Please help, thanks.....

mrlimbeng · 24 August 2006, 07:06

I have a waverider470 RHIBs total weight abt 331kg.
overall length is 4.7m and using a mercury obm 50HP.
May i know what is the actual or top speed(knots) i can reach?
What is hull speed and what is the hull speed for my boat?

Can someone post me the formula to calculate?
Thanks!!

Poly · 24 August 2006, 14:04

Unless I am mistaken - hull speed doesn't really apply (meaningfully) to a RIB. It is a term used to describe the "maximum" speed that a displacement boat can achieve (when displacing water the longer the boat the faster it can go). RIBs are planing craft so in theory have no actual maximum speed - put a bigger engine on it and it will go faster (or the boat will be uncontrollable / fall to pieces!).

As well as considering your boat and engine you need to consider the prop that you are using when you think in terms of maximum speed.

What speed is your engine spinning (typically somewhere in the 5500-6500 rpm range) at full throttle?

What is the gearbox ratio (usually its roughly about 2 - but you need the actual number to get it accurately).

So this means the prop would spin at roughly 2750-3250 rpm. (Actual no. comes from actual gearbox and actual rpm)

You then need to know the pitch of the prop you are using. This is usually in inches. Under ideal (purely theoretical) conditions each turn of the prop would move the boat forward by the amount of its pitch. So e.g. a 15" prop spinning at 3000 rpm will try to move the boat forward 45,000 inches every minute! That is 1250 yards per minute. There are about 2025 yards in a NM. so that is 0.62 NM per minute which is 37 knots. Now this assumes 100% efficiency, ie. that the prop moves through the water without any slip. In reality this is impossible to achieve. A well set up rib is typically expected to have about 10% slip. so if you got 33-34 knots under these conditions you would be considered doing well.

This is all theoretical and you will note takes no account of the prop diameter, the length of your boat, its weight or the size of your engine. There are some tools on the web for estimating these but they will just be estimates. The above calculations suggests all you have to do is stick a bigger prop on to go faster - in reality it is not as simple as that - obviously it takes more work from the engine to move a bigger pitched prop so the engine revs will probably fall.
HTH

mrlimbeng · 25 August 2006, 07:04

Thank you Mr Polwart

I have once came across your reply last year in oct on power calculation in which you have mentioned that:

Theoretical Boat Speed [in knots] = (RPM * 60 * Pitch [in inches]) / 72913.2

I have checked through that the model of my obm is mercury 50HP and model is 10.125Dx15P. Does the 15P actually means 15 inches in pitch?

If i used the above formula to calculate assuming the speed for my obm is 60% of actual speed (5500rpm), i will get about 3300rpm,
The theoretical speed will be approx 40.7 knots.

Does the weigh of my RHIBs will affect this speed? as it seem that my boat does not goes that fast at all.
Assume the weigh of the rhibs is 331kg , how will it affect the speed if i add more weight.

Thanks for the help.....

benc · 25 August 2006, 08:15

The size of the propellor is normally engraved on it I believe. But its sometimes on the inner rim, so you have to take the prop off to see it.

What speed are you getting at the moment?

mrlimbeng · 25 August 2006, 16:35

Hi..
I can only approx the speed about 12 knots.
But the more people i carried, the slower i get.
I assume the current and wave must have some impact on the speed.

I have a friend which having the same kind of boat but he is using the water-jet type obm 45HP (can't remember the model) and it seem faster than mine.
Is water-jet obm 45HP can run faster than 50HP propeller type?

jyasaki · 25 August 2006, 18:18

12 knots certainly doesn't sound right. Call it a 15 foot boat, 50 horse; should see at least 30mph, assuming everything's working right (based on other boat/motor combo's.)

Does the boat get up on plane? My RIB, at 12 knots, is still wallowing along, off-plane and pushing a ton of water. Too much prop pitch will make the motor work harder to get up on plane, and may well keep you from getting up at all. Once the boat is up on plane, more power is available for speed (rather than popping the hull out of the water.)

A friend of mine is running a Zodiac Pro 16 (actually, I think it's about 15.5 feet) with a Merc 60 4-stroke, so it's probably a pretty close scenario. He's turning (I think) a 12 or 13 pitch prop. He had problems when pitched too high, and also when forgetting that the thing is inflatable (meaning the tubes are not supposed to drag deflated in the water when running - he's not exactly the brightest person on the planet.)

If you have a tachometer on the boat, run it up wide open, and let us know what rpm you're achieving. That'll give a pretty good clue as to how your propped. If you don't, and inductive-pickup auto tune-up meter (that picks up the spark from a plug wire) may be an easy way to kludge it up.

Also, if you trailer the boat, pull the plug so you're sure the hull is not full of water.

The more people=less speed thing is normal. Takes energy to move more weight. That detracts from speed. Too many people (or gear) and you'll be right back to the "can't get on plane" thing (been there, done that.)

jky

Rogue Wave · 25 August 2006, 18:39

The hull is a pretty good design and it will compare favourably with other ribs in it's class. you should be seeing at least thirty knots with your rig and closer to 70 with a 70hp.

I am presuming that by waverider you mean a gemini waverider?

Ribshop on this forum is an expert on these boats and this is a link to gemini's website. http://www.gemini-inflatables.com/in....asp?boatID=23

12 knots is not right, can you get hold of a GPS spo that you can accurately measure your speed, but it sounds to mee as if your hull is full of water, so whip the boat out and check thevoid between decks and check that the outboard is functioning properly I'd start with a compression test. It could be the prop but that figure is way off the mark can you post a photo of the prop and engine

Rogue Wave · 25 August 2006, 22:31

this thread may also be of use to you.

wavecrosschris · 25 August 2006, 23:58

4.7m rib with a 50 - certainly on or above 30knots no more than 35knots. My 4.5 ribeye with a 60hp won't do much over that.

mrlimbeng · 26 August 2006, 05:17

Thanks for your comment & info guys....

Indeed mine is gemini waverider and it seem unable to get up on plane as mentioned by mr jyasaki. I have reach a speed of 5500rpm on my tacho gauge onboard (maybe i have carried too much load and this is the reason why the speed is so slow)
I will check on the status of my rhibs again

I need to know if those velocity speed gun able to track the boat speed? My friend have one and might need his help to take down my speed when i try out my top speed next week.

once again thanks....

Rogue Wave · 26 August 2006, 07:16

Quote:

Originally Posted by Rogue Wave

The hull is a pretty good design and it will compare favourably with other ribs in it's class. you should be seeing at least thirty knots with your rig and closer to 70 with a 70hp.

should read 40 with a 70 hp

if you are seeing 5,500 revs and going very slowly then I would definitely check out the void

jyasaki · 26 August 2006, 19:10

5500rpm and 12 knots? Doesn't sound right at all. Usually, if you are unable to get up on plane, the motor will bog down well into the 4000's, probably even lower (don't ask how I know this.)

Does the speed (as low as it is) seem to be linear with the rise in rpms, or does the thing accellerate to 12 kts and stay there while revs rise?

I'm thinking possibly a spun prop hub. If you have a buddy with the same engine, see if you can borrow his prop for a bit.

A waterlogged hull is a possibility, but I would think that if the overall weight was the problem, the revs would be much lower.

Dunno.

jky

Rogue Wave · 26 August 2006, 20:27

you may well be right!

Poly · 26 August 2006, 22:02

Quote:

Originally Posted by mrlimbeng

The theoretical speed will be approx 40.7 knots.

Does the weigh of my RHIBs will affect this speed? as it seem that my boat does not goes that fast at all.
Assume the weigh of the rhibs is 331kg , how will it affect the speed if i add more weight.

Thanks for the help.....

Remember this is the theoretical speed with NO slip on the prop etc. Realistically 10% is the best you would get so about 35knots (as others have said from experience) is about the best you can expect. BUT you should be getting much more than 12 knots. jky and RW both have practical suggestions as to what may be wrong so hopefully one of them will fix it.