Tohatsu SP (propane) throttle question

Jul 28, 2018
57
Catalina 22 TX
I have a new propane outboard, the Tohatsu Sailpro. On the first test run, after the break-in, I noticed that the RPMs judging by the engine sound do not increase after the half-way mark on the throttle.
This is with a full propane tank, not a small bottle. The "choke" was in/off. (The engine has a high-rev limit and buzzer -- it isn't that high.)

Could it be:
* A phenomenom due to the high-thrust prop?
* Due to the boat's inherent hull speed limit (C22)
* Lack of gas pressure
* A throttle adjustment issue

Or, is the sound not enough to go by:
 
Last edited:
May 24, 2004
7,129
CC 30 South Florida
The prop has to be matched to the boat. The displacement and hull characteristics of different boats will require different pitch props. The correct prop size will allow the engine to reach maximum rated RPM without exceeding the limit. Sounds like the OEM prop is too high pitched for your boat. Most engines come propped for the planning hulls of powerboats.
 
May 27, 2004
1,964
Hunter 30_74-83 Ponce Inlet FL
Yeah, but he bought a "SailPro" outboard! You'd think the dealer would have asked a few questions.
The only way to determine the facts in this case are to determine boat speed at different sounding RPM's during similar conditions on the water.
Or, take it to the dealer and have him check the HP at different actual RPM's.

I'm interested in this thread because I'm fed up with E10 Gas ruining my Mercury carb over and over again.
 
Last edited:
Jul 28, 2018
57
Catalina 22 TX
That's right, this is the SailPro model which comes with a prop intended for displacement hulls of about this length.
And yes, the E10 and carb problems are exactly why I went with propane.
I'll do some boat speed tests next weekend.
 
Last edited:
Nov 6, 2006
9,885
Hunter 34 Mandeville Louisiana
First thought that comes to mind is whether Tohatsu compensated for the power decrease that comes with using propane as fuel; usually a bit under 10%.. They may have included the same prop from the gasoline engine and the engine cannot get to enough RPM to make its rated power?? I'd check prop first; there are 4 pitches available for that motor.. A digital tachometer check of the max RPM underway would be useful.. I don't find enough useful info from Tohatsu to draw a meaningful conclusion.. I do note that the engine itself is the same whether gasoline or propane, and the gear ratio is the same for both engines.
It could be timing or fuel pressure or a bazillion things, but I'd start at tachometer and propeller.. I see their max operating range is 5000-6000 RPM.. and max torque is developed at 4100 RPM.. so if the motor is not getting to about 5000, it is not making rated output..
Not recommending this one, just using it as an example:
https://www.amazon.com/Signstek-Pro...ocphy=9025394&hvtargid=pla-599105885964&psc=1
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
Check out this table. It is from the owners manual. It may help answer part of your question.
Prop Table.PNG
 

nat55

.
Feb 11, 2017
210
Gulfstar 1979 Gulfstar 37 BELFAST
FYI, ethanol free gas is available in many markets, check here for your state. https://www.pure-gas.org/

The town dock here in Belfast Maine only sells Ethanol free gas now.........
 
Jul 28, 2018
57
Catalina 22 TX
Great replies - this is very helpful! I will check the RPMs too... Question: How do I check prop RPM in the water with an optical tach? I can use an induction/ignition tach, but that's before the gear ratio, isn't it?
 

Harlan

.
Jun 4, 2004
99
Oday 34 Niantic
Agree that ethanol is a problem. There is ethanol free gasoline available, at least in CT, at home centers, etc called True-Fuel. It is sold in quart and gallon containers. Available as plain gas or in a 50:1 premix. Downside is price of over $20 per gallon. But if you only use a gallon or two a year (that's all I use in my dinghy) it is a lot cheaper than carb rebuilds or buying a new engine.
 

Gunni

.
Mar 16, 2010
5,937
Beneteau 411 Oceanis Annapolis
Question: How do I check prop RPM in the water with an optical tach? I can use an induction/ignition tach, but that's before the gear ratio, isn't it?
Yes, you use an ancient mystical tool called “mathematics”. Nearly lost art! Engine rpm * gear ratio = prop rpm.
 
Jul 28, 2018
57
Catalina 22 TX
Yes, you use an ancient mystical tool called “mathematics”. Nearly lost art! Engine rpm * gear ratio = prop rpm.
Of course. I'll be measuring the engine RPM. I was just curious about kloudie1's suggestion of an optical tach.
 
Nov 6, 2006
9,885
Hunter 34 Mandeville Louisiana
The RPM you will be looking for is at the flywheel.. Gear ratio does not affect that.. on most engines, you can see the flywheel turning after the cover is off.. A piece of reflective tape would help the tacho to get a reading..
Propane is pretty much ethanol free..
 
May 27, 2004
1,964
Hunter 30_74-83 Ponce Inlet FL
Nat55, Yeah, I heard that...
And you will understand my outrage when after two brief uses of a brand new mercury outboard over two months, it stopped starting/running due to, (wait for it) according to the merc dealer:
"Varnish residue caused by E10 gasoline." And "Warranty doesn't cover that!"
Oh, did I mention that I only buy gas from the local marina who touts having ethanol free gas?
And, since it is used only on my dinghy, I guess I could live with a few less horses.
 
Dec 2, 1999
15,184
Hunter Vision-36 Rio Vista, CA.
I purchased a power washer a couple of years ago and it came with a Honda 4 stoke engine. Included with the unit was a bottle of "Ethanol Shield" with explicit instructions to use the product with every refill. It has been trouble free. I also use it in all of my gas powered tools with great success.

The product is available a Walmart and many other locations. The product is a lot cheaper than tearing into the fuel system every year. About $7 for a small bottle.
 
May 24, 2004
7,129
CC 30 South Florida
How come we can design a lawnmower engine that runs perfectly on E-10 Gasoline but not an outboard motor? Seems to me some US manufacturer could come up with a reliable engine since the foreign manufacturers just seem to ignore the problem. I'm not amiss to the safety and technical design factors for an outboard engine but I'm sure that whoever can fix the problem will corner the market in short order.
 
Mar 29, 2017
576
Hunter 30t 9805 littlecreek
Can you try that motor on another boat maybe a neighbor dink or something to see if it rev up better
 
Jul 28, 2018
57
Catalina 22 TX
I did some speed and time tests today:

RPMs:
WOT: ~4600 RPM (10% less than the 5000 RPM minimum from the prop table in the manual, above posted by Steve Dion)
1/2 on throttle dial: ~4500 RPM (fits my original question about not much difference between 1/2 and full)
Idle in FWD gear: ~2800 RPM

Boat Speed (SOG via GPS, hull speed is 5.9kts)
Only a few data points -- the only conclusion I can draw from this is that the motor/prop combo MAY be able to hit hull speed at 4600-4700 RPM. Need more data points to tell.
4630 RPM, 5.9kts
4600 RPM, 5.1kts
4500 RPM, 5.3kts
4100 RPM, 4.6 kts
2900 RPM, 3.9 kts
2800 RPM, 3.8kts

Run Time:
Using two Coleman bottles simultaneously in a Coleman "Gemini" carrier, the engine ran at 4000 RPM for 28 minutes. After that, the RPMs began to slowly decrease down over the next 36 minutes as the pressure decreased and the throttle was untouched. It would have run longer if I hadn't stopped it when it got down to 2700 RPM. In an earlier test, it idled for well over an hour on one bottle.

Finally, I thought it might not start after depleting the pressure to this point -- but it started on one pull even so. It purrs at idle, BTW.

Thanks for any input, observations, advice!