Overpowering a RIB

Looking at getting a RIB for diving and quick missions etc. I have my eye on an Aquapro 3.2 alloy RIB which is rated for 15hp.
I have a 30hp 2 stroke on the back of my big boat as an Auxillary that has had bugger all use and was thinking maybe I could run it on the RIB to save going out and buying another outboard.
Weight wise, the 30 2 stroke weighs about 58kg, a four stroke 15 weighs around 52kg (Mercury 15hp 4s) so I don't think that an extra 6kg on the transom is going to break the bank?
Power and torque will be considerably more than a 15hp 4 stroke but surely "not being an idiot" can mitigate any extra stress on the transom?
OK idea/shit idea?

MAYBE change the prop to a less aggressive pitch?? Will slow it down but give more grunt on big boat.

15hp 2 stroke Mercs are reasonably cheap and certainly go well.

what about just buying a slightly bigger RIB ?

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Originally Posted by dannyb

what about just buying a slightly bigger RIB ?

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A 3.2 with the pontoons deflated is about as big as can go and still just cart it round on back of the ute not have to stuff around with trailers etc

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Originally Posted by Micky Duck

15hp 2 stroke Mercs are reasonably cheap and certainly go well.

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Big boat actually had a 15hp Merc on it as aux when we got it but with the boat loaded up etc it struggled a bit to push it round if there was decent current or strong chop and waves etc so we put the 30 on it to make it a bit safer if it had to be used in the event of main dying

All the surf lifesaving boats run 30hp outboards there around 3.6m from memory, For a boat that would roll up into a bag once you pulled the floor boards up we never had any transom issues

Steve Schmidt designed and built some of the first RIBs... the Naiads built in Marlborough. Fibreglass at first. The original prototype was 2.5m that he used as a plug for the mold... but he wanted to test it

It had a 25hp on it because that is what he had on hand. It sat bum heavy in the water.

He let me take it out for a ride on Picton harbour. Being a little gung ho, when my confidence had built I opened her up ... and it started to chine walk violently, did a 180, there was a huge splash and stopped dead in the water because I had let go the throttle and was laying at the bow half in half out.... luckily enough still right side up.

Caitiously returned to shore. Steve was taller than I and could wedge his elbow into his hip and keep the engine from wobbling if it started walking. Smart as I thought I was, I found a 2 foot piece of heavy plastic pipe that fitted tight over the throttle and gave me more leverage.

"Round two" Throttle wide open, chine walked, big splash and in an instant I was in the water. This time however the throttle was held half open by a bent piece of plastic pipe and the little Naiad was doing 15mph sweeping circles around me in Picton harbour. At about its third or fourth pass I latched on to the life line... stretched my arm and ended up back in the boat.

On another occasion I have been belting along in an over powered aluminium pontoon dinghy, travelling way too fast, a small gust of wind caused the boat to lift up completely lout of the water, the bow had gone past vertical above me, We dropped a little and the transom hit the water while we were still moving forward and the bow crashed down to to the surface and I was laying battered and bruised in the bow... aluminium can be quite hard.

I would not grossly overpower a tiller steer dinghy. 15 hp up to 20 hp maybe but not 15 to 30, because ... 1) the weight makes the bow much higher as well as the stern lower, which make climbing in and out awkward and 2) you will be tempted...

Thanks for the daily laugh.... The tempted thing is why I suggested changing prop....so it doesn't have ability to push it faster than the 15 would.,which is probably fourty KMPH anyway.

My son runs my IBC [Tauranga] RIB 3.8 with a 30hp Yamaha. Simon retubed it and made some mods to it including installing a rocket launcher. My son sneaks into a honey hole in Tauranga Harbour... too shallow for usual boats. Fills up on the incoming tide. Did that on Saturday. The alternative if you want to spend is an Arancia. Nice boats. Mate bought one new and did all of Northland from Tutukaka to Kaitaia. Bloke on YT has a 3.6 which he goes all around Fiordland on it and Rakiura.

https://www.youtube.com/watch?v=UYMo64MpgEU

Could you just limit the amount by which you can open the throttle?