Rib Engine sizing help!!

[playspot1]

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

[Keith Hart]

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

[Alan Mckewan]

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...

[playspot1]

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.

[batfalcon]

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.

[playspot1]

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!!

[Jono Garton]

Graeme

This would solve our prop situation!!!!!!!!!!!!!

[Pete7]

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.

[playspot1]

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!

[playspot1]

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.

[Pete7]

Mike, the Merc 90 two stroke should be fine for the boat as its the same weight as the 75 hp, the difference is is probably in the carbs and ports.
Fuel should aslo be about 6 uk gallons per hour slightly less if your lucky because I doubt if you are going to use full power. A nice stainless steel prop will improve performance if you can guarrantee not to run aground (see previous posts on props) Not sure how big a US gallon is but, 1 uk gal is equal to 4.5 litres, so you will be using 27 l per hour at speed. Probably easiest explained by one red merc fuel can every hour or just under anyway. But please take two at least, with seperate fuel lines heavy cylinders and careless divers do not go well with rubber pipes. The merc will thank you if you have a good filter as well. Merc do one but the better option is a CAV glass bowl and filter that normally fit diesel engines. Your not supposed to use with petrol but they work fine and have a very fine filter and you can see the water or colour of the fuel. The standard engine one isn't really up to it and esily over welmed.
Additional fuel always useful when you have lost a diver and have to search for an hour. We learnt that lesson watching another rib in Northern Ireland many years ago. Ended happily after five hours when the divers where found 20 miles away Great to here your using metric, esp for diving the calculations are soooooo much easier for air consumption and pressure etc. Petrol here in the UK is 73p a litre at a garage and we saw 93 p a litre ! at a waterside pump over Easter. The reason its so expensive is 90% is tax
20 UK gallons will cost £65 and we can use 30 easily in a days diving, plus the 2 stroke oil. Running about 1.7mpg with the 150 Mariner at the moment.
Do please fit an echosounder its the only thing that stops you running aground and worth its weight in gold. Also gives you an accurate depth prior to deploying divers, and lastly it finds your wrecks. Lowrance or Eagle have an excellent rep with UK divers and just work year in year out. But choose carefully the more expensive the greatr the power and better bottom definition. Its al down to power of the unit and it really is quite suprising what the more powerful sets will show up. I have the Lowrance LMS 350a which is great and miles better than budget sounders. Hope this helps and safe diving, Pete

[Pete7]

Mike, here is the link I was looking for:
http://www.outhill.com/ its a bit slow and took a couple of attempts to load, regards Pete

[playspot1]

Ok, one last question and I'll be done with this thread! I promise!

As I have calculated, fuel is is roughly 1/3 the cost of where most of you are. (more or less) Yes, I feel lucky! To this end, I was planning on buying a 2 stroke 90hp. I might still be swayed to the 4 stroke 90hp, but I'm not sure.

Of course the added benefit is that they are cleaner and more fuel efficient. Of course I would be getting a Merc, and they are still a little new in this. (Yam Suzie, or Honda are priced out of my range, the Merc is the only one that is ALMOST within pricing grasp) The Merc 4 stroke will cost me about 4200 pounds. That is about 20% more than the 2 stroke. Of course the 4 stroke also weighs 80 more pounds.

Is it worth it with longevity and performance issues if the fuel consumption is not factored in? Until April 15th, there is a extended warranty on these motors that gives 5 years of coverage to the original owner.

Input is appreciated.

[Brian]

My humble opinion is that if you can save 20% by going 2-stroke AND save 80lbs in weight, then surely it's a no-brainer?
I think all up weight is the true gating factor here.

[Rene]

Hi

About the engine sice that Pete was talking about. I bought my RIB from Pete, with a 115 HP Yamaha. And with 6 divers 2 tanks each, you will have to place that right in the boat to get it "out of the hole" even with the 115 HP.

I would recommend a console !!, if you can get hydralic steering it's a nice touch

Rene

[Rene]

Sorry about this.....

This was meant as a reply to "RIB Engine sizing help!"

Must have pressed the wrong button.

Need to look more on my mouse movement

Regards
Rene (Sorry about this) Nielsen

[batfalcon]

I didn't realize that you were talking about jets. As far as prop engines I do have my doubts. A friend of mine had an accident the year before last due to a broken screw. The one that bonds the engine with the flexible rod from the steering wheel. When the screw broke the torque of the engine turned it all the way to the left forcing the boat to do a sharp left turn and throw the skipper in the sea. Luckily he had his kill cord attached so the boat stopped after making a full circle. He was twice as lucky as the boat didn't hit him making the 360 turn.
Without the steering system he had to attach something on the engine in order to steer the boat. Using his hands was impossible to hold the engine straight so he had to use something like a steering rod. Using some ropes he managed to attach his fish gun on the engine's cover and made his way back, controling the remote control's lever by his foot.
So I think that at the time of the acceleration it is much to difficult to keep the engine alined. Maybe it's a little easier when the boat has a constant speed, after it has been out of the hole.

[Pete7]

Rene, what are you doing about the engine, you were thinking of changing ?

This might be of interest:

http://www.outboardsdirect.co.uk/products.html

Regards Pete

[Rene]

Hi Pete

Well I was thinking about that, but I talked to some people from Yamaha on the boat show here in Copenhagen, and they had one of there people to have look it over, and now the problems are gone....

I had another shop to look at it last year, and after they had it in for service, it was a nightmare to get it started... I toold the Yamaha people about this on the boat show and they must have called the shop where I had the boat in last year, because Yamaha called me and told me that they would look it over for free..

I don't know what they did to it, but I didn't pay anything, and now the problems are gone.

So the next issue here is to get a bigger boat , I just have to convince my wife about that too

I would like something like the 8.5 meter scorpion, but I'm not settled on anything yet.

They Osprey is a super RIB, but with 6 divers it's getting crowded

Regards
Rene

[playspot1]

Actually I WAS talking about prop drive. I used the example of the jet, but I've done both prop and jet in large engines. Really the whole key is probably the torque of the engine steerer tube. (if that's what you call it) If the engine is "loose" on the back of the boat, it's not really a big deal with a steering wheel, but it could be with a tiller. If the engine is snugged up a bit, it makes it easier to hold. Also, realize that thed larger the motor, generally the longer the tiller. This starts to use a longer leverage principle, and becomes very manageable, even with a large prop driven motor.

I have run boats that are very loose on the back, and swing freely from side to side. (You know, the guy who is trailering his boat without hit O/B fixed, and every turn, or lane change, the engine swings from one side to the other. This does make it more difficult, as you have to exert more force. This engine looseness is pretty irrelevant with a steering wheel, as the steering box makes it very easy to hold the engine aligned correctly. Since it is common to have between 31/2 to 4 complete turns of a wheel lock to lock, very little change happens with a pretty good turn of the wheel. With a tiller handle, very little engine shift prompts a significant change in course angle. Subsequently, making the engine more difficult to turn takes away the hazard associated with a tiller, while not reducing steering input. Running full out with a tiller, you can change course probably 30 degrees with only a 6 inch shift of the tiller.

Now, don't get me wrong, thanks to you, and others advice I will be putting remote steering on my boat. This is more so because of my concern of the weight to power ratio of the craft. Unburdened, in a medium chop at full speed, you would have nowhere near the control with a tiller that you would with a steering wheel. The problem would come from hitting a wave and skipping sideways a bit, while trying to hold your body and tiller fixed so as not to over steer the craft. I think I would have difficulty with this. With a larger RIB, one that wouldn't have a tendancy to bounce laterally as much, and weighed more, I would still consider a tiller a fine alternative, but not a craft this small.

As far as your friend goes, I can imagine that it WOULD be extremely difficult, almost impossible to control an engine not set up with a proper style tiller, especially if is swung "loose". Properly set, the torque of the engine shouldn't make the engine move off course by itself. Of course if you hit a wave, untended, the engine is bound to swing one way or another. For safety sake, I never let go of the tiller, but typically, once running, my hand is just resting on the handle. The throttle should stay put, as should the engine.

Thanks for the help, I do appreciate it.

Mike