4.2M with a 9.8 Nissan RIB?

[slowstang305]

Ive got a 4.2M ZEFIR RIB powered by a 9.8HP outboard 4 stroke Nissan. I know this size boat is best with a bigger engine but thats the motor it came with. I have not put it on the water yet, never ran it. Will it be ok with a 9.8? I know most people would suggest 30-40HP+ but that is not an option. I just need this boat to be able to go 8-9 knots. Possible? How do you think this boat will handle? What top speed do you think I will be able to reach?

Thanks!

[Polwart]

Originally Posted by slowstang305

Will it be ok with a 9.8? I know most people would suggest 30-40HP+ but that is not an option. I just need this boat to be able to go 8-9 knots. Possible? How do you think this boat will handle? What top speed do you think I will be able to reach?

It might just... to get there prop selection will probably be critical.

I'd expect you to manage 4-5 knots with no trouble at all - that will be the hull's maximum "displacement speed". After that the boat has to lift out the water to start planing. I think 10HP would be enough to keep you on the plane, and to maintain 9 knots but the question is can it get you over the hump necessary to get on the plane. This will be very sensitive to how the engine is trimmed, how the boat is loaded (how many people and how much stuff as well as how it is distributed).

It would manage it easily with 20HP (and probably get a top speed of something like 18-20 knots (I can do 22 knots with a slightly smaller / lighter boat and 20HP 2 stroke). I think you'd manage OK with 15HP, and with some skill and playing around lightly loaded with 9.8 HP you might just get away with it.

[Polwart]

Oh - some supplementary thoughts

(1) if you are sitting in that struggling to get on the plane mode then your fuel consumption might be quite high (e.g. a 15 or 20 HP engine sitting at 10-11 knots will probably be burning less than a 10HP at full throttle struggling to manage 9knots).

(2) at this size, and with forward/aft weight distribution (and weight in general) you are more likely to succeed with a 2 stroke than a 4 stroke (because they are lighter and have better "acceleration").

[slowstang305]

Thank you Polwart for your quick response. Hopefully I have some time next week to put it into the water and see. I mainly bought it to cruise the waterways here in Miami and just need it to basically cruise at slow speeds while maintaining a quiet ride with good fuel consumption. I would have loved to get a bigger motor for it, I may just have to in the future. Honestly, 4-5mph is pretty good for me would just love to see this thing plane up.