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Old 23-09-2021, 08:45   #1
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Dinghy planing expectations

On my summer cruise I got my first experience driving a rib with a big engine. I have a Highfield CL310 (2018 generation) and a Suzuki 20hp 4stroke. With a pretty full 3 gallon tank in the bow locker, myself (~250lb), my buddy (~200lb), and my dog (34lb), we were able to get up on plane pretty easily and zip around.

Later, I picked up my sister and a couple of her friends (probably ~400lb total). So with me and one of them sitting right behind the bench seat, and the other two and my dog up in the bow, we were unable to get up on plane. We tried shifting our weight around a little bit back and forth, but could not find any position that helped.

Should I expect to be able to plane with that type of load? or does that sound normal? Would it help to add a hydrofoil fin to the outboard? Or does this require a different propeller pitch? Would a slightly larger dinghy (CL340) work better?
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Old 23-09-2021, 09:03   #2
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Re: Dinghy planing expectations

I think the 340 might do better, but you are 4 people and a big dog.... I think you are way over your 400 lb estimate. 4 people at only 150# each average is 600 lbs alone.

I could get a heavy double floor 10 ft RIB up with 3 people some gear and a 2 stroke Yamaha 15.
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Old 23-09-2021, 09:09   #3
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Re: Dinghy planing expectations

Sorry, I meant that my sister and her two friends were probably ~400lbs, then myself at 250, and my dog at 34.
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Old 23-09-2021, 09:14   #4
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Re: Dinghy planing expectations

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Sorry, I meant that my sister and her two friends were probably ~400lbs, then myself at 250, and my dog at 34.
The waterline will help a bit, not sure if enough or not. When I had trouble getting on plane I found it helped to have weight back in the middle then have one lunge forward popping it up on plane. But that's hard with that many.

I'd try two in the middle seat facing aft. Then one squatting right in front of the middle seat facing forward. At the right time have that squatter lunge forward to the bow.

Not sure how the engine block size runs at 20 HP, but you could benefit from the highest HP in the same block weight if a 25 or etc is the same block as the 20.
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Old 23-09-2021, 11:37   #5
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Re: Dinghy planing expectations

You're probably close to 800 lbs when you include the motor, fuel and any sundry items you might be carrying.


That's a lot of weight to plane in a 10' inflatable.


Adjusting the engine's trim position and using prop with less pitch will help but might not be enough.
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Old 23-09-2021, 11:42   #6
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Re: Dinghy planing expectations

If it feels like it can almost get up on the plane, a hydrofoil can help. Doesn't hurt to try one - much cheaper than buying a bigger motor or bigger dinghy....
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Old 23-09-2021, 12:08   #7
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Re: Dinghy planing expectations

Thanks guys, I think I'll look into a hydrofoil as a first attempt.



What is the effect of changing the propeller? Does a shallower pitch increase power but reduce top speed? I'd be willing to sacrifice some top speed if it meant I could get up onto plane easier with the boat loaded more.
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Old 23-09-2021, 12:12   #8
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Re: Dinghy planing expectations

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Originally Posted by Muaddib1116 View Post
What is the effect of changing the propeller? Does a shallower pitch increase power but reduce top speed? I'd be willing to sacrifice some top speed if it meant I could get up onto plane easier with the boat loaded more.

Less pitch may help there. By allowing the engine to rev higher while trying to plane you'll get a bit more usable power to get you over the hump (just make sure you're not over-revving at WOT once you're up on plane). A prop with more blade area may improve your low speed thrust a bit as well and help get over the hump (due to less slippage and cavitation under high load).
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Old 23-09-2021, 12:15   #9
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Re: Dinghy planing expectations

Lower pitch will increase takeoff and top speed, but don't overdo it or you can overrev your motor.

Another option is MAXI tabs - many people swear by them: https://www.maximarine.com/products/lifters/index.html
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Old 23-09-2021, 12:30   #10
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Re: Dinghy planing expectations

I tried a hydrofoil on my 15hp. It almost seemed to hurt rather than help really. Not sure why. maybe the drag of the foil in the water? You know how big drag is, if your HP is marginal, than yeah, it may hurt rather than help.
But I think you can get the most changing motor tilt and less pitch. I've not found pitch to matter a lot on a dingy type zooming around, already plenty of HP and thrust, but when you need to step up on that plane with marginal HP, then yes.
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Old 23-09-2021, 12:47   #11
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Re: Dinghy planing expectations

I like the idea of the maxi lifters. I'm not finding many pictures or reviews of them, but they claim that once the mounting brackets are installed that the lifters can be taken on and off easily, which sounds great as far as not interfering with stuff when I lift the dinghy in my davits.


If they work as advertised, the only downside I could see is that they would get in the way of dinghy wheels, if I ever decide to add those.
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