Tohatsu 3.5 hp cavitating?

[susqueduck]

I have a Tohatsu 3.5 hp 2 stroke short shaft mounted on a Bankes Intruder sneak boat. Above 1/4 throttle I see no increase in speed. I'm not sure but the motor seems to be cavitating. I have it tilted all the way in towards the transom. I had this same motor on a boat with a side mount transom and it did the same thing...I thought it was just the side mount setup, but it does it on my new boat as well. The motor has less than 2 hours on it and it has done it since day one.

 

[TOHATSU GURU]

Re: Tohatsu 3.5 hp cavitating?

It might be that you have reached hull speed with both boats. There is a point at which your boat will max until it gets on plane. You probably have more power than you need or not enough, depending on what you are trying to accomplish.

 

[susqueduck]

Re: Tohatsu 3.5 hp cavitating?

The sneak boat that I'm using it on is only 12 ft. and weighs 180lbs. I don't think that I am overpowered. I suspect that the motor is not sitting correctly on the transom. I just do not know how to fix it. Would trimming out away from the transom help cavitation?

 

[TOHATSU GURU]

Re: Tohatsu 3.5 hp cavitating?

Not really. I suspect that with a boat weight of 180 lbs. Your total weight is such that the engine is pushing the boat as fast as the boat can go at 1/4 throttle...Unless you get a larger engine and get the boat up on plane. <br /><br />Of course if you have the engine mounted incorrectly that could be the problem too. The horizonal plate above the propeller should be even to 1 1/2 inches above the bottom of the boat. You can play with the tilt angle rod and see if it makes a difference.

 

[susqueduck]

Re: Tohatsu 3.5 hp cavitating?

The anti-cav. plate or "ventilation" plate is dead even with the bottom of the boat. I though this plate was supposed to be below the boat bottom by 1 inch or so..not above?<br /><br />Will trimming out from the transom help?

 

[beepx2]

Re: Tohatsu 3.5 hp cavitating?

What attitude does the boat have at the max speed: is it stern down/bow up, putting up a big wake, or flat? If the stern is "squatting" you may be at or near displacement hull speed.<br /><br />You might get a little more speed by moving weight forward.<br /><br />I just checked the operator manual for this motor and it says to mount it so that the anti-cav plate should be 1 - 2 inches below the bottom of the hull.

 

[TOHATSU GURU]

Re: Tohatsu 3.5 hp cavitating?

Meant to say below...Not above

 

[mercrewser]

Re: Tohatsu 3.5 hp cavitating?

It would be impossible for an engine of such low horsepower to cavitate. Ventilate, maybe, but you would see it around the propeller, and hear the engine wind up with excessive rpm. I think its fine.

 

[susqueduck]

Re: Tohatsu 3.5 hp cavitating?

Cavitate, ventilate...not sure which but it does it. And it is definitely not fine.

 

[mjbrueck]

Re: Tohatsu 3.5 hp cavitating?

What everyone is saying, is that there's a world of difference between hp required for planing and hp required for hull speed. To reach hull speed (probably around 4 mph for your hull), you need about 1 hp for every 400 to 500 pounds. So heavily loaded, 2 hp will reach hull speed, and lightly loaded less than 1 hp. To plane, you'll need closer to a hp per 50-70 pounds. So with just one small person on the boat (total weight with boat and motor around 350 pounds), you'll need around 5 hp to plane. <br /><br />At displacement speed, your boat creates a bow wave that it needs a lot of power to get over. Your boat requires relatively little power to ride in the trough, but as you give it more power it starts to climb the backside of the bow wave. That's why as a boat comes up on plane, the bow initially rises and then drops. <br /><br />Most likely, on your boat, the first 1/4 throttle is at displacement speeds, and the next 3/4 throttle is trying (but not able) to climb over the bow wave.

 

[tgissel]

Re: Tohatsu 3.5 hp cavitating?

I have a 3.5 tohatsu short shaft also, and it will almost plane my 15' v-hull with just me in it. One thing I did is remove the orignal tiller and make an extended pole for it, to try and get the boat to flatten out. <br /><br />These motors have a pretty good punch for their size. Here is a picture of my fishing boat (green) used for a lake cleanup a couple of years ago.<br /> http://tinypic.com/view/?pic=nw0zzo <br />With all of that weight it still went close to 4 mph. <br /><br />Tohatsu also makes a replacement prop made out of aluminum, I can not really tell a difference between the orignal plastic versus aluminum, but I am sure it does not flex as much. I will get the pitch off of it tomm.<br /><br />Good luck!