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Old 05 April 2002, 12:30   #1
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Country: USA
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Rib Engine sizing help!!

Hello there from Oregon, USA!

RIB use here is very limited, and information is hard to come by, so I would appreciate if some of you could give a piece of advice on adequate engine sizing for a RIB I have just purchased. The RIB I just purchased is a 5m Polaris, used Canadian Rescue boat, without engine or console. It is common practice here to operate boats with tiller handles to provide more deck space, and that's my current intent. What size engine should most likely be sufficient to get this boat up on plane with 6 loaded divers on board, including two tanks apiece? 6 divers with full gear and tanks will be VERY rare. Most of the time it will be 4 divers, maybe occasionally 5. I am not looking for high speed, simply enough power to get out of the hole, and on top of the water. I am still in debate on the 2-stroke 4-stroke issue, mainly due to power issues and the added weight of 4 strokes. I know that responses will be slanted due to differing boat styles, but a relative guess would be great. The boat is supposedly rated for a 90hp, but Polaris says that their current boats are only rated to 75hp. (I still haven't picked this boat up) Is a 75hp 2 stroke enough? Will a 75hp 4 stroke, with added weight, and supposedly a little less power work?

Thank you so much for your help! I appreciate any light that you could shed on this is.

Mike
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Old 05 April 2002, 12:46   #2
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Country: UK - Scotland
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Hi Mike

I've no idea of the answer to your question but I think your expression: "I am not looking for high speed, simply enough power to get out of the hole, and on top of the water", is just great.

Knowing the experience on this forum you should have some answers soon.

Regards

Keith (a two stroke for lightness man) Hart
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Old 05 April 2002, 13:06   #3
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Country: UK - N Ireland
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The price of gas where you would suggest a 2 stroke!
I previously used a 70Hp Yamaha on a 5m Tornado and it would easily get on the plane with 8 on board, albeit with no dive gear. Provided that you dont put too big a prop on I would think anything over 65HP should be OK, unless the Rib is particularly heavily built. One more thing, its going to be very hairy to drive with a tiller on your own with this sort of power...
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Old 05 April 2002, 13:52   #4
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Alan,

Thanks for the reply. I suppose that we are fairly lucky here that the price of gas is fairly inexpensive. Also, this boat is more intended for short trips, not extended touring, so the fuel isn't too much of an issue.

I imagine that this RIB will handle significantly different than other boats that are heavier. I have previously operated boats by tiller up to 200hp, but those are indeed heavier than this type of craft. (Common shallow river running boats here) I doubt that I will spend much time solo in this craft, as the main intent and purpose is so we can access more dive sites with our group of friends. Does a RIB typically skip around more so because of the lighter weight? Perhaps my best bet is a Mercury 75 as that model is easy to switch from tiller to remote. ???????

I wouldn't be against adding a small steering console, but it has been difficult to find something here in the states that doesn't cost a small fortune.

Thanks, and I still look forward to hearing others opinion on this matter.
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Old 05 April 2002, 15:29   #5
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Country: Greece
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Boat name: SUN KISS II
Make: Nuova Bat 9 Falcon -
Length: 5m +
Engine: Outboard Mercury 115
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If you intend,

to buy a new engine, I don't think that you must break the bank to buy a small console. I recommend this type of driving, since it's more comfotrable AND SAFER. Nobody's hands can hold still an accelerating 200 hp engine. Personally, I've used the tiller / remote on a 50 hp 2 stroke Yamaha and I think that I was NEAR my limit.
Any way if the console is an expensive issue, why don't you try to build one yourself (using plywood) ? It is not a difficult task.
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Old 05 April 2002, 16:00   #6
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I think you are right that a console would certainly be a much easier way to go, and most likely I will indeed do so.

With regards to the tiller though, I would have to disagree with you. In fact I HAVE run a 200hp with a tiller handle on many occasions. I have run boats professionally in Alaska waters constantly with a tiller, as do many others. The boats that we run here and in Alaska are generally aluminum hulled, most commonly referred to as "sleds". Some utilize props, some with jets. The hull designs vary from deep V to semi-v to almost flat hulled. The tiller handle is optimum for keeping boat space available to move around, and work.

The picture I've included is of a typical shallow vee boat with a 200hp jet with tiller. This would also be used with a prop, but of course the jet allows us to run up rivers in 4-6 inches of water. I have never had any problem controlling or "holding on" to a tiller while accelerating in any craft so far. Perhaps RIBs are not as stable, but that doesn't seem to be the general consensus, rather, the opposite seems to be held by most.

Anybody else out there have experience with higher power motors, tiller handled, on RIBS? Keep in mind that I am not after long runs to far destinations in heavy seas.

Thanks!!
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Old 05 April 2002, 17:34   #7
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Jets

Graeme

This would solve our prop situation!!!!!!!!!!!!!
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Old 05 April 2002, 19:18   #8
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Mike, first the bad news, you are not going to get six twin set divers in a 5m rib. I have only done it once, and ppls feet were hanging outside the boat, it wouldn't plane with a 55 hp engine and wasn't safe. We only went half a mile on a dead flat sea but not good news and never again.
Have a look at this thread which Rene and myself dicuss a 5.95m with a 115 yam 2 stroke which did take six divers in comfort and safety: http://rib.net/forum/showthread.php?s=&threadid=617 .
Four divers is a better option and close to my previous rib a Ribtec 5.35 which actually measured more like 5.20. fitted with a Evinrude 70 hp two stroke it took four divers and ran at 36 mph for 4 UK gallons per hour. Quite acceptable me thinks.

The console is a must. It makes an excellent steering position and if space is at a premium then have a stand up one with the fuel tank underneath to save space. Great for mounting GPS and Echo sounders on, I would not be without one even for diving close in. Indeed in the UK before Ribs started to become popular in the late eighties we put consoles into true inflatables. Suggest you have a look at some of the rib manufactures world wide for examples although there is one firm in the UK whoes name I have forgotten who make lots of consoles many for the larger Rib firms as a sub contractor.
The Merc 75 hp 2 stroke will run at 6 UK gallons per hour for 3/4 throttle. Hope this helps but see if you can get a slightly bigger boat say 5.5m with the 75 hp, there is a big difference. Shout if you want more info. Regards Pete.
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Old 05 April 2002, 20:25   #9
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Pete,

Thanks for the response!! I have sketched out the arrangements of tanks and gear, and it works great....as long as the divers don't plan on coming along. I know you are right about the six divers not being appropriate for this size boat, and about the only place it would happen is in this little protected inlet, that the dive site is about a kilometer off shore. (see how I used that crazy metric system there instead of our archaic US measurement standards? Impressive, huh?) In reality it would be more of a floating slow moving platform so we wouldn't have to surface swim if indeed I ever thought it would be ok, and then we'd probably only take 1 tank and leave the rest on shore.

As far as the console goes.. I've just found one that I will be putting into the boat. So that settles that.

I've also found the motor today. It will be a 90hp Mercury. (2 stroke)

As for boat size... Once again, the myth is shattered that size doesn't matter! As this is my first boat(RIB anyway), and used, and CHEAP, I got what I could. There's not much options here for used RIBS. Most are either tenders, or, well, that's really it. This RIB is in quality shape, and was used by the Canadian Coast Guard, so at least I feel comfortable with the quality. Indeed I plan on moving up to a larger RIB, most likely new, in the future, but I have to start somewhere. I am a licensed Master Captain, and I am going through my divemaster program (well, just starting) and will be using this small boat to scope out more dive areas with friends before making the move into full dive charter business. It has come to my attention that dive charters are much easier than the fishing charters that I have been running most of my life. Schools of fish move, dive sites usually don't.

Thanks for all the help folks, I'll post a pic later of one of the few RIBs in Oregon. I'm going to feel real unique in my Coast Guard Orange lifelined RIB!
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Old 05 April 2002, 21:36   #10
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Pete,

What will a Merc 90hp 2 stroke burn per hour? As I said, long runs are not really what will normally be happening with this boat. It's on a trailer, and most dive sites are fairly close to launches. Cost for fuel here is lower than there so I hear.

What is fuel cost in the UK? I've tried to convert US Gallon to UK gallon, as well as the US dollar to Brittish pounds, but I don't know if that is accurate. 1 US gallon of fuel here in US dollars averages about $1.40. Our gallon is about .83 of the UK if I'm not mistaken, and our dollar is about .7 of Brittish pounds sterling. So.... 20 UK gallons here would cost about 23.55 pounds. How does that compare to your neck of the woods?

I really enjoy all the information everyone has posted. Thank you all.
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