Jack Plate for a 14ft V-bottom with Short Shaft Transom

[cchildr84]

I have a 78 Starcraft Seafarer 14ft boat with a short shaft transom. I have two motors that I use interchangeably with this boat. (1) a Mariner 9.9hp and (2) an Evinrude 25 hp. The anti-cavitation/anti-ventilation plate on the mariner sits flush with the bottom of the transom. However, on the evinrude it's approximately 1" below, thereby causing drag which I want to eliminate.

So my question is, what is the best method for jacking the evinrude up. As a cheap experiment I'm considering getting some 3/4" x 3/4" or 1"x1" square tube from home depot, as I've only got about that much space on the top end of the transom to allow the clamps to remain on the transom. The other alternative is to get one of the manual jack plates with an 5" offset.

What are you guy's opinion, what have y'all been successful with? Your thoughts and suggestions are appreciated.

 

[steelespike]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

The old time outboard racers would put a stick in place to hold the motor while while tightening the clamps.
It really wasn't needed once the motor was secured.
I say raise it 3/4 or what ever works and see what the results are.
Of course the jack plate would let you raise the motor to the fine point where ventilation becomes a problem.
Then you could consider a prop like the Solas Amita or Rubex or turning point (Rascal?)and maybe get a little more height.
The set back also gets the prop in "cleaner" water that may also allow slightly more height.

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

The old time outboard racers would put a stick in place to hold the motor while while tightening the clamps.
It really wasn't needed once the motor was secured.
I say raise it 3/4 or what ever works and see what the results are.
Of course the jack plate would let you raise the motor to the fine point where ventilation becomes a problem.
Then you could consider a prop like the Solas Amita or Rubex or turning point (Rascal?)and maybe get a little more height.
The set back also gets the prop in "cleaner" water that may also allow slightly more height.

Yeah, the issue I believe I've run into with the potential of the jack plate and being able to raise the motor where needed, is that my 25 HP in an 81 year model with the split gear case and non through hub exhaust. The highest pitch prop I've been able to find on this motor is a 12" pitch, which is what I have on there now in order to keep the RPM down below max rpm's. With a jack plate, and the prop being in cleaner water, wouldn't I need to go to a higher pitch?

 

[kfa4303]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Hi cchcild. I ran into a similar issue with my vintage 2-stroke. While you can certainly buy fancy adjustable jackplates, I simply made a fixed transom riser which extends a few inches above the stock transom. I made mine out of scrap aluminum and lumber, then bolted it in place with stainless hardware, but you can buy professionally made versions as well. My anti-vent plate now sits level with the keel rather than dragging a few inches below. While I didn't notice a huge increase in speed, the steering is much more responsive and she seems to hop up on plane a little more quickly.

 

[steelespike]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

I believe you may be able to find props at Propco Not sure of name.
The motor is rated for a max of 5500 rpm make the changes and check your rpm.
with a 12". An induction tach like a Tinytach will get your rpm. A gps for speed.
if its a 2.15 ratio a 12" prop at 10% slip would produce about 26 mph.at 5500.
What is your total weight ?Motor is about 105,boat about 200 tank of gas about 40,battery about 50,you?Gear? a friend ?
According to the NADA list; a Sea Farer is likely a 74 or75.

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

I believe you may be able to find props at Propco Not sure of name.
The motor is rated for a max of 5500 rpm make the changes and check your rpm.
with a 12". An induction tach like a Tinytach will get your rpm. A gps for speed.
if its a 2.15 ratio a 12" prop at 10% slip would produce about 26 mph.at 5500.
What is your total weight ?Motor is about 105,boat about 200 tank of gas about 40,battery about 50,you?Gear? a friend ?
According to the NADA list; a Sea Farer is likely a 74 or75.

The boat is deffinately a 78 Sea Farer, I have a title and HID# that identifies it as such. But as you say the boat itself weighs about 200#. With the 12" prop, this past weekend myself, a friend, gear and motor, load was 750-800#. And we were moving around 24-25 mph at 5000 RPM. That was as high as the rpms would go. I do have a tiny tach on the motor, so that's how I'm tracking RPM's.

So Im curious to know if the 11" prop would be better with a heavier load. Would the heavier load reduce the RPM's on the 11" prop. If it's just myself then maybe the 12" prop would be better. Thoughts?

 

[steelespike]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

If I recall correctly the rpm range is 4500-5500 so your presently well within the range,
You appear to have good performance as is.
Is your hole shot good with that load?

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

If I recall correctly the rpm range is 4500-5500 so your presently well within the range,
You appear to have good performance as is.
Is your hole shot good with that load?

I have no basis for comparison on the whole shot, but I suppose it's not to bad. If I had to take a guess I'd say it takes a good 20-30 yards to plane out. It looks like my hopes of reaching 30mph with this motor are going to fall 2-3mph short. Oh well, it's a great motor and I enjoyed re-building it.

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Well, I say no basis for comparison, but it's obviously a lot better than my 9.9hp. Haha.

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

The old time outboard racers would put a stick in place to hold the motor while while tightening the clamps.
It really wasn't needed once the motor was secured.
I say raise it 3/4 or what ever works and see what the results are.
Of course the jack plate would let you raise the motor to the fine point where ventilation becomes a problem.
Then you could consider a prop like the Solas Amita or Rubex or turning point (Rascal?)and maybe get a little more height.
The set back also gets the prop in "cleaner" water that may also allow slightly more height.

Well I have adjusted the motor as high up on the transom as I can without it coming off, and it's steel slightly below the keel, I think that the proper word. I'm gonna take it out this week end to see if there any difference, even if it's minimal.

 

[kfa4303]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

If you run out of transom, you may need to get a transom extender of some sort. You can make your own, or buy one like in the pic below

 

[steelespike]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

The boat is deffinately a 78 Sea Farer, I have a title and HID# that identifies it as such. But as you say the boat itself weighs about 200#. With the 12" prop, this past weekend myself, a friend, gear and motor, load was 750-800#. And we were moving around 24-25 mph at 5000 RPM. That was as high as the rpms would go. I do have a tiny tach on the motor, so that's how I'm tracking RPM's.

So Im curious to know if the 11" prop would be better with a heavier load. Would the heavier load reduce the RPM's on the 11" prop. If it's just myself then maybe the 12" prop would be better. Thoughts?

Calculations indicate your slip is 22% I think perhaps the tach is off high. Typical slip is 10 to 12%.
Is your prop dinged up or possibly from 1981? Is your speed by gps?
An 11" prop would help hole shot and probably add up to 500 rpm.
Is your prop shaft simple pin drive,no splines?

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Calculations indicate your slip is 22% I think perhaps the tach is off high. Typical slip is 10 to 12%.
Is your prop dinged up or possibly from 1981? Is your speed by gps?
An 11" prop would help hole shot and probably add up to 500 rpm.
Is your prop shaft simple pin drive,no splines?

I have two props, both of which are brand new. The 11" prop is a michigan wheel #012016 and the 12" prop is a michigan wheel 012030. The prop shaft is a 13 spline shaft with the plastic end cap/cone with a cotter pin. I'm using a gps app on my phone, and when comparing speed in my car it's dead on accurate. The tiny tach I bought is brand new as well. I have it set to the 180 degree setting as when it was on the 360 degree setting rpm's were way to high at right at double. Also, it's non-through hub exhaust if that makes any difference.

 

[steelespike]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Your gear ratio is 1.75 5000 rpm with a 12" prop @ 25 mph calculates to 22% slip.
Its possible that the load may increase the slip.It would be good to get some numbers lightly loaded.
Sometimes the tiny tach will pickup stray pulses if the "antenna" wire is too long inside the hood.
It should be on the plug that gets the wire outside the cowl as soon as possible.
We know your speed is accurate we know the ratio is 1.75 and the prop is an excellent 12" If the tach is accurate at 5000 then your running a 22% slip.
If we assume the tach is wrong and use 12% slip the rpm is 4375.
A lightly loaded test run will get us a more accurate indication of prop efficiency.

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Your gear ratio is 1.75 5000 rpm with a 12" prop @ 25 mph calculates to 22% slip.
Its possible that the load may increase the slip.It would be good to get some numbers lightly loaded.
Sometimes the tiny tach will pickup stray pulses if the "antenna" wire is too long inside the hood.
It should be on the plug that gets the wire outside the cowl as soon as possible.
We know your speed is accurate we know the ratio is 1.75 and the prop is an excellent 12" If the tach is accurate at 5000 then your running a 22% slip.
If we assume the tach is wrong and use 12% slip the rpm is 4375.
A lightly loaded test run will get us a more accurate indication of prop efficiency.

Well maybe this will help, this weekend I went out and put the 11" pitch prop back on. With me and my brother plus all gear load is probably somewhere in the range of 850-950#. We were running at 25mph at around 5300rpms per the tachometer. While my brother was getting the trailer, I ran by myself and less 300# I was running at 27mph at 5500RPM. Thoughts.

 

[Sea Rider]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Have you tried your long shaft engine sitting on your short shaft transom at wot ? Have you experienced water splashes at back transom ? where they over the engine/transom or just side transom out splashes ? Boaters will say you can't use it that way due to the eternal best cav plate height match endless discussion, but you can with a little add on device.

If you're achieving 5500 rpm lightly loaded or with you alone and that range is the max rpm factory stated for that engine stay with that 11" pitch prop. Max rpm will drop accordingly when addding more weight, besides most boaters don't run full wot all the time, just to plane faster and then reduce throttle accordingly to maintain a perfect plane with less fuel comsumption.

 

[cchildr84]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Have you tried your long shaft engine sitting on your short shaft transom at wot ? Have you experienced water splashes at back transom ? where they over the engine/transom or just side transom out splashes ? Boaters will say you can't use it that way due to the eternal best cav plate height match endless discussion, but you can with a little add on device.

If you're achieving 5500 rpm lightly loaded or with you alone and that range is the max rpm factory stated for that engine stay with that 11" pitch prop. Max rpm will drop accordingly when addding more weight, besides most boaters don't run full wot all the time, just to plane faster and then reduce throttle accordingly to maintain a perfect plane with less fuel comsumption.

My motor is technically a short shaft motor, but a little on the long side if that make since. I've moved the clamps up as high on the transom as possible with out it coming off. Now the anti cav./vent plate is approximately a 1/2" below the keel. I did notice, that by myself it seem to plane out a little quicker than before. I would say that the splashes you mention are at the anti cav plate, and are somewhat small.

 

[steelespike]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Your numbers are a little better.Slip is still high at 17%.Speed is reasonable at 27 though the 25 was rated at 30-32 on a reasonably fast boat. The 18 was rated at 27 on a reasonably fast boat.
Some aluminum boats are hard to push with all the rivets seams and convolutions.
Some jons have up to 5 keels for extra strength.
I don't think raising the motor will add much speed at this point.A jack plate would probably get it in "cleaner" water, might improve slip and thus a whisper more speed.

 

[Frank Acampora]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom



here are a couple of photos showing how I jacked up my 55. Now, obviously with you 25 you won't need as much bracing but it should give you an idea. As for high pitched props, you really should go on ebay and look for bronze props. These old school props usually come in higher than standard pitch and if you can find an Oakland Johnson (OJ) Prop they perform remarkably well. You just need to do your homework to be certain it will fit your engine..

 

[Sea Rider]

Re: Jack Plate for a 14ft V-bottom with Short Shaft Transom

Cchildr84

Can you post a pic of your 25 horse showing side of lower unit, cav plate, prop area to visually check your current lower leg shape.

Happy Boating