the water when running on plane. Engine mounting height is generally adjusted to permit the anti-ventilation plate to ride just above the surface of the water stream flowing around the gear case when the boat is on plane and the engine trim has been adjusted to set the A-V plate parallel to the surface of the water stream. See the many pictures in the article linked below. A-V Plate Pictures http://continuouswave.com/ubb/Forum4/HTML/006491.html The height to which the engine can be raised depends on several factors: --the propeller used, and its ability to maintain a good grip or bite --the location of the cooling water inlets and their ability to provide good water flow to the cooling system --the overall boat handling that results The typical approach to engine height is to raise the engine until a negative outcome is reached, that is, the propeller loses grip, or the cooling water flow declines, or the boat handling becomes awkward. At that point, your engine is too high and should be lowered one hole.