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Old 04 May 2008, 18:48   #1
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Country: UK - N Ireland
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Reprop Redbay - speed increase?

Hi Guys,
Got a few questions to throw at you and I would be very glad if you could help me out. I may be buying a second hand redbay stormforce rib. The only thing holding me back at the minute is speed. The boat is 8.5 metres long and I would guess weighs under 2.5 tonnes including engines (is this a reasonable estimate?) It has twin 144hp steyr sterndrives fitted to bravo 1 legs. Having talked to a few people including the guy that designed the boat they think it would only do speeds in the low 30 knots e.g. 32, 33.

My questions are;
Do you think this is a right estimate of speed?
How much speed is it possible to gain by optimally propping the boat? I know someone who repropped an 11m redbay and gained over 6 knots - is this unrealistic?
Do trim tabs reduce or increase top speeds (Not too worried about planing at low speed etc.) and what would be the best ones to get?

Thanks in Advance

Conor
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Old 04 May 2008, 20:37   #2
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Quote:
Originally Posted by chorscroft View Post
...Having talked to a few people including the guy that designed the boat they think it would only do speeds in the low 30 knots e.g. 32, 33....
That would be my guess too.


Quote:
...Do trim tabs reduce or increase top speeds...
Trim tabs are not really a boat speed item, they are more a boat stability item. They have an effect on speed by dint of them causing the boat to run better in the prevailing conditions and also by their drag but they are not going to increase the boat's basic maximum speed.
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Old 05 May 2008, 04:47   #3
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Country: UK - N Ireland
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Engine: Yanmar 300 / etec 50
Join Date: Mar 2008
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And what about repropping the boat - how many knots do you think I could squeeze out of it then?
Is it impossible to tell without being out and reading the rev. counter?

Regards

Conor
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Old 05 May 2008, 08:27   #4
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Country: UK - Wales
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Have a test drive and take a hand held GPS.

I would say you might get a bit more in an ideal setup - maybe 35 - 40 mph. Remember it is a heavy boat and twins aren't as efficient as a single. Props can make a big difference but all top speed and crap acceleration is not good. Trim tabs aren't supposed to increase speed but if a boat is stern heavy they can help by lifting the stern slightly. Trim settings on the legs are also very important. I would say the best trim tabs - especially where space is limited like yours - are the QL boat trim system - providing you have the the right kind of transom they are much easier to fit and less to go wrong.

It all depends what you want it for. To be honest in flat calm water even 60mph becomes boring after a while. RIBs are meant for the rough and the Atlantic/Irish Sea is pretty wild most of the time. I promise you 20kts in the right conditions is VERY exciting.

It looks an awesome boat and will thrive in the rough!!!
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Old 05 May 2008, 10:21   #5
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Originally Posted by chorscroft View Post
And what about repropping the boat - how many knots do you think I could squeeze out of it then?
Is it impossible to tell without being out and reading the rev. counter?

Regards

Conor
In general terms, you can't get more speed out of a boat by a simple prop change unless the current props are a mismatch. It is sometimes possible to realise a gain by a change of prop style but there is usually a compromise somewhere in the boat's performance. Speed isn't everything, especially in a boat like a Redbay because they are built to perform well in other circumstances.

The bottom line is that the 8.5mtr Redbay has a particular hull type and weight, you're driving it with about 280HP through two drives so your speed is going to be in the low to mid 30's.
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Old 05 May 2008, 11:28   #6
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Codprawn;
I am getting a testdrive next monday. The boat is currently fitted with a Lowrance Globalmap 2000 Navigator and a Lowrance Globalmap 300 Plotter will one of these do speed - I don't have a handheld gps and I assume these would be more accurate.
How much acceleration would you have to lose to get top speed up to about 38 knots?
Having talked to the designer he informs us that because the boat was taken from a 9.1m mold the bow sits too high - (I assume this is the same as sternheavy?) If it was sitting like this in the water would trim tabs increase speed and by how much?
Are trimtabs a better solution that keeping a heavy anchor, dinghy and other heavy stuff in the bow locker?
Yeah I know its not slow but our last boat did 40knots and I would like to be able to keep pace with the other redbays when they go WOT on the flat.
Terrific boats in the rough - Have driven the 9.1m and the hard, flared bow, twin engines and long length make it fabulous.

jwalker;
You say there may be a compromise in performance - can you elaborate on this? Is it what codprawn was saying about loss in acceleration or will it actually affect handing and performance in the rough?

Thanks again guys - terrific community on this forum!

Conor
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Old 05 May 2008, 17:10   #7
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They won't be more accurate than a handheld - just the same - I didn't realise it had them built in though it obviously has!!!

To get that sort of speed you may well sacrifice acceleration but even more important than that is grip in rough water - you don't want to lose too much of that. Then again maybe a change to 4 blade props will erneable you to have your cake and eat it? Prop selection is a really black art and with turbo diesels and their narrow power band things get even more complicated........

You need a nice big anchor up front but trim tabs are definitely the way to go rather than adding weight at the front just for the sake of it. Before you do though you can get a few hefty blokes to move around and see what effect it has on speed. As to how much trim tabs would help it's almost impossible to answer.
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Old 05 May 2008, 17:21   #8
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Originally Posted by codprawn View Post
...Then again maybe a change to 4 blade props will erneable you to have your cake and eat it?
You'd best look at the pictures again, Codders.
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Old 05 May 2008, 17:22   #9
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Another thing you may want to look at - these days many engines in a range are similar with different chips governing power. The 144hp is basically the same engine as the 163hp. Marine engines have different ratings for pleasure and commercial use. The 163hp is developed 200rpm higher so the engine won't last as long flat out - but the power is always there if you want it. At lower revs the life is the same.

In other words a simple chip change or fiddling with the governor could give you a nice power boost - just watch the gearboxes!!!
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Old 05 May 2008, 17:48   #10
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Country: UK - N Ireland
Town: Cushendall
Boat name: JanJack /
Make: RedbayBoats / Brig
Length: 8m +
Engine: Yanmar 300 / etec 50
Join Date: Mar 2008
Posts: 81
But if the boat does ride stern heavy you think trimtabs will give me an increase in speed?
How much including fitting would a set of trim tabs set me back?
Do many rib and boat shops let you just try out props or is it a matter of buying what you think is the right prop and hoping for the best?
Are there any places that lets you buy a prop and send it back again if it isn' t the right one?
When you say you may sacrafice grip in the water what do you mean by this? - coming off waves when the leg lifts out?
It has 4 bladed props at the minute - does this increase grip at the expense of speed?
When you talk about sacrificing acceleration how bad is it? Because I think it would be worth it if say I gained 4 knots top speed and it only took me another 5 seconds to get on the plane - is this way out and what would be a reasonable estimate?

Thanks again codprawn - you're a fountain of knowledge!

Regards

Conor
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