Mini Jack Plates

[scout-j-m]

I have a 1648 Alumacraft with a 15" transom and as all of my OMC short shaft outboards sit on there, the anti-ventilation plate sits about 1 1/2" below the bottom of the hull. The boat is rated for a 35hp outboard.

Can anyone tell me who all makes a manual "mini" jack plate aside from Bob's Machine Shop? Theirs is very nice but more than I would want to spend at $200 plus it only has a couple inches of vertical adjustment. I already purchased plans for a mini version from Dillon Racing and am leaning towards building my own using those plans, but I am having trouble finding aluminum angle at a reasonable price online or locally. It seems the total cost for me to build my own would be about $100 and I would think a smaller version could be obtained for retail for not too much more.

 

[Scott Danforth]

your forgetting labor, etc. your labor is free.

plus, companies like Bob's buy material at a much higher quantity to get a discount than you and I can do on our own.

Bob's is probably the best bang for the buck.

however why not shove a 2 x 2 under the clamp frame on the motor to raise it up 1-1/2"

 

[Trawler13]

Try T&H Marine they have several options available at most marine stores...#MJ1DP, #TH65213DP, #JP4FADP...thmarine.com
Good Luck

 

[scout-j-m]

your forgetting labor, etc. your labor is free.

plus, companies like Bob's buy material at a much higher quantity to get a discount than you and I can do on our own.

Bob's is probably the best bang for the buck.

however why not shove a 2 x 2 under the clamp frame on the motor to raise it up 1-1/2"

I can raise it up a bit more but not quite enough. The current outboards I'm running on it are OMC 9.9/15's and their clamps are right under about a 1" strip of aluminum that caps the transom. I can probably move it up about an inch although I will still need to secure the motor to the boat with a cable or something just in case.

I have shimmed up my 9.9hp with a 3/4" board before but I didn't see any noticeable improvement. However, my 15hp performs considerably better on this boat so I may see better results with trying to raise it up the full inch of room I have.

 

[scout-j-m]

Try T&H Marine they have several options available at most marine stores...#MJ1DP, #TH65213DP, #JP4FADP...thmarine.com
Good Luck

Thanks. I will look into them.

 

[gm280]

I guess I'm missing something here. If you are merely using a 9.9/15 HP OB, what performance are you really looking for? In that class of OB's getting the hull to plane is about it. When you get into larger engines, then I could see a jack plate accessory. JMHO

 

[scout-j-m]

I guess I'm missing something here. If you are merely using a 9.9/15 HP OB, what performance are you really looking for? In that class of OB's getting the hull to plane is about it. When you get into larger engines, then I could see a jack plate accessory. JMHO

1) Just anything anything that would help performance. With the 15 hp and an 8" prop it runs right at the top of the WOT range at 6900 rpms with just me in it in and does about 20 mph. With a 10" prop it turns about 6500 rpms and does 22-23 mph with me solo. I'd like to go up a prop size from the 8" though so I can cruise at lower rpms without lugging the motor too, so any increase in rpms would help.

2) I use this boat to fish a lot of creeks and backwaters so I deal with shallow water and logs/stumps/timber. If I could raise the motor 1 1/2" to get it level with the bottom plus raise it even more with the offset from a jackplate, those extra 2-3" would be very beneficial.

3) I have an old 35hp that I am fixing up to also use. I use the 9.9/15's right now because the two closest lakes to me have 10 hp restrictions. I'm not sure if even a 30-40 hp is large enough to see a ton of benefit from a jackplate but nonetheless the first two reasons would still apply.

 

[gm280]

1) Just anything anything that would help performance. With the 15 hp and an 8" prop it runs right at the top of the WOT range at 6900 rpms with just me in it in and does about 20 mph. With a 10" prop it turns about 6500 rpms and does 22-23 mph with me solo. I'd like to go up a prop size from the 8" though so I can cruise at lower rpms without lugging the motor too, so any increase in rpms would help.

2) I use this boat to fish a lot of creeks and backwaters so I deal with shallow water and logs/stumps/timber. If I could raise the motor 1 1/2" to get it level with the bottom plus raise it even more with the offset from a jackplate, those extra 2-3" would be very beneficial.

3) I have an old 35hp that I am fixing up to also use. I use the 9.9/15's right now because the two closest lakes to me have 10 hp restrictions. I'm not sure if even a 30-40 hp is large enough to see a ton of benefit from a jackplate but nonetheless the first two reasons would still apply.

WOW, I honestly have to ask, are those RPMs you posted true with a known RPMs with a good tach? If so I think you are way way over revving your engine. I would go to a lot larger pitch prop to bring those RPMs down a lot. You don't want more RPMs you want to run that engine near its WOT range. So what does the manufacture specify for your engine(s). And I can see a use for a jackplate on a 30-35HP engine. But I honestly don't see much benefit with 15HP or less myself. An inch or inch and a haft to get into shollower water is the only issue I could see if that would actually make a differece. JMHO

 

[scout-j-m]

WOW, I honestly have to ask, are those RPMs you posted true with a known RPMs with a good tach? If so I think you are way way over revving your engine. I would go to a lot larger pitch prop to bring those RPMs down a lot. You don't want more RPMs you want to run that engine near its WOT range. So what does the manufacture specify for your engine(s). And I can see a use for a jackplate on a 30-35HP engine. But I honestly don't see much benefit with 15HP or less myself. An inch or inch and a haft to get into shollower water is the only issue I could see if that would actually make a differece. JMHO

I am using a Hardline inductive tach. It seems spot on as when I run the numbers through a prop calculator I get reasonable prop slip numbers (~10% with the 8" and ~20% with the 10"). I can still verify it if necessary with my timing light with built in tach though.

As to the WOT range, I have the factory service manual and it states that the WOT range for my 1992 15hp Johnson is 5500-7000 rpms while it is 4500-5500 for the 9.9 hp model. I was skeptical at first too since I know other 15hp models in the 80's have a max range of 6500 rpms, however, I searched around a good bit online and never saw anything or anyone who disputed it and a few who endorsed it. So far the motor seems to run fine at extended WOT periods up to 6900 rpms. I have also checked it after continuous use with my IR thermometer and the head and block stay right around 130-145 deg although the intake port covers do get hotter (like 180 deg if not more) I guess due to the extra friction and being so far away from the water jacket.

Back to the jackplate issue, the only thing I would expect performance-wise on the smaller motors would be to possibly get the prop slip down while using a larger pitch, thus maybe giving me some better efficiency paired with lower rpms (relative to the 8" prop) so I can maintain a decent speed but have better fuel economy. It probably won't matter or help but at least the increase in motor height would be nice.

 

[gm280]

I am using a Hardline inductive tach. It seems spot on as when I run the numbers through a prop calculator I get reasonable prop slip numbers (~10% with the 8" and ~20% with the 10"). I can still verify it if necessary with my timing light with built in tach though.

As to the WOT range, I have the factory service manual and it states that the WOT range for my 1992 15hp Johnson is 5500-7000 rpms while it is 4500-5500 for the 9.9 hp model. I was skeptical at first too since I know other 15hp models in the 80's have a max range of 6500 rpms, however, I searched around a good bit online and never saw anything or anyone who disputed it and a few who endorsed it. So far the motor seems to run fine at extended WOT periods up to 6900 rpms. I have also checked it after continuous use with my IR thermometer and the head and block stay right around 130-145 deg although the intake port covers do get hotter (like 180 deg if not more) I guess due to the extra friction and being so far away from the water jacket.

Back to the jackplate issue, the only thing I would expect performance-wise on the smaller motors would be to possibly get the prop slip down while using a larger pitch, thus maybe giving me some better efficiency paired with lower rpms (relative to the 8" prop) so I can maintain a decent speed but have better fuel economy. It probably won't matter or help but at least the increase in motor height would be nice.

Okay scout-j-m, I stand corrected if your engine manual states 7000 RPMs. I have never seen an OB that had that high WOT range myself and that was why I was saying anything. I didn't want you to over rev and blow the engine. Accept my humble apology. :sorry:

 

[scout-j-m]

Okay scout-j-m, I stand corrected if your engine manual states 7000 RPMs. I have never seen an OB that had that high WOT range myself and that was why I was saying anything. I didn't want you to over rev and blow the engine. Accept my humble apology. :sorry:

Ha, no problem. Honestly, I'm still a little skeptical myself. I don't know why it would be 500 rpms higher than other years which are essentially the exact same engine and drive. Either way the 15hp is rated at 6250 rpms so it's not like it would be "lugging" the motor to run it there. 6250 is about what it runs with the 10" prop and two people.