Short shaft on 20" if no plane ??

[jddenham]

Silly question I guess:

I have a 15' tri hull with 20" transom. I removed the outboard and sealed up the holes.

I'm wondering if I could put a 9.9HP short shaft on the back and be ok for putting around since it obviously won't plane?

What kind of speeds do you think I might get? I'm just hoping to be able to get out in the river to fish and have enough power to get back up river to the launch.

 

[tashasdaddy]

Re: Short shaft on 20" if no plane ??

well the first problem is if the 9.9 will be deep enough to get water to cool. it will not go fast, as it will have trouble to get water to grip. i really suggest not. if you really want to fish, get an adjustable motor mount for a gas trolling motor, so the 9.9 can do what it was built to do.

 

[fishingdan]

Re: Short shaft on 20" if no plane ??

Unless you want to go really slow, you should put the engine on an outboard bracket. These are something like $70 and are available in lots of places. As noted above, you will have a problem with the engine getting sufficient cooling water once you pick up any speed. The bracket will improve the situation some because you can position it lower and a little farther back.

You will only be able to go at "hull" speed. Since it is a short planing style hull, you will probably top out at 5-6 mph. If the river has a strong current, you might struggle.

 

[jddenham]

Re: Short shaft on 20" if no plane ??

It has an adjustable kicker bracket also. Looks old but should handle a small motor.

I will try it out on there, that way I can get optimal depth. Having the motor a little high might be beneficial as I can get a little slip to get rpm's up if I am underpropped.

I will make sure that the water pick-up is adequitely submerged of course.

 

[jddenham]

Re: Short shaft on 20" if no plane ??

You will only be able to go at "hull" speed. Since it is a short planing style hull, you will probably top out at 5-6 mph. If the river has a strong current, you might struggle.

I was hoping for closer to 10mph. I think my 55lb electric motor pushes it along at 4-5mph. I suppose that hull speed has a limit no matter how much power there is, until planing is acheived.

 

[Chinewalker]

Re: Short shaft on 20" if no plane ??

Keep in mind that instead of drawing water from the free & clear area under the hull, the propeller will now be drawing water from the narrow area between the lower unit and the flat of the transom. Good chance it will ventilate by drawing air from the surface in the process. I wouldn't do it unless you could use a bracket as described above or lengthen the motor to a longshaft...
- Scott