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

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

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, that and not having the engine gear ratio, kept me from being able to make a comparison. BoatTest.com doesn't seem to test these small combos, either.

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.

I'd redrill the transom, sit the motor on a 2x2 or even a 2x4 to get enough height for the cav plate to be level with the bottom of the hull. The extra foot is just a big brake holding the boat back. Clean up the prop, same for the bottom of the hull, no dents or dings. Try to get the engine to rev higher.

Prop slip angle is one of those mystical numbers that means nothing if the boat isn't set up right.

Jamie

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?

10% slip is not unrealistic. My big boat (22 foot center console/250 Suzi) has a prop slip of 6% at cruise and at WOT (on glassy smooth water). She is a big heavy deep-v boat to boot.

My understanding is that it is substantially easier to get low slip numbers and the weight of the boat and/or the underway wetted hull fraction decreases. This makes intiutuve sense, prop slip occurs because of the forces opposing the forward motion of the boat. If you decrease those forces, in this case boat mass and drag on the wetted portion of the hull, it is easier for the boat to move forward and the prop slips less.

I'm curious now to figure out what the slip is on my tin boat. I've got a tiny tach in the garage that I'll have to install...

Go Fish said:

I've got a tiny tach in the garage that I'll have to install...

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I have one like this: https://trailtech.net/tto_tach-hour.html

Picked it up this last winter when I was working on a new to me vintage snowmobile. Worked great for idle and wot rpms on the 25 merc. Have to try it on my Nissans, yet. Has setting for 4 stroke, 2 stroke, and 2 strokes that fire top and bottom of stroke.

Just hope the internal battery lasts a long time.

this is what i did . don't know about the slip , made a couple mph difference in speed gettin it up . now i run right at 30

if you have 10 inches in the water and you raise it up to 8.5 you have a 15 percent less drag ,less drag more fast, more fast more better

flatboat said:

if you have 10 inches in the water and you raise it up to 8.5 you have a 15 percent less drag ,less drag more fast, more fast more better

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That...

I think it's intuitive that you should visit bass boat central and read up on thier forums. The glass guys are obsessive with speed and really go into detail with the engine setups for their hulls.

Jamie

I have a 99 merc 2 stroke 25hp. I cant help you with prop slip but I will say that I was running a 12 pitch prop and was getting 31mph WOT by myself and 31mph with 2 people and an extra batt and cooler. The extra 200-250lbs did not make a difference in GPS top speed at all.

I got a 15 pitch prop expetcting to run 35mph. NOPE! Still running 31mph WOT loaded or unloaded. The only difference the 15 pitch prop made was that I could run midrange on plane 18-25mph at much lower rpm and my range was improved greatly. WOT I wasn't screaming like I was with the 12 pitch too. Time to get on plane wasnt affected one bit either.

My cav plate is a little lower than I would like (about 1.5 inches below the hull) I just cant raise it anymore. I have a mini jacker and its already mounted extra high to compensate for the longshaft motor.

I would advise you to get a mini jacker and mount that motor with the Cav plate flush or .5" above the hull. You might get to 30mph if you do. Much higher than .5 or 1 full inch above you will experience cavitation in turns at speed. I even cavitate every now and then when Im showing off how I can spin a jon boat like a jet ski. LOL

My boat is only a 1542 NOT a 1648, but I would think that the extra drag of my motor being low would put us about even considering the extra drag created by your extra surface area with 1 foot of length and 6 inches of width on the 1648. I bet you can get 30mph if you raise it to even or .5" above the hull. I would definately do the jackplate and don't waste your money on a new prop. A mini jacker is cheaper than a prop anyway!

Mind you I have no proof of this, only based on my experience with my boat which is not of equal size to yours.

reedjj said:

I have a 99 merc 2 stroke 25hp. I cant help you with prop slip but I will say that I was running a 12 pitch prop and was getting 31mph WOT by myself and 31mph with 2 people and an extra batt and cooler. The extra 200-250lbs did not make a difference in GPS top speed at all.

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That has the same gear ratio as mine: 2.25

The only way that works out, is you are way over the 6Krpm max. At 6K, theoretical top speed is only 30mph. At 31mph you are going faster than the prop is spinning???

With a reasonable slip of 10%, says the engine must be turning 6800rpms.

https://go-fast.com/Prop_Slip_Calculator.htm

I am going to do some additional weight test runs before trying to raise any further. I will add a 2nd and 3rd person, and see what sort of magnitude is of the impact. At this point, expecting less of an impact than what I see on V hulls. When I do the runs, I will repost data. Probably a couple of weeks out.

In the meantime, I received a new 4 blade prop to test on my i/o this weekend.

I checked out what my slippages was for the first time,today.
My boats a 1652 with alittle jet tunnel(hull).
I have a 30hp mariner/yamaha(87),found out that it has a 1.71 gear ratio and only supposed to have a 4600 WOT rpm?(h)
The rpms have me alittle concerned as I run 2 different pitch props.
With the 13,used 90% of the time...I get 5500rpm,about 29mph(gps)...gives me around a 26% slip.
The 11 pitch ,rpms go up to 5650...around a 15% slip.
I'll take the gps out today,it's been awhile.
Can't beleive the WOT for my motor is only 4600rpm,also can't believe it hasn't blown up,it has been used..used.

FuzzyGrub said:

reedjj said:

I have a 99 merc 2 stroke 25hp. I cant help you with prop slip but I will say that I was running a 12 pitch prop and was getting 31mph WOT by myself and 31mph with 2 people and an extra batt and cooler. The extra 200-250lbs did not make a difference in GPS top speed at all.

Click to expand...

That has the same gear ratio as mine: 2.25

The only way that works out, is you are way over the 6Krpm max. At 6K, theoretical top speed is only 30mph. At 31mph you are going faster than the prop is spinning???

With a reasonable slip of 10%, says the engine must be turning 6800rpms.

https://go-fast.com/Prop_Slip_Calculator.htm

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GPS don't lie!

I don't have a tach. But when I run WOT with the 12 pitch she is screaming! So much so, my motor guy recommended that I switch to the 15 pitch so it wouldn't be revving so much. So your right it does over rev. It is much smoother, quieter, and economical withe the 15pitch. BUT... I ran it over a log and folded 2 of the blades so its back to the old 12 for now.

I always figured it had a rev limiter! As long as Im not bouncing off the rev limiter Im good to go! LOL

reedjj said:

GPS don't lie!

I don't have a tach. But when I run WOT with the 12 pitch she is screaming! So much so, my motor guy recommended that I switch to the 15 pitch so it wouldn't be revving so much. So your right it does over rev. It is much smoother, quieter, and economical withe the 15pitch. BUT... I ran it over a log and folded 2 of the blades so its back to the old 12 for now.

I always figured it had a rev limiter! As long as Im not bouncing off the rev limiter Im good to go! LOL

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I'm not saying the gps is lieing. But, negative slip is not possible. So, your rpms have to be over 6K, and/or prop has cupping, that would make it act like it has more pitch, etc.

PS: I don't think there is a rev limiter. I haven't come across that in the Merc service manual, anyway. If it did, than it would kick in around 200 rpms above max, so 6200rpm. That would make best case for your numbers, which would equal zero slip.

Zum said:

I checked out what my slippages was for the first time,today.
My boats a 1652 with alittle jet tunnel(hull).
I have a 30hp mariner/yamaha(87),found out that it has a 1.71 gear ratio and only supposed to have a 4600 WOT rpm?(h)
The rpms have me alittle concerned as I run 2 different pitch props.
With the 13,used 90% of the time...I get 5500rpm,about 29mph(gps)...gives me around a 26% slip.
The 11 pitch ,rpms go up to 5650...around a 15% slip.
I'll take the gps out today,it's been awhile.
Can't beleive the WOT for my motor is only 4600rpm,also can't believe it hasn't blown up,it has been used..used.

Click to expand...

Yea that does seem low, but the gear ratio would seem to support it.

What kind of tach are you using? If the pole setting is wrong, it could account for the higher rpm reading. If so, that would improve your slip numbers.

It's a tiny tach.
Since I put it on acouple years ago I never knew if it was working right.
To get any readings that seem somewhat accurate,I only have 1 wrap around the spark plug wire.
Until I read that my WOT should be 4600rpm,I thought the 5600 ish,was fine.
If it was a newer outboard I'd worry abit more,I'm running this one to the ground,was only $300.
The tiny tach I receved doesn't have any pole settings.
Told them what outboard I had and this is what they sent me.
Says I idle at 1200rpm(seems high)but the WOT seemed good...

try this one
https://www.rbbi.com/folders/prop/propcalc.htm

Zum said:

It's a tiny tach.
Since I put it on acouple years ago I never knew if it was working right.
To get any readings that seem somewhat accurate,I only have 1 wrap around the spark plug wire.
Until I read that my WOT should be 4600rpm,I thought the 5600 ish,was fine.
If it was a newer outboard I'd worry abit more,I'm running this one to the ground,was only $300.
The tiny tach I receved doesn't have any pole settings.
Told them what outboard I had and this is what they sent me.
Says I idle at 1200rpm(seems high)but the WOT seemed good...

Click to expand...

If your tach was correct, I'd think you'd be slaming the gears during shifting.

Brought the gps with me...average 28mph at 5400rpm,thats with a 10.5 pitch prop.
Works out to around 9-10 slippage.
Didn't try the 13 pitch.