Tohatsu 30hp top speed

[Steverob1983]

Hi all, iv bought a 2006 tohatsu 30 short shaft on a excel HD385 RIB. It's topping out at about 20mhp and just doesn't seem or feel as quick as it should. Anyone give me some kind of idea what speeds I should be seeing from this setup?
Thanks

 

[pvanv]

Year is irrelevant; we need model/serial. Assuming you have an MFS30B, 3-cylinder EFI. Put a tach on it and see what the WOT RPM is. If too low, verify transom height and trim, and see what pitch prop is installed. That's a fairly sluggish, heavy RIB, and you want all auxiliary weight (fuel tank, battery, etc.,) as far forward as reasonable, because it is so stern-heavy.

 

[Steverob1983]

It's a M30A4 twin cylinder. I'll have to buy a tiny tach or similar to check rpm but it doesn't sound as if it revving as high as it should for a 2 stroke. Was told when I bought the setup should be good for 30-40mph.

 

[pvanv]

If this was purchased in North America, it was not new in 2006; by then the 4-strokers took over thanks to the EPA.
You want 5150 to 5850 WOT RPM. To achieve that, you need the best possible trim (motor dead vertical at speed), and best transom height (water passing between the antiventilation plate and the splash plate at speed), and the best prop pitch. You are likely over-propped. Figure maybe 200 or 300 RPM per inch of pitch. What pitch is the current prop?

 

[Steverob1983]

It's a European engine. I'm from Uk and was new in 2006. Current prop is a 13 pitch, but I have no idea the wot rpm. Hight looks perfect on plane and wot. Been out today and found a new problem. Was on river and perfectly flat calm. Wot and on plane. Every 30 seconds or so engine wound pull down and to one side causing boat to jump a bit and then would settle flat again until it did it again 30 seconds later.

 

[Sea Rider]

Some issues :

-How loaded was combo on initial wot water test ?
-Was OB factory delivered with a 13 pitch prop ?
-Were there any water splashes back or over transom or prop aeration at wot ?

Can't possibly know if current prop is doing its homework right without installing an induction tach. Would recommend a Hardline [FONT=&quot]HR-8061-2 model.[/FONT]

Once installed will know at which rpm is OB missing, not running even along max wot revs achieved. To get the max out of any Rib need to check if OB sits at the Tip Top Transom Spot as Sibs, Ribs brands for that matter aren't built with exact same transom heights to match well a Tohatsu OB. From there can go for a nice prop maximization for OB depending on load to run at least middle to max wot rpm range.

When at plane with OB sitting fully vertical (90?) although water flow can pass right in middle of both horizontal plates, skimming right under upper water deflector plate will achieve better prop grip and thrust, less prop aeration issues on other than non flat water cond and much better dime close turns at speed.

Have sold and installed many M30A4-S/ OB's, all were factory delivered with a 13 pitch props which find it's a complete factory mistake. On light applications, can run as much as 800-1000 less wot revs from max 5850 rpm. Ideal would have been to be delivered with 12-11 pitch props.

Happy Boating

 

[Steverob1983]

Last run was loaded with just my self, 2 children and full 25l of fuel. No gear other than life jackets. Boat was very fast dropping onto plane. Outboard standard prop is 13 pitch. No water splash back or over transom, no prop aeration and water line skims just below upper deflector. When I bought the engine hydrafoils had already been installed. I was going to test it with them off this weekend to see if they were maybe causing the problem with OB pulling down and to one side but didnt get chance. Prop has a few small dings in it. Ordering a tach this week, Was hoping to get the boat sorted before I take it to the coast for the week in 2 weeks but not going to get chance to test it with the tach on it before we go.

 

[Sea Rider]

Congrats, nice to hear that the Tohatsu is sitting and worlking at the Tip Top transom spot, Just curious, how did you checked that ? Which non ethanol, ethanol octane fuel is OB burning ?

Once OB sits spot on on transom, hydrofoils are useless, it's just a matter of distributing deck weight evenly, go full throttle and combo will plane near instantly. If hydrofoils does not run parallel to water level once combo is on plane will achieve excessive water drag and slow combo down a bit, besides are huge garbage collectors, remove and throw them away...

Once tach is installed, report max wot rpm achieved on flat calm, no windy water cond...

Happy Boating

 

[Steverob1983]

Checked it only way i could,stuck my head over transom while wot. Using unleaded straight from the garage pumps. 95 octane.

 

[Sea Rider]

Checked it only way i could,stuck my head over transom while wot. Using unleaded straight from the garage pumps. 95 octane.

Way to go, don't understand why is it so darn difficult for other boaters to check if OB sits at the Tip Top Spot by that method, prefer losing precious boating time by hearing if prop aerates at speed, at tight close turns, wild guesses. To each his own crazy theme..

Happy Boating

 

[preventec47]

I am in a similar situation as you but in the opposite direction. Too bad we are not neighbors. I am in Atlanta, GA and you I think are in the UK.
I have a 30HP on a small race boat with an 11 pitch and I am over revving by about 1000 rpms. We should swap props but shipping is
likely more than the new purchase cost. I have used TinyTach a few times in the past on various motors and if you call them they
have a special version for outboards with special programming and a pickup induction line in shielded cabling and they will custom
make any length of cable you need to get the readout to your cockpit. When I find some free time I plan to research prop availability
and try and upgrade to a 14 pitch stainless steel prop as I heard one or two prop makers make them with 14 as a max pitch.
Scott in Atlanta

 

[Sea Rider]

Scott,

You say that OB is over revving to 6850 wot revs from max 5850 factory stated. So that's 500 wot rpm for 1 less in pitch. So the Rule of Thumb that states + 200 wot revs per one less pitch does not always applies on light to medium light applications.

Happy Boating

 

[Steverob1983]

Finally had chance to get it in sea with tachometer on it today. WOT 4300 rpm. Time for a new prop.

 

[pvanv]

You need less pitch. You want 5800 RPM.

 

[Steverob1983]

I'm seeing that lowest pitch I can get is a 9 which will give me approx 5500 rpm? With this pitch will it make it a lot harder to get onto plane? at the minute it's straight onto plane with no hesitation

 

[Steverob1983]

Found an 8 pitch but it's a different diameter

 

[pvanv]

Also go through the carb to be sure that you are delivering full power to the prop. Less pitch will usually cause a slower hole shot, but in your case, since the motor is lugging, you may not notice it, since you will achieve the middle of the torque curve quicker.

 

[pvanv]

It might. Also check the carb and sync to confirm that you are developing full output.

 

[pvanv]

It might. Also check carb and sync to confirm you are developing full power.

 

[Sea Rider]

According to my experience installing Tohatsu 2 strokes 30 HP-OB's, some of them are being factory delivered with slight less timing advance than the usual 25?. Check that timing mark stops at 25? on timing plate when at full throttle. With OB-Off, move shift lever to forward, and full throttle. Butterfly must be found fully opened to 180? and timing advance mark stopped at 25? at middle mating surface.

If with a 13 pitch, try a 11 with same diam, don't go towards 10-09 pitch props, OB will over rev badly. Those props are intended to push heavy loads and your 3.85 Mtr Rib is considered a light boat, if over revving with a 11, just throttle bit less. Will achieve a incredible hole shot provided that deck weight is evenly distributed along carrying more load. Report your findings.

Grease prop shaft well before installing new prop..Impossible to know if a 11 pitch will make for OB to rev at it's max 5850 wot range, can only check that on a wot run as loaded, but will perform much nicer than with a 13 for sure.

Happy Boating