Max Power for RIB Dinghy Question

[Kenomac]

The center console and seat is what we wanted, most of the time it’s just the two of us, two Brompton folding bikes and some groceries. The dinghy and console weigh in at under 200 pounds, the motor will weigh 165 plus controls. Total weight: 385 or so.

[belizesailor]

Originally Posted by a64pilot

You no longer have to be on the upper lobe to have a hyper auto. We aren’t. My Wife paid for her car on a School teachers salary.
Mopar and even GM build automobiles that will outperform the majority of the European performance sedans now.
I don’t know about Mopar, but GM has for 14 years now, although most don’t know.

Only point is, your either mature, or your not. My last street bike which was old, would accelerate to 60 mph in 2.3 sec, 100 in 6 and had a top speed of 186 MPH. I rode it for more than a decade without incident, although from a logical perspective, you can’t justify a vehicle with that kind of performance potential, it has no need whatsoever, it’s purely for enjoyment of performance

I was referring to IQ, not socioeconomic status...plenty of immature idiots at all socioeconomic levels. [emoji16]

[boatpoker]

Originally Posted by Kenomac

I’m guessing you failed to fully understand post #1. In fact, the dinghy with a larger motor would be able to properly tow the mothership, which might actually prevent a serious insurance payout.

I’ve been in several situations where more power is beneficial in emergency situations.

I did read and understand the OP. Any insurance claim involving a dinghy with a motor over the rating on the serial plate would be denied. Having done hundreds of insurance claim investigations, I'm pretty sure of that.

[Kenomac]

Decided on an awesome Mercury 40hp 2 stroke with electric tilt, start and controls following an extensive search. If I need to upgrade the dinghy, I will.

Here's a good example of why I need more horsepower.

http://www.cruisersforum.com/forums/...ay-187339.html

[a64pilot]

Ken, I’m not arguing with you, but if you want it to be more of a powerful tug, you want a big diameter, low pitch four blade prop. You will sacrifice some speed, but it will pull like a tractor then.
For just pulling power, likely a 20 HP with the right prop will output a 40 with a speed prop, but of course with the proper prop, the 40 easily wins.
A go fast prop just cavitates when you throw power into it, and don’t achieve much forward motion

[Thalas]

Originally Posted by a64pilot

Ken, I’m not arguing with you, but if you want it to be more of a powerful tug, you want a big diameter, low pitch four blade prop. You will sacrifice some speed, but it will pull like a tractor then.
For just pulling power, likely a 20 HP with the right prop will output a 40 with a speed prop, but of course with the proper prop, the 40 easily wins.
A go fast prop just cavitates when you throw power into it, and don’t achieve much forward motion

I’ve been looking at the Yamaha F25 as it appears to be the lightest 4 stroke 25 right now. If my priority is pulling up a wakeboarder over getting to the dock super-fast, are you saying the larger diameter/low pitch prop is better? Is 25hp on a Wakefield RIB sufficient to get a 190lb adult out of the water on a wakeboard?

[a64pilot]

As a general statement props are like a single gear vehicle, you can gear for speed and have little pulling and acceleration, or for power and you will be “Gear bound” meaning it’s easy to hit the rev limiter if your not careful.
Many years ago when I was a kid, we had the first “Tower of power” Mercury 100 hp engine around.
We had two props, a two blade that made the boat fly, and a lower pitched three blade that we honestly could pull 5 or 6 skiers out of the water with.
It takes about three minutes to swap a prop, one nut and cotter pin, that’s all.
You may want a power prop to pull a skier, and a regular prop for when your not pulling things.
I have never ridden a wake board, but from looking it seems you only need about 20 MPH or so? I’m older, we either slalomed or barefooted if you were real adventurous.

[Thalas]

Originally Posted by a64pilot

As a general statement props are like a single gear vehicle, you can gear for speed and have little pulling and acceleration, or for power and you will be “Gear bound” meaning it’s easy to hit the rev limiter if your not careful.
Many years ago when I was a kid, we had the first “Tower of power” Mercury 100 hp engine around.
We had two props, a two blade that made the boat fly, and a lower pitched three blade that we honestly could pull 5 or 6 skiers out of the water with.
It takes about three minutes to swap a prop, one nut and cotter pin, that’s all.
You may want a power prop to pull a skier, and a regular prop for when your not pulling things.
I have never ridden a wake board, but from looking it seems you only need about 20 MPH or so? I’m older, we either slalomed or barefooted if you were real adventurous.

Thanks for the great info. I can't imaging barefoot waterskiing.

I've never swapped out a propeller on a dinghy as I've only chartered cats thus far. Didn't realize how easy it was. I guess I'll look into the costs now as there *was* a case on a charter where we had some longer dinghy commutes for one reason or another. Woulda been nice to have some speed then. Generally, I don't really care about speed and was mainly interested in a more powerful motor to tow friends and push wayward boats when bad sailors anchor poorly.

[cyan]

Originally Posted by Thalas

I’ve been looking at the Yamaha F25 as it appears to be the lightest 4 stroke 25 right now. If my priority is pulling up a wakeboarder over getting to the dock super-fast, are you saying the larger diameter/low pitch prop is better? Is 25hp on a Wakefield RIB sufficient to get a 190lb adult out of the water on a wakeboard?

A 190lb guy (me) can indeed ride a wakeboard behind a 10.5ft RIB with 25hp. However, you will probably be plowing water for a while before you stand up, requiring upper body strength. You'll feel worn out after only a few runs, and you'll require more beer as a result. It is also pretty hard for a beginner to learn with this combo in my experience, as quicker acceleration gets you out of the wobbly, unstable phase in less time. I think 40hp in general might be a better minimum for women or beginners, although skinny teenagers seem to pop up just fine, probably with even less than 25hp. You won't be throwing any sort of a fun wake, but exotic tricks are not really the point with our group. You'll want to limit the number of passengers, but one up front seems to help with the leverage. (and nobody cares about flagman laws when you are the only boat for 100 miles)
Final note: 25hp is plenty to pull kids on a planing inflated raft.
YMMV

[Uricanejack]

Truthfully I don’t know just guessing really.

First guess I have a 50 4st on the back of a 14ft double eagle. So boat is not that much bigger. So my ball park guess it should be fine and dandy.
If the weight is the same logically the boat will sit at rest in the water just the same so no weight problem according to Archimedes.

The only difrence is powere. Which brings up the question why do you want the power.
If it’s to go faster, ok so long as you are not an aggressive driver. I’ve nearly flipped a high powered rib. Addmitdly I was driving it very aggressively. Tight turn and lots of power. Normal driving should be ok. 40 hp the risk of flipping if driving like you stole it.
If you want to tow. The pitch will not be right for the job. It will be pitched to be efficient going faster at higher rpm.

A 10 hp saildrive with a much coarser pitch designed to give power at lower RPM might be more effective.

Or do you just happen to have a good but used 40 available and don’t feel the need to get a 25.

I would guess it will work fine. With a conservative operator.

What the heck give it a try and let us know how it works out. If we don’t here from you again we can assume it was a serious error.