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Old 13-02-2015, 15:56   #16
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Re: horsepower to plane a 10' RIB

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Originally Posted by atoll View Post
the rib is only rated for a 9.9 hp max............

hydrofoils might work better!
Well then, that's so far overkill that I think it looped back around to "yeah, that works!"

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Old 13-02-2015, 18:31   #17
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Re: horsepower to plane a 10' RIB

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Originally Posted by colemj View Post
Are you sure you are under-propped? If it takes a lot of time and hp to get onto plane with 4 people, it sounds like you may be over-propped, not under.



We have the same outboard with probably the same standard prop. Our 12' 350lb dinghy will take all 20hp to slowly bring 4 "American" people on plane. I recently borrowed another prop that was 1 inch less in pitch - and with this prop the dinghy popped right up on plane with 4 people instantly and stayed on plane at 1/2 throttle.



Mark

I say I'm under propped as with one aboard it'll bang the rev limiter at 3/4 throttle or so.
So with one or two aboard I'm underpropped.

But, with four aboard, it's all she can do to get on plane and I can't hit the rev limiter, so with four aboard, I'm correctly propped, maybe even a little overpropped.

Only point is with a boat this small, an extra 400 lbs, makes a huge difference in power required to plane. I've had four aboard with an average weight of at least 200 lbs each, probably more.

I think it hits the rev limiter at about 30 mph, which is plenty fast enough for me, I mean just how fast do you need a dink to go? When the weather gets warmer, I may clock it with my phone, the Seatow app has speed as a part of it, just haven't bothered to yet.

A good lightweight dink, with two 125 lb people aboard if it's a good hull, just might plane with a 6 hp? BTW a light chop gives the least drag, smooth glassy water is very draggy. You wouldn't believe the increased distance required to get a float plane flying on glassy water


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Old 13-02-2015, 18:55   #18
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Re: horsepower to plane a 10' RIB

I have a rule of thumb I use when people ask me how big an engine they need. If they want the boat to plane, the engine needs to be at least 2/3 of the maximum horsepower that the boat is rated for. They'll be even happier if they put 100% of the rated horsepower.
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Old 14-02-2015, 07:07   #19
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Re: horsepower to plane a 10' RIB

Quote:
Originally Posted by a64pilot View Post
I say I'm under propped as with one aboard it'll bang the rev limiter at 3/4 throttle or so.
So with one or two aboard I'm underpropped.

But, with four aboard, it's all she can do to get on plane and I can't hit the rev limiter, so with four aboard, I'm correctly propped, maybe even a little overpropped.

Only point is with a boat this small, an extra 400 lbs, makes a huge difference in power required to plane. I've had four aboard with an average weight of at least 200 lbs each, probably more.

I think it hits the rev limiter at about 30 mph, which is plenty fast enough for me, I mean just how fast do you need a dink to go? When the weather gets warmer, I may clock it with my phone, the Seatow app has speed as a part of it, just haven't bothered to yet.

A good lightweight dink, with two 125 lb people aboard if it's a good hull, just might plane with a 6 hp? BTW a light chop gives the least drag, smooth glassy water is very draggy. You wouldn't believe the increased distance required to get a float plane flying on glassy water
Got it. I read your post as saying that you were hitting the limiter with 4 people on board.

When I was testing the lesser pitched prop, we would sometimes hit the rev limiter with 1 or 2 on board. The fact that this prop seems to work better with 4 people than yours does may just be due to the longer waterline of our dinghy allows it to get on plane faster.

Our dinghy seems to go 20kts at WOT whether it is hitting the rev limiter or not. At 1/3 throttle with two on board it does 15kts and sips gas - so that is where we run it.

Mark
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Old 14-02-2015, 08:59   #20
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Re: horsepower to plane a 10' RIB

I have had both 2 stroke and 4 stroke on our rib. I was surprised at how much better the 2 stroke popped it onto a plane. Not scientific at all, but seems the 4 stroke needed 5 more hp to get it planing the same way.
Once on a plane the 4 stroke was great (quiet, better on gas).
15 hp 2 stroke was probably best. 3 adults and a bit of gear no issue.
Your milage may vary
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Old 14-02-2015, 09:41   #21
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Re: horsepower to plane a 10' RIB

All things being equal, 2 stroke outboards have much better torque than their four stroke cousins. When Mercury built the Verado outboards a few years ago the reason they supercharged them was to get parity torque wise with the 2 strokes.


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Old 14-02-2015, 09:46   #22
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Re: horsepower to plane a 10' RIB

I agree with HopCar but prefer to say 100% of the rated power.

We have a 9.9 Yamaha 4 stroke and it will only plane with my wife, myself, and no groceries or equipment. The boat is rated for 15 HP.

I really wish I had a 15 HP two stroke because the 15 HP 4 stroke is just too heavy.
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Old 14-02-2015, 09:58   #23
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Re: horsepower to plane a 10' RIB

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I really wish I had a 15 HP two stroke because the 15 HP 4 stroke is just too heavy.

20 HP Suzuki = 99 lbs, 5 lbs per HP
Other manufacturers will or have followed I'm sure


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Old 14-02-2015, 10:42   #24
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Re: horsepower to plane a 10' RIB

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Originally Posted by Rick Williams View Post
We just bought our first RIB: a used 10' 6" Avon with a hard fiberglass bottom.
It came with a 4 stroke 6 HP suzuki that just plows the water, doesn't come close to planing with two people.
We need suggestions on what size outboard will reliably push the RIB on a plane with two not too large people.
Thanks
9.9 might do it but 15 is close to the same price and generally the same engine, in my experience. Probably run like a scalded dog.
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Old 14-02-2015, 11:02   #25
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Re: horsepower to plane a 10' RIB

Go for the 15hp outboard - it is the same displacement as most 9.9hp engines so is also the same weight - but more efficient. Don't forget that sometimes you'll be glad of the extra power. Depending on where you intend to sail the make and type can be important. We sail in remote area's and two strokes dominate (No.1 Yamaha Enduro range, No.2 are Tohatsu). If I were cruising closer to 'civilisation' then I would go the 4 stroke route. Out in the bundu we can always get spares for our Yam Enduro's - we have a 25hp on our main duck, a 15hp on the kids duck and two Yamaha Malta's (3.5hp) for tootling around and which also get the most use.
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Old 14-02-2015, 11:55   #26
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Re: horsepower to plane a 10' RIB

15 hp (2 stroke). Got us up fine.


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Old 14-02-2015, 16:29   #27
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Re: horsepower to plane a 10' RIB

I have tested RIBs and engines for many years while working for West Marine. When two-strokes were common, there was an 8HP engine from both Nissan and Tohatsu (and perhaps others) that was extremely lightweight (around 70#) and it was wonderfully smooth. For a lightweight RIB and two people, it was very hard to beat.

The larger engines from Nissan and Tohatsu were 9.9HP and 15HP. They were identical except for a cam which controlled the throttle butterfly. Either of these engines worked well, and would plane a RIB with three people, but they were substantially heavier (around 100# as I recall).

With four strokes, the situation gets more complicated. There have been two generations of four-stroke engines since their introduction about a decade ago. (The following information varies by brand, of course, but it's generally true.) Several vendors came out with first generation engines that were really heavy and just seemed peaked by comparison to older two-strokes. Then came some lightweight engines from Mercury and others, and these appeared to solve both the pollution problem and the performance problem.

Bottom line: I think you can get most well-designed RIBs to plane with two people with an 8HP, but if you can handle the extra weight, a 9.9 or 15HP would be more versatile.

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Old 14-02-2015, 17:36   #28
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Re: horsepower to plane a 10' RIB

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Originally Posted by a64pilot View Post
20 HP Suzuki = 99 lbs, 5 lbs per HP
Other manufacturers will or have followed I'm sure


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Well that's awesome! We were looking at the Yamaha's at 111 vs. our 87.

How hard is it to start that 20 Hp manually?
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Old 14-02-2015, 18:09   #29
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Re: horsepower to plane a 10' RIB

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Well that's awesome! We were looking at the Yamaha's at 111 vs. our 87.



How hard is it to start that 20 Hp manually?

It's not hard, but I don't think my wife could.
really unusual engine, it's electronic fuel injected, but has no battery. So before you get half a pull on the rope, the computer has to boot up, fuel pressure has to be built up, injection and ignition occur etc. You rarely have to pull the rope twice. Being fuel injected means of course there is no choke, you just pull the rope, nothing else required.
So far I've had no issues with bad fuel clogging anything, my hope is since the fuel is sealed without any contact with air in the engine, that the injectors won't suffer from clogging like the carburetors do.
I'm conducting a little experiment, which I hope does not come back to be a problem. In October I brought the dinghy and motor home, they have been sitting in the back yard since then. I did nothing to preserve the engine. I figure in March I'll try starting it, if it goes four months with fuel in it, I think the fuel issues we have all become accustomed to will be a thing of the past.
Fingers crossed


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Old 14-02-2015, 19:44   #30
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Re: horsepower to plane a 10' RIB

You need to be going a decent distant to need to plane. Having said that my 10' RIB w/ and 4 stroke 8hp will plane easily with one and supplies and a clean bottom, less easily or not at all w/ 2 and supplies and some chop. But the fact is I don't need to be speeding on a plane and when I am anchored in Newport RI for example I think there is a speed limit which precludes planning.

The 4 stroke is about 100 pounds and requires a lifting crane/hoist to get it onto the rail. We tow always but without the motor. I don't think the larger HP is heavier however... the bore may be different.

If you need to plane regularly with more than 1 and some supplies.. go for 15hp.
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