SIBS - Outboard Pairing

[keithb7]

I'm learning more each week that I get out on the water in my 3.3M SIB. My boat has a soft inflatable floor. My boating buddy has a rigid aluminum floor. His SIB is also 3.3M. Both boats are rated for max 15 hp.

It seems to me 15 hp and its relative top speed can't be fully utilized unless you are on smooth water conditions. On rough water these sibs bounce around a fair bit and beat you up. You likely can't rip a 7.5 hp outboard wide open in rough conditions, let alone a 15 hp. I think I've found a sweet spot with my 3.3M sib with an 8.0 to 10 hp-range 2 stroke, with a 9" diameter, 9" pitch prop. I've come to realize faster isn't always better with these sibs. For me sibs are about storage convenience. Access to remote lakes. Portability. Having a light weight engine is a huge part of the convenience. The light weight two-stroke In the 8-10 hp range suits me very well.

The proper inflation psi has a huge effect on the boat's handling at higher speed. The faster you go the trickier it can get. Payload weight distribution, outboard engine weight and HP rating offer more variable factors to the boat's stability at speed.

I'm fortunate to have multiple small outboards, props and a couple of boats to experiment with. I've gained a fair bit of experience quickly this spring. So far I've settled on my 1990 8.0 Merc 2-stroke a my top choice for my soft floor 3.3M SIB. Still more experimenting will be done. I was out last week with my 7.5 Merc 2 stroke. It performed pretty good also. It had a 7" pitch prop. I want to try it again soon with a 9" pitch prop.

 

[cwella]

I'm learning more each week that I get out on the water in my 3.3M SIB. My boat has a soft inflatable floor. My boating buddy has a rigid aluminum floor. His SIB is also 3.3M. Both boats are rated for max 15 hp.

It seems to me 15 hp and its relative top speed can't be fully utilized unless you are on smooth water conditions. On rough water these sibs bounce around a fair bit and beat you up. You likely can't rip a 7.5 hp outboard wide open in rough conditions, let alone a 15 hp. I think I've found a sweet spot with my 3.3M sib with an 8.0 to 10 hp-range 2 stroke, with a 9" diameter, 9" pitch prop. I've come to realize faster isn't always better with these sibs. For me sibs are about storage convenience. Access to remote lakes. Portability. Having a light weight engine is a huge part of the convenience. The light weight two-stroke In the 8-10 hp range suits me very well.

The proper inflation psi has a huge effect on the boat's handling at higher speed. The faster you go the trickier it can get. Payload weight distribution, outboard engine weight and HP rating offer more variable factors to the boat's stability at speed.

I'm fortunate to have multiple small outboards, props and a couple of boats to experiment with. I've gained a fair bit of experience quickly this spring. So far I've settled on my 1990 8.0 Merc 2-stroke a my top choice for my soft floor 3.3M SIB. Still more experimenting will be done. I was out last week with my 7.5 Merc 2 stroke. It performed pretty good also. It had a 7" pitch prop. I want to try it again soon with a 9" pitch prop.

Great write-up! It’s interesting how much you can learn just by spending time on the water and trying different setups.

I think you make a good point that more horsepower does not always mean a better experience, especially with smaller SIBs. A 15 hp engine might look ideal on paper, but real conditions, comfort, handling, and safety matter just as much. In rough water, having a lighter and easier-to-manage setup can make a big difference.

I also agree that proper inflation, weight balance, and choosing the right prop can greatly affect how these boats perform. Finding the right combination of speed, stability, portability, and efficiency is more important than just trying to go faster.

Your 8 to 10 hp setup sounds like a great match for a 3.3M SIB, especially for exploring smaller lakes and enjoying the convenience of an inflatable boat. It’s great that you have different engines and props to test and compare.

I’m interested to hear how your 7.5 Merc performs with the 9 inch pitch prop. It’s always helpful to learn from real-world experiences like this.

 

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