Petrol inboard RIB why not...?

jonoritchie123 · 20 July 2024, 15:35

Hi,

I use a RIB up the west coast of scotland and Northern Ireland. I have the option to purchase a 7.4m RIB with an inboard petrol engine.

A diesel inboard makes sense because of fuel availability up here and efficiency at cruise but Turbo lag can be less user friendly than the petrol equivalents.

My question is - why no RIBs with petrol inboards?
just because outboards are easier to work with?
space saving? - this is a fair point on a 7m
I have read and seen the explosive risk of petrol but why is that different for an inboard to an outboard? .

The engine is a mercruiser 5.7l 350 Mag(300hp) so she's certainly go. Fuel efficiency looks like 35lph at 29 knots, 70lph at 42 knots.

Most of my trips will be trailering to north coast ireland, 60 -150nm round trips for a day or weekend. some free diving, fishing and family rock climbing on the islands of Scotland. So not huge amount of hours per year but short on time.. hence diesel sems less of a big concern for me

Am I mad to go for an inboard petrol?

Thanks

Pikey Dave · 20 July 2024, 16:38

Our first RIB had an inboard petrol. An OMC V6 4.2litre Cobra. We had it 10 years before it succumbed to corrosion & the sump rotted out. Great while we had it & this was the time before 4 stroke outboards came on the scene. So at the time it was relatively economical/quiet/smoke free. It was a pig to work on due to the confines of the boat (it was a 6.4 Osprey). I wouldn’t have another, due to the weight & space that it took up and the aforementioned maintenance issues. In it time it was the dog’s danglies, but it’s had its day.

beamishken · 20 July 2024, 19:42

I've had quite a few inboard boats, mostly diesel & the maintenance is not to be underestimated, the drives are the Achilles heal & can lead to some big repair bills if your not doing the work yourself. Inboard ribs dont handle like outboard boats there heavier & feel less alive & that's not just because of turbo lag, even petrol inboards are less agile than outboards. Add to the mix that petrol engines are usually raw water cooled giving them a life expectancy of 10 -15 years before they rot out & often replacement manifolds every 5 years or so the maintenance starts to add up. Diesels can also be costly & replacements are hard to find second hand & new packages are extremely expensive if the worst happens & you burst an engine (north of 40k for 200hp)
A few years ago we did some trips in company with a couple of friends they had a single engine regal 2150 21ft cuddly cabin sports boat with a single 260hp petrol mercruiser, we had a searay 290 sundancer 29ft 4 berth cruiser with twin 300hp diesels & when we did the sums the twin diesel boat was running cheaper per mile than the single petrol boat.
Petrol filling facilities dock side in scotland & Ireland are a rare luxury, diesel is far easier to find. A petrol inboard has all the disadvantages of inboard & none of the benefits of a diesel. In short if I was looking for a boat in that size it would be an outboard every time unless I wanted the fuel availability of diesel

jonoritchie123 · 20 July 2024, 22:20

Thanks, the vessels was a race boat so only its in very clean condition, stored inside so no corrosion risk yet but good points.

jonoritchie123 · 20 July 2024, 22:21

Thanks!

beamishken · 20 July 2024, 22:50

Quote:

Originally Posted by jonoritchie123

Thanks, the vessels was a race boat so only its in very clean condition, stored inside so no corrosion risk yet but good points.

It doesn't matter where the boat is stored they rot from inside out a raw water cooled engine is pumping seawater around the block which rots the engine away from the inside, external condition can look perfect but the block & heads corrode away unseen

willk · 21 July 2024, 09:04

Quote:

Originally Posted by jonoritchie123

I have read and seen the explosive risk of petrol but why is that different for an inboard to an outboard?

Am I mad to go for an inboard petrol?

Because it's not the engine that's exploding, it's the petrol vapour in an enclosed space. Watch this:

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

Voodoo · 22 July 2024, 13:38

I have a Ribtec 7.4 with an inboard petrol. Mercruiser 350 Mag on a Bravo Leg. its a joy to drive and always feels safe and planted whatever speed we are doing. plenty of weight in the back makes sure of that. Had it 4 years now and still love taking it out...we even go shopping in it sometimes rather than drive to a supermarket...just because we can I guess..

jonoritchie123 · 22 July 2024, 20:48

Quote:

Originally Posted by Voodoo

I have a Ribtec 7.4 with an inboard petrol. Mercruiser 350 Mag on a Bravo Leg. its a joy to drive and always feels safe and planted whatever speed we are doing. plenty of weight in the back makes sure of that. Had it 4 years now and still love taking it out...we even go shopping in it sometimes rather than drive to a supermarket...just because we can I guess..

Wow, thats fortuitous! thanks for posting.

im looking at a osprey vipermax with the same engine and bravo duoprop.

Could i ask what top speed and cruise speed do you get and a rough fuel burn?

Have you had any issues with the fuelin bilges/do you have blowers to vent the area?

Do you have a rough maintenance cost vs outboard? im not going to do 100s of hours every year so i wasn't too concerned.

thanks

John

Voodoo · 23 July 2024, 08:29

Hi John, PM sent and a couple of pictures to your phone.

Nos4r2 · 24 July 2024, 22:44

Quote:

Originally Posted by jonoritchie123

Wow, thats fortuitous! thanks for posting.

im looking at a osprey vipermax with the same engine and bravo duoprop.

Could i ask what top speed and cruise speed do you get and a rough fuel burn?

Have you had any issues with the fuelin bilges/do you have blowers to vent the area?

Do you have a rough maintenance cost vs outboard? im not going to do 100s of hours every year so i wasn't too concerned.

thanks

John

I have the same engine and drive in a hardboat.
Check you've got access to the starter motor without taking the exhausts off or repairs will get expensive.

Kristoff · 29 July 2024, 09:44

I don't see many (any) valid reasons to go for inboard (sterndrive) petrol engine in a rib these days.

If you absolutely need a clear transom for some sort of work related purpose, maybe, but then if its a work boat why not a diesel?

Modern 4 stoke outboards are cheaper to maintain, and thus more reliable, because the maintenance actually gets done. And ultimately, reliability is what you want.

Voodoo · 29 July 2024, 12:14

I think it all comes down to a mix of personal choice, fuel availability, inboard / outboard / petrol / diesel they all have plusses and minuses. Easier to build outboard boats (I'm guessing) just hang a lump on the back, with or without a standoff bracket. With outboards you can lift them out of the water, but you have a top weight swingimg around at speed and some saddles aren't the best...although they have improved over the years, as have transom materials and design.....Inboards you have the sterndrive to worry about, but the weight is in a good place....like I said personal choice really and what is for sale at the time you want to buy..