Formula for Engine Mounting Height v. Set Back

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Author Topic:   Formula for Engine Mounting Height v. Set Back

Paul Graham posted 01-15-2009 11:51 PM ET (US)
I have a 78 Montauk with a 75 hp Honda, I would like to set my motor back 4 to 6 inches on a fixed jack plate. I do not know how much I will be able to raise the motor with either set back, is there a formula for this?
tedious posted 01-16-2009 07:49 AM ET (US)
Paul, I believe the formula is 2" back = 1" up.
Tim
Plotman posted 01-16-2009 11:42 AM ET (US)
Yeah, but remember that most transoms are angled past vertical somewhat - so that you need a certain amount of "lift" in the jack plate just to keep the engine from ending up below where you started.
Tohsgib posted 01-16-2009 11:51 AM ET (US)
My 70 Suzuki was on a 6" setback and lifted up approx 4.5" and ran sweet.
Tom W Clark posted 01-16-2009 11:58 AM ET (US)
12" back, 1" up.
Tohsgib posted 01-16-2009 12:30 PM ET (US)
Actually if you are talking about engine brackets it is 6" back 1" up so a 30" setback you can run a 5" shorter shaft. With Jackplates it is much more extreme. My 115 is up about 5" up on my Revenge and runs normal, just like my Newtauk did.
Paul Graham posted 01-17-2009 10:54 PM ET (US)
Tohsgib, did you have that set up on Montauk, how did it affect the steering cable entering the steering tube? I would think the cable not route thru the cut out in the transom but perhaps would have to route up over the starboard side of the higher part of the transom.
jimh posted 01-18-2009 01:25 AM ET (US)
[This discussion about performance has been moved to the PERFORMANCE discussion.]
jimh posted 01-18-2009 01:57 AM ET (US)
The set back bracket may be fixed, but you will still have the engine mounting holes as a means of adjustment. This will give you a 3-inch range of vertical adjustment.
L H G posted 01-19-2009 01:55 AM ET (US)
Mercury's Performance Rigging Manual says:
1/2" up for every 6" of setback
combined with, if applicable:
1/2" up for every 5 mph of top speed capability OVER 40 MPH.
As an example, my 19 Outrage has a top speed capability of 50 MPH, with the 150HP Merc engine mounted on a 6" setback jackplate. The formula gives 1-1/2" up, which when combined with a 21" Laser II prop, works perfectly with great holding and acceleration.
Tohsgib posted 01-19-2009 11:56 AM ET (US)
Yes I had it on my Montauk and Newtauk. Cable length was not an issue and was easier to install. Don't remember the exact setup though. Again you can raise it up and keep going until you feel you went too high for your driving habits. Just bring a couple 3/4" wrenches with you and keep going up an inch at a time. People can speculate and show you charts and graphs about what is "recommended" but I did it the old fashioned way...trial and error and my results are 4+" up on a 5-6" setback on a 17' hull and my 70hp Suzuki 4 stroke.
Tom W Clark posted 01-19-2009 12:24 PM ET (US)
Dvid made a good point. With Whaler's typical 15 degree transom, you also go down as you move the motor back.
For every 6" you move the motor back the motor goes down about 1-1/2". So Nick's motor is either only up 2-1/2" or he raised it on his bracket 5-1/2" to get the motor up 4".
outragesteve posted 01-19-2009 02:04 PM ET (US)
I rigged my Montauk w/ a Gil 12" Mod-VP style bracket. I raised the engine 6" and it was perfect. With my Merc 115/6 I gained over 10MPH. In sloppy water the boat ran like it was much "longer". I had to go 4" up in prop pitch to keep the max RPM @ 5800-6000. I miss this boat!
hauptjm posted 01-20-2009 10:47 AM ET (US)
I never thought of the phenomenon of having to change the pitch on one's propeller because of moving the engine further back and up. Has anyone else experienced this? Is there a formula?
I'm considering some changes to my set-up in the future and didn't know there would be any material change in the prop department.
Tohsgib posted 01-20-2009 03:41 PM ET (US)
For Tom et al...My 4+" is where my cav plate is above my keel. so I have really no idea how high th eplate itself is. There for I am probaly correct in that I am up about 3" if the correct setup is 1.5" above the kel on a transom mounted engine. My Revenge transom does not have such an extreme angle and she is up about 4" above the keel as well.
jimh posted 01-20-2009 09:13 PM ET (US)
In response to the inquiry about a formula for computing the relationship between engine setback and mounting height, I notice that:
Tom W Clark says: 12-inch back, 1-inch up,
and L H G says: 1/2-inch up for every 6-inch of setback
I believe these two fine gentlemen and Boston Whaler experts are saying precisely the same thing, albeit in a slightly different way. Has that happened before?
Tohsgib posted 01-20-2009 11:24 PM ET (US)
What do you really mean Jim? Yes it is the same but......
brisboats posted 01-21-2009 09:22 AM ET (US)
I "faithfully" believe it does happen.
Brian
number9 posted 01-21-2009 05:11 PM ET (US)
Please correct me if wrong.
One can mount the motor three holes up giving approximately a 2.25" range, not 3" as reply stated.
Tom W Clark posted 01-21-2009 05:16 PM ET (US)
Large Mercury outboards have five sets of bolt holes allowing 3" of vertical adjustment
Large OMC/BRP, Yamaha, Suzuki and Honda have four sets of bolt holes allowing 2-1/4" of vertical adjustment.
All large outboards have their bolt holes spaced 3/4" o.c.
L H G posted 01-21-2009 06:51 PM ET (US)
There is a lot of VALUABLE information if you click the LINK in the first post:
http://continuouswave.com/ubb/Forum4/HTML/006432.html
Note that the Merc 90's and 115's, all varieties, have a much longer shaft length than standard, and probably need to be mounted much higher than one would expect. I wonder why they designed those engine that way?
number9 posted 01-22-2009 02:34 PM ET (US)
Tom, thanks for that info about the number of Merc holes. FYI my '05 Honda rigging guide indicates the BF200/225 also have five. Reason I wrote approximately 2.25" is that Yamaha and Honda give the C/L distance of 0.73" between holes.

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Author	Topic: Formula for Engine Mounting Height v. Set Back
Paul Graham	posted 01-15-2009 11:51 PM ET (US) I have a 78 Montauk with a 75 hp Honda, I would like to set my motor back 4 to 6 inches on a fixed jack plate. I do not know how much I will be able to raise the motor with either set back, is there a formula for this?
tedious	posted 01-16-2009 07:49 AM ET (US) Paul, I believe the formula is 2" back = 1" up. Tim
Plotman	posted 01-16-2009 11:42 AM ET (US) Yeah, but remember that most transoms are angled past vertical somewhat - so that you need a certain amount of "lift" in the jack plate just to keep the engine from ending up below where you started.
Tohsgib	posted 01-16-2009 11:51 AM ET (US) My 70 Suzuki was on a 6" setback and lifted up approx 4.5" and ran sweet.
Tom W Clark	posted 01-16-2009 11:58 AM ET (US) 12" back, 1" up.
Tohsgib	posted 01-16-2009 12:30 PM ET (US) Actually if you are talking about engine brackets it is 6" back 1" up so a 30" setback you can run a 5" shorter shaft. With Jackplates it is much more extreme. My 115 is up about 5" up on my Revenge and runs normal, just like my Newtauk did.
Paul Graham	posted 01-17-2009 10:54 PM ET (US) Tohsgib, did you have that set up on Montauk, how did it affect the steering cable entering the steering tube? I would think the cable not route thru the cut out in the transom but perhaps would have to route up over the starboard side of the higher part of the transom.
jimh	posted 01-18-2009 01:25 AM ET (US) [This discussion about performance has been moved to the PERFORMANCE discussion.]
jimh	posted 01-18-2009 01:57 AM ET (US) The set back bracket may be fixed, but you will still have the engine mounting holes as a means of adjustment. This will give you a 3-inch range of vertical adjustment.
L H G	posted 01-19-2009 01:55 AM ET (US) Mercury's Performance Rigging Manual says: 1/2" up for every 6" of setback combined with, if applicable: 1/2" up for every 5 mph of top speed capability OVER 40 MPH. As an example, my 19 Outrage has a top speed capability of 50 MPH, with the 150HP Merc engine mounted on a 6" setback jackplate. The formula gives 1-1/2" up, which when combined with a 21" Laser II prop, works perfectly with great holding and acceleration.
Tohsgib	posted 01-19-2009 11:56 AM ET (US) Yes I had it on my Montauk and Newtauk. Cable length was not an issue and was easier to install. Don't remember the exact setup though. Again you can raise it up and keep going until you feel you went too high for your driving habits. Just bring a couple 3/4" wrenches with you and keep going up an inch at a time. People can speculate and show you charts and graphs about what is "recommended" but I did it the old fashioned way...trial and error and my results are 4+" up on a 5-6" setback on a 17' hull and my 70hp Suzuki 4 stroke.
Tom W Clark	posted 01-19-2009 12:24 PM ET (US) Dvid made a good point. With Whaler's typical 15 degree transom, you also go down as you move the motor back. For every 6" you move the motor back the motor goes down about 1-1/2". So Nick's motor is either only up 2-1/2" or he raised it on his bracket 5-1/2" to get the motor up 4".
outragesteve	posted 01-19-2009 02:04 PM ET (US) I rigged my Montauk w/ a Gil 12" Mod-VP style bracket. I raised the engine 6" and it was perfect. With my Merc 115/6 I gained over 10MPH. In sloppy water the boat ran like it was much "longer". I had to go 4" up in prop pitch to keep the max RPM @ 5800-6000. I miss this boat!
hauptjm	posted 01-20-2009 10:47 AM ET (US) I never thought of the phenomenon of having to change the pitch on one's propeller because of moving the engine further back and up. Has anyone else experienced this? Is there a formula? I'm considering some changes to my set-up in the future and didn't know there would be any material change in the prop department.
Tohsgib	posted 01-20-2009 03:41 PM ET (US) For Tom et al...My 4+" is where my cav plate is above my keel. so I have really no idea how high th eplate itself is. There for I am probaly correct in that I am up about 3" if the correct setup is 1.5" above the kel on a transom mounted engine. My Revenge transom does not have such an extreme angle and she is up about 4" above the keel as well.
jimh	posted 01-20-2009 09:13 PM ET (US) In response to the inquiry about a formula for computing the relationship between engine setback and mounting height, I notice that: Tom W Clark says: 12-inch back, 1-inch up, and L H G says: 1/2-inch up for every 6-inch of setback I believe these two fine gentlemen and Boston Whaler experts are saying precisely the same thing, albeit in a slightly different way. Has that happened before?
Tohsgib	posted 01-20-2009 11:24 PM ET (US) What do you really mean Jim? Yes it is the same but......
brisboats	posted 01-21-2009 09:22 AM ET (US) I "faithfully" believe it does happen. Brian
number9	posted 01-21-2009 05:11 PM ET (US) Please correct me if wrong. One can mount the motor three holes up giving approximately a 2.25" range, not 3" as reply stated.
Tom W Clark	posted 01-21-2009 05:16 PM ET (US) Large Mercury outboards have five sets of bolt holes allowing 3" of vertical adjustment Large OMC/BRP, Yamaha, Suzuki and Honda have four sets of bolt holes allowing 2-1/4" of vertical adjustment. All large outboards have their bolt holes spaced 3/4" o.c.
L H G	posted 01-21-2009 06:51 PM ET (US) There is a lot of VALUABLE information if you click the LINK in the first post: http://continuouswave.com/ubb/Forum4/HTML/006432.html Note that the Merc 90's and 115's, all varieties, have a much longer shaft length than standard, and probably need to be mounted much higher than one would expect. I wonder why they designed those engine that way?
number9	posted 01-22-2009 02:34 PM ET (US) Tom, thanks for that info about the number of Merc holes. FYI my '05 Honda rigging guide indicates the BF200/225 also have five. Reason I wrote approximately 2.25" is that Yamaha and Honda give the C/L distance of 0.73" between holes.