SHAFT LENGTH ??? IS BIGGER , BETTER ???

[Royally PO'd]

Has anyone had experience with using a 25" lower unit shaft , where the pontoon came with a 20" shaft outboard ? I'm wondering how much difference it really makes with the prop deeper in the water, without the vee hull. This is a single outboard of 225 hp, to replace a 150 hp.. Please hold your responses to semi informational ----- I know you guys !!! lol

 

[HavaToon]

I’d say it depends on what you are trying to accomplish. Personally I have the 20” and even had the motor raised a hole, probably raise it a little more. I found that it provided a flatter more comfortable ride. It also provided a couple mph on the top end and less trim.

With the prop deeper in the water I think you would be possibly better at carry heavy loads and water sports. But I haven’t noticed any loss in capabilities personally and I did the opposite.

 

[Royally PO'd]

Thanks for that, Wiz, I appreciate the input...

 

[MissHavasuCig]

I have a short shaft Verado and it was buried when I got the boat. I raised the motor one hole and picked up 7MPH. I just noticed this past week that they were putting long shaft motors on all of the new Bennigtons at Sun Country. Even raised all of the way, I think the long shafts would be too deep. I saw one cruising the Channel a few days ago, and the bottom of the motor hood was at the waterline. Way too deep IMO. I would not put a long shaft motor on my boat.

 

[Justfishing]

The transom height determines the shaft length needed. If your boat is designed for a 20in shaft, a 25 in shaft will bury the prop to deep and kill your speed. You will not be able to raise the motor enough. The cavitation plate should be just out of the water on plane.

 

[Royally PO'd]

Thanks, all. Good input.

 

[TCHB]

I have long shaft Honda motors on the back. They had to be long shaft because Honda only makes them long shaft due to one being counter rotated.

If you have a single they make a short and a long but for twin applications.

 

[Justfishing]

On some tritoons it seems a 20" shaft sits low in the water. Prop height correct just the when the boat is at rest the powerhead is to close to the water. Then when the boat bounce up and down on a rough day the lower edge of the cowling is being submerged.

This could be why Bennington is going to a longer shaft motor.

The basic premise of the OP was proper depth of the prop. It can get a little trickier with a poonton. You run into planeing hulls vs displasment hull

A stanard pontoon or even tritoons is a displacement hull. That is the boat pushes through the water displacing the water. There comes a point where great increases in hp are need to gain a little speed and the top speed is limited. A standard v hull starts as a displacement hull. We have all felt the boat lug as it picks up speed and then suddendly takes off when it gets on plane.

A planeing hull rides on top of the water at planeing speed. It becomes a much more efficiency hull. In the pontoon world the advent of lifting strakes, pontoon design and placement has changed them to a planeing hull. You need the added features to plane and get speed and efficiency.

The thing I find is that the front of a tritoons can get heavy with passengers. People like to ride in front of the driver on these boats.

All of this makes some challenges in prop selection. You have to marry the torgue curve, rpms and prop features ( bow are stearn lift). WOrk with a prop shop to get you the right prop or props

 

[ONE-A-DAY]

Higher is always faster, as high as you can go until water pressure starts to decline, then drop it down a notch from that point.

 

[Royally PO'd]

The transom height determines the shaft length needed. If your boat is designed for a 20in shaft, a 25 in shaft will bury the prop to deep and kill your speed. You will not be able to raise the motor enough. The cavitation plate should be just out of the water on plane.

Bingo, my motor-builder / guru says the same thing.. Static , the cav plate should just be in the line of the lowest wetted surface. Too high = cavitation, too low = wallowing ...

 

[TCHB]

The transom height determines the shaft length needed. If your boat is designed for a 20in shaft, a 25 in shaft will bury the prop to deep and kill your speed. You will not be able to raise the motor enough. The cavitation plate should be just out of the water on plane.

You are right on point. My engines are set that the bullet on the lower unit is one inch below the toon.