Planing a dinghy with 3.5hp

[mikedefieslife]

With a 6hp motor and 3m dinghy you should be able to plane effortlessly no matter where you sit. Unless there is a mechanical issue with the engine/prop, my guess would be on wrong prop diameter and pitch.

My Yamaha 6 (2 stroke) has a 8.5" x 6.5" prop.

[Stu Jackson]

Originally Posted by Panope

>>>>>>>>>>>>>>>>
No need for tiller extension/moving forward.

Note the motor plate that I constructed that RAISES the motor about 1 inch (I did this for more speed/effiecency).

I bought PVC pipe large enough to fit around the throttle handle. Only YOU can measure what you have.


Steve's right, raise the engine. I did it with a piece of 1" square wood glued to the top of the transom.

[Rdubs]

Originally Posted by mikedefieslife

With a 6hp motor and 3m dinghy you should be able to plane effortlessly no matter where you sit. Unless there is a mechanical issue with the engine/prop, my guess would be on wrong prop diameter and pitch.

My Yamaha 6 (2 stroke) has a 8.5" x 6.5" prop.

Friends - thanks much for the helpful input. Two things based on the suggestions:
1) While I hope a brand new motor wouldn't have a problem with that rubber propeller bushing that Steve pointed out, anything can happen. Seeing as how by myself the motor would race at full throttle as if all the power was being wasted, that would seem to indicate a slipping bushing. But: when my friend joined me in the boat (he's about 200 pounds), and the inflatable was sitting deeper in the water, at full throttle there was no slipping. The boat wouldn't anywhere near plane with 400 pounds of people in it, it would just chug through the water, but could the bushing be the problem if it does it with only me in it but not with additional weight? Or if the bushing was the problem, wouldn't it slip regardless of how much weight was in the boat?

2) I checked online and it looks like the OEM propeller is 7.5" diameter by 7" pitch. Mike's prop on a similar model but 2-stroke is 8.5" x 6.5" pitch. I want to try a different size/pitch prop to see if that might be part of the problem. Any recommendations on what to get?

I'm also thinking about buying a replacement OEM prop just in case the bushing is the problem, but I wonder if that would be the problem since the revving problem doesn't happen with two adults in the boat.
Again thanks.

[Rdubs]

Originally Posted by Stu Jackson

I bought PVC pipe large enough to fit around the throttle handle. Only YOU can measure what you have.


Steve's right, raise the engine. I did it with a piece of 1" square wood glued to the top of the transom.

Sorry gents I'm trying to understand this. I would have thought that raising the motor would make the problem worse, since down deeper there is more water pressure so the propeller gets a better bite without cavitating. Up higher, wouldn't not there be less water pressure therefore less bite as well as getting closer to the boat's bottom where the flow into the blades isn't as smooth?

[waterman46]

Originally Posted by Panope

I have a very similar dinghy (10 ft., high pressure floor) and same outboard (short shaft Yamaha 6). Planes effortlessly with one person, nicely with two people, and will stagger onto a plane with three (lightweight) adults. No need for tiller extension/moving forward.

Note the motor plate that I constructed that RAISES the motor about 1 inch (I did this for more speed/effiecency).

Assuming the angle of your engine is adjusted properly (as mentioned in above posts), I would be surprised if the problem was propeller ventilation.

These engines do not use a "shear pin" to protect the drive train from damage in case of propeller strike. Instead, the propeller has a hub that is separate from the blades. A rubber(?) bushing keeps these two propeller components connected under normal operation. However, sometimes, these hubs "slip" at high power settings (I have had this happen), resulting in high engine RPM, and little thrust.

Good luck,

Steve

Steve, thanks for the info on the rubber bushing design used on at least some Yamaha props. Shear pin, though it has to be replaced and a spare or 3 always carried, would seem to be a more reliable method of prop and motor protection. So does anyone know if the new 2 to 15 hp Yamaha range also provide the option of shear pin installation along with a solid body prop (no bushing)?

[lancelot9898]

I'm still waiting for the money to present itself prior to buying both the dinghy and motor, but wanted to thank all for the info. Since there is a discussion about the prop, I thought I would relate what I did years ago on that 2 stroke 5 hp Nissan to get it to plane with 2 people aboard. The original prop would not plane it so I order another prop with a higher pitch but with the same diameter. While it took longer to get up on speed, it did eventually plane the 10 foot Archilles with wooden floor. Now just the other day on a ytube channel a fellow was talking about getting the dinghy up on plane with a 6 hp tohatsu. He claims it was achieved by reducing the diameter and keeping the same pitch. Maybe both ways will work?

[Panope]

Originally Posted by Rdubs

Sorry gents I'm trying to understand this. I would have thought that raising the motor would make the problem worse, since down deeper there is more water pressure so the propeller gets a better bite without cavitating. Up higher, wouldn't not there be less water pressure therefore less bite as well as getting closer to the boat's bottom where the flow into the blades isn't as smooth?

You are correct. I only mentioned raising my motor to indicate that the shaft length (short shaft) is adequate and that you should NOT need to get a long shaft.

It would be best if we compared transom heights. Unfortunately, my inflatable is rolled up and in storage...

Steve

[Rdubs]

Hi all- just closing the loop....was able to get my inflatable up on plane. The problem was two things:
1) I was sitting too far backwards. When I used the tiller extension and sat more forward, it planed immediately. Fairly fast with a 6hp motor.
2) previously I was just sitting on the bottom of the inflatable. This was creating a huge lump in the hull bottom. Planing works best when weight is distributed evenly over the water surface contact area. So I installed the seat that came with the dinghy and sat on that. It spread out my weight over a much greater surface area and made it plane even better.

Not only was I able to get it to plane with the 6hp Yamaha, I also got up on plane with my little backup 2.5hp. Just barely, and not as fast as the 6, but was very impressed.

[Hardhead]

Dinghys are a great feedback system. Quick reaction based on load distribution, etc. It's good experience to have. Every dinghy has a little different qualities from another. Glad you got up on plane even with the 2.5 - good light little motor.

[Group9]

Originally Posted by Hardhead

Dinghys are a great feedback system. Quick reaction based on load distribution, etc. It's good experience to have. Every dinghy has a little different qualities from another. Glad you got up on plane even with the 2.5 - good light little motor.

So true. Our old dinghy was an 11' Boston Whaler. With our 9.8 Nissan, we could barely get it to plane. We sold it and bought a 13.5 Dell Quay Dory (a Boston Whaler knockoff of sorts). It is bigger and weighs more, but the 9.8 easily planes it.