# Raising Motor Question



## fishnpreacher (Jul 28, 2018)

My 16ft Carolina Skiff has always porpoised when I trim the motor up. I'm running a 40hp Mariner, 15p prop, and running 32-33mph at WOT, 5400 rpm. Anything up causes it to porpoise. I really haven't been too concerned about it. But after seeing the difference it made in changing the prop, I've started to consider possible fixes. The motor is mounted as low on the transom as it will go, and the cavitation plate is about an inch and a half below the hull. Will raising the motor cav plate flush with the hull help the porpoising significantly?
If and when I raise the motor, do I need to place a shim on top of the transom for the motor to rest on, or just let the bolts hold the weight? I won't have to redrill the transom, since there are several mounting holes on the motor, and its at the lowest setting. 
What material would the shim need to be? 
Thanks!


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## Copahee Hound (Dec 21, 2017)

My old 18 jvx had the cav plate level, but I can’t remember if there was anything on top of the transom or not.

Trim tabs are worth their weight in gold, but a cheaper option is the nauticus smart tabs. Though they’re not adjustable on the fly like real tabs


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## fishnpreacher (Jul 28, 2018)

Copahee Hound, did your 18jvx porpoise when trimmed up?


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## JC Designs (Apr 5, 2020)

If your anti cav plate is below the hull bottom, it is likely the cause of your porpoising. Ever ran a shovel through the water? Same thing happens when that flat plate is below the surface!


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## BrownDog (Jun 22, 2019)

Another vote for raising the engine.

I did not use a shim on my old boat when there was a gap at the top of the transom.


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## Copahee Hound (Dec 21, 2017)

fishnpreacher said:


> Copahee Hound, did your 18jvx porpoise when trimmed up?


Only if I trimmed up too much


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## LowHydrogen (Dec 31, 2015)

Cav/vent plate should be a minimum of .5" above the planing surface but 1" would prob pick you up some speed. Run it as high as you can without blowing out to increase speed and efficiency, the more you're dragging along in the water the more you rob performance. Check it in turns though because you're running a super flat slick bottom boat that's already prone to sliding in turns.

Motor doesn't have to rest on anything the bolts will hold it (think jack plates and race boats). 

To do it, lower your tongue jack all the way down, put blocks under the skeg, lightly loosen the slider bolts and pull the top two bolts, now crank up the tongue jack until the motor is where you want it (aligned with a bolt hole) and tighten everything back up. That's the safest way for it to be a 1 man job in my opinion.


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## fishnpreacher (Jul 28, 2018)

"Check it in turns though because you're running a super flat slick bottom boat that's already prone to sliding in turns."

She slides now, first time was a sphincter puller. Got sideways pretty quick.

Thanks for the input guys. So, I'll be OK with letting the bolts hold it? 
And I need to be slightly above the hull with the cav plate. Rainy day work day!


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## fishnpreacher (Jul 28, 2018)

Got it done today. I followed LowHydrogen's advise (thanks, man!) Easy! I measured before I started and I was slightly over 1-1/2" low. I raised the motor as high as I could go without drilling new holes, and now the cav plate is level with the hull bottom. I'm running it tomorrow and I'll let y'all know how it goes.


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## lemaymiami (Feb 9, 2007)

I've done more than one motor that way (the exact way LowHydrogen describes) - works like a charm, and I was working with motors much bigger than what's typically found on a micro-skiff...


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## hunter4626 (Aug 7, 2016)

lemaymiami said:


> I've done more than one motor that way (the exact way LowHydrogen describes) - works like a charm, and I was working with motors much bigger than what's typically found on a micro-skiff...


I've got a similar problem-although it comes after 2-3 years of running it. All of a sudden she's starting to porpoise, the load is exactly the same as it always was,loaded the same way-but this year it started porpoising I've trimmed up, down and doesn't seem to help.I used to be able to trim under way-but the hydraulic trim seems to have weakened- (I'm running a 20 HP Zuke) I can't trim it down under way anymore.Could the fluid be low or the trim motor shot?


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## fishnpreacher (Jul 28, 2018)

Well, I just got home, and it's the old good news/bad news scenario. Good news it helped the top end +2mph, and probably fuel consumption,if I have sense enough to throttle back. Bad news, the porpoising is better, but not fixed. Looks like a hydrofoil in my future. Question, what do you recommend for a hydrofoil stabilizer? One piece vs 2 piece, any brand to stay away from, any brand better than others?


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## LowHydrogen (Dec 31, 2015)

Not saying you won't eventually need a stabilizer, but I'd bet money you can fix this without one.

Porpoising is almost always caused by motor height+angle.

I'm going to PM you my #. Easier to talk about than write a novella.


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## NativeBone (Aug 16, 2017)

If you decide a hydrofoil is the way you want to go, I have a Bobs True Tracker for 25-40hp motors that may fit your motor. NO cost other than you pay for the shipping, prob $10-15 bucks or less. 



fishnpreacher said:


> Well, I just got home, and it's the old good news/bad news scenario. Good news it helped the top end +2mph, and probably fuel consumption,if I have sense enough to throttle back. Bad news, the porpoising is better, but not fixed. Looks like a hydrofoil in my future. Question, what do you recommend for a hydrofoil stabilizer? One piece vs 2 piece, any brand to stay away from, any brand better than others?


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## fishnpreacher (Jul 28, 2018)

Native Bone, that's mighty fine of you! I'm gonna try moving some batteries and weight around and see if that helps my problem. I'll keep you in mind if I can't get it straightened out. Thanks!


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## Shallows (Mar 29, 2020)

Copahee Hound said:


> Trim tabs are worth their weight in gold, but a cheaper option is the nauticus smart tabs. Though they’re not adjustable on the fly like real tabs


The non-adjustable type tabs come on most all JV Carolina Skiffs, so have to assume the porpoising is a known issue with those hulls.


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