Do I need a Jackplate

[mrtool2u]

I need/want more speed. Boat is a 19 foot aluminum Spectrum,with a 1993 Johnny 120 hp. w/trim/tilt. I am currently running approx 42-43..(gps) I have run OMC aluminum props,,ballistic,,Stiletto and a unknown stainless,,and my results were very minimal,,like within 2 mph with all props.I lifted the motor to be approx 1 1/2 inches for cav plate above bottom of boat,,with minimal results.I talked to a guy today that seems to think that most of the reason is because of the material of the hull,,,aluminum. He said that aluminum will flex,making air pockets which will scrub off speed.I stood on the bank as my nephew went by at wot with motor trimmed up,and I was amazed at how much boat was still in the water,like 16 feet worth.I have not tried a Raker,but I have heard a lot of good about them,,would this be a possibility..If not,,,would a jackplate help me with my performance,,and if so,,can you give me some real world expectations.If a jack plate is recommended,,would a manual one be worthwhile.

 

[phatmanmike]

Re: Do I need a Jackplate

a jackplate is never a bad idea

 

[mrtool2u]

Re: Do I need a Jackplate

Mike,do you have any experiences with a plate,,I would really like to hear some success stories that other guys have had.

 

[FlyBoyMark]

Re: Do I need a Jackplate

Another factor you have not considered is how much the boat ACTUALLY weighs with all the gear and "BOB's" in the boat.....This is very important in the final computation. All the data on pitch and rpm is moot unless the exact weight is known too...Have you also tried trimming the bow way up too, to point of instability? AS you trim the bow way up you may have to drop the engine a 1/2" at a time to test if you can get enough bow lift. There is a point at which trimming up to far for the "jack" height will ventilate the prop before you can get enough lift. Weight distributuion in the boat is important too, not too much weight in the bow area..Power Lift makes a very nice manual jack plate (5-6" backset)for $130.00 mail order. Uses all existing bolt holes with no drilling and is made of aluminum with stainless hardware.

 

[BillP]

Re: Do I need a Jackplate

Does your boat have rivits on the bottom? If so I wouldn't waste too much time trying to make it go fast with a jackplate (I owned one on a flats boat). One of my cousins experimented by "de-riviting" a 16' al skiff and welding it back. Speed increase was huge. Before that he tried different motors, props, etc. Rivits made a "drag wall" that was big.<br /><br />You can use existing motor mount holes to raise the engine and find out if a jackplate is what you want. I think your speed is "there" now unless you can substantially lighten the load.

 

[mrtool2u]

Re: Do I need a Jackplate

Bill,,yes boat is riveted,,and I know what you are saying about the drag with rivets. I did raise the motor,to 1 1/2 inches with cav plate above keel of boat.It made very minor differences,,but I did notice some blowout compared to having the plate even with bottom of boat. It just seems like if I could get a bit more bow lift,I could pickup speed.Flybob,,what do you mean by trimming the bow up,do you mean jjust trimming the motor,and if so,yes I have experimented with different trim ranges.

 

[BillP]

Re: Do I need a Jackplate

Originally posted by mrtool2u:<br /> Bill,,yes boat is riveted,,and I know what you are saying about the drag with rivets. I did raise the motor,to 1 1/2 inches with cav plate above keel of boat.It made very minor differences,,but I did notice some blowout compared to having the plate even with bottom of boat. It just seems like if I could get a bit more bow lift,I could pickup speed.Flybob,,what do you mean by trimming the bow up,do you mean jjust trimming the motor,and if so,yes I have experimented with different trim ranges.

This is one situation you may want to try one of the foils. I have one on a 17'ob semi vee hull and the cavitation plate is mounted well above the boat bottom. The foil lets me trim the motor way higher before blowout. It gets the bow up and provides a few mph increase. Foils are cheap and easy to try.

 

[mrtool2u]

Re: Do I need a Jackplate

Bill,what kind of foil are you running.I have a doel fin,but it is currently is off the motor.

 

[BillP]

Re: Do I need a Jackplate

Mine's also a Doelfin. It can't hurt to try it but make sure the foil is above the boat bottom so you don't get drag at planing speeds. All you want it to do is capture water and keep it around the cooling water intake and prop at full throttle. I get zero handling issues from the fin but keep in mind that the motor's small trim tab gets less effective when the motor is trimmed real high. Not a problem on mine but something to be aware of.

 

[sorrydog]

Re: Do I need a Jackplate

The jackplate will move your center of gravity back making it easier to achieve bow lift which it sounds like would possibly help. However, moving weight to rear can lead to porpoising problems.<br />It will also help by letting you adjust motor height to the best possible spot.<br />I have no idea about a speed prediction. My experience with an aluminum boat is that they do pretty well up to a certain speed and then to make them go faster starts to require major HP increases. If you got up to 45, I would be happy with that. If you to do make sure you have enough steering/ control cable slack for the extra setback.<br />Hydrodynamics in Louisana makes a very nice manual plate called a Rapidjack that can be adjusted in the water.<br /><br /><br />As for the prop - I tried a raker once on a aluminum boat like yours (a 18' Bluefin, which became Spectrum) and it didn't seem to like. My guess is a raker is a good semi-surfacing thru hub, but takes a lot of HP to turn if it is buried and they are known for being poor accelerators. Whose knows though as prop behavior is hard to predict- if you can try one without buying then try it. I ended up with a Laser2 on mine. I think a Laser2 is a great all around prop but you might have to have the hub drilled out to fit your OMC.

 

[quantumleap]

Re: Do I need a Jackplate

43mph isn't all that bad for that set-up.

 

[FlyBoyMark]

Re: Do I need a Jackplate

MrTool..try getting some weight off the boat or moving it aft then make a WOT run and trim the the bow up till it porpoises(unless it ventilates before you get that far), then a tick down and note speed. A jackplate in your case will stabalize the boat at a higher speed without porpoising because the leverage point of the motor has been moved aft with the center of gravity also.(which means more leverage to lift bow) A "dolfin" will really kill your speed off...

 

[mrtool2u]

Re: Do I need a Jackplate

Sorry dog,I have plenty of cable,,and enough steering cable.I would be really happy with 45 by the gps,,,best I have done so far is 43.I have never been able to get this boat to porpoise,so I am hoping that the jackplate would not change that.