What Size Jackplate On A Small Fibreglass Bass Boat

[rv02201]

Ordered a new boat that will probably be ready for delivery in 3 weeks. It is a 4.5meter (15ft) fibreglass bassboat.

We are wondering about what size of Jackplate to install with a 70hp Yamaha outboard. We currently have a 5" plate but considering to change it for an 8".

The Demo boat was tested with a 125hp Mercury and a 6" Jackplate but we thought the motor struggled to plane the boat quickly. The motor was probably not 100% setup but we thought that size motor should have plained quicker on that small boat We think the 8" Jackplate could help or eliminate some of that. Would our thoughts be correct.

To view some pictures of the boat you could Google "Bass Hooker 455"

[J Francho]

Assuming the engine height is set correctly, increased setback won't improve hole shot.  Go down one pitch with the prop, or go to a 4-blade prop in the same pitch, and see if that doesn't help.

[rv02201]

Would there be any benifit going for the 8" plate over the 5"?

[J Francho]

I'm not sure it would make a difference on a boat that small.  It might even make the handling worse.  Generally, jackplates are used on boats bigger than 18' or in special circumstances where engine hight needs to be raised higher than the transom allows.

 

I'd talk to the manufacturer, and get some referrals from other owners, and see what they are successfully running on their boats.  I know I've learned a TON from other Bullet owners about proper set up for my Bullet.

[BKeith]

There is something I not really understanding about this situation. The boat you went on a demo ride in had a 125 Merc. I don't know of a 15ft boat on the planet that's rated for a 125. Then my next problem with your deal, if a 125 was struggling with, why in the heck would you order a 70. Now, if you took a demo ride in a totally different size and style boat than what you bought, what benefit was the demo ride. It dang sure didn't show you anything about the boat you bought, other than it floats. Kinda like going for a test drive in a Mustang GT, to buy a Fiesta.

As for the jackplate. The VERY FIRST thing you have to do is check with the manufacturer and see if one is even allowed on how much if so. Many don't allow them on certain hulls and if you install one, it WILL VOID the warrantee. I would have doubts about an 8" being allowed.

A general rule of thumb for figuring engine offset (size jackplate) is to level the boat on the trailer with the tongue jack, trim the motor until anti-cav plate is level, then measure from the bottom edge of the pad to the front, leading edge of the motor. 1 1/4" - 1 1/2" per foot of hull length is usually a very good (desired) offset. So, for a 15' boat you are looking at 18" to 21". You subtract the measurement you get from the desired offset and that gives you the size jackkplate. Too much offset and you will have a bad problem with the boat porposing at mid speeds and coming off plane. If you get the jackplate up to the height that gives you the best speed, it will usually take away from hole shot, unless you install a hydraulic unit that lets you run it down for hole shot and run it up for max speed. Which, with a 70hp motor, you are probably looking at $200 per mph gain.

Another thing, unless you are running a high rake, stainless steel, performance prop, you truely really wasting your time and money anyway. Which you are probably pretty much doing that anyway for what small gain you will see.

The next thing is going to be your steering cables, since I have doubts you will have hydraulic steering on a 15' boat. Most factory installed cable lengths will not be long enough to give more than 6" of setback and many won't even reach with 6". So you may be looking at new cables right off the bat.

[rv02201]

Keith

I know this is hard to believe but this is in fact a 15ft boat with a 125hp merc on. The people that are doing the marketing on this boat mentioned that they are going to test a 150hp aswell. It is that thing of "the bigger the better". They are however not marketing the boat to be rated with a 125Hp. It is rated for 60HP-115Hp.

 I am not very good with Facbook and those things but it is the only way I could post some images for you to see the boat. Please try to access the link that should take you to 4 pics of the boat.

https://www.facebook.com/media/set/?set=a.519850781431547.1073741825.100002198882039&type=1&l=d92b222b81

 

Thanks for the information on the Jackplate. This gives us a good idea on how to measure for the correct Jack and offset.

 

To your comment about us taking a ride in a demo with different motor etc. This was the first time this boat was on the water and we wanted to see how it performed with a much larger motor than what we intended it for. Our logic was that if it is handling that weight of a 125hp we would be well setup with our 70hp.

 

The 125 was not completely setup correctly as I mentioned and I think they had the incorrect prop on there (think 22p Prop) which made the performance of the motor questionable. It was also a brand new motor and is probably still running in. Make no mistake that once the boat has plained it was running very well for a small boat but we just thought that in take off we did not feel that spark of performance.

 

In your oinion would you think we would go overboard with a 8" as we saw some similar small boats in Japan with a big body outmount and probably a 5"/6" Jackplate on top of that and it seems to perform very well (60hp Merc)?? 

 

We will however utilize your guided calculations to determine the size. Thanks again

[BKeith]

OK, it's a 125 on a 15' hull.  Not sure where you are located but there are a number of states that give you a rather healthy fine for over powered boats.  Insurance companies usually will not insure one that's more than 5% over powered.  Which I guess in your case will not be a problem.   I do have a problem with taking a demo ride in a boat that has almost twice the HP as the motor I'm going to have.  There is just absolutely no way you are going realize the preformance loss that boat is going to suffer with a 70, when you tried it with a 125.  Apparently you are not in the US and don't have to worry about Coast Guard ratings.  At todays standards, a 15' boat would probably be rated for about 85hp max.  Putting something on the market with that kind of over powered setup is a personal invite for some massive law suits the first time somebody gets hurt by one of them. 

 

Back in the early 80's I bought a 15' ProCraft with a 70.  It struggled to get on plane with two people in the boat and if there were three, one had to sit up at the bow until it was on plane.  The ran about 45 mph at best with just me in it.  I put a 150 Merc (which was about a 115 at todays ratings) on it and with two people in it, it ran almost 70 two people in it with no jackplate or real setup, and shot out of the hole.  Have no idea how fast it ran with just me because poor ballance made the chine walk was so bad you couldn't drive it.

 

Any way, back to your's.  Don't forget the steering cable issues (I hope they had dual cable or hydraulic steering on that one you tried) because most 15' will only come with single cable steering and it's most likely going to be too short to install much setback.  Don't forget the fact a high rake, performance prop is going to be a must, and that's going to be another $500.  Don't even think just because an aluminum or standard prop the right pitch is going to work.  Well, yes it will "work" but the performance will suck so bad, you totally wasted the money of intsalling a jackplate.  One of the main functions of a jackplate is to get the motor back so it has better leverage to lift the bow and get the boat running on the last two or three feet of the pad.  If you don't have a prop that's going to help provide that lift, it's kinda sinseless to install a jackplate to try and get it.  

 

Next is going to be the hull design.  If it's not a riser hull, with a good pad, it's a waste anyway.  If you are not aware of what a riser hull is, look under most any large bass boat built today, you will see a the pad that's about a foot wide, then it starts going out in steps so as the boat gains speed, the water starts lifting the hull more and more out of the water and with enough hp and speed, it finally gets up and running on that smaller pad area of the hull.  The easiest way to know you are getting fully on the pad, it will usually start chine walking where it starts falling of the pad from side to side.  Then you have to learn how to drive it so it's stays balanced on the pad.  However, with a 70hp motor, I don't think you will ever have to worry about getting anywhere near the chine walk stage because that usually doesn't come into play well into the 60's or lower 70's.

 

I've gotta quit typing so much, I'm liable to give somebody the fool idea a know something about the dang things.

[rv02201]

We are located in South Africa and I am not aware of these types of laws here as you described it.

 

Thank you very much for your help and the willingness to share your knowledge.

 

We got onto the demo to basically observe how it is lying/sitting in the water in terms of boyance and nothing about the performance.

We have a lot of confidence in the hull design that it would be able to handle rough waters. We also wanted to get a feel on how functional the layout of the decks was

 

In about 3 weeks time we will be starting to setup the boat we ordered with the 70 Yami and will post a picture of it again at that time. 

 

For interest sake: Are you a boat dealer or just a boat fanatic that you have so mich knowledge about these things??

 

Thanks again

[BKeith]

Just a bass fisherman and a speed freak. 

 

Cars, boats what ever, I have this thing about making them go fast.  Back in the late 60's I had a 16' Allison with twin 135BHP Merc racing engines on it that was running 81 mph.  I have a 20' Javeline Renegade basss boat that will run 83 with two people, full gear and livewell, but thats with one of my hotrod Johnson 3.0 V6 engines I build that putting out 327HP @ 6,200 rpm.  I've built two stroke race engines and had hot rod boats since the late 60's.  I've built and raced cars since the early 60's.  My first real hotrod was a 1955 Ford I built in 1964 with a 63, 427 Ford engine that I bored an stroked to approx 460 C.I. and have no idea how much HP it had because I didn't have a Dyno, but it was "A Bunch".  I could haul 40 gallons of moon shine (that's how I made my spending money back then) at a time and there was not a cop car or ATF Revenuer that could touch it, but several tried.  I probably painted that car a 1/2 dozen times, they would get wise to it, and I would paint it a different color to throw them off again.

[james 14]

Until I drove the boat with the 70 I wouldn't even buy it much less try and think about what jackplate I want. If the 125 struggled getting it on plane there is no amount of setup that will make a 70 work.

[South FLA]

Cool little boat! Would you be able to post a picture of the hull?

[rv02201]

_{I have attached some additional pictures of what I have about the hull on the attached link. https://www.facebook.com/media/set/?set=a.519850781431547.1073741825.100002198882039&type=1&l=d92b222b81}

_{Once we have recieved our own boat I could add more specific pics.}

[J Francho]

Thanks for the pics.  Looks like a riser pad hull with sponsons.  I'm guessing the sponsons in the rear are necessary with that size motor.

[South FLA]

Looks like a fun little boat that would fly with a light modified engine and lithium batteries with proper weight distribution.  It should be a jack rabbit out of the hole if set up correctly! Scary at speed though I am sure!

[BKeith]

Yep, I looked at those last pictures and it's hard to tell about the hull. Not sure if that rib is just a reinforcement rib or if it's the start of one of the steps on a riser hull. If they put a high performance hull under it, that little boat, with the a good setup, would be almost suicidal with that 125 on it, and to think they are thinking a 150.

Can you imagine just how fast that little thing could do a bat turn if someone chopped the throttle with it up and flying the hull. That short of a hull would come around so fast, it would launch the person/people in it into the next time Zone. I've seen 20 footers throw people through the sides.

[rv02201]

It surely looks like a riser hull if I understand the terminology correctly as the centre part of the hull of about a foot or more, is running a bit deeper than the rest of the hull.

You are correct. When we tested it the turning of that little boat was very impressive.

[BKeith]

A "bat turn" is not a normal turn.  It's when the front of the hull bites into the water and all of a sudden the boats does a 180.  Not something you really want to be part of at a high speed.