Long Shaft on a Short Shaft RIB

[Lake1]

I?ve got a 11 foot inflatable designed for a 15 inch short shaft motor. Maximum 15 HP.

If I mount a 20 inch longshaft 15 HP, will the anti-cavitation plate plane up and raise the back of the boat out of the water by 5 inches, or will it just stay submerged and the boat run flat?

Would the long shaft cause squirrely handling or some other problem?

Thanks in Advance!

 

[boatdogboat]

Re: Long Shaft on a Short Shaft RIB

I have a 11.2 mercury inflatable and a 15 hp merc long shaft. The boat jumps onto plane and goes so fast it scares me. When i purchased it the saleman said a long shaft will most likley not plane. Boy was he wrong. Boat 100lbs., motor 100 lbs. me 200lb. WOW it flys....































w

 

[Lake1]

Re: Long Shaft on a Short Shaft RIB

Do you get any back spray over the transom at speed?

 

[mudslinging79]

Re: Long Shaft on a Short Shaft RIB

found this if interested...
http://www.dillon-racing.com/jackplate/index.htm

 

[seacamp]

Re: Long Shaft on a Short Shaft RIB

Do you get any back spray over the transom at speed?

I'm new at this; can I assume that I get transom backsplash w/my 11' RIB
and 15hp because it is longshaft; and only solution is raise transom
mount/i.e prop, or boat w/higher transom ? short of selling motor ?
thanks

 

[seacamp]

Re: Long Shaft on a Short Shaft RIB

I'm new at this; can I assume that I get transom backsplash w/my 11' RIB
and 15hp because it is longshaft; and only solution is raise transom
mount/i.e prop, or boat w/higher transom ? short of selling motor ?
thanks

 

[wa2712nj]

Re: Long Shaft on a Short Shaft RIB

As a previous poster pointed out, you can always use a jack plate to raise the motor up 5 inches.

I've been wondering the same thing you have - I've got a "spare" 9.9hp longshaft sitting right now, and the type of boat that I'm looking at for it will probably have a 15 inch transom.

I've read conflicting things, from "no problem" to "handling disaster". Middle ground seems to be something along the lines of "it is ok as long as you don't push the boat onto a plane" or "can cause a lot of backsplash over the transom". I'm pretty sure it won't be a total disaster, and will just try it. If backsplash is a problem, I'll build a jack plate.

ymmv,

-dm

 

[TOHATSU GURU]

Re: Long Shaft on a Short Shaft RIB

It's a handling disaster. Anyone who says differently lacks experience with operating one correctly and does not realize how wrong they are. Can you do it? Yes. Ship to shore, not on plane, no difference. Running at planning speed will throw water over the stern and into the boat, you will have reduced speed at WOT and turning is....Different

 

[surfga]

Re: Long Shaft on a Short Shaft RIB

When we were in Jamaica, we rented a small RIB with a 20" Yamaha 25 hp with a 5" riser. It performed great and was very responsive, more than my boat at least.

I personally have a 13' SIB with a proper 15" 25 hp outboard and it rides very nice.

Here's a pic from Jamaica:

Close up:

 

[trendsetter240]

Re: Long Shaft on a Short Shaft RIB

If I mount a 20 inch longshaft 15 HP, will the anti-cavitation plate plane up and raise the back of the boat out of the water by 5 inches, or will it just stay submerged and the boat run flat?

Try it and see for yourself. I've run 20" shaft motors on 15" transom boats and had no problems. Your results may differ. The boat I had planed easily and wasn't bad to steer at WOT despite what others may say. The biggest difference I noticed was an increase in drag and about 5mph lost from top speed.

 

[Lake1]

Re: Long Shaft on a Short Shaft RIB

All, Thanks for the input. I tried out a 5 HP long shaft 4 stroke and actually got the boat to plane. Boat was well behaved though a bit sluggish. No excess spray over the transom.

 

[ddrieck]

Re: Long Shaft on a Short Shaft RIB

You could add a 5'' vertical extension or an 2 pc adjustable manual jack...

http://www.dhcampingandfishingsupply.com/cmcstsepl.html

 

[steelespike]

Re: Long Shaft on a Short Shaft RIB

You may get a lot of spray and handling may be unpredictable but it will work.Some RIB have trouble with ventilation(cavitation) your long shaft should eliminate that.Though again could be unpredictable.

 

[mikebarry380]

Re: Long Shaft on a Short Shaft RIB

I have a long shaft 9.9 johnson on an 11 ft dink works great till you get to the beach. No water coming over the back but i am sure that the prop would have more fun near the surface.
A riser board bolted to the transom sounds like a great idea.