Do I want to install a jack plate????

[xberet]

I just refurbished a '64 Fabuglas Fury, 14' V-hull with an '81 Johnson 70 hp 3-cylinder. The motor does NOT have power trim, the boat has no trim tabs and no hydrofoil on the motor. If I use the holes/pin to change motor angle I can go from horrible nose plowing and really scary cornering habits to porpoising. Just as it begins to porpoise it's handling and ride get nearly perfect. But there is NO staying in between. Once it starts to porpoise it will do so until you drop it all the way off plane. The boat jumps on plane instantly, I have an 18 gallon fuel tank and battery installed immediately forward of the splash well so the only weight forward of that is two bucket seats and me (160# on a good day). Fuel load makes no difference. The prop is a 4 blade Comp prop 12.8"x17 pitch. I know I'm over the HP rating for this boat, but only by 15 hp and I have to think that if I were really over powered I could at least hold the nose up. Now here's the interesting part.... the boat will run 31 mph nose plowing (but it's painful). As you tilt out the motor, it picks up 5-6 mph and will hold that even though porpoising. Smooth water instantly adds another 3-4 mph. The motor does sit slightly too low and I understand I need to correct that, so my first thought is to add a jack plate to get not only the motor height adjustment but for the set back also (I have enough steering cable to do so). I've been reading all of the hull setup posts here I could find and while they get a little confusing, it seems like if I raise the motor and get rid of the porpoising when the motor's kicked out, I'll be closer to where I need to be... am I understanding what I've read????

 

[BillP]

Re: Do I want to install a jack plate????

Jackplates are mainly for speed or shallow water running and not to stop porposing or adjusting bow up attitude. Power trim is best for stopping porpoising. Tabs can stop porposing but may scrub speed to control hard cases. I'd look for a power trim setup for your motor before spending money on anything else. There are jillions of 70hp 3 cyl omcs around and a used pt unit should be easy to find. You can alway raise the motor on the transom without the cost of a jackplate. <br /><br />Your boat bottom may have a hook in the back section and that causes difficult trim problems. It's like having a huge built in trim tab. This is commom on old boats that used tilt trailers with short tilt bunks.

 

[Maximerc]

Re: Do I want to install a jack plate????

I have run a pretty wide gammet on the porpoising problem here .. the one thing that may make a diffrence is getting a 3 blade prop , it has been suggested here that the 4 blades have less "lift" . I would look at the bottom for some hook like suggested too. My Baja will hop like mad above 40 mph .. if I trim it down I get the same nose plow like you. if I let her hop I can run over 53 mph and hold on!! Smart tabs are a good fix. I however am going to go for a complete rebuild ..my floor is a bit soft and I feel that the rear of my boat is heavy from water soaked transom or foam. I agree too that you should try raising the motor with no jack plate just shims

 

[walleyehed]

Re: Do I want to install a jack plate????

Raising the motor is only one advantage of a jackplate. Depending on the set-back amount, it can also change the C.G., and CAN eliminate porpoise.<br />As for the comp prop, that wouldn't be my first choice, but generally, 4-blades lift better than 3.<br />I see a couple of things that may help...smart-tabs would help greatly, but still would work best with power trim-n-tilt, as BillP said.<br />Your experience of one extreme to the other, leads me to believe set-back would help-at least 6" worth.<br />I'd consider a good alum. prop, maybe even SS beings you spent tons of time refurbing, and keep the compprop for a spare.

 

[NathanY]

Re: Do I want to install a jack plate????

I think this would be your best bet. It would give you the set back you need.<br /><br /> http://www.basspro.com/servlet/cata...arClassCode=2&hvarSubCode=6&hvarTarget=browse <br /><br />Also it may be that your motor is sitting to high on the transom. See where the cav plate is in relation to the bottom of your hull.

 

[Maximerc]

Re: Do I want to install a jack plate????

I could have swore that someone said the three blade lifted better than a 4 .. sorry bout that .. I had a compuprop .. I loved it for all the diffrent lakes I go on. but I felt that it did not "hook up" like a good SS blade.

 

[xberet]

Re: Do I want to install a jack plate????

Thanks for the replies, the expertise and helpfulness here amaze me! I do have to respond to a few of your suggestions with an admission about one of my major boating flaws. I grew up on a lake, always had 100' or so of water under me and I'm having a horrible time adapting to the Mississippi. I simply cannot get my brain to accept that I've only 26' or less to play in and I seem to consistently run out of water. For that reason I've gone with the Comp prop as opposed to something more expensive. So I probably will go with the other suggestions and hope that in the process this old brain of mine will slip into gear so I can run a good prop without ruining it. I think my first step will be hitting the local boat shops for a used factory trim unit so I can correctly control my motor angle. When I install that I'll correct my motor height issues and go from there. As a note, I did check the hull today and there is no sign of hook, matter of fact, someone along the line has put longer bunks on the trailer so the hull is well supported. So at least I don't seem to have any major problems to overcome there. Thanks again for the replies and I'll let you all know how things progress!

 

[walleyehed]

Re: Do I want to install a jack plate????

Good suggestion on the link, Nate.<br />If a plate like that is installed, the engine will need to be mounted higher (the relation to the hull bottom) than if it was mounted directly to the transom....that's the beauty of these gadgets!! so many things you can fine-tune and get the most from your set-up.

 

[Jdeagro]

Re: Do I want to install a jack plate????

exberet;<br /><br />What I hear you saying is that when the motor is trimed down the prop is so far under the boat that it lifts the stern when crusing and the bow plows badly. If you bring the motor out (probably to the middle) the excessive stern lift goes away, it picks up speed, but it wil porpoise. All perfectly logical!<br /><br />The Jack plate will allow you to raise the motor, but that can be done without the Jack plate. Moving the motor (weight) further back will make the boat balance worse, and the increase the porpoising. Raising the motor may help with the porsoising but this is primarily a balance issue.<br /><br />As others have said, the Smart Tabs will eleiminate the porpoising. I suggest you get the motor height correct, and since you do not have electric tilt trim , put Smart Tabs on to improve the overall handling and performance. Worst case is if the Smart Tabs did not do the trick, We owe you a refund, as they are performance guaranteed.

 

[walleyehed]

Re: Do I want to install a jack plate????

nautijohn, smart-tabs would help smooth out the entire set-up, no doubt. My point here with set-back is adding leverage to stop the porpoising.<br />It has worked for me, and you remind me, that don't mean it will work on all of'em...Had I not installed mobster-tabs on mine, I wouldn't be capable of the speed I am now without some super-wild traits.<br />I do honestly feel he would kill 2 birds with one stone by using the CMC plate, smart-tabs would be another tool to compliment the set-up by fine-tuning (by itself, I might add-no buttons to push) The combination of both would be a vast improvement over the existing condition.<br />He needs T-N-T, I feel set-back would help get that prop back in some clean water and allow higher running heigth (better efficiency) Add the smart-tabs, and yea, he's got a new boat!

 

[Jdeagro]

Re: Do I want to install a jack plate????

Walleyehed;<br /><br />I I have never been able to understand why moving the motor back helps performance. I certaily understand moving the motor up and down. But moving the axis point further aft (especially beyond the transom) is beyond me. I have heard a number of explanations but It still escapes me.<br /><br />The main reason for the porpoising is that the boat is already stern loaded, why then increase the load by moving it further back?<br /><br />I am happy to have as much input as I can get!

 

[walleyehed]

Re: Do I want to install a jack plate????

The boat isn't actually pivoting on the apex of the transom/hull point, it's more fwd than that, so by going back with the engine, we gain more distance (leverage) to hold the hull in a more stable running condition. They porpoise with a light engine too, but move it back and again, more leverage to stabilize.<br />I'm not sure there's a boat out there with a mono-hull that wouldn't benefit from the engine being mounted back farther. If you start looking at some of the new manufactured boatsthey are moving the transom back at an angle, or adding a stepped hull. <br />The handling charactoristics are better in most cases.<br />The weight issue can be too much forward or aft, but ya still have greater leverage with the engine back farther. Remember, the "axis" is not really the transom line. BassBoats and tunnel-hulls are close to that point, but still not that far back.<br />Trim requirements are much less with set-back, as the leverage of the thrust is much more usable....in a sense we are letting the prop be more efficient, by requiring less trim. Now, add the smart-tabs, and how could you have a more perfect set-up????

 

[Jdeagro]

Re: Do I want to install a jack plate????

Walleyehed;<br /><br />Thank you for the explanation, I will read it a few times and try to let it sink in. I understand that the axis is further forward depending on the overall balance, but when you take the motor back the axis moves back? No?

 

[xberet]

Re: Do I want to install a jack plate????

You guys are making my head hurt....LOLOL... not really but you are WAY past my learning curve on this thing. I am really leaning towards a jack plate and factory power trim. Somewhere in the dim recesses of my brain is the memory of an old family boat... a 21 foot Lone Star aluminum cabin cuuiser. Had a 50 hp V-4 Johnson, handled horribly. Remember this was in the 60's, not all the trick stuff we can use today... my dad built a set back transom that set the motor back about two feet and the boat instantly became one of the smoothest and fastest boats on the river at the time. Ran 26-28 mph, would pull two skiers out of the water, the change was dramatic. Also, if I raise the motor, I don't want to drill more holes so the jack plate would let me use the existing holes to mount everything up. I'll let you guys go back to your discussion now and I'll just watch in awe......

 

[walleyehed]

Re: Do I want to install a jack plate????

The hull is working for us in the aspect of what portion of the hull is the "wetted" area. The forces change when the wetted area changes.<br />This area changes with trim more than any other factor when on plane-this is where the longer arm comes in...it requires LESS positive trim to "lift" and maintain a certain running position-stability is enhanced because the trim angle is not fighting as hard against the direction of movement-more direct thrust.