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Old 11-12-2018, 06:13   #1
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Planing a dinghy with 3.5hp

I plan on getting a 10 ft inflatable floor dinghy and because of weight restrictions on the outboard, using only a 3.5 hp. Is there anyone out there that can get the boat on plane with such a small outboard? I prefer getting a 6 hp motor, but the weight is close to 60 lbs and I don't think I can handle that without a crane. There are some 2 stroke outboards on sale on Amazon which weigh in at 40 pounds, but unsure of the quality. Didn't even know that it was possible to buy here in the US. Any comments about these outboards made in China?
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Old 11-12-2018, 06:17   #2
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Re: Planning a dinghy with 3.5hp

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Originally Posted by lancelot9898 View Post
I plan on getting a 10 ft inflatable floor dinghy and because of weight restrictions on the outboard, using only a 3.5 hp. Is there anyone out there that can get the boat on plane with such a small outboard? I prefer getting a 6 hp motor, but the weight is close to 60 lbs and I don't think I can handle that without a crane. There are some 2 stroke outboards on sale on Amazon which weigh in at 40 pounds, but unsure of the quality. Didn't even know that it was possible to buy here in the US. Any comments about these outboards made in China?
Well, I can plane with a 4hp if it is just me and the dog or one of the kids, but any more than that doesn't want to come up. Mine is only an 8 foot dink too. I think you will have a hard time getting on plane.
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Old 11-12-2018, 06:19   #3
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Re: Planning a dinghy with 3.5hp

I used to have a 9ft inflatable with a aluminium hull which would plane with a 3.3hp 2 stroke Mercury. Could only have myself onboard and would have to sit in the middle to keep the bow down.
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Old 11-12-2018, 06:30   #4
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Re: Planning a dinghy with 3.5hp

I think a longer dinghy might come up on plane easier than a shorter one. Also these small outboards come with plastic props and I don't think I can order a higher pitch prop like I could with metal ones. I did have a 5 hp Nissan that with the higher pitch could plane with me at 200 and my wife at 120 and this was with a wooden floor 10 foot inflatable weighing more than the inflatable floor model that I'm looking at buying. (60 lbs vs 110 ?)
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Old 11-12-2018, 07:20   #5
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Re: Planning a dinghy with 3.5hp

We have a nesting Spindrift hard dinghy (10') which we can easily get it up on plane w/5 hp Tohatsu (2 stroke; 44#) w/2 people and gear. We tried using the same motor on a 9' hypalon air floor and could not get it on plane w/2 people and gear. Probably would want an 8 hp to get the inflatable up on plane w/the same load

I refurbish small outboards (hobby) and over the years have tried 2.3, 3, 3.3 and 4hp light weight 2 stroke motors on the nesting dinghy to see how they would work. With the smaller motors they moved the boat OK at WOT, but not up on plane (again w/ the same load). I really wanted the 4 hp Evinrude to be able to do the job since it had 2 cylinders and weighed only 37#, but unfortunately did not do it. The 5 hp has plenty of power at ~3/4 throttle to get the nester up on plane (again same load). It's also nice to have some extra power in reserve if needed when you have more than the standard load (4 vs. 2 people, scuba tanks, etc.).
Based on my experience, the 3.5 will be great to get in/out/around the anchorage, but not enough if you want to have an "explorer" dinghy. Also if it bouncing around you'll want the engine hoist to safely get the motor on off the mothership. The newer 3.5 hp motors (4 strokes) are over 40# (w/o fuel) so they will be a hand full to get on off the boat in a sloppy anchorage.


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Old 11-12-2018, 09:11   #6
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Re: Planning a dinghy with 3.5hp

Ive got to say I love my Torqeedo 1003. Weighs ~ 30 lbs overall, comes in three pieces which are easy to assemble in the dinghy, and you can get a spare battery if you want. Theres a readout on the tiller telling you how much juice you have left.

No planing, but I am happy to forego that if I can avoid carrying gasoline. To store, just hose it off and put it in its case. Done!

Also, its quiet, so you can actually converse in the dinghy.

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Old 11-12-2018, 09:14   #7
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Re: Planning a dinghy with 3.5hp

We have a 9ft inflatable floor Achilles with a 6hp Honda and it struggles with the two of us to get on plane. Once it does it is scary as can be. Kind of reminds me of one of those round saucer sleds. I may not be pumping the floor firm enough.
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Old 11-12-2018, 09:17   #8
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Re: Planning a dinghy with 3.5hp

My 8' 6" Avon (wood floor, inflatable keel) will plane reliably with a Nissan/Tohatsu 3.5hp 4-stroke long shaft if it's only me onboard. I have gotten it on plane with 2 people but only in very flat water and a fair amount of juggling of seating positions.
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Old 11-12-2018, 09:17   #9
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Re: Planning a dinghy with 3.5hp

After over 30 yrs turning wrenches on most everything you can imagine, I can tell you with certainty that you can NOT TRUST most ANYTHING made in china , especially anything engine -driven , They are , without exception , Made with inferior materials and workmanship , There`s a REASON chinese knockoff stuff is so Cheap !
If Ya do decide to go that route , DONT FORGET YOUR PFDs & PADDLES --- LOL !!!!! ----
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Old 11-12-2018, 09:18   #10
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Re: Planning a dinghy with 3.5hp

Inflatable floor? Unlikely.

AB Rib with alloy hull yes but only 8 with one person in it.
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Old 11-12-2018, 09:18   #11
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Re: Planning a dinghy with 3.5hp

My 3.5 hp Tohatsu will easily plane my 10 portabote with one adult and gear on board. It will plane with two adults in calm conditions.

I doubt youll be able to plane with most inflatables using a 3.5 hp, but it might be possible with the right inflatable boat; say one with a high pressure floor.
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Old 11-12-2018, 09:33   #12
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Re: Planning a dinghy with 3.5hp

Inflatable hulls generally have less than optimal forms to avail ease and efficiency of planning.

Might I suggest that you utilize a kit to enhance the basic dinghy's capabilities in the form of a LARGE trim tab addition. A discussion with a manufacturer regarding how your craft might work with various propulsive power plants [motor hp and pitches of propellers] should be enlightening.

Linked below is to one such manufacturer which provides information and videos. I have no affiliation or specific knowledge of any of the trim tab companies, just placing this information as an alternative to all you "go fast" dinghers who desire the use of a light weight outboard motor.

If any of y'all have experience with the use of large trim tabs please advise as to the performance enhancement.

All the best.

MAXI MARINE - Detachable Planing System for Inflatable Sportboats/RIB (Lifters - special trim tabs)

I always get a chuckle at the fundamental concept that sailors who are fine with cruising long distances at relatively slow speed and with tacking far from their direct line when headed windward, then desire to travel fast for short distances between their yacht and a nearby shore destination.

Copied from the referenced website:

LIFTERSTM radically improve the performance of inflatable boats, lowering initial planning speed while increasing cruise speed and acceleration 25% to 45% and planing weight capacity up to 30%, while decreasing fuel consumption by over 25%. Boats having insufficient power or carrying heavy loads can now achieve plane or achieve higher planing speeds. Lifters are specifically designed and patented for Zodiac type inflatable boats to yield maximum performance.

The speed and weight carrying capacity of a planing hull is directly related to it's planing surface area and driving power. Unlike small trim tabs, Lifters add more that 5 times the planing area, or a substantial 4.2 sq. ft. (12.5" x 24" x 2) of additional hull planing surface as shown below. At low planing speeds the additional surface area at the stern creates more than a proportional amount of added lift by pushing water down, opposed to the forward "V" section which pushes water to the sides. The additional lift far aft of the center of weight (boat, crew and motor) lifts the stern and lowers the bow for more hull planing surface area and upward lift rather than backward drag. Lifters also generate less drag and more lift then trim tabs by having a lower angle of deflection or attack and more surface area further aft. At high planing speeds when less surface area is desired because of higher viscous skin drag, the ventilation slots at the front of the Lifters suck air into the mini step which decreases the planing surface area and drag. This technology is similar to the latest side ventilated stepped hull technology used on race and recreational boats, but more importantly, Lifters dramatically increase the performance and comfort for fishing, hunting, scuba diving, tending, towing, rescue and other boating activities.
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Old 11-12-2018, 09:41   #13
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Re: Planning a dinghy with 3.5hp

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Originally Posted by lancelot9898 View Post
I plan on getting a 10 ft inflatable floor dinghy and because of weight restrictions on the outboard, using only a 3.5 hp. Is there anyone out there that can get the boat on plane with such a small outboard?
I've tested dozens of inflatables with dozens of engines. It's difficult to get a 10' inflatable on a plane with a small engine due to the limited surface area, and to prove this, just try to get a 9' inflatable on a plane with the same engine. The lowest horsepower planing boat that I have experienced was an 11' (3.40) West Marine PVC boat with an inflatable floor that had a 5HP Mercury single cylinder four stroke engine. I didn't expect it to plane, but it did with one heavy person aboard. Note that most 4-stroke small engines (4, 5 and 6HP) are essentially the same engines with different maximum RPMs, so it's possible that this would have worked with a 4HP engine.

Little boats dig themselves into a hole as they go faster, and can't climb out of the hole without a lot of weight forward and a larger engine.

It goes without saying that the interior of an inflatable gets dramatically larger with each additional foot of length. Very little of the increased LOA is use "outside" the interior; virtually all of it goes inside (and a little increased beam helps, too.)

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Old 11-12-2018, 09:47   #14
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Re: Planning a dinghy with 3.5hp

Lance
I have your set up, avon inflateable floor, 10 with 3.3 HP Johnson, 25lbs. It wont plane. I just use for 1 mi or less shore trips. It will carry 4 adults (low in the water) But I am in no hurry. I think you are right about the new motor weights. You just cant manhandle 40lbs onto a mount. As you say a 4-5 HP will give you a neutral and reverse with a handle throttle. Its enough to almost justify a crane addition. There is a small, Older 3.3 Johnson on ebay right now but its pricey at $399. I would stay away from Chinese. You cannot nuy new 2 strokes in the USA. I dont know if those on the net would ship to USA. When its all said and done its about motor weight vs body weight..Good luck.
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Old 11-12-2018, 09:50   #15
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Re: Planning a dinghy with 3.5hp

I have a 10' inflatable with air floor that will only plane with me in it with a 3.5hp Johnson 2 stroke. I have to sit in the middle of it to plane out and I weigh 185lb

I also have an 8hp 2 stroke that will make it plane out with 2 185lb people in it
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