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Author Topic:   Is my motor/prop too low?

JRDIII posted 09-04-2012 10:37 AM ET (US)
I recently bought a 1962 13-footer with a 1992 Johnson 40 hp. The boat is in decent shape, and I plan to work gradually on restoring it. In the meantime, I just want to enjoy it. The motor runs pretty well, although it idles rough and will sometimes cut off when going from idle into gear. Probably needs a thorough carb clean and/or a new power pack. But once you get it going it runs great. It gets up on plane quickly and the motor is very smooth at full throttle.
My question is about the prop placement, which seems to me like it may be too low. Out of the water with the motor down, the anti-cavitation plate is just about even with the bottom of the hull, which is where it should be. But when the boat is at rest on the water, almost the entire lower unit is submerged. On plane, the anti-cavitation plate seems to be under the surface, although it's hard for me to safely tell. At full throttle on smooth water I am maxing out at 27-29 mph according to GPS. From everything I've read, this boat with a 40 hp ought to be in the low- to mid-30s. The motor is mounted in the top holes of the motor bracket, and the bracket rests on the transom. Should I consider raising it to the next set of holes down, or even two sets of holes down? I would have to drill new holes for the bottom bolts, which is not something I really want to do if I can avoid it.
I have some video from last weekend where you can get an idea of the motor position. FYI, the prop appears to be original and is 17 pitch. Can any of you experts get enough of a view from this video to suggest whether I should make an adjustment or just leave it as is? Thanks.
http://youtu.be/1fpJDIun2Yw

dgoodhue posted 09-04-2012 03:55 PM ET (US)

quote:
Out of the water with the motor down, the anti-cavitation plate is just about even with the bottom of the hull, which is where it should be.
The cavitation plate even with the bottom of the boat is NOT where it should be. You should raise the motor 2 or 3 holes up depending on prop and redrill the transom for the lower holes. I image you currently have it mounted with the blind holes.
JRDIII posted 09-04-2012 07:57 PM ET (US)
I don't think it's in the blind holes because I don't see any blind holes on my motor mount. Here's a photo of how it's mounted:
http://www.flickr.com/photos/jrdiii/7933423688/
JMARTIN posted 09-04-2012 08:21 PM ET (US)
http://www.flickr.com/photos/jrdiii/7933423688/
John
dg22 posted 09-05-2012 10:30 AM ET (US)
I have a similiar setup and raised mine an inch and it is a little faster now but the only downside is I do get more cavitation in the tight corners. I do need to re-prop so I'm hoping a SS prop will give me more bite in the corners. I have a 1973 50 hp Evinrude (large gear case like yours) with a 19 pitch prop and my top speed is around 33 mph and my RPM's are at 5300 with me alone in the boat.
dgoodhue posted 09-05-2012 01:55 PM ET (US)
I agree it's not blind mounted. Is that lower bolt, a lag bolt? It still should be raised up. Your top speed trimmed out should be mid 30's.
The neutral to forward stalling is more likely the carbs needing adjustment or cleaning rather than power pack. I find many marine mechanics don't spend he time to fine tune the idle. It's seems like the older carb 2 strokes are little more finicky these day with the 10% ethanol gas that almost everyone uses.
JRDIII posted 09-05-2012 03:46 PM ET (US)
Not sure what you mean by lag bolt, but the top and bottom bolts, nuts and washers appear to be the same. The bottom bolts are going through an adjustable hole in the motor mount. Unfortunately, they are at the very bottom of the adjustable hole, so I won't be able to just slide the motor up. I'll have to drill new bottom holes. Looks like it's been done numerous times before on this 50-year-old transom.

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Author	Topic: Is my motor/prop too low?
JRDIII	posted 09-04-2012 10:37 AM ET (US) I recently bought a 1962 13-footer with a 1992 Johnson 40 hp. The boat is in decent shape, and I plan to work gradually on restoring it. In the meantime, I just want to enjoy it. The motor runs pretty well, although it idles rough and will sometimes cut off when going from idle into gear. Probably needs a thorough carb clean and/or a new power pack. But once you get it going it runs great. It gets up on plane quickly and the motor is very smooth at full throttle. My question is about the prop placement, which seems to me like it may be too low. Out of the water with the motor down, the anti-cavitation plate is just about even with the bottom of the hull, which is where it should be. But when the boat is at rest on the water, almost the entire lower unit is submerged. On plane, the anti-cavitation plate seems to be under the surface, although it's hard for me to safely tell. At full throttle on smooth water I am maxing out at 27-29 mph according to GPS. From everything I've read, this boat with a 40 hp ought to be in the low- to mid-30s. The motor is mounted in the top holes of the motor bracket, and the bracket rests on the transom. Should I consider raising it to the next set of holes down, or even two sets of holes down? I would have to drill new holes for the bottom bolts, which is not something I really want to do if I can avoid it. I have some video from last weekend where you can get an idea of the motor position. FYI, the prop appears to be original and is 17 pitch. Can any of you experts get enough of a view from this video to suggest whether I should make an adjustment or just leave it as is? Thanks. http://youtu.be/1fpJDIun2Yw
dgoodhue	posted 09-04-2012 03:55 PM ET (US) quote: Out of the water with the motor down, the anti-cavitation plate is just about even with the bottom of the hull, which is where it should be. The cavitation plate even with the bottom of the boat is NOT where it should be. You should raise the motor 2 or 3 holes up depending on prop and redrill the transom for the lower holes. I image you currently have it mounted with the blind holes.
JRDIII	posted 09-04-2012 07:57 PM ET (US) I don't think it's in the blind holes because I don't see any blind holes on my motor mount. Here's a photo of how it's mounted: http://www.flickr.com/photos/jrdiii/7933423688/
JMARTIN	posted 09-04-2012 08:21 PM ET (US) http://www.flickr.com/photos/jrdiii/7933423688/ John
dg22	posted 09-05-2012 10:30 AM ET (US) I have a similiar setup and raised mine an inch and it is a little faster now but the only downside is I do get more cavitation in the tight corners. I do need to re-prop so I'm hoping a SS prop will give me more bite in the corners. I have a 1973 50 hp Evinrude (large gear case like yours) with a 19 pitch prop and my top speed is around 33 mph and my RPM's are at 5300 with me alone in the boat.
dgoodhue	posted 09-05-2012 01:55 PM ET (US) I agree it's not blind mounted. Is that lower bolt, a lag bolt? It still should be raised up. Your top speed trimmed out should be mid 30's. The neutral to forward stalling is more likely the carbs needing adjustment or cleaning rather than power pack. I find many marine mechanics don't spend he time to fine tune the idle. It's seems like the older carb 2 strokes are little more finicky these day with the 10% ethanol gas that almost everyone uses.
JRDIII	posted 09-05-2012 03:46 PM ET (US) Not sure what you mean by lag bolt, but the top and bottom bolts, nuts and washers appear to be the same. The bottom bolts are going through an adjustable hole in the motor mount. Unfortunately, they are at the very bottom of the adjustable hole, so I won't be able to just slide the motor up. I'll have to drill new bottom holes. Looks like it's been done numerous times before on this 50-year-old transom.