Repowering 7.5m rib

[747sp]

I am considering the purchase of a 7.5m RIB from a rescue service. I am a Rib newbie, although I have some basic boating experience learned in my teens (quite some time ago). I am going to do my Powerboat 2 when/if I get the boat.

I'm afraid that if this goes ahead I may have many questions. Up first is the choice of possible power.

I plan to use this boat on the Alde / Deben / east coast. I live in Kent and may also use it in the Thames estuary. It will also get towed to Mallaig every now and again as we have use of a house in Knoydart.

Said rescue service seem to have wanted redundancy and inversion abilities in their choice of two 2 stroke engines. Initially, I was thinking one big 4 stroke (?150), avoiding the problem with mixing fuel etc.

I'm told two engines actually only produce about 75% of the combined HP (presumably because they are sharing a relatively small area of water?). Increasingly, I am finding twin engines attractive as it will obviate the need for a back up and the boat was designed with two in mind.

So my question is - given the weight differential will my potential boat carry two 90 HP Four strokes?

[Nos4r2]

What's the boat? It makes a difference to how much power you need.

2x90hp should be fine, but there's a specific rating for most boats, and it probably won't go very well with a pair of 90s. I'd personally be looking at a single 200 or maybe a pair of 150hps.

[747sp]

It's an Atlantic 75.

[Xk59D]

Why do you want twins?

Most will say get a big single and an aux, it is a better setup for the majority.

Unless you have twin batts, twin fuel tanks and fill them up at different petrol stations you don't have proper redundancy anyway.

Twins also cost more to run and service so give it thought.

A single 200 would do me if Hull is rated to that

[oracle]

I've got a pair of 90hp 2 strokes on my 7m and she goes pretty well.

30 knots with 8 divers and lots of cylinders aboard (which is what she's propped for, grunt not top end)

I still get 35 knots when normally loaded, though.

[Nos4r2]

Personally (and I'd love an Atlantic/Pac/Arctic because the seakeeping is amazing) I wouldn't have an Atlantic of any size for a leisure boat without at least a 200 on it, and particularly not if I was going to tow it any distance on a trailer unless I had a serious tow vehicle.
IMO you'd never break one,but there's a trade-off for that. Heavy, thirsty and slow for their size without huge motors on.

[Last Tango]

I take it that it doesn't have any engines on it just now, because if it has, the price of replacing them would buy a hell of a lot of petrol?

[Iron Dials]

The Atlantic 75 was specifically designed as a twin engine boat, I'd be slow to reconfigure it without the benefit of experience from people who already have done it- which of course is why you are asking the question I suppose.

As for the twin versus single engine debate. More drag with 2 legs in the water and yes 75 x2 is not equal to a single 150.

The boat is already set up to support 2 engines independently and if you're looking at operating it in lonely areas where backup is scarce (without calling for another Atlantic 75 for help) then twin engines seem v attractive to me.

Its the minority view but machines do break down. So to me it depends on your intended use.

[747sp]

I'm thinking the big single 4 stroke route for some reason will be more reliable, cost less to run and will be less hassle (fuel mixing / fuel economy). This boat could be parked up for weeks at a time between trips.


If im going to have to take a small Aux engine anyway who don't I stick with the way the boat was designed and go with twins?


I note NoSr2 has suggested 2 x 150's - which suggests you think weight won't be an issue.


Interestingly, the next generation Atlantic 85 was pushed along by twin 4 stroke 115's

[Tim M]

Twins every time if you can justify the additional expense of fuel and servicing. I've driven a lot of twin engine boats, and have experienced a fair number of engine issues, but never one caused by dodgy fuel (good filters needed whatever your setup). The reassurance of simply being able to switch one off, trim it up and head back in to check it out is a massive bonus, particularly when working commercially, or in a really busy or dangerous stretch of water. People bang on about separate fuel tanks and filling up at different petrol stations: I have never had an issue caused by the fuel that was put in. A lot of vessels share a single fuel tank. I worked on a large vessel (triple 2500hp engines) which had a single tank - lots of big fuel filters but never saw any issues on that front.

Stick with twins if you can!

[747sp]

OK - thanks very much for all the input.

I've searched the web for something about the Atlantic 75's Hull Rating but have drawn a blank.

Assuming I go for the extra expense of twins - I really need to know if there is some formula to work out whether the boat will take the weight of two four strokes - as per the bigger Atlantic 85.

Put it another way - given the additional weight of fours stroke engines - how much more power would I need to push it all along?

115 / 130 / 150?

[Poly]

Initially, I was thinking one big 4 stroke (?150), avoiding the problem with mixing fuel etc.

mixing fuel has never struck me as a particularly challenging task. That said many 2 strokes do it for you anyway.

I'm told two engines actually only produce about 75% of the combined HP (presumably because they are sharing a relatively small area of water?).

yes, and no. Search for posts on twins by 9d280. A simple rule of thumb like that falls down depending on the engines you are comparing! Some might be better than that, some might be worse.

Originally Posted by 747sp

I am considering the purchase of a 7.5m RIB from a rescue service. I am a Rib newbie, although I have some basic boating experience learned in my teens (quite some time ago).

However, I am going to suggest that for the majority of RIB newbies, an ex rescue service atlantic is probably not the best boat you can waste your money on! They are designed with one very clear thing in mind, and that is not what most leisure users want/need. If you wanted a camper van would you buy an ex ambulance?

[747sp]

Despite the comment about my pride and joy being, basically, an ambulance i've gone and done it. no doubt the comment about this being a money pit will be more than accurate. I have been surfing the 'tinternet for a couple of engines - any suggestions as to dealers - there don't seem to be many about. I found Bill Higham. I'm beginning to think its a little like bic pens. Where DO all the used outboards go?

[Wightdiver]

Originally Posted by 747sp

Despite the comment about my pride and joy being, basically, an ambulance i've gone and done it. no doubt the comment about this being a money pit will be more than accurate. I have been surfing the 'tinternet for a couple of engines - any suggestions as to dealers - there don't seem to be many about. I found Bill Higham. I'm beginning to think its a little like bic pens. Where DO all the used outboards go?

Try Solent Outboard Exchange

[747sp]

many thanks...

[willk]

Originally Posted by 747sp

Where DO all the used outboards go?

Unlike the Bics, the outboards do NOT slip away to a parallel outboard dimension to do outboard oriented things. They get wrecked by numpty owners, neglected and forgotten - eventually to be broken for parts.

Bics have it handy.

[GuyC]

Originally Posted by Poly

If you wanted a camper van would you buy an ex ambulance?

Yep.

Congrats on the boat purchase.

I'd go for the twin set up if I could, primarily for the redundancy it offers in remote areas.

[747sp]

What a superb Camper Van.

[GuyC]

I hasten to say, it's not mine but ambulance motor home conversions can be pretty good.

My non-ambulance conversion is attached.

[747sp]

I like the look of that cover - where can I get one for my Boat? While i'm on - how would I decide about the length of shaft that's appropriate? Yamaha seem to do engines for at least 4 different transom heights....