RIB with jet turbine

[rapanui 2]

I want fit a jetski motor with turbine on a RIB,I am looking for reliable engine (rotax?) simple no turbo or injection what brand of jet will perform in the 120 HP range, the rib is a very light deepvee 6 m all carbon fiber (fibreglassing is no problem zone for me),why? sleek form,low draft,safer to keep long in the caribbean(no target for thieves).what is the best jetski for my goal?

[a64pilot]

Most any will work, by turbine I assume you mean the jet pump?
Jet drives are inefficient and require large motors to achieve the same result as an open propellor, jet skis have to go that route of course for safety.
You will lose a huge amount of interior space with motor

[rapanui 2]

what power jet drive equal a 90 hp propeller outboard motor?

[tkeithlu]

19 foot jetboats use about 140 HP.

I'm concerned, although you didn't ask, about the engineering here. Is that RIB hull designed to take a jet drive?

[Training Wheels]

Might be easier just to get a jet drive outboard.

[jamhass]

Or just use a gas turbine.

I saw a gas turbine jet ski in Oakland CA. Beautiful installation, and its not the only one in existence!

[Dave_S]

I think if you are going with a jet you might need a larger diameter jet than a jetski has and more torque from the engine than a Rotax or a NA 4cyl engine might make.

My current ski has around 170hp, I don't think it would be as good as a 90hp outboard on a 6m lightweight boat.

My previous ski was a supercharged 4cyl. Terrible fuel consumption and around 300hp with lots of torque, purpose made engine but the jet diameter I think would be too small and I think the power V's drag on a larger hull will over power the impeller at low speeds.

Maybe talk to a jet boat shop rather than a jetski shop they might have more suitable knowledge.

[Bean Counter]

We used to have a Sea Doo Jet Boat, a Sportster with single 85HP two stroke Rotax. It Was 14ft fibreglass and carried 4 people on the plane.

It was not particularly fast, less than 30 knots.


The RIB Jet Brand has something like a 60HP petrol Rotax jet in it. There is even a diesel jet version - again not very fast but acceptable for big boat tender purposes. The other jet Rib to look at is Williams and Avon.

[Pete7]

Originally Posted by tkeithlu

19 foot jetboats use about 140 HP.

I'm concerned, although you didn't ask, about the engineering here. Is that RIB hull designed to take a jet drive?

Just picking up on this comment, how do you plan to go from a deep v hull to have an area were the grate will fit to pick up the water? There will also be a loss of stern buoyancy at rest when you fit the jet.

Presumably you want petrol as a fuel, but can you buy a carb petrol engine in that hp either new or in mint condition? they will all be FI now surely?

Pete

[rapanui 2]

thanks for all the reply, the hull will be specially designed for the water intake, mainly to cut the bottom of the deep vee,do not forget that combination is made for 2 goals, less risk to be stolen in the carib and low draft and a sleek shape,currently I have a 150 hp yam 2 stroke very reliable engine but top prey to be stolen in the caribbean (as the best engine ever built!!)

[HankOnthewater]

Maybe pose the initial question on this forum:
https://www.rib.net/forum/

Maybe the process could be:
- how heavy is the total setup (loaded)
- how fast you want to go, in what seastate
- and then decide (hopefully by some kind of calculation), how much thrust you need, and then how many horses you need
My gut feel is that 120 Hp with jet drive might not be enough for weights over 1200 kg and speeds over 30 knots

[rapanui 2]

thanks for reply, actually my 7,5 m rib weight 735 kgs including 150 hp 2 strokes yamaha,empty no fuel, actually I reach 48 knots full speed i do not need that speed,35 knts will be perfect,and the hull will be reduced in lenght to be carried on the boat at 6 m ,I think the hull will be lighter because no heavy reinforcement at the back wall,do you know what power loss on a jet than a outboard engine?

[HankOnthewater]

Originally Posted by rapanui 2

thanks for reply, actually my 7,5 m rib weight 735 kgs including 150 hp 2 strokes yamaha,empty no fuel, actually I reach 48 knots full speed i do not need that speed,35 knts will be perfect,and the hull will be reduced in lenght to be carried on the boat at 6 m ,I think the hull will be lighter because no heavy reinforcement at the back wall,do you know what power loss on a jet than a outboard engine?

Hmmm, no I do not know how to calculate the required HP. All I know is that I used many types of Ribs in the size from 3 to 11 mt and HP from 3 to 900 HP. Only one of them had jet propulsion (with x2 400 HP yanmar diesels).

You are right, 30-35 knot top speed is fast enough as faster than that in rough seas will be very hard on the boat and on the bones of the crew.

A jet drive is heavier than a plain outboard, with this weight penalty plus less efficient drive, that makes an estimated 10-15% lower performance, but don't quote me on those figures. I am not a naval architect. Just an estimate.

With a 20% drop in HP (150 to 120 HP) and an estimated 10% drop in efficiency, that makes 30% lower performance, that might give you then just over 30 knots..... if the figures are correct.

[Dave_S]

Originally Posted by rapanui 2

thanks for reply, actually my 7,5 m rib weight 735 kgs including 150 hp 2 strokes yamaha,empty no fuel, actually I reach 48 knots full speed i do not need that speed,35 knts will be perfect,and the hull will be reduced in lenght to be carried on the boat at 6 m ,I think the hull will be lighter because no heavy reinforcement at the back wall,do you know what power loss on a jet than a outboard engine?

I could very easily be wrong but I believe you will need more hp than you think and you may be headed for disappointment. By the time you load all your gear and people in the boat, I think you will fall short of 35kn. From my experience just as a jet ski rider not a professional and looking at HP ratings Vs performance of skis I've riden, I think there must be greater efficiency losses through the jet than is being allowed for.

If your 2.5 times or more the weight of a 140hp ski with a max speed of that ski is 55kn I don't think it looks good for you to reach 35kn.