Dinghy outboard planing issues

[brico]

I bought new 10' inflatable with fibreglass bottom and it is 140lbs weight. I also got bigger (more powerful) 9.8 HP Tohatsu 4 Stroke outboard which is also heavier. The propeller is marked 8.5 diameter x 7" pitch. The engine is working excellently. I even wired the stater which came with it but the PO did not use.

The issue I have is that the boat is struggling to get on a plane with two people one sitting all the way in front and after a while it jumps right out and goes very fast. With me alone in the boat, holding throttle and tiller and unable to sit in the front, I simply cannot get on the plane. Would the differently pitched prop help? Do I need those plastic fins to bolt onto the lower leg to help it to get on plane?

Another issue I noticed about it is that I have to fight the tiller to right in order to be able to go straight. The engine is mounted in the centre of the transom and tilted back down against the transom, so it is not raised/lifted even though it has 5 different positions it is in the lowest one.

Any tips and advice is much appreciated.

 

[SEVEN]

Try one of these wings. This add was from West Marine

 

[petewentsailing]

I had a similar problem with a 9' Whaler. Went down to smallest prop Suzuki recommended for the engine, and added a fin, as mentioned on other post. Solved it for us.

 

[cenger]

You can try putting on a Whale Tail. I installed one on my 15hp Tohatsu, and it helps my 13’ hard bottom inflatable plane more quickly. A Whale Tail is easy to install and relatively inexpensive (~$75) on Amazon.

 

[carolinacoast]

after adding fin, ballast like sand in gallon jugs in anchor locker can help (forward CG). biggest prop with lower pitch (reduction) should get more torque at lower end. would reduce top end, but sounds like that is not the goal.

 

[Scarlett]

The steering problem is caused by not having your weight in the very center of the dingy. If you sit off center it will pull to one side

 

[Hat52MY]

Is 9.8HP, really enough to move approx 540lbs (10’ RIB, engine, gas, gear & 2xPeople) on plane? My 12’ RIB has 40HP @ 1200lbs loaded, but if I use the ratio above (9.8hp/540lbs), it would stipulate 21HP, which would be a considerable struggle to get on plane. Have you considered bumping up the HP?

Another option to consider is a tiller extension, so you can sit closer to bow and get the weight out of the stern.

(9.8lbs/540lbs) = (21hp/1200lbs)

 

[VieEnRose]

Can you get some negative trim from the motor? Put the fuel tank up front?

Is the motor mounted high enough?

 

[Photolomy]

Is 9.8HP, really enough to move approx 540lbs (10’ RIB, engine, gas, gear & 2xPeople) on plane? My 12’ RIB has 40HP @ 1200lbs loaded, but if I use the ratio above (9.8hp/540lbs), it would stipulate 21HP, which would be a considerable struggle to get on plane. Have you considered bumping up the HP?

Another option to consider is a tiller extension, so you can sit closer to bow and get the weight out of the stern.

(9.8lbs/540lbs) = (21hp/1200lbs)

I have an 11 ft inflatable with 9.9HP mercury and it planes ok.Sitting forward and moving the gas can to the front helps with one person. But it will even plane with 3 people, just barely. 18.5 mph with one, 17 mph with two, and 15.5 mph with 3. It is hardest to plane with 3 people. 3 or above, we just take our time.

 

[brico]

Thanks everyone for chipping in with tips. I'll get those fins, tiller extension and possibly prop pitch adjusted.

I do not want to add gallon jugs of sand or any form of ballast on the boat, that would defeat the purpose. 10' dinghy leaves 8" space at each end of the transom of my 37 Hatteras. I do not need bigger one.

I've added 22% HP over my 2 stroke Mercury 8HP and 19% length over 8.5' inflatable I had earlier which planed easily and had those fins fitted.

I've never tried old dinghy and outboard without those fins. They may be the cheapest solution to the problem.

 

[Phasma2128]

Had the same experience. I solved it by making sure the pontoons were inflated to the maximum pressure. If they are even a little soft you will struggle to get on plane. If I was alone I simply leaned far forward until it popped out of the water. I was then able to sit back. Try it. It works