cutting the transom on an inflatable

[saintsfan]

I am having a problem on my new inflatable(dinghy),its an 11' with inflatable keel and a 8 hp merc shortshaft ob,the dinghy is rated to 15 hp and calls for a short shaft ob,but with this 8hp I cant seem to get on a plane without the motor cavitating like crazy,everytime the bow starts down the transom raises and it starts cavitating, this weekend at catalina I had 2 people with 2 sets of gear in it and no cavitation problems, I think because it was lower in the water.So I was thinking of cutting a slot in the transom to lower the motor down 1-1.5 inches. what do you guys think??

 

[Tom Winters]

Well it sure sounds like you need a long(er) shaft outboard.
Unless you are really, really good at this stuff, leave the transom alone. You lower if and you're likely to fill your boat with a nice following sea or shore break.

 

[Roughwaterjohn]

I would not cut the transom. If the outboard prop is below the bottom of the boat, it is the right length, and lowering it more would not benefit. The only way your outboard would be too short is if the majority of the prop was above the bottom of the boat.

It sounds like you need to add a pair of plastic planning wings to the bottom of your outboard. West Marine and most boating stores have them. They are adjustable for different size outboards and screw or bolt onto the exisiting small metal planning tabs that are part of your outboard.

I use an 11' inflatable with a deep vee rigid hull and a 35 HP motor as a tender for my boat. It wouldn't get up on plane either unless I had a couple of divers hanging over the bow like dead deer. I added the plastic wings and it goes up on plane and stays there even at part throttle.

My inflatable requires a long shaft motor because of the rigid hull. Most inflatable keels use a short shaft as you mentioned. Check prop depth to determine if that is correct.

John A.

 

[Tasarsailor]

Standard setup for outboards is to have the cavitation plate just about level with the bottom of the transom. If that's how your's is setup then just try to redistribute weight in the boat when you aren't runnin a full load.

The wings that attach to the cavitation plate help a lot on small boats.

Good luck
Tony

 

[saintsfan]

honestly It looks like it is where it should be,but all I know is when the boat is lower in the water there is no cavitation

 

[Wildcard]

I had the same problem with my 14 foot inflatable keel with a SS 30 HP on it. The wing made it even worse. It would run great out then cavatate back in with the same load. I had considered doing the same thing, lowering the transom.
Try moving weight around or even adding a couple of sand bags for when your light.
Try the wing, they arn't much.
Then try the cut down as a last resort.
I sold mine and never got the problem fixed.
Good luck!

 

[ProfessorAronnax]

Don't cut or chop transom. Throw some weight in the bow. Also, make sure your boat is completely inflated-should be rock hard.

 

[divinginn]

sounds like you need more weight distributed in the back

 

[saintsfan]

Don't cut or chop transom. Throw some weight in the bow. Also, make sure your boat is completely inflated-should be rock hard.

I actually let air out and it didnt cavitate as bad,once again engine is lower in the water.I may buy a hydrofoil tomorrow I have to go to westmarine anyway

 

[Aquanautchuck]

You don't say if you bought it new from a dealer or if it is just new to you. In any case I would run it by a dealer. I bought my C-5 Zodiac new from Inflatable Boat Works in Portland and they have been great on answering any questions about the height of the mtr or other questions.