First boat... Need help with low rpm/speed

[emoney]

Are you sure the motor is the correct shaft length? I feel like the motor being too low wouldn't rob you of 2,000 rpms either. Out of curiosity have you tried the boat with just you and nobody else aboard? I would try moving it out as suggested above with just you in the boat and see how that helps. If it doesn't get you within a few hundred RPM of whatever the manufacturer says is WOT, then I think there's another issue. Have you done a compression test and confirmed it's firing on all 3 cylinders?

 

[Smgbad]

It's a combo of the trim angle and engine height. Setup right, the boat should plane out and that will raise your rpm along with speed. Right now, with the way the trim is set, you are losing at least 20% of your engine power, Raising the engine will give you another 10% increase in speed and power and also raise the rpm some. The prop selector says you should be running a 13P prop with a 1200# load and 420# boat @ 30 mph. I would not lower the prop pitch at all. If you can't get on plane with at least 25 mph, then you have engine problems and/or a really bad hull problem.

Good info. The hull looks like new except the little bit of wear on the transom, so I don't think that's the issue. The motor idles great right at 960 according to the idle... What in the engine would cause it to be 2k rpms low? I'll try to take it to the boat ramp this week to see how well the simple tilt adjustment may have worked. It'll be just me so less weight as well. I'll touch base afterwards. Thanks for sticking in there with me through all these questions

 

[Smgbad]

Are you sure the motor is the correct shaft length? I feel like the motor being too low wouldn't rob you of 2,000 rpms either. Out of curiosity have you tried the boat with just you and nobody else aboard? I would try moving it out as suggested above with just you in the boat and see how that helps. If it doesn't get you within a few hundred RPM of whatever the manufacturer says is WOT, then I think there's another issue. Have you done a compression test and confirmed it's firing on all 3 cylinders?

Not sure. That is the motor the boat came with, already installed. I'll go later this week after work and get back with the results. Also, I don't think I have the tools to do the compression test. I'll check YouTube for instructions.

Also, you mentions firing on all three cylinders... This is a 2 cylinder 2 stroke I thought. Am I wrong?

 

[WesNewell]

The NS40C is a 2 cylinder. The D model is 3. And you have a 20" transom, which is why it's so low mounted all the way down.

 

[Sea Rider]

Smgbad

You have a PM, check your Inbox to have a clue how should that motor needs to be installed for max performance on all water conditions..

Happy Boating

 

[Smgbad]

Changed the spark plugs and made sure they were gapped correctly. The guy I got it from said he'd just done it and it looked like he had gapped them to nearly 1.4.

Last nights test run... It did run a bit smoother with the trim angle set to the 3rd from the farthest, which was the most even with the boat bottom (and most 90 degree). With my friend and his son (combined 190 lbs) sitting in the front it got up on plane and got to 24 mph over the course of about 30-45 seconds of WOT. When my friend moved back towards the center (and then to the rear) I could physically hear the rpms dropping the further back they walked, and we dropped to about 13(ish) mph.

Then I tried it with the trim set to the second from farthest back and the performance dropped noticeably.

So then I did a run with just me in the boat with it back at the third from farthest trim setting... Got up on plane and was cruising very easily at 27-28 mph on 5 different runs. Tach got up towards 5k finally.

So thats where I'm at with it. I hate to have to do all thats involved with raising the engine 2-3", but if thats what it's going to take to be able to cruise at speed with 1 or 2 passengers, I guess that's what I'll do. I was hoping it's close enough now that a prop change would help out. It's currently a 3-blade 11".

 

[WesNewell]

It's really hard to tell just how low the engine is mounted from the picture. Moving mine up 1.5" gave me a 10% increase in speed.Yours looks to be 2-3" too low. Ideally, the av plate should be even or just above the yarrdstick in the last photo. I would suspect you to see another 10-15% increase in speed and probably a smoother ride, and a slight increase in rpm. And as you found out, proper weight distribution is also very important.

 

[Sea Rider]

Thanks for posting your wot results, going to ask you a big favor, make another wot test with your friends sitting at bow, motor trimed to 90 deg, if matched at third hole perfect. When at full wot on flat calm water cond, have someone visually check at which lower leg plate was the water flow crossing by to go from there... This is the trim parameters "Factory Recommended" for a M40C motor :

Good to have experienced for yourself that a combo running at wot with the trim set at any other trim position than at 90º achieves a lousy water performance....The spark plug gap for that motor is 1.0 mm tight, once the plug plug gap has been correctly set, the feeler gauge must remain tight. 1.4 mm is way too much...

Lets avoid idiotic motor comparison, to each motor/boat brand their own proper installations....

Happy Boating

 

[WesNewell]

Not true. Brand doesn't matter. The av plate on all of them are in the same place just above the prop.

 

[Smgbad]

Yep. Next time I take it out I'll be sure to take some film of where the plates are in respect to the water level and get back with you guys.

 

[WesNewell]

It's pretty obvious where the av plate will be. It will be 2-3" below the water line, meaning the leading edge of the lower unit is 2-3" lower than it needs to be causing excessive drag. If you can get the engine mounted higher you can expect to see a 10-20% increase in speed. So instead of 24 mph, upwards of 29 mph with the same load and an increase in rpm probably within the max rpm range.

 

[Sea Rider]

Yep. Next time I take it out I'll be sure to take some film of where the plates are in respect to the water level and get back with you guys.

OK take a picture or small video shot from side of the lower leg and report if the water flow is crossing under which 1-2-3 plate's heights, motor must be trimed to 90º and deck load well distributed...

Happy Boating

 

[WesNewell]

Youtube had lots of vidros on engine height. This is a 40 hp similar to yours.
https://www.youtube.com/watch?v=v4GGNEe10r0

 

[JimS123]

Its a little boat with a little engine that is not intended to optimized with a transom bracket with many holes to chose from. With that setup raising the engine is not a practical option.

The simple answer is that when you are that close to the max capacity of the rig you will experience problems like you describe.

If it were me i'd be looking for a bigger boat. If that isn't an option, replace the prop with less pitch and give it a try.

 

[Sea Rider]

Youtube had lots of vidros on engine height. This is a 40 hp similar to yours.
https://www.youtube.com/watch?v=v4GGNEe10r0

That's the "most idiotic way" of testing any motor on any transom by turning head side way towards the motor when the real thing is to visually checking what's going on at back of the lower leg to have a clue at which height is the water flow passing by at plane.

I know if there's a splash going on at back transom raise the motor 1-2 notches, if with prop aeration lower the motor 1-2 notches, right ? but happens that's a Suzuke 40 HP clamp screw motor powering anything else except a toon, so those parameters in real boating world might not fully apply. To each motor brand powering whatever floats their own install.

Next time post true examples of a toon powered by same OP's Tohatsu M40C motor.. Similar is not exact same as both motors could have taller or shorter lower leg heights if compared between.

Happy Boating

 

[JimS123]

Not rocket science. This is a clamp-on motor, not a bolt-on that has the capability of up and down adjustment. Run it in its "normal" position and change props to get the spec RPMs at the "normal" load.

In this case the "normal" load is a bit heavy, so an abnormal prop pitch may be required.

 

[WesNewell]

It has 2 thru transom bolts and 2 clamps for the top. It's not just a clamp on engine.

 

[Sea Rider]

It has 2 thru transom bolts and 2 clamps for the top. It's not just a clamp on engine.

Sorry mate, that Nissan M40C it's 100% a Clamp Screw Motor, those 2 through transom bolts you mentioned are just to secure the motor to the transom, a good passer by thief deterrent. That's a heavy and bulky motor and as a precaution should be bolted, due to motor vibration the clamps may become loose, operators never looks back towards transom and motor might be diving into the deep blue as has happened that I know twice with total motor loss..

The Install specs for said motor doesn't specify that the AV plate must be set even with boat's bottom, that's a tech fallacy you guys swears by it. Wrong page, different movie...check

Let the OP report his findings to go from there....

Happy Boating

 

[WesNewell]

only a moron would mount a 40 hp engine using just 2 clamps, when there's two bolt holes in the mount to mount the engine. And they aren't there just for thief protection And I'm not your mate. And to mount the engine av plate 1.2-2" below the hull bottom is losing at least 10% of the engines power. It's been proven hundreds of times. I don't know of one person that will agree with that other than you.

 

[Sea Rider]

Bolting or not bolting a 40 HP motor on any transom it's the sole responsability of each owner if whether being a moron or not. There is a better way to secure a clamp motor to the trasom specially if will be removed and stored after use, so to each their own boating requirements.

Sorry, that 1.2-2 inch below the hull information was an illustration excerpted from the Tohatsu Installation Parameteres, ain't mine, so don't speak on my behalf. Said years back that each HP motor brand has their own installation tramsom match height parameters stated on each Owner's Manual, which you guys don't read and with astonishing conviction deny.

Where did you get a 20% power reduction, that's from own personal experience with said motor, repetitive word of mouth hearing or six o'clock tea time talk ? that's with water flow passing under upper or over lover AV plate at full wot with motor trimed to 90 deg. ? please instruct me and the boaters that are following this intresting OP's post, will be much pleased to hear your own tech version.....

Happy Boating