Glen-L.com

Do any of you run jack plates? I am considering a manual plate with adjustable setback in order to further fine tune the beast for speed when she is complete.

I run a TH Hijacker (4 inch set back), manual adjust.

Yes, you can increase your speed running a jack, but you really need to put things into perspective.

A boat with an adequate motor that is kept tuned and a properly matched prop is just about as good as it gets in terms of performance.

Adding a fin, jack, trim tabs or any of the thousands of items claiming to increase performance do so but the amount is quite small.

To get a 15% performance increase (read top speed) you need to increase your horsepower by 50% - you can't get even remotely close to that with "add on's".

A correctly adjusted jack plate may (MAY) result in a 5% performance increase, but usually somewhat less than that, so if your rig is already doing 50 mph you may see 52 on a good day.

A jack's real benefit is seen in terms of years. Since you fiddle with it and find an optimal engine placement which in turn reduces a little bit of drag you end up burning less gas (again though I'm talking a few less ounces per hour, not gallons) which over many seasons does result in many hundreds of gallons saved if you boat alot.

I rig outboard powered boats and personally believe that any motor greater than 125 horses should be on a jack as opposed to being bolted right to the transom (if done at initial purchase) but when asked if the cost is justified when done as a retrofit it always comes down to "how much do you run your boat".

If you boat less than 100 hours a year it is unlikely that it will ever pay for itself in terms of fuel savings.

If you are simply looking for more speed, save your money. It will not produce any difference that you will actually notice. Even if you do the install yourself a low end plate will run you a couple hundred bucks - so is each extra mile per hour worth 100 bucks a piece?

The only sure way to make a noticable difference in your top speed is to remove your current motor and replace it with one that has significantly more horsepower....

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Graham

Yes, Plywood is "real" wood

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it

I gotcha, thanks for the clarification. My goal for this boat build is 50 MPH, so I figure anything I can do to inch me towards that mark can't hurt right? I understand what you're saying about the cost/speed-gain ratio not being so good; however, I am a welder/fabricator by trade so I've definitely got the material and skill on hand to fabricate one. So this is actually why I am asking, as it seems like a fairly straightforward design that could net a few MPH if nothing else. So, if I'm going 48, and the jackplate will allow me to go 50, and I can do it without spending any money, why not right? Goal met!

Absolutely, I firmly believe that "every boat" could benefit from a jack plate and trim tabs.

However, I just caution against unrealistic expectations. If you read the advertising hype on many of these products you would think they could convert a 30 mph hull into one that could easily hit 60.

Unfortunately it's guys like me who install these things that get the brunt of the disappointment when there is "no noticable change".

But if you are taking the perspective that you want to "finely tune" your entire rig in order to allow it to perform at it's maximum potential peak in term of speed, fuel economy and handling that is alot different than "Joe boater" who is looking for a bolt on 10 mph increase in top speed (which just doesn't exist)....

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Graham

Yes, Plywood is "real" wood

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it

ToneDef,

What boat are you setting up??? Sorry I forgot

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Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

No worries lol, I'm building the XP8

For an 8 foot+ hydro, just make "shim sticks" to get you at your best possible motor height for maximum mph.

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As Conan the Barbarian says, " It's not how hard you can hit, it's how hard you can get hit and remain standing".

But what about setback? That's something else I'd like to mess with that I could incorporate into the jackplate. Not to mention shim stacks are ugly IMO, although I could shim to find the best height, then build a jackplate to that height.

ToneDef,On a boat that small,a jack plate will have a lot of effect on balance and leverage,placing the motor back away from the hull.

I set-up a 14' outboard SK with a 150 on a jack plate and had lots of operational problems and ended up installing the engine flat on the transom.

With the plate,I got maybe 3 MPH more top end,but the boat wouldn't stay on a plane below about 30MPH.The boat was much more usable and friendly without in this case.

If you decide to try one,here are some plans to make a small one for little tunnel/race boats:

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

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

As noted, a common issue with jack's is "undesireable" side effects such as poor hole shot, slip in a turn and sucking air when the lower unit breaks the surface.

Although there are no hard and fast rules, in general, for each 4" of setback you raise the motor about 1/2" higher than where you would normally mount it. Additionally, most outboard mfg's have mounting height instructions for each of their models. Since I work on Merc's (in addition to rigging), I have am familiar with their data.

On a rig "capable" of reaching 50 mph, Merc already recommends mounting the outboard 1/2" higher than normal so if you also ran a 4" setback you would raise the motor so that the cav plate is 1" above the lower edge of the transom instead of directly in line.

If top speed is your only goal you go with alot of setback and mount as high as you can (while still keeping the waterpump intakes in the water). However, doing so results in a boat that only likes "very flat water" and only likes to go in a "straight line".

I stuck with a minimal setback (4") and fiddled with the vertical adjustment quite a bit. My primary goal was not an increase in top speed but instead, better fuel economy at cruise. With some trial and error testing I got my 140 Evinrude to save .3 (point 3) gallons per hour (or about 1 liter/hour) going from 7.6 down to 7.3 gph at cruise at about 22 mph measured with gps and floscan.

In Canada where gas is about a buck a liter (that's about 4 bucks a US gallon) saving a dollar an hour for every hour on the water is significant since that particular rig sees 200 to 300 hours run time a year.

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Graham

Yes, Plywood is "real" wood

A "professional" is someone who gets paid for their work - it doesn't necessarily mean they are good at it

Also,

Probably not in your situation but in most others it would throw the boat off kilter due to higher center of gravity but most high perf boats on the larger scale run 15" mids, sometimes 12" ROS. Not like you would wanna pull a sharp turn off at over 40mph anyways.

Good points on this page, a definite reason to bookmark it for future reference.

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Tuffy with '75 Merc 650 SS.