# jack plate blow-out



## fishhawk (May 16, 2011)

THinking about getting a 4" set back jack plate that should put my prop perty dern "skinny" My question is....by running skinny like this how does it effect performance in rough water ?


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## Pappy (May 16, 2011)

Depends!......(Don't ya' love that answer)? Will depend on lots of variables. How high did you mount the engine, what kind of load do you have onboard on a particular day, Where is the load placed, how rough is it, how fast you are trying to get through it, what heading in relation to the waves, etc. There are several threads in here that have been started on jack plates, you should read through these first. Properly set up you won't find lots of differences. Properly set up are the key words. The higher you go the more modifications there are that have to be made to accomodate the added engine height. Propeller cupping and re-cupping or replacement with mods to the replacement may have to happen. Weight transfer to accomodate the rougher days may have to happen. How much time, effort, and mmoney are you willing to put into it! At this point noone even knows what boat and engine we are talking about. Information is key here!


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## fishhawk (May 16, 2011)

thanx pappy, I fully understand the part about weight displacement, wind direction,quartering into the waves, etc, believe you me I have been in some very hairy situations and most of the time I find it benificial to gas it, get on top of the waves and hang on!! I run a little 'ol 1446? lowe lake jon with a 35hp 'rude short shaft, my cavitation plate is level with the bottom of this boat, so I can go up a little.What I don't want is prop blow-out on tall waves. Seems like to me it would cause problems but i haven't heard any one say!


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## fishhawk (May 16, 2011)

pappy I forget to tell you about motor height.I haven't got the jack plate yet but I'm guessing it will bring my mtr. up no more than 2".


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## Pappy (May 16, 2011)

Two inches from where??? Again, information is key here. A short shaft Johnson (15") is a different length than a short shaft Merc is a differen length than a short shaft Yamaha and so on. Plus, they all have different locations where they take in water to cool the engine. There is no one answer here so going up no more than 2 inches means nothing at this point. Find those threads mentioned earlier and read up a little. You could be absolutely right but at this point................???


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## crazymanme2 (May 16, 2011)

I think we need some pics here so we all can see whats goin on [-X


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## hkmp5s (May 17, 2011)

Pappy brings up a good point about the water intake for your cooling system. Your motor can still be peeing but not have enough water for proper cooling at WOT.
Thats why a pressure gauge is a must in my book for when you add a jack plate.

Back to your original question, Blow-out can happen but thats the beauty of the jackplate, you can adjust to conditions.
I'm going to add one on mine some day just for the added versatility. I'm just waiting for a smoken deal.


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## Riverjet502 (May 17, 2011)

I run a 1648 with a 20 inch transom Crestliner jon boat. '78 35hp Jonny long shaft tiller. Mounted on a CMC 130 tilt & trim plate with @ 7inches of setback... Motor sits 2.5 inches higher off the transom. Running 13 pitch...Runs awsome..(motor pulls 5250 rpms).. The jon luvs the trim helps reduce the wetted surface big time. Haven't had any cooling issues. (looking over the transom while running water intake is always below the surface) I can vent the prop with to much trim or tuck it in for rougher waters... picked up 9mph per GPS... I want to try a 14 pitch and see where it tachs out at next...


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## reedjj (May 17, 2011)

I have always been told that if your Cav plate is even with or just slightly above the bottom of the hull you are in good shape. Some even go up 1"inch above the hull when the motor is trimmed level. When you get going you can trim it up and when you slow down or get into some chop or a turn you can trim it in if it cavitates.


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## Pappy (May 18, 2011)

Okay, in Fishhawks case we can assume he has measured the cavitation plate when it was parallel to the bottom of the hull. So we have a starting point. We also know that he has the water intakes on the side of the gearcase as opposed the bottom of the cav plate so his engine can be mounted higher and still pull plenty of water. In addition to the fact that no two engine manufacturers build engines that are exactly 15" or the fact that water inlets are located differently by different manufacturers the fact is that boat builders are even worse at building 15" transoms! I have seen over 1.5" difference in production 15" transoms between boat builders. This is why it is continually stressed to measure where the cavitation plate is in relation to the bottom of the boat......so we can establish a starting point. 
Riverjet states that his engine is around 2.5" above the transom but we don't know how high that particular builder made that transom do we so we do not know how high the cavitation plate was to begin with. 
Fishwawk, Extablish a baseline. Get your speed and RPM with the boat set up as is. Try to limit variables in load so you can go back and duplicate your numbers if you have to. in general you can place your jackplate with the engine cavitation plate around 1 - 1 1/2" above the boat bottom as a starting point and be safe. Run your numbers at this new height. Raise the engine above that point 1" the first time and check your speed, cooling water and RPM. If you are cooling, gaining speed and some RPM then go up another 1/2" and check again. Your limitations will be seen in the form of either running out of cooling water or when RPM goes up and speed stays the same or drops. If you have not met one of these "walls" keep going up until you do.Then back down. If you are still cooling and feel that there is more performance left on the table then this is the point where propeller work comes in to play. You need to keep your RPMs reasonably close to book numbers but the propeller can be cupped to limit slippage (RPM going up and speed staying the same)and continue to go up in height and gain performance until you meet the cooling wall. Make sense? Moderators - At some point in time we need to make a sticky on this question.


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## BaitCaster (May 18, 2011)

I have a mini-jacker. My cav plate sits about an inch above the bottom of my boat. I have only taken the boat out once since installing the mini-jacker. I did notice some cavitation in tight turns and in choppy water, but nothing I would consider a "blow-out".


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## fishhawk (May 19, 2011)

thanx, I know what you mean pappy, Makes sense,That was a lot of good advice. Not meaning to be rude but actually all's I wanted was some in-put on people using the mini-jacker or any other brand small manual j.plate in rough water. Yes I do see how different setups float differently and it would be hard to say if mine would be a problem so it sounds like I just need to bolt it on and see. I have a tac. but not a water pressure gauge, where might a good man pick up one of those? I have like 4 different props for this 'rude so I should be able to make it work.


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## Pappy (May 19, 2011)

Always nice to support your local dealer. You can pick one up there or online.


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## hkmp5s (May 20, 2011)

fishhawk said:


> thanx, I know what you mean pappy, Makes sense,That was a lot of good advice. Not meaning to be rude but actually all's I wanted was some in-put on people using the mini-jacker or any other brand small manual j.plate in rough water. Yes I do see how different setups float differently and it would be hard to say if mine would be a problem so it sounds like I just need to bolt it on and see. I have a tac. but not a water pressure gauge, where might a good man pick up one of those? I have like 4 different props for this 'rude so I should be able to make it work.



I'm going to have one put on later this year along with a tach. Please keep us updated after you get it lined out.
I'm sure you will be pleased with the performance of it. I can't think of a reason not to do it.


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## fishhawk (May 20, 2011)

The only reason I wouldn't like it is because of prop blow-out in bad weather. I have been known to stay out too late in stormy weather or go out when i probably shouldn't, but like bait caster said it wasn't too bad in " choppy water so I might as well try it. I think most of the replies i recieved were refering to adjustable jack plates or even hydrolics but not the case here.


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