Jon boat outboard motor mounting height problem

So I came across one of the “too good to be true deals” and bought a 14′ duraboat v-hull Jon boat. The Johnson 25hp tiller motor’s propeller is a few inches too short and when getting to around 8-10mph it makes a dip in the water and loses any thrust and just spin freely. I’ve been looking up options and what I’m seeing is either cutting the transom to mount lower, buying a jackplate and mounting upside down, buying a adjustable mount (which to feel safe about weight limit wound be around $400) or trying to find a prop extension for it. Any suggestions on which route is best?

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The good thing is they made that motor forever. There is a long shaft kit for it, I think. But it sounds like you have a spun prop hub. Mine just went out. I never knew there was rubber in there but there is and it gets old.

Didn’t know this was a thing either but watching a few videos and seeing how they act it is a possibility. Spun prop hub kits are pretty cheap so might replace anyways but I still feel like I need to get it deeper in the water. To be more precise with my original statement, it seems to spin freely across the top of the water. Propeller seems to be spinning good just not under enough water to gain thrust

How tall is the transom on that boat?

If that is my boat, I’m taking a sawzall to that section, smoothing off rough edges, reattaching motor and going fishing.

20″ transom

Take another picture with a straight edge on the bottom of the boat out to the lower unit. Have you tried different pin locations to see if it helps?

Working out of town til tomorrow, I’ll take more pics when I get home. I’ve tried every pin position but only changes the tilt. The pics do look misleading but I’ve held a level flush with bottom of the boat and the plate above the propeller is around 2&1/2″ above the bottom of the boat.

Sounds as though the prop isn’t far enough down in the water and cavitates when a certain amount of load is put to it. Looking at the pictures your most economical fix is to take a 3″ cut out of the transom, then seal the exposed wood with a heavy coat of thinned spar varnish so it soaks deep into the wood and then a couple coats of spar varnish straight from the can.

Pin location as it sits is a moot issue. The cavitation plate is above the bottom of the boat and needs to be an inch under the last portion of the hull in the water.

Michael Tabor wrote:

or trying to find a prop extension for it.

A spacer kit would be the “best” option. I would contact places which sell used parts, such as Tim’s Outboard Tell him what you have and he can probably fix you up with all the pieces you need to convert your motor to a “long shaft”.

The difference in transom heights for this age motor is 5 inches. Standard heights were 20 inches and 15 inches. Cutting 5 inches out of your present transom may create an unknown structural weakness.

There’s a support bar on the inside, I could cut about 3″ and be flush with it but 5″ wouldn’t even be an option. From what I’ve read…the plate above the propeller just has to be flush with the bottom of the boat and I’m only around 2-2&1/2″ from it. Am I wrong? I know it’s a short shaft and it’s a 20″ transom, so I really need to get 5″ lower?

Michael Tabor wrote:

There’s a support bar on the inside, I could cut about 3″ and be flush with it but 5″ wouldn’t even be an option. From what I’ve read…the plate above the propeller just has to be flush with the bottom of the boat and I’m only around 2-2&1/2″ from it. Am I wrong? I know it’s a short shaft and it’s a 20″ transom, so I really need to get 5″ lower?

I have no idea if 3 inches lower would be enough. Just saying that motor was designed for a 15 inch transom height. One thing about it, cutting the transom is pretty much permanent and hard to put back if it does not work out for you.

True! That’s why I haven’t jumped on cutting it yet. Thanks for all of your suggestions! I think I’m going to shop around for an extension and if I can’t find one I’ll consider cutting it. Will post pics and results once a solution is found

You may still come up short if you do the cut. After studying the picture on the right I think you’re still going to be short.

Sawzall will cut thru the support bar as well. Cut low enough, attach new plywood strip with bolts to act as new support bar. Don’t overthink a jon boat.

Deuces wrote:

Sawzall will cut thru the support bar as well. Cut low enough, attach new plywood strip with bolts to act as new support bar. Don’t overthink a jon boat.

Cut low enough and water will run over the transom too!! You have no idea how much freeboard this boat now has, or how wide it is or how much load it carries. Yes, other boats have been built with 15 inch transoms, however, they were designed for a 15 inch transom height. There is a reason this boat was built with a 20 inch transom.

Lowering the available freeboard may well create an unsafe condition.

**UPDATE**
Found a 5″ gear case extension kit with everything included (5″ gear case, drive shaft, shift rod, water tube and bolts) from http://www.timsoutboard.com/ for only $100 plus shipping and got it in like 3 days! THIS DID THE TRICK! Boat fly’s across the water and handles great! Took a few YouTube videos and learning a lesson of not letting the impeller key fall in the propeller housing but got it done in about 2 hours. Thanks huntingdave for the link to Tim’s. I couldn’t be happier with the setup now

I like reading success stories on a Friday afternoon. Good stuff !

S.R.

Steve Root wrote:

I like reading success stories on a Friday afternoon. Good stuff !

S.R.

I really appreciate everyone’s time and suggestions leading to me finding the solution so I figured I’d at least take a moment to update and share the success! Thanks again everyone! I’ll update pics later when I get home

Very nice! Glad you got it tuned in.

Steve Root wrote:

I like reading success stories on a Friday afternoon. Good stuff !

S.R.

I’ll wait to tell my “fail” story til Monday then…

Glad it worked out for you.

Good deal! Appreciate it when guys who start a post finish it off with what they found out. That’s pretty cheap for that kit.