Reprop Redbay - speed increase?

[chorscroft]

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

[jwalker]

Originally Posted by chorscroft

...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.

...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.

[chorscroft]

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

[codprawn]

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!!!

[jwalker]

Originally Posted by chorscroft

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.

[chorscroft]

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

[codprawn]

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.

[jwalker]

Originally Posted by codprawn

...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.

[codprawn]

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!!!

[chorscroft]

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

[jwalker]

Originally Posted by chorscroft

...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.

Why is it that they don't actually know the boat's speed?

I know someone who repropped an 11m redbay and gained over 6 knots - is this unrealistic?

Very. At the speeds and weights you suggest for your boat an increase of 6knots is approximately equivalent to an increase of 100hp.

[Poly]

Conor,

It sounds like you have already spoken to the guys at Redbay about this, and didn't like what you were hearing. Whilst no one is infallable, and certainly there might be ways of pushing the boat a little bit faster by sacrificing something else - if the designer/builder/salesman isn't convinced then that says something. If anyone has a feeling for what that boat is capable of it must surely be them (or the current owner - who can't really be trusted as he is trying to sell it).

The detail in your post suggests the designer even gave some serious thought to the speed rather than the first number which tripped off the tongue.

But you might be better asking how often Redbay owners in your intended cruising area actually spend time doing > 30 knots anyway. Even on flat calm days its probably not ecconomical, esp with rising diesel costs.

[codprawn]

Think of driving a car in say 3rd gear - pretty good acceleration but top speed not so good. Now think of 5th gear - great for top speed but useless if you need to accelerate out of the way of a falling tree!!!

In rough seas you won't be going very fast but you need instant response and the higher the pitch of prop the worse it gets. To be honest I reckon Redbay will already have got the best compromise they can out of those engines.

Trim tabs generate lift at the stern - this will make the hull run flatter and lift the arse end - this MAY overcome the extra drag they create but no guarantees. Still triim tabs are a good thing to fit anyway - if your bow is light it is good to keep the nose down into a head sea.

http://www.motortechmarine.co.uk/pro..._tuning_plans/

£453 inc vat - expensive but cheaper than a dual ram Bennet or similar and easy to fit - to the right hull. You will have to check they will fit first - how much people will charge you is anyones guess!!!

I would buy the boat IF the price is right and look into chipping the engines - it should be possible but the marine market is much smaller than the car market so finding one may not be easy.

[jwalker]

Codders, you are recommending these tabs again, have you actually fitted yours to your boat yet?

[Richard B]

Originally Posted by chorscroft

The boat is 8.5 metres long and I would guess weighs under 2.5 tonnes including engines (is this a reasonable estimate?)

You may be surprised how heavy that RIB is... with the inboards, the wrap-around console and covered foredeck it may tip the scales at well over 2.5 tonnes, but impossible to guess as it all depends on the lay-up (ie the thickness of the GRP).

Here's a hand grenade: http://www.rib.net/forum/showthread....ighlight=steyr

[Poly]

Originally Posted by Richard B

but impossible to guess

not really - the redbay site lists the hull weight (ex engines) as 1171 kg. I assume that is ex. the canopy etc too - as it seems to be a general wt for all the 8.4m hulls. The engines are roughly 500 kg in total. redbay list the max loading as 1077 kg, so I guess about 2.75 tonnes all in, fully laden.

[chorscroft]

The original owner of the boat just commisioned redbay to do the hull and he the. Fitted the engines himself. Those boats were uwully fitted with the slightly heavier 165hp yamahas. This is why the boat may not have been tweaked correctly.
Codprawn - where would I go to find out more about the chips; the engines are around 1998 would they still have this feature?

Regards

Conor

[chorscroft]

Jwalker
They don't know the boats speed because the broker mainly deals in yachts - will find out actual speed on monday.

Polwart
The exact same hull design has been fitted with twin 240hp and did over 50 knots - the hull is capable it's just about drawing every last inch of power out of it.
As a current redbay owner myself I know that we would often cruise at around 32 knots. Although the boat is almost surely capable of this it wouldn't be doing it at economical revs.

codprawn
Is there a compromise to be made like say 4th gear XD how rapidly does the loss in acceleration/gain in speed come with each inch in prop size changed?
I have quite alot of experience in ribs - will I be able to tell driving it if the bow sits too high and if trim tabs would help?
Do those tabs have the same effect as other ones? All the 11m boats have bennet ones is there any real advantage?

Richard B
Don't worry have read that thread and it hasn't significantly altered my view on steyrs - I will see how they go on the testdrive.

Thanks in Advance

Conor

[Andy Gee]

A set of tabs will help you get the bow down and at times will give you better speed as the boat is "running" at a better angle.
32-34 knots would be right for this boat with the engines fitted, my old 9.1 had twin 165hp Yamahas and did 35-36 knots flat out in perfect conditions, but more importantly could maintain a better rougher weather speed than anything else, so it is a bit of a trade of. I had around 70kg of chain in her anchor locker to help her head get down .
I have had 41.? knots from the 11m with very little fuel on board and flat weather and can get around 38 knots when full of fuel.
Andy

[Poly]

conor,

am I missing the point? you feel that this boat with these engines might be too much of a compromise to get the top speed you want. whilst redbays don't exactly grow on trees they are hardly hens teeth either. is this boat particularly cheap or something? otherwise if it doesn't tick your boxes walk away and wait for one that does.

I wouldn't be spending 40k+ on a boat, with the hope that I could rechip the engines to get it go faster unless I absolutely knew it could be done (i.e. steyr told me so). likewise if a 500 quid set of tabs would get an extra 20% top speed then why wouldn't the previous owner have put them on.

Not saying these things can't work - but if it doesn't perform how you want on the test sail then walk away unless you "can't afford not to".