Jackplate set back pros and cons

Hi all. I think I've finally got my jackplate materials all sorted out and how to make it, but I'm a little unclear about the issue of set back. As I understand it, when the motor is on the jackplate a couple inches behind the transom it effectively lengthens the boat by the same amount while at the same time raising the motor higher in the water. From what I've read, this is usually a good thing inasmuch as it reduces drag to aid in top end speed while also improving handling. I believe it will also allow the motor to take in water more efficiently by placing it in a less turbulent ("cleaner") pocket of water behind the boat (not sure where I heard/read that?). Do I have that right, or is there something I'm missing? The motor is a '66 20 hp and it will be set back about 2" and up another 2"-3" (guesstimate) and I want to be sure it always has plenty of water for cooling. Also, what size SS hardware should I use to bolt it through the transom? I was thinking 1/2" with several fender washers on either side to distribute the load and lots of good 'ol 5200. The plate itself is going to be made from 1/4" aluminum C-channel and a piece of sealed non-PT 2" x 6" for the motor itself to clamp to. It will look something like the pic below. It's by no means to scale, but should give you an idea of what I'm shooting for. Thanks for the help.

Re: Jackplate set back pros and cons

Setbacks under 6" mean very little.

When you get to about a foot of setback, engine height is affected by about 1" or so, and the tendency to porpoise goes up exponentially. On low HP applications where the engine is not actually lifting a big planing hull out of the water, it's almost a non-issue.

Re: Jackplate set back pros and cons

Cool. Thanks Philster. That's kind of what I thought. It's only a 14' tinny and an old 20 hp Johnson. Nothing fancy. Overall set back will only be about 2" with about 2" rise. It probably won't even make me noticeable faster, but perhaps I can get on plane a little quicker and draft a few inches shallower, which is handy down here where even the beach in only knee deep and full of oysterbars, etc... Just clamping the motor a few inches higher on the transom has already helped a bunch, so I'm hoping another 2"-3" more will be just right and should put my anti-vent plate just a smidge above the keel. Thanks again. I'll keep y'all posted and try to remember to take pics. I just gotta find something that can use to cut thick aluminum fairly easily. That or I got A LOT of hacksawing to do.......

P.S.
Is there a blade I can put on a circular saw that can cut aluminum?

Re: Jackplate set back pros and cons

I used to put a wood blade on backwards to cut aluminum roofing but that is thin material, your better off using a sawzall.

Re: Jackplate set back pros and cons

Real simple bullet proof answer. Ask Mr. DHadley. If he doesn't know no one knows. But don't PM him, get on the forum and address your question to him. He told me today that he tunes in the site about 3 times per day so you shouldn't have to wait long for your answer.

Mark

Re: Jackplate set back pros and cons

Hello again. I think I figured out an alternate design that will only require one cut, which I think I can handle even if I use a hacksaw. Again, it's not to scale. Just a mock up. The aluminum I have to work with is basically a 1/4" plate that has a "C" already bent into each end. I realized that by simply cutting off one end I would be left with sort of a "J"-shaped piece that I could then invert and attach as seen in the pic. Do y'all see anything that might prevent it from working? Thanks again.

Re: Jackplate set back pros and cons

Gonna attempt to put this ^^^ together tomorrow (got the bunks on my trailer sorted out today. what a PITA!!!!). I'm a little leery of drilling in the transom, but I'm gonna measure twice, or three, or four times and hopefully only have to dill once. Do y'all see any reasons why it wouldn't work? Thanks.

Re: Jackplate set back pros and cons

IMHO the latest design is not strong enough to handle the power of your motor. I would be concerned it will break that adapter plate. I liked the first version better! The link below shows you what the racers do and I think we should pay attention to their labors.

http://www.dillon-racing.com/jackplate/index.htm

EDIT: The fastener hardware size is specified on the drawing in the link.

Re: Jackplate set back pros and cons

Thanks Willy. I think I'll stick with the first design too and use the "C"s to my advantage. I did a little test cut with my hacksaw and it wasn't too bad. I really only need to make 2 straight, 12" cuts and drill the holes. I think I should end up with about 8" of the C channels bolted to the transom which should leave 4" left above to mount the wood to. It will essentially look like the one in the link only fixed rather than adjustable. I can always try various trim/tilt and clamping positions to tweak the performance. Thanks for the advice. I'll try to take pics of my progress (or failure). I don't think it'll really make me much faster, but perhaps it will get me on plane a little quicker, handle a little better, and get me into a little skinnier water.

Re: Jackplate set back pros and cons

I cut the the 3/8" aluminum for my jack plate using a 40 tooth carbide blades on my circular saw and on my miter saw. Just have to go slow. Had to cut the 4" fixed jack plate I had in half first and then space the halves apart with some 3/8" x 6" wide flat plate I had purchased a while back. Used 1/2" carbide router bit with 1/2" shank in my 3hp router to cut the 6 adjustment slots, but that takes some very advanced router techniques and is definitely not for a newbie. In hind sight, if I hadn't already had the materials I would of just purchased a 6" adjustable jack plate although mine turned out much cooler looking.

-- Jeff

Re: Jackplate set back pros and cons

If you just want to raise the motor, take a look at these:

http://cook-mfg.net/cart/index.php?main_page=product_info&cPath=10_13&products_id=13

Re: Jackplate set back pros and cons

kfa4303 said:

Hi all. I think I've finally got my jackplate materials all sorted out and how to make it, but I'm a little unclear about the issue of set back. As I understand it, when the motor is on the jackplate a couple inches behind the transom it effectively lengthens the boat by the same amount while at the same time raising the motor higher in the water. From what I've read, this is usually a good thing inasmuch as it reduces drag to aid in top end speed while also improving handling. I believe it will also allow the motor to take in water more efficiently by placing it in a less turbulent ("cleaner") pocket of water behind the boat (not sure where I heard/read that?). Do I have that right, or is there something I'm missing? The motor is a '66 20 hp and it will be set back about 2" and up another 2"-3" (guesstimate) and I want to be sure it always has plenty of water for cooling. Also, what size SS hardware should I use to bolt it through the transom? I was thinking 1/2" with several fender washers on either side to distribute the load and lots of good 'ol 5200. The plate itself is going to be made from 1/4" aluminum C-channel and a piece of sealed non-PT 2" x 6" for the motor itself to clamp to. It will look something like the pic below. It's by no means to scale, but should give you an idea of what I'm shooting for. Thanks for the help.

View attachment 155168

Click to expand...

When dealing with a 1966 20 HP motor and not a racing boat or hull.............. I would save the money and effort since the gain will be minimal at best if any at all. There is not going to be any noticeable change in handling.

If you are looking for more speed....... look for a bigger boat/motor or strip any excess junk and people you carry along. OR get a newer 20 HP motor.

Your plan is to use PT wood near aluminum? Big NO! The chemicals in the PT will eat the aluminum.

If you move forward with this....... I would like to see GPS number's before and after.

Re: Jackplate set back pros and cons

UPDATE:

Well I got it all done! and because I already had the aluminum and 5200 etc... it only cost about $15 in SS hardware and A LOT of sweat, mostly because it 900 degrees down here in the summer!!! I decided to go with/keep my original design and make 2 sets of "C" channels out of the 1/4" aluminum plate in the pic below. I essentially just cut the ends off which gave me the 2, 12" "C"s I needed and left me with a nice flat piece of 1/4" aluminum plate about 12" x 18" leftover to use in the future. I then mounted the channels in a manner very similar to the transom raiser in Mr. Demeanor's post. There's a bout 8" of the 12" channels bolted to the transom, which left about 4" above the transom and they're spaced about 9"-10" apart. After I got all the aluminum pieces cut (PITA btw. b/c I didn't have very good tools for the job :/ Thank god for sawzalls!) and drilled the holes in the transom (scary) I then test fitted everything. I could hardly believe it when it all went together on the first try! Although, my first bolt hole was a bit askew which put one of the channels a smidge out of plumb, but it's unnoticeable. I then painted a 2 " x 6" block in good ol' Rustoleum Professional Oil Based Enamel, let it dry overnight and put everything together using SS hardware. It's not the prettiest thing in the world by any means, but it is ROCK SOLID with 0.0% flex or give to it. It's amazing how much stronger 1/4" aluminum is than 1/8". The AV plate on the motor is now level with the keel even though it looks a little higher in the pic for some reason. I still have several vertical inches with which to raise the motor, if need be as well. As I stated in my previous posts, I don't really expect to gain much in terms of speed per se, but perhaps I'll get up on plane a bit more quickly and stay there with less throttle which may save a bit of gas. I also hope it improves my handling a bit and helps me to get into some skinnier water than before. We'll see. Unfortunately, I don't have a GPS to do any official comparisons with so I'll have to base my sea trial results on feel. I hope to get out on the water by the end of the week. I'll post the results as soon as I have them. The job itself would've been a lot easier if I had metal working equipment, but I had to make the best with what I had. Thanks again for all of help, input and tips!

Re: Jackplate set back pros and cons

...here are some pics with the motor mounted and stuff.

Re: Jackplate set back pros and cons

Keep a watchful eye on your water intake or you will be looking for another motor

Re: Jackplate set back pros and cons

For sure! I tried to make sure that it would still be well submerged. It was several inches below the waterline before, so hopefully it will still be good. I actually want to add a tell tale to the motor so I can more easily glance over my shoulder to check the water flow while underway rather than having to look at the back of the motor, but that's another adventure for another day... As you said, the transom riser may not make much of a difference, but it was a fun project and cost less than $20 thanks to the materials I had on hand. I just wish I had some metal working tools. They would've made life A LOT easier and the lines a lot cleaner. Thankfully, I think I'm done with major projects on the boat for a while, there's really nothing else I want/need (famous last words!). The next big thing would be a bigger motor (35-40 hp)(a '57 35 would be my dream motor), but that's a long way away. Ironically, I sold a '66 33 hp that would've been perfect, but c'est la vie. I like where she is right now. I'm gonna try to get my hands on a GPS to get some numbers for my own curiosity. It's so hard for me to tell how fast/slow I'm really going while on the water :/ I know I can putt-putt and idle along in the manatee zones all day at walking speed, which is about 4-5 mph and WOT "feels" like it's about 15-20, but that's just a guesstimate. If I could milk 25 mph at WOT I'll be more than happy. Btw...are there any tips or tricks for determining approx. speed while underway? Thanks again for all the help. I'll keep y'all posted on the sea trials......

Re: Jackplate set back pros and cons

kfa4303 said:

I like where she is right now.

Click to expand...

That's what it's all about! Good work as usual.

Re: Jackplate set back pros and cons

Thanks Willy! If all else fails I can put everything back the way it was. No harm no foul, but I hope I get a little bit better performance. She's come along way from the '59ish skiboat/runabout she started life as to the tiller drive/grab bar set up I have now. No where near as pretty as so many of the other boats here on the site, but I've learned a lot tinkering on her and she's brought me home every time.....so far.

Re: Jackplate set back pros and cons

sds