Proper mounting of 50HP outboard on my skiff

[longshanks]

Howdy folks, from British Columbia

I'm the owner of a 15' heavy duty aluminum skiff that I purchased last year. It is a very beamy hull, with a 6 degree vee hull, to be used as a versatile fishing skiff for rivers and lakes. The hull has a small tunnel in the last 18" of bottom, to direct 'clean' water to the prop/jet, and to permit operation in shallow waters.

I bought the hull with a 60HP outboard jet, and I have recently purchased a merc 4-stroke 50HP prop motor for the boat, so I can swap engines and use in lakes or rivers or ocean.

I am aware that for optimal performance, prop motors' cavitation plate should be mounted flush with the bottom edge of the transom (in my case, the top of the tunnel). With the 20" prop leg, the cavitation plate of the prop motor will sit approximately 2" above the bottom of the transom/top of tunnel.

My question is, should I cut down the transom before mounting the prop motor, or is it worth trying the motor first, to see if the prop functions ok in this 'high' position? I hear that some hull/motor combinations work ok with this configuration. The hull works great with the jet, and I'd rather not cut away the transom, unless it is needed for prop use.

thanks in advance for your thoughts

cheers

 

[jtexas]

Re: Proper mounting of 50HP outboard on my skiff

Have you considered a jack plate or setback bracket of some sort?

 

[tashasdaddy]

Re: Proper mounting of 50HP outboard on my skiff

i'm not sure, but be ready for a surprize in performance. the 60hp jet was about equal to a 40 hp prop engine, due to loss of effeciency in the jet.

 

[longshanks]

Re: Proper mounting of 50HP outboard on my skiff

thanks for the replies guys. I am considering making up some sort of setback plate, that would allow me to drop the motor an inch or two, and put it 3-4" back from the transom. Because it is a tiller steer motor, I need to determine if the handle is long enough to permit it. If this works out, I could avoid cutting the transom, which is a large bonus.

Regarding performance, l am hoping for some increases, as my 2-stroke jet is around 40HP at the jet, and very noisy. BUT, it runs in 5" of water! The 50HP 4-stroke should be much nicer to operate in lakes/ocean environment, where I spend about 50% of my time. All in all, if I can make it work properly, I'll have a versatile little boat!

cheers

 

[bassboy1]

Re: Proper mounting of 50HP outboard on my skiff

You should be okay. I would try it first before fabricating a setback plate.

 

[ondarvr]

Re: Proper mounting of 50HP outboard on my skiff

Give it a try, the tunnel may direct more water to the prop, just make sure it's still pumping water. My 19' bay boat has a small tunnel and my 200hp is mounted higher than I thought it could be, the AV plate is about 2.5 inches above it and I think it could go higher.

 

[atticus3600s]

Re: Proper mounting of 50HP outboard on my skiff

Give it a try the way it is. Make sure the engine is getting water at that height and watch your water pressure on plane. It it performs fine without the set back than leave it as it is and have fun8)

 

[longshanks]

Re: Proper mounting of 50HP outboard on my skiff

well that's some advice I can live with. I will give it a go come spring (2' of ice here now).

I found some similar guidance at:

http://sites.mercurymarine.com/portal/page?_pageid=126,49901,126_49907&_dad=portal&_schema=PORTAL

Judging by the way the outboard jet intake sits so close to the top of the tunnel, I am expecting the prop leg to be similarly close to the back of the tunnel. Most of the diagrams on the linked merc site show a foot or more of space between the bottom of the transom, and the prop leg, whereas mine will be closer to 3 or 6". ie - I expect the AV plate will be out of the water at planing speed.

I should also ask, because this is a tiller outboard with no controls, I assume that if the water telltale is peeing steady while the boat is on plane, that water pressure is ok?

I'll report back on what I find out, thanks alot for the information.

cheers

 

[roscoe]

Re: Proper mounting of 50HP outboard on my skiff

You will likely be ok without any modifications.

Anti ventilation plate SHOULD be out of the water when on plane.

If you add a plate to create setback, you would normally raise the motor an inch.

Try it the way it is.

A water pressure gauge can be added to your motor.
You just need someplace to mount it. It connects to the engine via an 1/8" plastic tube.