Calculated Prop Slip on Jon Boats - What is a reasonable number?

[John_S]

I picked up a new project boat a little while ago. It is a DMI 1648 mod-vee jon boat (welded seams, but rivited supports), with a '98 25HP Merc 2 stroke.

With the merc prop (13P), I am getting 5600 rpms (wot rpm range 5-6K) with a gps speed of 26mph. This is two adults plus some fishing gear. Manual trim settings were tested, and best seems to be parrellel with bottom, but about 1.5" below. Raised it 0.75" with no measurable improvement. Water still going over top of cav plate. I cant raise it any more without drilling holes in transom or using a jack plate.

Using a calculator, and specified gear ratio, I get a 16% slip. That seems a little high, but this is my first jon boat. Speed seemed reasonable to me, based on semi-v and v experience. I have seen some people reporting speeds of 30-31mph with similiar boats and motor combos. Many people don't seem to use tachs on small motors, which makes such claims slightly suspect.

My next testing will be with added weight to make sure I will still have decent performance after adding trolling motor, battery, front and rear decks, etc.

 

[steelespike]

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Assuming a bare bones boat with a clean smooth bottom speeds around 30 are possible.
26 is in the ball park. 13" prop seems reasonable.
Some jons are stubborn about keeping the bow up and some have 4 or 5 keels hardly designed for speed. Slip calculation is only as accurate as the input.16% is a little high 10 or 12 acceptable.Is the prop in excellent condition.Aluminum prop in silty water where contact with soft sandy bottom is likely may actually wear to undersized diameter over time.

 

[John_S]

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Boat has a clean and straight bottom. There are 4 longtitudal "ribs" or strakes tthat run full length of boat. A fifth center one only runs partially down the length.

Prop is not worn or knicked. It is a merc alum 3 blade prop and has part number.

Pictures can be seen: http://www.joefishin.com/JonBoatProject.html

My thinking was, this type of boat planes quickly, but maintains much more wetted area than typical v's. There is no power trim, to try to raise the hull out of the water. When I tested the next trim rod position out, it did raise the bow more getting on plane, but had random air being sucked into prop. Top wot speed was unchanged.

 

[John_S]

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

It has also been suggested to raise the motor even farther. Clamps won't work any higher, and would have to drill. Don't want to end up with multiple hole in transom. So, looked at the possibilities with the slip calculator. Will raising that 1-2 inches gain me 400rpms? Theoretical speed would be 33mph, assuming a very low slip of 10%, would yield at wot speed of 30mph. Seems optomistic??? Have others seen that much of an improvement?

 

[steelespike]

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

I don't think raising the motor will net 300 to 400 rpm. though the less motor in the water the better. I would do a careful test at the present height see if it is sensitive to venting in turns.
Many times motors end up with the plate about an inch above the bottom.With all those
"ribs" I wonder if they may disturb the water enough to cause some venting issues as you raise it further.The 30 mph I mentioned was with an ideal setup/boat motor combination.
I've seen a test with a new 25 2 stroke direct injected not a Merc and a 16 ft aluma craft,not a jon, 280lbs,2 men 6 gal gas battery and gear a little over 31 mph. Of course tested with as close to ideal professional setup as possible.

 

[John_S]

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Years ago I played with allot set-up stuff with a 10' semi-v and 10hp sporttwin. No gps or speedo and no tach. Was all seat of the pants. Ventilation was an issue when that raised combo was put into a tight turns.

With a jon boat, not sure if you could do a sharp turn. Have to slow down, and it is a wider turning radias than a V.

I think I will do the additional weight tests and see how that effects the current performance, before making a final decision on raising engine further. I suspect the jon boat can handle additional weight better than a v, but we will see.

 

[steelespike]

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

The old Evinrude with the old style gear case with the split case can't run as high as most of todays motors.

 

[John_S]

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

Re: Calculated Prop Slip on Jon Boats - What is a reasonable umber?

The old Evinrude with the old style gear case with the split case can't run as high as most of todays motors.

But she will plane a boat faster than any modern 10hp!