9.9 hp on 10' inflatible

[clownfishsydney]

I read somewhere that you need 5 hp per person on average to plane. This is also my experience based on the two RIBs we own. We have a 3.4 metre RIB (say 11.3 feet) with Mercury 15 hp 2 stroke. It will just plane with three persons, lots of gear and full fuel. Changing the prop from 9" to 7" made it worse, so I have just changed back. I reckon get 15 hp, probably same weight.

[Wild Bird]

The Tohatsu 9.8 two stroke is the lightest 10hp in the world. 26kg. This are robust and reliable engines. We have one on a Caribe 9 ft rib. The addition of a plate to the engine makes a large difference to the ability to get on the plane easily. We would be without it. Our rib is the Caribe light weight model that has a V at the front and a flat section at the rear. This is a very dry boat and works very well with this engine. We plane with two very easily and with some additional weight but it won't plane with three onboard. The fuel consumption of the Tohatsu is very good even if ran hard most of the time


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[howy33]

I have a 2017 West marine 9.6 dbl floor aluminum hypalon with a 9.9 Merc 4 stroke w/fins(added) fully inflated tubes. I am at my wits end trying to get it up on plane with 2 people. By myself and gear=300lbs no prob 5800rpm 17mph. 2 people and gear=450lbs hanging over the bow 4000rpm 6mph no plane. I am getting ready to purchase my fourth prop. Currently on a 4 blade 8.9x7 Anita, going to 8.9x5 Anita. If this doesn't work I will sell the POS.

[Cheechako]

Originally Posted by howy33

I have a 2017 West marine 9.6 dbl floor aluminum hypalon with a 9.9 Merc 4 stroke w/fins(added) fully inflated tubes. I am at my wits end trying to get it up on plane with 2 people. By myself and gear=300lbs no prob 5800rpm 17mph. 2 people and gear=450lbs hanging over the bow 4000rpm 6mph no plane. I am getting ready to purchase my fourth prop. Currently on a 4 blade 8.9x7 Anita, going to 8.9x5 Anita. If this doesn't work I will sell the POS.

Hmmm.. so this is a RIB with a double aluminum floor? Must be pretty heavy.
How heavy is the dingy without the engine?
The 4 stroke engine is heavy too I imagine?
I always felt like doelfins "dragged". They angle the dingy maybe, but not sure they are a benefit to get on plane... is the drag more than the benefit if you have marginal HP?
In a "near plane" situation I always started up to plane then I lurched my body forward (or had the passenger) and got it up.

Sounds like you need a lighter dink or 15 HP engine to me. I used a 15 HP on a 10 ft double floor heavy fiberglass RIB and it screamed onto plane. (but was 2 stroke)

[howy33]

106lb rib and 88lb motor I believe a 2 stroke has more torch.

[Cheechako]

Originally Posted by howy33

106lb rib and 88lb motor I believe a 2 stroke has more torch.

Well, hmmm, that's not too bad. I think my 10 ft double floor Caribe was 130#! My Yamaha 15 hp weighed about what yours does also...
I wonder if the 4 stroke just doesn't have the real HP... or maybe the extra waterline length helps a lot.
My same motor was way too much on the 9 ft Caribe I bought after that dingy.

[howy33]

Originally Posted by Cheechako

Well, hmmm, that's not too bad. I think my 10 ft double floor Caribe was 130#! My Yamaha 15 hp weighed about what yours does also...
I wonder if the 4 stroke just doesn't have the real HP... or maybe the extra waterline length helps a lot.
My same motor was way too much on the 9 ft Caribe I bought after that dingy.

I read that a longer rib might do it. Do you thin going to a 5 pitch from 7 pitch will do it? it is almost there. I tried jumps forward up and down. Added tiny tach to check wot. I need to try current prop 8.9x7 by myself.

[Cheechako]

Originally Posted by howy33

I read that a longer rib might do it. Do you thin going to a 5 pitch from 7 pitch will do it? it is almost there. I tried jumps forward up and down. Added tiny tach to check wot. I need to try current prop 8.9x7 by myself.

I've only changed dingy props a couple of times. Usually when one got damaged or spun the rubber in the center. I would often buy a spare prop at a good price regardless of what pitch it was. I never sensed any difference after changing. I guess I just had plenty of HP. So I'm not sure about how effective the path of changing props on a dingy is.
I also changed props to get "out of the hole better" on my 24 ft V8 motorboat once also. I went with a slightly different prop recommended. It really seemed the same as it was after.
I know props can make a difference, but maybe it's not huge. Let's see, if you want to jump out of the hole better you need less pitch right? It might be worth a try.

Your prop isn't spinning on the rubber hub is it? I guess not with all your changes...

[dfelsent]

Are you shifting your weight forward to get up on a plane? Makes a huge difference.
Hit the throttle then when it squats everyone moves forward to flatten tout then moves back as speed picks up.

A previous poster mentioned this.