Tohatsu 2stroke 30hp overreving

[bblagonic]

Hello to all,
My colleague is having a problem with his Tohatsu 30hp, 2 stroke, engine.

The problem is that while reaching certain boat speed (abt. 10 kts), the engine starts to overrev.
However, he states that once the boat with the same engine used to work normally and reach 19kts!

Following has been checked:
1. The prop hub is good,
2. Gears and clutches...all good.
3. The engine has been tested in the workshop with load on complete range of rpms...all good.
4. The antiventilation plate is on the level of the bottom of the hull (not on the bottom of the keel)
5. Fins have been installed on the antiventiation plate but the problem persisted. (Fins are removed now.)
6. Different mounting heights of the engine have been tested as well as the trim angles. The problem is always there.

Facts:
1.The hull is a semiplaning type with long and wide keel (keel height is 6inches while the thickness is 4inches). However, the keel is slightly rouned and narrowed last 4 inches before the prop.
2.The keel is really wide for my opinion. 4 inches of keel versus 9.9 inches of three bladed prop diamater.
3. Engine cooling system use to have problems and a lot of soil deposits. (The water intakes are on the same level as the keel and I suppose that beacuse of being in keel's shadow, a lot of air is cooming on the intake port.
4. Starting from zero speed, the boat and the engine interacts weel, the engine is gaining roms and boat the speed but at certain boat speed the egines starts to overrev and there is no and gain of boat speed.

Possible corrections:
1. Remove part of the keel...abt.20inches...on the boat aft
2. Try with XXL engine leg in order to have the prop and the water intake bellow the top of the keel.
3. Try the engine brackets moving the engine from the transom for abt. 10 inches aft.

I know that it is confusing that everything worked normally with the same hull and the engine but all has been tested so far and the only thing to try now is to do something with the hydrodynamic. (The only problem that is present from the day one of this boat is poor cooling performance of the engine which I suppose is due to the bad position of the water intake sucking the water in the keel's shadow.)

Any ideas?

Few photos are attached.

Thanks!

Bojan

 

[GA_Boater]

Welcome aboard.

If this motor on this boat used to work well, why do cut up the keel, add fins or do anything else except fix the motor?

You say the prop hub is good. How did you check the hub?

For the cooling problem - Have you changed the impeller?

 

[bblagonic]

Several props have been tested. I have alse marked the shaft and prop housing. No any angle deviation. Impeller is new.

 

[GA_Boater]

Is this the same boat with the same motor and prop, and it used to run well?

So what has changed?

 

[bblagonic]

It has to be something related to the overheating sensor which didn't functioned well previously and allowed the engine to reach max.rpms. Now, the engine has been serviced and checked several times and all malfunctionings have been corrected.
I will make a test with hand tachometer and see exactly what is happening with the rpms of the engine in the moment when it starts to suddenly change the rpms. Will let you know.

 

[WesNewell]

Prop ventilation? looks like an inverted mod v hull so maybe lower the engine 1 hole. And what are those black things on the prop shaft housing right in front of the prop. Surely they aren't there when running? They could cause cavitation.

 

[jimmbo]

Prop ventilation? looks like an inverted mod v hull so maybe lower the engine 1 hole. And what are those black things on the prop shaft housing right in front of the prop. Surely they aren't there when running? They could cause cavitation.

Are you talking about the 2 bolts that hold the bearing carrier on?

 

[jimmbo]

Looking at the pic from the side, it looks like the prop hub is considerable smaller in Dia. than the gearcase housing. Was the prop changed, and then the problems began? Also the prop seems to be sticking out of the gearcase more than I usually see. Either of the two conditions can lead to exhaust gases flowing into the blades instead of going through the hub.
That said, that keel is going to be a problem, period. Considering its depth, I would start a taper on it about 5 ft in front of the Transom

 

[GA_Boater]

The keel can't be an issue if the same motor used to work fine, as the OP says.

 

[Fed]

In addition to the previous comments it looks to me that the motor is mounted crooked, could the keel shadow be running down the Starboard side of the gearcase?
That would also account for the trim tab being set on the "wrong" side.

 

[bblagonic]

Prop ventilation? looks like an inverted mod v hull so maybe lower the engine 1 hole. And what are those black things on the prop shaft housing right in front of the prop. Surely they aren't there when running? They could cause cavitation.

The black things are flushing mufs and the motor is mounted at its lowest hole.

 

[bblagonic]

In addition to the previous comments it looks to me that the motor is mounted crooked, could the keel shadow be running down the Starboard side of the gearcase?
That would also account for the trim tab being set on the "wrong" side.

The motor is mounted in the boat center line. Will check if it is possibile to move it slightly PS or SB. Will check the trim tab, too.

 

[bblagonic]

Looking at the pic from the side, it looks like the prop hub is considerable smaller in Dia. than the gearcase housing. Was the prop changed, and then the problems began? Also the prop seems to be sticking out of the gearcase more than I usually see. Either of the two conditions can lead to exhaust gases flowing into the blades instead of going through the hub.
That said, that keel is going to be a problem, period. Considering its depth, I would start a taper on it about 5 ft in front of the Transom

Original prop has been changed with Solas prop due to that the original one had incorrect dia x pitch size. I will compare the prop hub sizes, original vs. Solas. My colleague says that the motor used to run well with the Solas one.

Thanks for opinions so far!

 

[Faztbullet]

I have had a cruiser do this and it was due to the bottom coat that was installed..It was to rough and cause turbulence and air to be introduced into cooling inlet and prop.

 

[Sea Rider]

Mmmm, whild guessing prop testing it's no bueno, time consuming along dumping $$$ into the deep blue...

To have in mind :

Was the over revving issue achieved when the combo was riding on flat, slight choppy, windy or rough water condition ?
Was the motor previously trimmed to 90º (fully vertical to water level) ?
Was the deck load evenly distributed when running at wot ?
Which OEM prop pitch was previously installed on the motor ? was it a 13 pitch ?

-On those 30 HP horses doesn't matter which brand prop you test if the entire motor is badly transom height matched. Top thrust is cancelled if the lower leg has insufficient water bed height to work with.

-As there is no posted wot rpm, assume that motor doesn't count with a installed tach whatsoever, so impossible to talk about having a superb top water performance with no RPM information.

-If with prop aeration on all 3 water cond, need to lower the motor accordingly till this cond (second pic) is met when on plane.





That combo needs to work at it's mas 5800 wot rpm range, with spot on transom/motor height matched, well trimmed, with correct deck load distribution as usually loaded to take the max 30 HP out of that motor. Testing lower leg heights must be done on "flat calm, no wind water cond".

Bottom Line : The only way to know if the Sweet Transom/Motor Height has been reach or need to be varied is by visual inspection, pull head out side of motor and check at which lower leg height is water flow passing by. Report your findings...

Happy Boating

 

[bblagonic]

Mmmm, whild guessing prop testing it's no bueno, time consuming along dumping $$$ into the deep blue...

To have in mind :

Was the over revving issue achieved when the combo was riding on flat, slight choppy, windy or rough water condition ?
Was the motor previously trimmed to 90º (fully vertical to water level) ?
Was the deck load evenly distributed when running at wot ?
Which OEM prop pitch was previously installed on the motor ? was it a 13 pitch ?

-On those 30 HP horses doesn't matter which brand prop you test if the entire motor is badly transom height matched. Top thrust is cancelled if the lower leg has insufficient water bed height to work with.

-As there is no posted wot rpm, assume that motor doesn't count with a installed tach whatsoever, so impossible to talk about having a superb top water performance with no RPM information.

-If with prop aeration on all 3 water cond, need to lower the motor accordingly till this cond (second pic) is met when on plane.







That combo needs to work at it's mas 5800 wot rpm range, with spot on transom/motor height matched, well trimmed, with correct deck load distribution as usually loaded to take the max 30 HP out of that motor. Testing lower leg heights must be done on "flat calm, no wind water cond".

Bottom Line : The only way to know if the Sweet Transom/Motor Height has been reach or need to be varied is by visual inspection, pull head out side of motor and check at which lower leg height is water flow passing by. Report your findings...

Happy Boating

Thanks for your tips. Will do as suggsted and report.

For my info...does the prop ventilation states for air coming in the propeller stream from the water surface in above the prop or we could say the prop is ventilating beacuse of a poor water flow mixed with air and induced by the keel in front of the prop, too?

 

[Faztbullet]

we could say the prop is ventilating beacuse of a poor water flow mixed with air and induced by the keel in front of the prop, too?​

This....

 

[Sea Rider]

Forgot to ask, was the water flow at speed exiting middle hull where the long keel is located comes out flat whatever shape it has (1) or comes out agitated, convulsed (2) ?

It's not the same fine tuning a lower leg height to work spot on on a flat water flow exiting middle rear hull compared to a non flat one (2) which will tend to give prop thrust issues.

Happy Boating

 

[jimmbo]

Things are not adding up. Boat is a semi-planing hull, so it never truly gets on plane. It apparently used to get 19 knots, now it gets only 11-11 before the prop blows out.

In one Paragraph
"Original prop has been changed with Solas prop due to that the original one had incorrect dia x pitch size. I will compare the prop hub sizes, original vs. Solas. My colleague says that the motor used to run well with the Solas one."

Are you saying the Origninal prop was a problem, or just the Solas, but the Solas used to be fine?

The Keel... Anything that can disrupt the flow of water in front of the propeller is of concern. Your buddy says the boat could attain 19 knots(22 mph). that would normally be fast enough for most boats to plane. and the keel should be leaving a trough of turblent water/vacuum bubbles behind it. I can't see how it could attain that speed.

 

[bblagonic]

Hi guys,
I have spent this morning in testing.

These are the results...

The boat gets speed and the motor turns till the point of abt.4500rpm vs. 9,5kts when it suddenly increase the rpms to 5600 and the boat loose some speed. (I have checked the rpms by hand tachometer)
After the speed vs. rpm test, I have immersed the camera and made a video of the exact moment when the rpms are jumping by 1000 rpm.
Seeing that amount of the exhaust gases before the prop, I started the investigation of the prop and the lower unit.

The owner's father at certain point had re-installed the original propeller without the trust washer. The results are machined (by 5mm each) both the edge of the original prop and the edge of the bearings support leaving a gap between them. (Today test have been done with SOLAS prop which is not damaged but the bearings support is well damaged/machined by at least 5 mm.)

Looking to the pictures attached, especially the one showing the exhaust gases in the water stream before the prop, do you think this could be the main reason of propeller overreving or I should concentrate to other things, too?

We will replace the bearings support and make a new test.

Few information...the motor is a 4 stroke and the it should produce 30hp at 5250-6250 rpm.

Regards,
Bojan