70hp on an Avon SR4?

[Alex r]

Hi there, just bought my first rib, avon sr4, just doing some work to her before i take her out, hopefully in the next couple of weeks, only slight concern is the engine (evinrude 70hp) - slightly over powered - been told as long as i am sensible it will be fine, the transom will be OK, good condition, any advice would be greatly appreciated, thanks alex

[Mextli]

Hi
70 HP on a SR4 with the original transom will NOT be fine. Besides, you have no idea of telling(exept drilling) if the transom is slightly wet.

[Alex r]

Thanks for your useful input

[willk]

Welcome to RIBnet Alex r, some of the SR4 nutters will be along shortly to assist. They know their stuff - have a think about their replies. I've moved your post to increase it's exposure a bit.

w.

[willk]

Originally Posted by willk

...some of the SR4 nutters will be along shortly to assist...

...or maybe not!

You've communicated to me by PM that your engine is a 1981 and possibly well under 70hp in actual output. No harm, 70hp is a lot to dial into an SR4 - they can get dangerously squirrelly with a 50hp, being prone to chine walking at higher speeds.

Older SR4s can have issues with weakened/rotten transoms. Applying a lot of power through one such as that will likely end in unhappiness. I presume you've had your transom checked for wet timber/movement?

Obviously we'd all be interested to hear how the 70hp Evinrude runs on your RIB.

[matt h]

A older 70 is overweight and overpowered, not a good recipe dude.personally i wouldn't.

[breezeblock]

Originally Posted by willk

...or maybe not!

You've communicated to me by PM that your engine is a 1981 and possibly well under 70hp in actual output. No harm, 70hp is a lot to dial into an SR4 - they can get dangerously squirrelly with a 50hp, being prone to chine walking at higher speeds.

Older SR4s can have issues with weakened/rotten transoms. Applying a lot of power through one such as that will likely end in unhappiness. I presume you've had your transom checked for wet timber/movement?

Obviously we'd all be interested to hear how the 70hp Evinrude runs on your RIB.

excuse my ignorance but whats chine walking

[beerbelly]

going to be a crazy fun boat until it goes wrong tho . seriously I wouldn't be running it flat out if It was mine it may out perform the hull and turn into a bit of a handful .steady away and it may be fine

[matt h]

Originally Posted by breezeblock

excuse my ignorance but whats chine walking

When at speed it jumps left to right. with that hull design it generally can start at 36 knots if weights and balance dictate.
an ideal weight and engine size is a lightweight 40hp - 50hp max

[breezeblock]

Originally Posted by matt h

When at speed it jumps left to right. with that hull design it generally can start at 36 knots if weights and balance dictate.
an ideal weight and engine size is a lightweight 40hp - 50hp max

ah gotcha had that on my old brig I think it was quite bad if the motor wasn't trimmed up enough

[SimonStDavids]

Hi Alex,

Was that the boat that was on eBay?

I don't know the weight of your 70 but I had a yam 55 on mine for a couple of years and with everything heavy forward it was pretty well balanced and that was one heavy lump!!

Have to agree with the others though. They are awesome boats but a little hairy after 30 odd knots and a 70 is going to push it VERY fast. As mentioned above they chine walk a bit in certain conditions and a fast stern heavy setup is going to make that worse [emoji22]. Use it sensibly and I think it will go ok and have awesome acceleration, and if it was mine I would try it out see how it goes and keep an eye out for 50. It wouldn't cost much to trade in your 70 for a nice yamaha 40 or 50. I know it is the cliche combo for the SR4 but now having finally got a 50 with ptt I can see why.

I think someone had a 60 on one few years ago but can't remember who or how it went? Think they ended up with trim tabs though to make more stable[emoji51]

[Mollers]

That old nail will be heavier than a planet, don't.

[Nos4r2]

Yep, I wouldn't either. My SR4 hit warp speed with a 50hp. You don't need 70hp, it's way too heavy and too much for the transom.

Besides, it'll drink fuel for no usable extra performance.

[Alex r]

Hi there again, thanks for all your posts, sorry for my sharp response, as regards to my rib i know 70hp is a bit much, weight wise it's about the same as a yam 50hp 4 stroke, mines probably got less hp than the 70hp, still doing some work on the boat before it goes on the water, see how it handles? was planning to take it safe, i will take your advice on looking out for a 40/50hp (any help there would be great) - either sell my set up (runs well for an old un) or keep on to it for my next (larger!) rib, again thanks again for your help, alex

[jonp]

Originally Posted by Alex r

Hi there again, thanks for all your posts, sorry for my sharp response, as regards to my rib i know 70hp is a bit much, weight wise it's about the same as a yam 50hp 4 stroke, mines probably got less hp than the 70hp, still doing some work on the boat before it goes on the water, see how it handles? was planning to take it safe, i will take your advice on looking out for a 40/50hp (any help there would be great) - either sell my set up (runs well for an old un) or keep on to it for my next (larger!) rib, again thanks again for your help, alex

As long as its not over the manufacturers maximum recommendation I would go with as much power as possible, you don't have to use it all. If outside the manufacturers max you could end up in trouble if an accident happens and someone gets injured.

Being able to run at lower rpm and still maintain good cruise speed will be far better on the fuel consumption.

If you carry heavy loads such as a few mates and dive gear like I do and have to cross bar crossings which are often extremely dangerous then the more power you have to sit on the shoulder of a big wave the better. Power can be your best friend working big seas. If you look at the swell in this shot in-front of our river entrance you will note a solid 2.5m swell with almost flat conditions once you pass the shore break. Trying to cross these bars without enough power brings many a boater undone.

If you find its to much power theres quite a demand over on the worldseafishing forum (UK) for engines around the 70hp.

[willk]

Originally Posted by jonp

As long as its not over the manufacturers maximum recommendation I would go with as much power as possible, you don't have to use it all. If outside the manufacturers max you could end up in trouble if an accident happens and someone gets injured.

SR4 is rated for 50hp, AFAIK?

[Alex r]

Ok, any idea's for a 50hp, best places to look? Price's etc

[spartacus]

Originally Posted by Alex r

Ok, any idea's for a 50hp, best places to look? Price's etc

Couple of 50hp engines here:

2005 Tohatsu long-shaft 2-stroke
Tohatsu - M50D2 3E3 Boat Engines for sale in Devon, South West | Boats and Outboards

Mid 90s Yamaha long-shaft 50hp, power-trim
https://www.gumtree.com/p/boats-kaya...ed./1197813516

[Mextli]

Weight and power wise a Yam 55 will be cool too. I'm running one on my SR4 with no problems concerning speed and seakeeping.
I've increased the thickness of my boats transom when I re-did all the wood though. A friend of mine owns an original 4M, with a 50HP engine as well, and his transom flexes slightly during sharp acceleration.

All Seariders like light engines more than heavy ones., weight limit is 110 KGS on the 4m, but that much weight will upset the balance and make the boat handle less than optimal.

[Alex r]

What would be the optimum transom thickness required for a 50hp +engine, thanks alex