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Old 31 October 2015, 14:48   #81
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Dubrus, is that a Mile or a Nautical mile you quote there ?
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Old 31 October 2015, 15:22   #82
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"normal" mile as i use google maps to check distances for days out, makes it easier.

some pics from previous trips attached. the 67 mile trip was a day out with last tango funnily enough, we were on/off plane a few times chasing basking sharks, hence the lower moving average.

The pic showing both chartplotters is the lowest speed it will keep whilst on the plane (3100rpm and 21mph), it will maintain 0.9 up until 3800rpm and then it goes to 1 litre then exponentially up from there to a max of 1.4 at 51mph. i normally cruise on my own at 24 mph which is 3400 ish.

if you get the trim set well with less fuel (i always have 200 plus litres onboard but i set the NMEA to show 175l max for reserve) you can eek it out to 0.8 but it is hard work to keep it there.

if you look carefully you will also see how crap the onboard fuel gauge is, as soon as you start moving it goes to half tank.

i have verified it is correct by refilling after days out and it is within 5% of the NMEA readings which is perfect.
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Old 31 October 2015, 18:29   #83
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Mmm, that's interesting.

Looking at my logs I worked out I'm getting 1.1 litres per "normal" mile at about 3200 rpm, which is around 33 miles an hour.

My boat is 8.5m with twin 175's on the back.

But that's at a steady cruise for an hour or more on a calm day. As soon as its rough that goes up.

The slowest I can go still on the plane is 17mph. But most of the time I cruise about at 40 mph and ignore the fuel flow rate

How come you use two Garmin's then, why don't you use the 750 to show fuel flow rates etc. I set mine up to show that for each engine.
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Old 31 October 2015, 18:41   #84
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Don't you mean 2.2 litres per mile for both engines?

I have 2 Garmins because they came with the boat.

The 750 shows fuel flow and maps normally, the 551 shows trip data when I'm out.
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Old 31 October 2015, 18:45   #85
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I've got 4 of the Ullman shock mitigation seats on my Rib. Flipping expensive, but they were fitted to the boat when I bought it so I didn't actually see them individually priced. If I did I may have thought twice.

They really do smooth out the whole ride - big waves, small waves all rendered comfy. Basically I only stand up to change the view, not to cushion the ride.
Yes, I have found myself standing when things were a little rough.

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You can live without them. But I'm glad I've got them fitted already. I've done whole days of cruising about in the Med in mixed conditions and never felt tired at the end. Last trip was a fast blast for 4h in a f4-5ish and ride quality didn't figure into it.

So you could go out in horrendous conditions just for the fun of it. But I think they suit longer distance / adventure seeking rough weather Ribbers, or those who need a comfy ride. More than just general messing about / cruising use.
I eventually intend to undertake some rough weather trips but I think the cost of the seating precludes it for now. If I save enough pennies then I may specify them when buying a new boat.

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However, there are other things you could spend the money on, like a decent engine and storage fees. They are a luxury, one I cant do without, but still a luxury.

Of course, after getting your boat and using it for a while you will soon figure out what you like and don't like. Then you can add them later. A while back there was a thread about some chap selling his second hand for a massively reduced price. (you really don't need to buy new ones)
Good point about second hand purchases.
Thanks Trimix (diving handle?)
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Old 31 October 2015, 18:55   #86
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Performance is a trade off as with most things.

The general rule of thumb with ribs is you want the max HP, or as close as possible to it, the hull is rated for whilst staying under the weight limit for the transom. The makers site will give you that info if you need it.
OK. Got that.

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As a guide, my 6.8 and df200 (ideally it should have 250hp on mine) gives 0.9 litres per mile at 3200-3800rpm which is 22-28 knots or there abouts. My old Avon adventure 5.6 with 100hp 4 stroke used half that in same cruise range, very different size and style boats though, should also mention the tech in the 2012 df200 is in a different league to the 100hp of 2006 vintage so new tech helps there for sure.
OK, I see what you are saying. Sheesh! Miles, HP, RPM, Knots and decimal fractions of a litre... I am going to have to go back to school and redo my maths. Any numbers in old money?

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Most tubes are hypalon in the size you will be looking at with the likely exception of ribeye playtimes being PVC unless custom ordered. It is dependant though in ribeyes case so you would need to check the actual boat.

Hypalon is what you ideally want unless something good comes along, it is a better material.
Thank you for the helpful advice
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Old 31 October 2015, 18:59   #87
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Just to put this this in perspective.....more or less any 6m deep V (mean deadrise 25 degrees) will get onto the plane easily with 90hp unless it is very, very heavily laden. (50hp/ton will get you onto the plane) One of the main users of RIBs in the early days was divers and there is a huge difference between getting a 6mtr boat onto the plane (referred to as the "hole-shot") with six burrly divers, twelve bottles + associated gear and getting it onto the plane with the family and a picnic box for an afternoon cruise.
Probably want a bit more power for flexibilty but remember the bills and the weight go up with HP.

The hole shot will not be an issue....what you want is an engine that's not flat out at cruising speed. Most of us probably cruise at around 20-25knots (I'll wait for the barrage ) so if your chosen engine boat combo will do 35knots flat out then it's not going to be getting tortured running for several hours at 25knots. If you're going to use it for a bit of water skiing then your skier is going to sap about 15hp for haul-out. (Once up, the drag will be about the same as having them in the boat)
Reliability wins over top speed every time.
Thank you Last Tango. An excellent short explanation and helpful to the newbie. I had to look up the term deadrise and spent a little time more trying to understand more about how a hull shape moves through water. What a great resource RIBnet is.
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Old 31 October 2015, 19:17   #88
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Jepho , I think Dubrus and Last Tango have given you some good down to earth advice .
Hello Bern. Yes, me too.

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If you go out in " adventurous " conditions in a 6M boat you and your boat will only be able to take so much of a battering . It is often said " If it's hurting you , it's also hurting the boat ! " .
That's a maxim I can understand.

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Most of us will have to swallow our pride and get home at displacement speed . Assuming your boat/engine combination is reasonably propped , at displacement speeds you will probably NOT need the maximum hp your transom can take !
I see... more maths.
I should ask... if the pitch of the prop is used to determine the absolute speed in RPM from an engine or the torque available to push the boat along? Perhaps it is both and a decision must be made as to ride efficiency over RPM. I am assuming that faster RPM changes the pushing force. I suspect that if a prop turns inefficiently (more cavitation?) it will cause some sort of performance drop off.

Perhaps you understand what I am trying to say and will stop me talking nonsense.

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It's probably the wrong end of the season , but if you can join either RIBnet or BIBOA cruise and get a ride on as many different ribs as you can until you find something that really suits you . ( Someone will be along in a minute to say that you will only get big Scorpions on a BIBOA cruise but that's not true )
I may have to leave that until next season begins but it is a good idea. Thank you for your input.
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Old 01 November 2015, 02:46   #89
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I see... more maths.
I should ask... if the pitch of the prop is used to determine the absolute speed in RPM from an engine or the torque available to push the boat along? Perhaps it is both and a decision must be made as to ride efficiency over RPM. I am assuming that faster RPM changes the pushing force. I suspect that if a prop turns inefficiently (more cavitation?) it will cause some sort of performance drop off.
.
You're over thinking this one.
All the maths in the world will only give you a headache and get you "fairly close".
Trial and error is what gets you spot on and it's likely, if you buy a second-hand boat, someone will have already fitted the best prop. If not, cheat...... just stick the question on here and someone will already have done the work for you.
You're generally looking for the engine to be reaching it's optimum max revs at full throttle with your normal load (varies but generally between 5000 & 6000 rpm) and there's a bit of scope there.
Replacing a prop takes about 10 minutes. Huge variance in price, you'll see them on ebay, but the last three second-hand stainless props have cost me between 130 & 150.
You'll find endless debate about the subject on here, all relevant, but don't get hung up on it, it's not a deal breaker. More important is, if the prop is damaged make sure there isn't other damage round the area.
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Old 01 November 2015, 05:55   #90
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Dubrus, good point - but it is 1.1 as I measured the fuel I put in the tank to fill up again, not 1.1 per engine

When you use the 750, do you use the Cruising range facility ?

I cant seem to get mine to show what I expect the cruising range to be. I would have thought its quite simple as you tell it the quantity of fuel you put in the tank and it measures the rate your using - so hey presto, it tells you the range left.

But mine doesn't seem to do that
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