Checked your RPMs lately?

[jacoboregon]

Do you just assume your motor on your inflatable boat is properly propped?

I have a 1989 tohatsu 40hp 2-stroke on a 13' bombard C3 with the factory 11" pitch prop.

I'm by no means a "Guru", but was a helicopter mech for 8 years in the great US Coast Guard and just 'felt' that my engine was over revving at wot.

I bought an inductive tach/hour meter of ebay for $21 and installed it. I'm running 6400 rpms at wot! The engine specs are 5200-5800 at wot. I'm glad I checked it.

I'm wondering how many engines out there on inflatable boats are over revving as they are usually propped for the general boat population and inflatables are so much lighter.

Just food for thought...

 

[Sea Rider]

Re: Checked your RPMs lately?

Yes you are right, most people run their light sibs at wot and don't have the slight idea what are factory min & max revs for, glad you post about that important tech issue to be aware of.

After installing the tach/hour meter and with engine on, the idle reading was weird, at 3/4 throttle my engine is reving at 9900 rpm, seems this tach/hour meter is picking both signals from both ignition coils and adding them, have placed pick up signal cables at different plug's cable lenght and shows this high rpm reading, have written Hardline to inquire if anything can be done. By the way Jacoboregon, which brand tach and model have you installed, does it read properly ?

Update: Hardline tech dept. said 1/2 reading same version is needed to work well as tach is picking waste induction from ignition coil, works well with 2 strokes 2.5,3.5,5 HP single cylinder engines and 4 strokes models.

Happy Boating

 

[jacoboregon]

Re: Checked your RPMs lately?

Sea Rider, the tach I used seems to work perfectly, with no irratic readings.

I'm not sure of the brand/model, but here is a link. (it cost me $22 with shipping). I'm very happy with it.

http://cgi.ebay.com/ws/eBayISAPI.dl...7283&ssPageName=STRK:MERFB:IID#ht_1762wt_1189

It does not need to be grounded and uses one wire wrapped around (4-5 times) the spark plug lead. It is programmable for different firing cycles and also for maintenance intervals.

 

[Sea Rider]

Re: Checked your RPMs lately?

Definitely a more sophisticated model, bought years ago a Tiny Tach and have same problem, a matter to divide by 2, there is no erratic reading, reads well but double counts, my idle speed is 900 but reads 1800. That link has well priced hourtachs compare to other brands.

Happy Boating

 

[jacoboregon]

Re: Checked your RPMs lately?

Yes, I almost got the tiny tach, but read several bad reviews so I decided for half the price or less I should give this one a try. I'm VERY happy so far!

 

[Sea Rider]

Re: Checked your RPMs lately?

Yes, I almost got the tiny tach, but read several bad reviews so I decided for half the price or less I should give this one a try. I'm VERY happy so far!

Now that you know you are over reving, how are you going to limit you max rpm achieved, just throttle down a bit ? Where have you installed your tach ? There's a empty place at front engine, probably 40C has same available space to place the tach.

Happy Boating

 

[Peter_C]

Re: Checked your RPMs lately?

Sea Rider, the tach I used seems to work perfectly, with no irratic readings.

I'm not sure of the brand/model, but here is a link. (it cost me $22 with shipping). I'm very happy with it.

http://cgi.ebay.com/ws/eBayISAPI.dl...7283&ssPageName=STRK:MERFB:IID#ht_1762wt_1189

It does not need to be grounded and uses one wire wrapped around (4-5 times) the spark plug lead. It is programmable for different firing cycles and also for maintenance intervals.

From here: http://forums.iboats.com/showthread.php?t=522318
"note that the tiny tach equivalent with the two buttons S1 and S2 is not water proof ....The front will curl up if left in the sun and the battery is prone to come disconnected ( 10% of production) They do have the advantage of suiting 1 stroke 2 stroke or 4 stroke motors ...... sold thousands... "
Edit: It could probably be made more waterproof by putting sealer on the bottom.

I have a Tiny Tach mounted with double stick tape to my tiller. So far it is out of the way and I can read it, although I have considered putting it in the same place but building a nice angled mount facing starboard, since we always sit on the starboard side tube to drive.

 

[jacoboregon]

Re: Checked your RPMs lately?

Now that you know you are over reving, how are you going to limit you max rpm achieved, just throttle down a bit ? Where have you installed your tach ? There's a empty place at front engine, probably 40C has same available space to place the tach.

Happy Boating

I've ordered a new prop to take care of the over revving. My present prop is 11.3" and I've ordered a 15" pitch to replace it.

For now, the tach is tie-wrapped to my tiller, but I plan to move it to the front of the motor eventually.

 

[fbpooler]

Re: Checked your RPMs lately?

A rule of thumb is 500 RPM engine speed for each inch of prop pitch, so tach out that 15 in prop to make sure it is not too much.

 

[jacoboregon]

Re: Checked your RPMs lately?

A rule of thumb is 500 RPM engine speed for each inch of prop pitch, so tach out that 15 in prop to make sure it is not too much.

Most references I know of indicate 150-200 per inch, but I'll certainly check with turning point.

 

[jacoboregon]

Re: Checked your RPMs lately?

Well, the rep at turning point states 150-400, depending on the prop.

 

[fbpooler]

Re: Checked your RPMs lately?

The 150 -400 is probably closer to what you will obtain if you don't go from plain to cupped. Your sweet spot is probably between 5500 - 5700 RPM with a light load.

Does not mean a thing for your rig, but a 14 ft fiberglass boat with 15 HP dropped RPM from around 7,000 to 6,400 by going from plain 8 in P to cupped 9 in P SST. Really increased the power from the motor.

Same boat, 15 HP:
9.75 in P, about 5,300 RPM
9 in P, about 6,000 RPM
Props were same design, ie, gentle cup aluminum.

Be interested to know what your RPM change is when you run the new prop. Sometime you luck out and hit the right spot on the HP/torque curve and good things happen. Had two boats in the past where that happened and it made me a happy man.

 

[jacoboregon]

Re: Checked your RPMs lately?

Well, I ended up getting the 10 7/8 x 11" pitch, as the hustler is more cupped and raked than the tohatsu 10.6 x 11.3". My rpms dropped to 5500-5700 range with this prop. Very happy with it.

As I looked more into getting the replacement prop, I realized that although the old 150-200 rpms per inch rule may be fairly accurate with flat pitched props, but the props with more rake and cupping are closer to the 400 rpms per inch.

I'm very glad I got the digital tach to check the rpm at wot.

 

[fbpooler]

Re: Checked your RPMs lately?

Man, you hit the sweet spot right on the nose. That is great news, and you should get everything out of that engine it is capable of.