Beating the dead horse-with a prop

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Apr 18, 2022
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Nashville Tn
Hello all,

I'm Jaxon, I found the forum several days ago on my search for the perfect prop, still haven't found it. I thought I'd register and see if I couldn't get some insight.

My boat: Lowe L1648 (16' jon, 48"w)
Engine: 06 Honda 20 short shaft (which, as far as im finding, is perfect for this boat)
New 9.25x10 3 blade
(top speed unloaded 6150RPM 25 mph- over revving by 150)
(top speed loaded, 5700rpm 23ish)
Old 9.25x10 4 blade, rough shape, Honda factory!
(top speed unloaded 5400RPM 22mph perfectly between the rec'd 5-6k)
(top speed loaded unknown rpm, 18ish)
Unloaded boat props work out to about 12% slip.
Primary boat use is fishing, low weight, low miles.
Secondary is hunting, loaded heavy, lot of water to cover.

Boat dry weight is about 300, curb weight of approx 550-600.
Total max loaded weight 1200, average weight is probably closer to 1k, 'no load' as id call it would be 750/800

The problem, old prop was a little beat up and was starting to slip (airerate? cavitate?) at top end when loaded (only when loaded, only at WOT) new prop runs great! So what's the problem? It only runs great trimmed waaaaay in (down, if 'in' means down) if I trim out one position (middle setting) I pick up 1.5-2 mph loaded, but can't keep it due to slip. When the motor is trimmed all the way out, the plates are dead in line with the bottom of the boat, unless I'm missing something that would be the optimum position, but I'm slipping top end way trimmed in of that. As far as all the prop calculators tell me, I should be using a 9.25x12 or 9.5x12. My logic tells me In should find a 10x10. A little help would be greatly appreciated.

PS: old prop had similar aireration at the same setting, a little less maybe, but I believe was still being run trimmed much too far in.

Some background, if you cared enough to read this far. Sometime around '04 my dad got tired of listening to his dad fawn over a jon boat and offered to split one, Grandpa died of cancer having not finished one season on the boat (I think there's one picture with him and the boat), sometime near... 07 maybe, someone stripped the boat of everything but the paint under cover of night. A year or so went by and dad bought another motor to replace the one someone (evidently) needed more than we did, put a carb on and was off to the races. (motor was dealer sold in OR then shipped to us in Hellinois, but now is on the boat in TN) Its been set in the same position my dad set it in, with the same prop, and only recently got put back to use when I got it from my brother who it had been given to but didnt use it. Hindsight says brother might have dealt with war related injuries a little better with some more time on the water in his life, but alas, spilt milk. This portion had no relevance to the rest of it, but if you're still reading, heed this warning, quit smoking, if you're gonna weld stainless don't breathe the plume, and check on your veterans a little more often. Mission 22.
Well to start with you aren't really comparing apples to apples. 4 blade props will slip less and will have less "effective" pitch than their 3 blade equivalent. As an example, a 19P 4 blade is going to run close to the same RPM as a 21P 3 blade. The 4 blade will probably plane out better, offer more grip in turns, and the 3 blade will offer a little more top end because of less drag. So you should've stepped up the replacement 3 blade a little if you wanted something close to the original, although that would not help your ventilation problem.

Additionally, prop diameter doesn't really mean a whole lot in this case. Pitch is what matters, and is what diameter is dependent upon (it decreases with an increase in pitch).

JMO but I'd say a replacement 10P 4 blade would be about right and should help your ventilation issue. If they make it for your application, the Mercury Spitfire is a kick butt 4 blade. Super thin and light, very little sacrifice on the top end and excellent grip.

It sounds like you would benefit from dropping the motor a little, but that may not be possible.
I am aware of the different performance metrics of 3v4 blades, I was taking the word of the prop shop with the 3 blade. I thought the 3 blade would only worsen the problem, which, somewhat true. Everything im seeing says more pitch, but how does that help the grip issue? I'm fine with all else about the new prop, even the rpm getting too high, as its never just me and no gear in the boat. I would really like to be able to run trimmed out a bit more, stop bashing the bow into the lake. Dropping the motor was in fact my first thought, then I looked into where its supposed to sit, which appears to check out as proper, at any rate the motor is tight against the transom so without jackplates it isn't happening anyway.

Edit: I'd be fine with going back to the lesser performance of the factory prop if I could make it not slip so bad. Thats my whole desire here, to do away with the slip. I've come to the conclusion that if I can't find a definitive recommendation im going to become a prop collector pretty quickly
PROP CALCULATOR - Run this one here: , also go for the email reply where they consult with you. Try their 4-blade 'pressed' aluminum Hustler propellers - awesome on smaller boats! I've used that calculator to rig a dozen rigs and the results were awesome in all cases. Plus they offer a 'prop exchange' for a $35 fee ... which was never needed.

HEIGHT - Ideally your OB vent plate should be from even to bottom of hull to 1" UP without the OB trimmed all the way out. Set the trim in a mid-position, as long as it avoids bow steer, then check height and see TRIM.

TRIM - Sounds like the hull stern is lifting whilst at speed, sounds like there may be a 'hook' to that hull that only causes issues at higher RPMs. Place a straight edge against the hull and see if you can find any areas where the bottom of the hull is up and away from the straight edge. Check to see how it sits on the trailer and add bunk supports if needed to fully support the stern garboard/transom transition.

For running, try adding weight to the stern, like battery and fuel, with OB trim at mid-position. Boats run better with the weight in the rear to stern anyway ...
Thanks for the reply, my computer wont let me visit that site due to security concerns or something. I've tried on multiple occasions. (edit, let me try another browser...nope, did not work)

Trimmed full out, the plates (2) split the bottom of the boat near perfectly, the top plate is in that 1 above the bottom range.

I am loaded motor, fuel, battery, driver at the last 2 feet of the boat, a 40lb troller on the bow, and one passenger on the center, for the purposes of this test. When the passenger shifts 18" back the ride improves drastically, however it is impractical to have 90% of the weight in the last 30% of the boat, were the trim in a more optimum place I believe we could load a little more sensibly.

I believe a correct prop could solve the grip, allow me to trim out, and thus correct the whole of the problem. Please correct me if I'm wrong.
I was remembering incorrectly, the top plate, to the bottom line of the boat is a whopping THREE inches difference.... suppose its time to look into jack plates...?
Jdero620 said:
I was remembering incorrectly, the top plate, to the bottom line of the boat is a whopping THREE inches difference .... suppose its time to look into jack plates...?
No idea #-o what you mean here, as there is only ONE ventilation plate on an OB ... that plate immediately above the prop. With 'normal' trim, that should be even to 1" high, I find higher to be better.

Me thinks we needs a picture to best advise you ...
Not sure the anatomy of an OB exactly, (as im sure you can tell by now), the lowest plate sits probably 1-2 below the bottom of the boat, while the top plate is about 3 above the bottom and skims the surface of the water on plane. 5" or so between them.

Not sure how to add a pic, as its not allowing me to drag one off the desktop. Google BF20D and you'll notice a bottom plate just above the prop, and one of a similar shape facing backwards. Appears the bottom plate is anti vent, whats the upper one about then?
Maybe this will work...

Note the image was taken at full out trim, usually it is nowhere near this level with the boat, if the boat is considered 0 degrees then the motor is pitched down 10 degrees where it runs most reliably. 7.5 is about where it gets the best speed but slips too much.

Greatly appreciate ya btw.
The cavitation plate is immediately above the prop and often extends rearward quite a bit. Look at your picture, it's set pretty dang close to where it should be, if not a touch low.

I wonder if Dale is onto something about there being a hook in the hull. The trailer bunks should extend past the transom a couple inches, and yours don't appear to. If not they will eventually cause the hook or indention he is talking about.
By the picture , to me the motor is too low. Perhaps try and shim the motor under the clamp with a piece of 3/4 or 1" stick of wood. As long as the clamps can still make good contact with the transom your ok. It's a possibility the cavitation plate is causing some turbulence because to me it doesn't make sense that the motor ventilates with the boat loaded. There might also be something wrong with the hull.

Sent from my SM-A526W using Tapatalk

Sounds like the OB wants to turns wheels, but it can't ... you're waaaaaaaaay too deep!

I had this issue on my current hull, which has a slight hook that I cannot correct and I solved it by 2 things ... OB height and a foil.

HEIGHT - Add shims to get that vent plate (it is NOT a cavitation plate ... ) up to 1" above the bottom of the hull, using a straight edge against the hull bottom.

FOIL - I now like these 'gull wing' hydrofoils, by Attwood, for use on small boats as the ends/tips are out of the water @ highest RPMs. Some Walmarts sell them, < $40. I won't run a small boat without them. If you can't find the 'gull wing' one, the SE Sport 200 foil is also highly rated. They also help resolve any trim issues.

SO. I shimmed it up 3/4, if that helps but doesn't solve ill get jack plates, but there is a slight concave (from the bottom) curve to the hull, its probably 3/8 at worst and is gradual over the full 8 or 10 feet that is the flat portion of the boat, it terminates at the rear over about 3 feet. See attached. Too, the weld at the transom v floor joint hangs below the flat portion by about 1/8 - 3/16. Appears as though there may be a hook, but would this really cause great issue?
Jdero620 said:
Appears as though there may be a hook, but would this really cause great issue?
Yes, hooked hulls can indeed ... but again, raise as suggested and give it a whirl. I don't think 3/4" is going to do it.

On the hull I had, I had to add a 3/4" thick fully epoxied large piece to the rear of the hull that went higher than the transom, to the height of the block that raised it up and sat on the transom cap. I through-bolted the engine on once I was satisfied, as otherwise the motor is well up (but not off) the regular transom.

Believe me, I've been boating for 50-years now on my own boats (my bros & I once owned/ran 14 rigs for a few seasons there) and we NEVER, ever had to mess with one like we did on this one ...
Well.... im satisfied with the days answer.

the same test conditions plus another passenger (60lbs)
unloaded no speed change,
loaded, max speed also no change.

I was able to trim out to the second to furthest position while loaded, and the highest position unloaded with very little slip. I think we still need to go up another 3/4 or so, so jack plates are in order. Another thing, while loaded with extra passenger, I was over revving by about 50. I've called the prop shop and they're gonna trade the prop I just bought for a 12 pitch.

Long story short, the prop the calculators recommended, and jack plates.

As far as I'm concerned, mystery solved. Jack plates set to arrive monday so I'll holler back when there's new data to share. I greatly appreciate the help y'all, hours upon hours of searching over days solved in less than 24 hours thanks to yall.
Keep in mind that with the added setback of a jack plate you may need to go higher than you think.

I second what Dale said about a hydrofoil. I had a 14' Alumacraft with a heavy 25hp Johnson on the back. Without one you could only run in the lowest trim pin and it would porpoise horribly at more than half throttle. Move the trim pin up and you were staring at the clouds. A hydrofoil totally changed that boat, handled like it was on rails and it would run about 30mph wide open.

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