Engine for a Cobra 5m?

selkie72 · 07 March 2012, 20:04

I have recently bought an old Cobra 5m RIB that was advertised locally here in Shetland. The boat itself is in remarkably good condition for it's age and hopefully should only need some repairs to the gel coat, cleaned up and painted before it is ready for a new engine.

As the transom plate was missing I contacted Cobra RIBS who informed me the maximum outboard for this model is 90hp. I was thinking more along the lines of 70hp.

Any advice on what the best engine and horsepower for a RIB of this size would be?

I intend to use the boat for inter island / offshore cruising around Shetland.

Cheers, Ryan

Xk59D · 07 March 2012, 20:10

70hp should be good on that setup if you have your eye on one.

generally speaking your better with the max hp/weight rated for your boat as it is much easier to knock the throttle back than get 90hp from your 70hp :-)

cheers

9D280 · 08 March 2012, 08:56

Only other thing to watch is the max transom weight. (and remember to add any Aux you might want)

Mine for example is rated for 60Hp. Problem is if I hang a random 4- stroke 60 on the back It will likely exceed the 110Kg weight limit...... Subtracting approx 10-15 Kg for an aux limits the choice of main engine somewhat!

And that, in a nutshell, is why I have a floating museum!

As you are probably aware, most outboards are built in groups of two or three. (e.g the Suz DT25/ 30, Merc Clamshell 50 / 60), so it's probably worth trying to get the catalougue for the 70 / 90 you are looking at - you may find for a given manufacturer they are the same physical lump so the 90 is a no brainer. Likewise comapring one brand to the other, for one the 70 might be the high Hp of it's 50-60-70Hp group, the other it's the low end of a 70-90-100Hp group, which means you end up lugging lots more Kg around for your HP.

If you are shopping around for a used engine you are more likely to be limited by what you can find, but it's food for thought.

selkie72 · 08 March 2012, 14:05

Thanks for the advice. As the Cobra is an old model I expect the 90hp rating was for a 2 stroke engine which, if I remember correctly, weighs about 120kg (I used to drive an Avon SR 5.4m + Yamaha 90hp 2 stroke for my work back in the 90's). The Cobra is quite beamy aft though, so should be able to take quite a heavy engine.

I've had a look at the specs. for the latest Honda and Yamaha engines. The Yamaha F70A weighs in at 119kg and the Honda BF75 is a lot more at 165kg, about the same as the BF90. All the online reviews I've seen for the Yamaha F70A are positive so that's a possibility. I've also just seen an advert locally for a 6 year old Honda BF90 for £2500 + VAT. Must be a from a commercial operator but might be worth a look?

Please let me know if anybody has any more thoughts on the above, will be an expensive decision, so I don't want to get it wrong!

clydeoutboards · 09 March 2012, 13:05

Hi

we are honda dealers and could do you a deal on a 90 ( same weight and only a few hundred pounds more than the 75 or we havea a 60, 70, and 90 2 strokes for sale. please call me on 0759 563 8185 if interested.

Dave

Daibheid · 12 March 2012, 00:06

I've owned two cobras under 6m, I'd watch the engine weight as you are quite right to be wary about 4stroke over 2 stroke engine weights. That said they do have a lot of buoyancy because of the wide beam so moving the battery and fuel tank(s) forward could help a lot and keep your anchor and chain in the bow under way..
I've always like the Honda 90 but I think it's heavy for the output and you might find a Yamaha F80 a better bet.
You'll find the Cobra very well built, very seaworthy and the tubes last extremely well with a little care and a lot of covering when off the water.

clydeoutboards · 12 March 2012, 07:41

Quote:

Originally Posted by Daibheid

I've owned two cobras under 6m, I'd watch the engine weight as you are quite right to be wary about 4stroke over 2 stroke engine weights. That said they do have a lot of buoyancy because of the wide beam so moving the battery and fuel tank(s) forward could help a lot and keep your anchor and chain in the bow under way..
I've always like the Honda 90 but I think it's heavy for the output and you might find a Yamaha F80 a better bet.
You'll find the Cobra very well built, very seaworthy and the tubes last extremely well with a little care and a lot of covering when off the water.

Hi small point but the Honda 90Hp is lighter than yamaha 80hp and has BLAST and has a 6 YEAR WARRANTY and is the only engine of that size that has a 2 year commercial warranty ( the yam has only 1), the big downside with the Honda is that good 2nd hand ones are v hard to get. lots of ex commercial ones with daft hours on them so be carefull.

Dave

selkie72 · 12 March 2012, 12:39

Thanks again for your comments. I reckon the Yamaha F70 would be the best option. At 119kg it's only 15kg heavier than the Yamaha 70hp 2 stroke that was originally on the boat. Will post some pics of the boat once I've carried out the minor repairs to the gel coat and given the hull a coat or two of paint.

Daibheid · 12 March 2012, 13:51

Quote:

Originally Posted by clydeoutboards

Hi small point but the Honda 90Hp is lighter than yamaha 80hp and has BLAST and has a 6 YEAR WARRANTY and is the only engine of that size that has a 2 year commercial warranty ( the yam has only 1), the big downside with the Honda is that good 2nd hand ones are v hard to get. lots of ex commercial ones with daft hours on them so be carefull.

Dave

@clydeoutboards: Thanks for the correction I bow to your greater knowledge and I'd hate to mislead someone - I've owned two F100s but never an 80. I've also had two BF90s and would highly recommend them on most counts with one possible exception that only applies if you're towing boards or skis. They will not take large throttle openings from idle and will bog and stall every time. I think US emissions settings caused it and I can only talk of engines from early noughties with carbs. I expect fuel injection has sorted that but selkie may be looking at my sort of vintage engines. You're dead right about trying to find a good used one. Those Honda 90s and the Yamaha 100s seem very popular internationally with small inshore fishing boats and can run up starship hours with little visual evidence. Verifiable history or a solid warranty is essential!

@selkie - I'd say you'll be on a winner with that engine if you can get one reasonable. I had use of a Valiant 4.9 with an F60 for a month and it was an excellent performer with loads of performance and amazing economy. The extra 10hp should cover the extra size/weight of the Cobra.

selkie72 · 19 April 2012, 06:51

Got a great deal on a one year old Honda 90hp 4 stroke, engine has only done 18 hours! Engine fitted and ready to go, doesn't look too big on the wide transom of the Cobra. Hopefully the weather will be reasonable this weekend for blast off!