Miss marked prop?

[The Ole Blue]

Hi everyone,

I’m kind of at a loss with this one. I’m running a 2006 9.8 Tohatsu 4 stroke. It’s on the back of my 1986 Meyers Vhull. It’s been moded into a mini bass boat. Cedar frame, 1/2” ply decking, carpet, batteries, ect adds around 200 pounds to the boat so its heavy and I don’t expect it to be fast.

here’s the problem I’m facing. At WOT I am achieving 4900 to 5000 rpm at around 14-15 mph with just myself in the boat. My best day was 5250 at 16.6 mph but this was a rare occurrence. It takes a little time but it does plane up and run pretty well at this speed.

I decided I should probably raise the RPMs a bit to be running optimal range of 5200 to 6000. I looked for a lower pitch prop and did not have many options available. I ended up buying a 8.9x7 pitch to replace the 8.9x8.5 that was on it. I installed the new prop and went for a test run. I stuggled to get on plane and managed 11.5 mph at 4400-4500 rpms max. Went back to the launch and swapped back to the OEM and went back up to 4800 range. I expected the prop change to raise RPM at WOT not lower them.

my question is do you think it’s possible that this prop is just not what it says it is or is it possible I have a different underlying issue with the OB that could be causing this?

it did come from Amazon and was direct shipped from China but it is also marked on the box and prop itself to be a 7P.
I made an adjustment to the mounting height yesterday and bought a couple spark plugs thinking maybe the plugs are foiled (they did look a little black on the threads). I will try to get back out today or tomorrow and go for a run then swap in the new plugs to see if it makes much of a difference. Just wanted to see if anyone had any input in the prop itself.

Thanks

 

[Sea Rider]

Welcome to IBoats,

To start with, is the newly acquired prop a 3 blade Tohtsu OEM one ? With motor trimed at 90 deg, deck load well distributed, when at plane (full throttle) on flat calm water cond at which lower leg height is the water flow pasing by ? is it passing right under the small upper plate or over the lower AV plate, need to pull head out transom to check that....



To take the max out of any Tohatsu motor will need to rev towards its max wot rpm range factory stated as loaded, this is the ideal motor height for Tohatsu OB's...

Happy Boating

​

 

[The Ole Blue]

Thanks for the reply.
It is a 3 blade but NO it is not OEM. Its an Off brand from Amazon. I was having trouble finding one in the pitch range I wanted and wasn't more than a month out.

As for the height of the motor I'm going to have to take a real good look or video next time I'm out. I looked last time and only noted that I could not see the AV plate but I didn't take note of the position of the upper plate. I will say that there seemed to be a lot of water splashing up around the leg of the motor

When I first put this motor on the AV plate was sitting about 2 1/2 inches below the bottom of the boat. at that point I couldn't get on plane, the stern would just dig in and I maxed out at 8mph. After that I added a Dole Fin which helped bring the stern up and allowed me to get on plane and cruise around 13. I then added a TH mini jacker which brought the AV to 1/2 inch below the boat. Yesterday I raised the plate another 3/4 inch and it put the AV about 1/4" above the bottom. I have more room to go up but I was trying to take baby steps.

I also have my suspicions that the fin could be adding alot of drag that could be slowing things down. I may remove it at some point during my next round of testing to see what happens.

 

[Chris1956]

Put the props side by side on the bench. Use a ruler to measure the blades from the bench fore and aft. You should be able to see a difference in the "twist" of the blade.

Also measure diameter, as that affects performance. Make sure the hub is the same size.

 

[The Ole Blue]

So I got out yesterday for some test runs. I did note that the water level is running about 1/2 below the upper plate just like the above photo shows

Ran with the OEM prop Averaged 11.5 Mph at 4600 RPM. New Prop 10.0 Mph at 4400 Rpm. I will say that it seems to accelerate a little faster with the so called lower pitch prop but over all still seeing a rev drop.

While I was out I tried a few other things. Stopped and replaced spark plugs and no difference. Stopped and removed hydro foil and no difference but was happy to know that the motor is mounted well enough now that I don't need it to plane but did notice the steering is much less stable.

I will pull the props and measure them this afternoon But I am really starting to feel like this is possibly a carb issue. It just seems as though performace is getting worse each time I go out.

 

[The Ole Blue]

I pulled the props and set them side by side. The diameter is the same and bub size is almost the same the new one is a millimeter larger in diameter sand slightly longer. I'm not sure exactly where I should be measuring from but the few places I did show a pretty big difference with the new prop being a bigger measurement.

Just looking at them side by side on the desk you can clearly see that the new prop looks to have much more twist than the other. You can see in the photos

 

[dwco5051]

If that is the new prop on the right it is obvious that it is mismarked. Here is a quick way to measure pitch.
The video makes it look more complicated than it really is;

 

[The Ole Blue]

I cant thank you guys enough for the input, That's a great video. I followed the instructions and it is simple. Sure enough I checked the OEM prop first and came up with 8.42" checked the new prop and what do you know 9.61". This explains a lot.

I started the return process though amazon but I will have to pay to ship it back and those estimates are 3 times what I paid for this thing so looks like I have a cool paper weight now.

Now Its time to go back to trying to find the prop I wanted. If anyone has a tip on where I can find one I'm all ears

Thanks again

 

[dwco5051]

I started the return process though amazon but I will have to pay to ship it back and those estimates are 3 times what I paid for this thing so looks like I have a cool paper weight now.


That would be about the ideal prop for a light 12' jon boat or v-hull. Since it is the same for Mercury or Tohatsu E-bay might be the way to go.

 

[The Ole Blue]

Thanks. I have actually found an option from Solas Amita at an 8.5"X7. Based on math with the .4 drop in diameter and the 1.5" drop in pitch I should see a potential increase of 400-500 RPM's which would bring me up to the 530-5400 range.

Does it seem like I'm on the right track here?

 

[poconojoe]

Interesting and well detailed thread.
Can't wait for the follow up.

 

[The Ole Blue]

Well I emailed the company yesterday asking if they know if this is an issue and looking for some assistance and the repose I got was....

Dear friend, Please see the attachment. The difference between 8.9x7 and 8.9x9.5 is clear at a glance. We are the largest propeller manufacturer in China. 8.9x7 is our very basic model. We have sold tens of thousands of them and sold them to more than 70 countries and regions. Please rest assured to use.

The photos they sent seem to support my claim but I am by no means an expert on the subject so maybe I am wrong.

Do any of you more experienced folks have an opinions on this?

Thanks again

 

[Chris1956]

Clearly those are different style props. Not sure what else I can say. The total surface of the prop blades looks close, but the rake of the new prop seems a lot greater. Obviously the pitch of the new prop looks to be greater.

Is it possible the original prop is not marked correctly or you read it wrong?

 

[briangcc]

Nah...if you enlarge the picture you can see the left prop is a 7P while the right prop is 9.5P.

Good job including the prop markings in the photos!

 

[The Ole Blue]

Those are the photos that they sent me to prove me wrong. They include the markings.

So I still had my suspicions of a carb issue. I pulled soaked and cleaned it. Drained and cleaned my tank and grabbed a couple gallons of fresh No E on my way to the lake.

it’s started and idled great as always. I had the OEM prop on and I rode around for an hour messing with trim settings and moving weight around. And was consistently running 13.5 Mph @ 4750 RPM. I put the Dol Fin back on and then I hit the sweet spot. I finally got up on a real good plane and was nice and smooth on the water. I rode around for another hours averaging 14.5 to 15.5 Mph @ 4900 to 5050 RPMs.
I also had a friend meet up with me. Big guy 270 pounds had him ride with me and Messed around till I got it at its best and was pushing 9.5 Mph. At 4400 RPM.

Put the other prop on and went out by myself messed around for an hour and the best I could do was 11.5 @ 4600.

With the OEM prop it was the best this boat had run since I put this little 9.8 on it and I am perfectly happy to have it run like this but I’m still just touching 5k at the best of times and I’m pretty sure that’s not good for the motor right?
So I’m back at square one needing a prop that will bring the RPM up. I don’t know if they measure this thing in some weird way at their factory but I keep coming up with the same thing pitch rating of 9.5
It’s definitely a different style and I’m thinking the the long steep blades of this 7 pitch might just be causing a lot of drag.

I ordered a 8.5x7 by Solas. We shall see what that one has to say for itself.

I apologize for the long rant but I have a lot of time invested in this new unusable prop now. But at least I can say that the old one gave me a real nice day on the water today.

 

[The Ole Blue]

Update:
Newest prop came in yesterday. Looking at the 2 7" pitch props side by side you can see the difference pretty clearly.

I went ahead and measured the pitch using the method above for all 3 props again last night just to make sure I am not missing anything.

OEM 8.9X8.5 Measured 8.45"
Amazon 8.9X7 Measured 9.57"
Solas 8.5X7 Measured 6.99"

I'm pretty confident in the measuring of these now I practiced on 5 or 6 different props I have around here and generally come very close to the marked pitch on all of them with the exception of the Amazon prop.

I hope to get out on the water this afternoon to see what this new prop results in

(Photos Right= Amazon. Left = Solas)

 

[The Ole Blue]

Got out for a ride this afternoon. New prop was pushing 15 Mph @ 5900 Rpm. Just myself in the boat.

Can definitely tell the difference out of the hole. Pushing it to its limits for sure, as I would see 6000 RPMs once in a while. But I do think it will preform well with another passenger. And should keep me in the mid 5k area, Which was my goal after all. Ill be headed to the Huron river chain of lakes in the morning with a friend and see how it performs.

Let this be a WARNING to anyone out there looking at Captains brand props from Amazon. They are not what they say they are for several reasons. Do yourself a favor and look right here at Iboats or any other reputable retailer.
Learn from my mistake.

 

[Sea Rider]

Out of curiosity, does the motor sits fully flat on the transom while trimed to 90 Deg ? Why is it that boaters likes going for other brand props when well priced Tohatsu OEM ones are available in different pitches to choose from which performs top. If the combo is easy to plane when at full throttle don't need the doel fin which will achieve extra water drag and slow the combo if does not ride parallel to the water level once the boat is on plane.

A 3 blade prop revving towards its full 6 K wot rpm range lightly loaded will make the day. If adding a boating mate up front for better deck load distribution and balance and depending on his weight should expect a 300 rpm min loss which will not affect much the hole shot achieved as when solo boating.

Happy Boating
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[The Ole Blue]

I'm not sure what you mean by flat against the transom. the transom has an angle and with my jack plate I have about 11" of set back at the prop. It does run at a 90 deg to the water surface when trimmed and planning however.

As for buying the cheap prop, It was more about time frame than cost. There was no availability at the time I would have been 3 weeks out according to estimates and impatience got the better of me and I was concerned with continuing to run over propped . Now there seems to be plenty of availability.

I have ran a few time with and without the dole fin. The boat planes easily without out it on. When it is on it runs parallel with the water surface and the water rides maybe a half inch over top of it. I do see a slightly higher top speed without it but the handling is much more stable and smoother with it, so for now I have decided keep it on for that reason.

Defiantly seems to be the right prop match for a second person on the boat. Hole shot is good and jumps on plane fine. Runs around 5600 rpms with the added weight. I think it's a little under by myself depending on the load I'm carrying. I've hit almost 62K a few times in good water conditions.

I have an 8.5X8 on it's way which I think will be just right for when I'm alone

 

[Sea Rider]

Some issues,

You said that the water flow is passing around 1/2 below the upper plate, in post 2 the water flow is skimming right under it, is it possible to lower the motor a tad via the jack plate or with removed jack plate for motor to sit on middle transom for the water flow to match post 2 pic ? Once there with the motor revving at near its max wot rpm range of 6 K, the combo will plane slight faster than now on flat calm water cond when at full throttle from a dead stop. Remove your current doel fin and use it as a paperweight or book stand LOL!!

The current pop installed with which you've done those water tests is the right one the boat needs, going for a one + pitch will lessen the wot rpm range a lot which will turn counterproductive when adding another boater and extra gear. On this scenario ideal is for the motor to run around its middle wot rpm range, not under with current prop as a 9.8 HP it's a very basic under powered motor used as a main for such application.

There's no issue running wot with you aone with current prop as its within the factory max wot rpm range...

Happy Boating​