jackplate question

[BayouRat454]

hi guys,i moved my center console forward when i re-painted and re-designed my 16 ft. modified sprint hull in the past month.i running a 225 looper with a 27 srx wheel.since i moved the console forward,the bow is not lifting as good as it used to.i know the srx is a stern lifting wheel and some bow lifting.i came down on the jackplate 2 notches,and it helped a little.the question is should i come down more on the jackplate or do you guys think i need a different wheel?anyone familiar with these wheels? i would like to keep it if possible. thanks

[FCnLa]

You say you moved you console forward, this moved the helm and seat forward, right?

If this is true you moved your center of gravity (CG) forward. Which will either need more trim or a prop with more bow lift. May be accomplished by adding cup to the prop.

Lowering the jackplate will let the trim be more effective, so to speak. How high is it? In relation to prop shaft center line to the running surface? When you last ran it before the modifications, was the jack plate in the same position? I have had mine so high that the trim became uneffective.

You may need more set back to change your CG back or closer to where it was before you did the modifications. Either spacers or a jackplate with more setback.

Got any pics? I'd like to see it. Sounds like a ball....16' with a looper!:)

[DSP1]

I've got 27 & 29 SRX - 28 chopper and 28, & 2x30 Mazco RE3's.

My experience in summary is that the SRX lifts the stern to the point where it's dangerous. The chopper is a good all rounder with a fair amount of bow lift - the mazco's, my favorites as you mayhave worked out, tend to lift the whole of the boat.

This is on a fairly light (approx550 lbs) 19 ft boat with modified looper

[BayouRat454]

( FCnLa and dsp1)
thanks guys for your post.yes the seat was also moved forward with the console.i had the jackplate as high as it would go which the centerline was 1 inch above the pad.thats where it ran perfect before. i was turning 6800 -6900 r.p.m. at 90- 92 m.p.h. depending on the humidity. since then i boosted the compression from 90 to 110 with a set of 93 heads a removed the air box. went from 62d to 72d jets and timing at 15 degrees at full advance.i really like that wheel.iguess i'll try to come down 2 more notches on the jackplate and see what happens.if i need another wheel(got any for sale?)your reply was greatly appreciated. thanks

[T-REX]

Put sum mo set back on it....It will help lift the bow and also lets the motor work less at holding up the bow(less motor trim)...U know how much my ole REX has...and it wuz for the very reason U have, and my ole REX iz a lot heavier than Yourz iz...U jus have to try diffrent props, choppers have the better bow lift, but in return they have the most wheel torque...@ 20" setback with a 30" chopper, I couldn't steer my bote @ wot...My bote runs it's best with a Cleaver prop, yours may not...try this...figger 27% of the length of the bote...say it's 5'(just an example) measure from the rear of the pad forward...put a floor jack under boat at this point(be shur and protect the bottom) Jack the boat off the trailer a couple of inches...U have to git in the boat, because U want it balanced in the configuration that U run it the most...Git G-Man ta help ya, cuz it's damn near impossible to do it by your self(but it can be dun, cuz I do everthang by my self)...when U move weight around to balance the bote, you are gunna be real happy with the performance and handling...to keep it handling good the whole day, U have to put the Fuel tank on the CG...Any way I can help, jus holler.....FCnLA, Jeffs bote is on the BY-U-Boyz site under "get togethers"...Jeff's bote iz the first pic...REX:cool:

[BayouRat454]

thanks Mike for the input.i checked the prop shaft height today. the center of the prop shaft was around 2 inches above the pad. thats where it ran its best at before i moved the console and seat forward.i dropped it today to 1/2 inch above the pad . i'm gonna try it tomorrow with gary. i was waiting on my new water temp. guage. let you know what happens. thank you.

[G-MAN]

ya adding mo plate would help. but that's moveing the moter back and when that happens the boat go's down. remeber a BIG LOPPER on a 16 foot. i think we just need to go and run it and see how much boat is in the water at W.O.T. you mite feel like you mite not have enought lift becouse of the change. but the boat mite be running more straight in the wind. maby just aidding a little cup would help.


boy that's one fine stream on side you'r house. ;) :D :cool:

G-MAN

(((((WHEN IN DOUT TRIM IT OUT)))))

[Hunter]

Just curious on something. I haven't seen any discussion on determining CGs. Maybe I'm not reading carefully enough; if that's the case, ignore this. Otherwise....

Pilots are very concerned about CGs. It kind of ruins your day when you're doing 120 or so with 3000' remaining on the runway and when you go to rotate, it turns out the nose won't come up because you've incorrectly loaded the plane and don't have the elevator authority.

Just how much of this effects the propensity of a boat to porpoise at moderate speeds until the aerodynamic forces generate a stabilizing effect? Where the heck is my CG anyway and what's a good point to be at?

Being stuck in Korea without my boat, there's little I can do but were I home....

I'd take my trailered boat to the local scale and weigh the entire trailer, the weight on both axles, and the caster wheel weight. Based of the principle of WEIGHT X ARM(in inches) = MOMENT, we can multiply the caster wheel weight by the number of inches from the hitch point (the datum point as I'd choose to establish it) and the distance from the trailer axle to the hitchpoint in inches times that weight and add those numbers (moments) together. Then, by dividing by the total trailered weight, one can determine how many inches from the hitchpoint the CG is located.

Hope that point isn't behind the axle! Anyway, by popping your boat off the trailer (I store on the municipal pond but a local boat dealer might let you drop on an empty trailer of theirs for a small fee) and performing the same weight measurements on the trailer alone and then extracting the trailer weight from the total weight you can not only determine your boats weight but the CG.

From this point, mark and measure the boat. Unless you've managed to somehow get the bow exactly over the hitch, you'll have to re-establish a datum point for the boat alone. Really, you can define the datum point from anywhere relative to the boat but the math's easier using the bow and since most of us don't have anything hanging off the bow but do have stuff hanging off the stern, the bow's my choice. It gets interesting from here.

As pointed out previously, anything removable and consumable can be placed at the CG point and not effect the static CG.

Here's where I go from idiot to total idiot. It would seem to me that you would want your CG to fall somewhere between your hull's contact patch with the water while planing. I assume that having the CG forward of the contact patch would be the breeding ground for porpoising as the boat struggls to keep picking up the bow. Still, I'd think you'd want the CG on the forward end of the contact patch to generate a dynamic stability.

Maybe I'm missing something on that. I do know that before I buy another prop or make another assumption on setback, I'm going to pursue the above thoughts, put a masking tape stripe on the side of my boat (along with a horizontal tapeline on the engine- convoluted, curved thing that it is) and make some video taped passes while varying the CG within reasonably usable limits.

Anyone have any other ideas?

[Gerben]

Just a few comments on this:
(with a limited knowledge of both worlds but hey)
Porpoising: the farther forward the cg is, the beter a "margin of stability" there is, the less porpoising there is.
More setback usually makes porpoising worse.

Just like plane aerodynamics, if the cg would be at or real close to the lifting point (mostly water contact for a boat, ehh MAC =mean aerodynamic chord or something like that for a plane) you'd have no stability or with a more rearward cg even negative stability.
Quite interesting in a plane and hard to achieve in a boat.

Now over to experts again..

Gerben

[Hunter]

I'm pretty sure we're on the same sheet.

All I'm getting at is that, during the period aerodynamic lift is establishing, a boat with an excessively forward CG (which I believe mine is) might be sensitive to wave action and set into an oscillation in which an effective bowlift generated by the prop becomes transitory - a parallel to stalling.