How fast should I be going?

Hey all, first post and look forward to learning from y'all.

First it's a 1648 Lowe big jon with a 1981 johnson 35 2 stroke, tiller. I have a 13p and 15p props for it depending what I'm doing with what load.

With me, 2 batteries, bow mount trolling motor, 6 gal gas in rear it takes 15-20 seconds to plane and top end of about 20-22. Regardless of load it lugs with the 15p prop. Quicker to plane on 13p like it should and of course motor runs out of revs before throttle. I've got no tach but can easy hear the motor.

My previous experience is of a 1452 polarkraft with a 25 evinrude and 13p prop that'd make that boat jump to 29 in less than a few seconds... SLIGHTLY lighter set up.

My logical side (small as it may be) says I should get to plane in 7 or less and reach 28-31mph.

I need yalls knowledge please. I think somethings is wrong with motor (low compression)..... What say y'all?

I'll start by saying I probably know less about props and such than many others on here.

When I had "jumping on plane " issues with a couple of prior boats...I first tried a "Dole-fin" (purposely misspelled) or similar add-on fin. It got me up on plane quicker, but cost me a couple of MPH.

I then experimented with Smart Tabs https://www.nauticusinc.com/smart_tabs_automatic_trim_tabs.htm

What a difference! The hole shot was great; the boat road better over waves and wakes; I could run at much slower speeds and still keep the boat on plane. Best thing I ever did for that heavy 14 foot Carolina Skiff.

I later added Smart Tabs to two other boats ...a 16-foot tunnel hull El Pescador and another 16 foot Carolina Skiff. Haven't needed them on my tinny, but will put them on if I find I need them.

Inexpensive....and they WORK. richg99

richg99 said:

I'll start by saying I probably know less about props and such than many others on here.

When I had "jumping on plane " issues with a couple of prior boats...I first tried a "Dole-fin" (purposely misspelled) or similar add-on fin. It got me up on plane quicker, but cost me a couple of MPH.

I then experimented with Smart Tabs https://www.nauticusinc.com/smart_tabs_automatic_trim_tabs.htm

What a difference! The hole shot was great; the boat road better over waves and wakes; I could run at much slower speeds and still keep the boat on plane. Best thing I ever did for that heavy 14 foot Carolina Skiff.

I later added Smart Tabs to two other boats ...a 16-foot tunnel hull El Pescador and another 16 foot Carolina Skiff. Haven't needed them on my tinny, but will put them on if I find I need them.

Inexpensive....and they WORK. richg99

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Well I had thought briefly about adding something like that but, I think that even without that, a 35hp should really push a 16' Jon boat much quicker as well as a better top end.
It's almost like its dropping a cylinder as soon as I go wot....as if the timer base is messed up, or a finicky rectifier.
Who knows. I'm gonna talk to a local shop who only does tin boat stuff and get their take on it.

I just figured SOMEONE had a similar set up and might could chime in with their results.

Thanks Rich! Btw, I like the idea of putting smart tabs on a jon boat, it'd make a HUGE difference

I'd think you'd be getting better out of it. I have a 16' 42" .100 gauge boat with a 2000 Johnson 25hp jet (same powerhead as yours) and it ran 22mph when I bought it and turned out the head gasket was blown not sure what it runs now after the new head gasket but if was getting those speeds with a outboard jet with a blown head gasket I figure you should be getting mid to high 20's pretty easy.
How high is your motor?

I think you should have better performance than that. Keep looking.....Something isn't right. Good luck.

ill assume its a short shaft engine which is what is required? should be running right at 30mph ,,, what dia prop,,, should be not much bigger then 10'' running a 14 p

Yeah I'd definitely say engine problem, either your mounting height or trim is funky or it's not making all it's power.

Lil' Blue Rude said:

I'd think you'd be getting better out of it. I have a 16' 42" .100 gauge boat with a 2000 Johnson 25hp jet (same powerhead as yours) and it ran 22mph when I bought it and turned out the head gasket was blown not sure what it runs now after the new head gasket but if was getting those speeds with a outboard jet with a blown head gasket I figure you should be getting mid to high 20's pretty easy.
How high is your motor?

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So the motor is a short shaft and it's directly on the transom. I think it's a short shaft, mid section is just over 15". My transom measures at 17". That may explain the prop cavitation while on plane if I hit chop or a wake.

riverbud55 said:

ill assume its a short shaft engine which is what is required? should be running right at 30mph ,,, what dia prop,,, should be not much bigger then 10'' running a 14 p

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I have a 13p and a 15p both 10" diameter. It is a short shaft I believe but I'm almost thinking I should run a 20" shaft and get a manual jack plate to lift it a couple inches.

My polarkraft (1452) was easy because transom was 20" exactly. 25hp 20" evinrude pushed it 30mph and popped out of the hole

If you are thinking you may need a long shaft based on a tape measure measurement from the transom to the bottom you would be incorrect.
A 15" transom measured that way will often be over 16" easily. The transom is at a 15 degree angle approximately.
You have to take a straight edge and lay it against the bottom of the boat, extend it several inches past the bottom and measure vertically from the edge of the transom to the straight edge.....you will have close to a 4" gap at the bottom. That is a true transom measurement.
Second....a 15" engine will almost never be exactly 15". To complicate matters, engine manufacturers will differ as well.

Pappy said:

If you are thinking you may need a long shaft based on a tape measure measurement from the transom to the bottom you would be incorrect.
A 15" transom measured that way will often be over 16" easily. The transom is at a 15 degree angle approximately.
You have to take a straight edge and lay it against the bottom of the boat, extend it several inches past the bottom and measure vertically from the edge of the transom to the straight edge.....you will have close to a 4" gap at the bottom. That is a true transom measurement.
Second....a 15" engine will almost never be exactly 15". To complicate matters, engine manufacturers will differ as well.

Click to expand...

Ok that makes sense then. I wasn't sure about that when I was getting 16-17" measurements.

Thanks everyone for your input. I'll start with compression test. I've got $ for a replacement so I may just replace it